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FAQs about Arrow Crabs 

Related Articles: Arrow Crabs, Crabs

Related FAQs: Marine Crabs 1, Marine Crabs 2Marine Crabs 3, Marine Crabs 4, Marine Crab Identification, Marine Crab Selection, Marine Crab Compatibility, Marine Crab Systems, Marine Crab Feeding, Marine Crab Reproduction, Marine Crab Disease, Micro-Crustaceans, Amphipods, Copepods, Mysids, Hermit Crabs, Shrimps, Cleaner Shrimps, Banded Coral Shrimp, Mantis Shrimp, Anemone Eating ShrimpCrustacean Identification, Crustacean Selection, Crustacean Behavior, Crustacean Compatibility, Crustacean Systems, Crustacean Feeding, Crustacean Disease, Crustacean Reproduction,

Maybe this Angler will eat your Arrow Crab... or vice versa.

Arrow Crab comp.  -- 04/03/10
Hello great and mighty WWM staff member!
<Howsit Tristan?>
I read through the FAQ's for arrow crabs, and as usual no matter what topic I read up on here, I end up more confused after reading through your site than I was beforehand. Your site has a lot of good stuff, but just too many conflictions or missing important details. :(
<Really? How would you improve the experience?>
I am setting up a 180gal tank that will be more geared more towards a predator tank really than anything else. I plan on having two Volitans lions, and the snails I have already (Nerites, Nassarius, dwarf and Florida Cerith), along with a brittle star or two later on as my cleanup crew once the lions get bigger. But, I want ONE big invert.
I was originally going to go with a reef lobster, but had some doubts and then looked into a arrow crab instead. From what I can tell, it seems that they can get big without actually getting big in mass, and they are aggressive and can defend themselves well which will come in handy against the lions. But will this combo work? (snails, lions, 1 arrow crab and a brittle star) Will the snails in general be okay as long as they don't get flipped over?
<As long as the Arrow Crab isn't "too" hungry... and doesn't get inhaled by a Lion>
Will the arrow crab harass and kill my brittle star?
<Not likely unless it's very hungry>
Are there any differences in behavior between a regular arrow crab and a decorator arrow crab?
<The latter tends to be more reclusive>
I want to get one large invert into the tank to keep things interesting but I seem to be having trouble picking one out. If the arrow crab isn't a decent choice, then what other large inverts could I look into instead?
<Perhaps a large/r hermit species...>
They must be large so that they won't be eaten by the lionfish too easily.
There will be a few frags worth of pop pom xenia as the only coral by the way. Thanks for your help.
<Welcome. Bob Fenner>

Arrow Crab Compatibility  11/2/2009
Hello - Are arrow crabs safe for reef tanks?
<Bill, the short answer is occasionally, but in most cases no. Without more information regarding your tank and its inhabitants it is impossible to say. In the mean time please read here:
http://www.wetwebmedia.com/arrowcrabfaqs.htm .
thank you bill
<Josh Solomon>

Arrowhead Crab Pulling Off Limbs 8/31/09
Hi There,
I have been reading through everything I can find on the topic, and have not yet been able to find anything concerning an arrowhead crab pulling off his own limbs. Tonight, as my whole family was sitting around the aquarium watching our fish eat dinner, our arrowhead decided to put on a show for us. One after another, he pulled off his legs and tossed them aside. He then managed to wrestle off an arm. Now he is hanging onto a fan coral just kind-of bouncing around in the current.
I captured him in the wild (I live in Haiti), and he's been doing fine for about a month now, but he might have just cracked and committed suicide tonight. Any ideas as to why an arrowhead would do this?
<Bizarre! Am only speculating... perhaps some sort of "ecdysis"... akin to some lizards discarding their tails, avoiding mortality due to predation?
Some sort of perceived water quality issue? Some behavior to replace these limbs via moulting? Bob Fenner>

Re: Arrowhead Crab Pulling Off Limbs 8/31/09
Alright, thanks. I think he got eaten overnight. My sea urchins were munching on the legs, it looked like, and my red legged hermit crabs might have finished off the body.
<Could be easily so>
I'm going to do a water change today just as a precaution. It's free for me since I live by the ocean ;-)
<Mmm, no; not free... A misperception... Your time is certainly worth something; the costs of transportation; the opportunity "cost"... i.e. what you might otherwise do with these resources. BobF>

Arrow crabs only keep dying ~ 01/12/09 Dear crew, <Franklin> I have a Oceanic BioCube 29 gal. It has one Sally Lightfoot, one Emerald Crab, one Coral Banded Shrimp, (evil reputation undeserved)one Peppermint Shrimp, several Blue Legged Hermit Crabs, several scarlets, and maybe five small unidentified hermits. Everybody gets along admirably. <Amazing.> My system has been up and running for a year. Successfully in large part to your site. My nitrate, nitrite, and ammonia, are all in acceptable range. My pH is 8.4, s.g. is 1.026,and my salinity is 35. I have 15 pounds of live rock, and 7 pounds of live sand. I have stock BioCube lighting, which I understand to be just slightly above bare minimum. I added a BioCube protein skimmer to the second chamber, which produces skimmate regularly. I change the filter pad once a week, and do a partial change every two weeks, around 10 percent. I do not use carbon. I add calcium and iodine supplements as prescribed by Reef Complete, and Kent Marine. Molting regularly, and well, has never been an issue. I also have Black Mexican Turbo Snails, Stomatellid snails, cowry snails, and several other hitchhiker snails that you have deemed beneficial. All doing excellent. Now on to the problem. Every time I add an Arrow Crab, or a large red hermit crab (white spotted,) they eat like expected for three or four days, molt successfully, stop eating, or moving, and then die. No sign of damage on them at all. I drip acclimate them over the course of hours, and tong feed them, and everyone else, a variety of foods regularly. My only guess is possibly these specific crabs are super sensitive to tap water where the others are not? <Not the case here.> I use Instant ocean salt mixed in a twenty gallon pail, turned by power heads for twenty four hours, and then stored in water jugs until needed. I then adjust temperature and add. everyone one else does great, just these two species always die. I tried changing supplier, same results. Any advice greatly appreciated. <Have you considered there may be a predator in your tank? The Sally Lightfoot is semi-aggressive and capable of killing/eating these crabs. My next question is your statement of "no sign of damage". Are you sure you are looking at the crab in question or the molt. Crabs are most vulnerable to predators right after a molt and I'm thinking the Sally Lightfoot is the culprit. If you decide to add another Arrow Crab, I would move the Sally Lightfoot to temporary quarters. Note that Arrow Crabs have been known to attack and eat Coral Banded Shrimp so consider this before buying. James (Salty Dog)>

Re: Arrow Crabs Only Keep Dying 1/13/09 James, thank you for your time, and your willingness to share accurate knowledge. <You're welcome.> In reply my sally is very small, and very lazy. It learned very quickly that I am willing to hand feed it as well as everyone else. Not a very hostile setup. It is definitely not the molt I see but the actual crab, they live for several days after molting, but just seem to fade away along with the red hermit (Clibanarius vittatus). Its frustrating, if everyone was suffering I'd say I'm definitely doing something fundamentally wrong, but only those two. I have tried with four arrows and three hermits. No longer wish to kill these interesting animals through ignorance, but cant seem to find any reason beyond awful luck. There are no unknowns in my tank, i.e.. mantis or murderous crabs. I understand both species to be very hardy, and considered beginner crustaceans. Any advice greatly appreciated. <I'm really at a loss here as to an accurate explanation for this, especially when it involves the same two animals that are easy to keep. Bob or other crew members may post their input here. James (Salty Dog)>

Stenorhynchus seticornis, comp.  4-3-08 Hi <Hello.>> My local fish store said an Arrow crab should be kept with other crabs or shrimp? <<It can be housed with other invertebrates but more than likely it shouldn't (read should not). This animal is definitely an opportunistic omnivore, smaller moving fish and invertebrates are at potential risk. In essence I'm saying you'd be doing no harm by keeping it alone.>> First I heard of it so wanted to check. <<First I have heard as well, and I disagree.>> Thanks <<Welcome -AJ>>

Anemone ID and Arrow crab predation query    01/13/2008 Thank you for this wonderful service and for devoting your time and effort! Tank parameters: *92 gal community reef (110lbsLR, 90lbs live sand, 4 small gobies, various softies, 2 green emerald crabs, 1 cleaner shrimp, 1 peppermint shrimp), *29 gal wet-dry sump/refugium with Caulerpa, mud, and 10lbs of LR (in line), *Lighting: 175MH, 4- 24W T5HO actinic, 12 LED night lights *Water chem: SG-1.024 , pH- 8.3 , nitrite- 0, ammonia- 0, nitrate- undetectable, calcium 410ppm, 78 degrees F I bought an anemone from my LFS yesterday and I am afraid I've walked into an unfortunate situation. I had been told by one of the workers that this anemone was a bulb-tip of some sort. So I put it on hold to research it for a few days. Upon return yesterday to purchase it, other workers told me it was a yellow Sebae anemone. I still bought it but now really wish I had gone back home to research instead. The column is a orange-red color fading to a yellow around the top with white/transparent tentacles. Sadly, from reading all the articles on here at WWM, I realize it's probably dyed and dying. Although, mine doesn't look like a Sebae to me...maybe because it's so far gone already?? I'd just like to know what it is so I can properly care for it / try to save it. <It looks like a bleached Long Tentacle anemone to me (hopefully Bob will correct me if I'm wrong).> On another note, I have a question about predation from arrow crabs: I am hoping to make a proper home for a mandarin goby in the next few months so I ordered a culture of copepods. I added them to the fuge but have seen nothing in over a week. I wrote the sender and they said to give it 3 weeks.... I'm doing that. In the meantime, I put my arrow crab (juvenile at this point) in the fuge to take care of a bristleworm population I have (he does have a much larger suitable tank to go to when he gets grows). <Hmmm, why do you want to "take care" of your bristle worm population? The vast majority of these worms are actually beneficial scavengers/detritivores.> Will the arrow crab, even if there are plenty of bristleworms to eat, harm the growing copepod population? <I suppose it's possible, if the crab is small and fast enough to catch them. But as the crab gets bigger, it will likely seek out larger prey (as big as small fish even). To be frank, they're not the best of live stock choices. Please see here: http://www.wetwebmedia.com/arrowcrabfaqs.htm> Thanks so much!!!! Erin <Best, Sara M.>

Re: Coral Banded Reef Shrimp (and Stenorhynchus) I have a 46 gallon tank, with a coral banded reef shrimp, arrow crab, three peppermint shrimp and three cleaner shrimp.  also have regal tang, two small clown fish and two small damsels.   All live in harmony, or so I thought until my Coral banded reef shrimp showed up one morning without any claws.  Is there any kind of explanation for this?  I find it truly strange that the "bully" in the tank got bullied by something else. <Not me... the most likely suspect is the Arrow Crab... as this animal gets larger, there will be other "missing parts", then whole organisms. Please see here: http://www.wetwebmedia.com/marine/inverts/arthropoda/crabs/swcrabs.htm and the Related FAQs (at top). Bob Fenner>

There Was Three... Now There Are None!  3/11/03 Hello all at the WWM,<Hey there Matt!  Phil on duty tonight!> for the last couple of weeks I've had 3 fish die in my tank. 2 Yellow Tailed Damsels and 1 Firefish. They all seem to die overnight and all are healthy right before they die. I don't know who's or what is killing them but I know I don't have any Mantis shrimp or any of that cuz I've already done fresh water dips on all my rocks and gotten all that stuff out. Plus I've already went through and inspected everything. But anyway last night my Firefish died and when I woke up I found him in the same place dead as the 2 yellow tailed damsels missing one of his eyes. This is the same spot the Arrowhead crab likes to hang out. I was wondering if the Arrowhead crab would be the thing killing my fish. I mean would be he goin after them and killing them while they rest on the bottom? I also have a Nimble Spray crab in my tank as well but he never seems to mess with nothing and actually acts like he's scared of the fish. So should I take the Arrowhead crab out? Cuz I only bought him so he would eat any small bristle worms and stuff.  Thanks for your help<I would really like to see some info on water quality...  I highly doubt the crab is the one killing the fish!  <<RMF doesn't>> I think the fish die and he brings them over to his area of the tank and "samples" them.  Please get back with me on the water levels, as I think this is the main problem.  Keep in touch! Phil>

Questions About Arrow Crabs... Not Compatible With Much! Hi. I have some questions about arrow crabs (Stenorhynchus seticornis) and even though I read through the FAQ and found out you guys are very much anti-crab I was wondering if you could help me out? I would like to know exactly what these guys are safe with. <Not much. http://www.wetwebmedia.com/arrowcrabfaqs.htm > How big do they get? <Big! Marine Inverts by Shimek states 6 inches but I have seen bigger. I just saw some big boys in the Bahamas and they were bold! Also saw a shockingly big one in a home aquarium back in September at a NJ reefers club meeting. Was in it's own 100+ gallon tank. I have a 75 gallon reef tank and a 10 gallon mini reef/frag tank. I love the look of arrow crabs and would like to get one for the 10 gallon tank. <Way, way, way too small for this creature. Shimek recommends a tank of at least 100 gallons.> From what I have read they're most likely not safe with feather dusters, shrimp, snails, and possibly fish correct? <Yup.> Are there any fish that could go in a 10 gallon that are fast enough to be safe with arrow crabs? Maybe a damsel? <If you're thinking cheap and easily replaceable.. but I wouldn't get too attached. These crabs can be very aggressive.> Are they safe with decorator or blue-leg hermit crabs? <Really... a decorator crab? They can get up to 12 inches across!> Any sea stars/mini brittle stars? <I think the arrow crab may think the mini brittle stars are spaghetti.> Macro algae? <Could be OK.> Corals (mushrooms, Ricordea, green star polyps, toadstools, zoanthids, Palythoas)? Will they munch on small hydroids? <Mmm, don't know, sorry.>  I'm sorry to ask so many questions. <It's OK.> I really like arrow crabs but I don't want to risk my beautiful mushroom collection or any of my other corals either. <I'm not sure what to tell you here, but I would not do this.> Thank you for your time. <You're welcome! Mich.>

Arrow crab eating Aiptasia? Shore    8/24/07 Hi all I'm a bit perplexed as to what happened to all of my Aiptasia. (Not that I'm complaining) I have had a saltwater tank for over a year now and have ALWAYS had these little pests! Thankfully they never grew to the plague proportions I've herd <Heeeee!> tell about. When I upgraded to a larger tank I thought that I had finally gotten rid of them but alas they followed me. I tried killing them with Aiptasia Control and it worked but they kept coming back (Probably in the filters) Anyway here's the thing. I added an arrow crab a few weeks ago and they are disappearing think that my arrow crab is eating them. <Oh yes> It's an adult wild caught specimen that came from Florida. I haven't actually seen the crab eating on the little pests but every morning when I turn on the lights I see fewer and fewer of them. The two or three Aiptasia that are still in the tank are missing "arms" Have arrow crabs ever been known to eat Aiptasia? <Certainly have. BobF>

Re: Worm that I cant identify and is this worm bad for my tank? Arrow Crab sel.  8/10/07 Thanks for the reply. <Welcome Ed!> I do have one last question. With the livestock that I have in my tank, how would an Arrow Crab do? <Well if your looking to get rid of most of you fish it would be a fine choice! http://www.wetwebmedia.com/stenorhynchus.htm I would not recommend.> I have read that this is a suitable predator for worms? <Well, is a predator for sure!> Also, what can go bad with a worm if it does keep growing? <While some are beneficial scavengers, others become predaceous on livestock.> Ed <Cheers, Mich>

Maroon Clowns or Gangsters?    5/4/06 Hi Gang! <Hello.> Thanks for running such a comprehensive an informative site! <Great.> I've got a 12g nanoreef. Set up and cycling went very well, water quality stabilized and I was finally ready for livestock. I added a pair of Maroon Clowns (one big, one small) a "bulb" anemone and an arrow crab. The anemone and the clowns quickly set up shop and the arrow crab lurked as they are want to do... <Mmm…I don't like the stocking choices, both inappropriate and overstocked.> All seemed well. After a few weeks I noticed the crab spending more and more time hanging out near the anemone. I watched and didn't notice him harassing the anemone at all, and he seemed happy eating the flora/fauna from the live rock and any left overs that the clowns didn't eat or stash in their anemone. My LFS guru told me that this was common in the wild for Arrows to hanging out near anemones... so I let it go. <Right….> Then one morning I found a couple of empty shells of crab leg on the bottom of the tank... I thought "hmmm... he must have molted." Looking through the rest of the tank, I found the crab tucked into a corner, badly beaten, broken legs and half of his body still attached to a chunk of shell that was on the other side of the tank. I'm a little dumbfounded, the most I could come up with is that the arrow started to molt, and the bull clown took the opportunity to nab the crab at it's weakest moment and 'teach it a lesson' to stay away from his anemone... Any thoughts or insights into what may have happened? <It is very clear what happened, mother maroon put out a hit on your crab…. a pair of maroons in the wild maintain a large territory, much larger than 12 gallons.  But just to cover all areas, what's your water quality like? I see no mention of it…. Adam J.>

Do arrow crabs molt?  3/18/06 Hi, do you know if arrow crabs molt? Obviously they molt from zoea to larvae etc, but is there a terminal molt for males/females or do females continue to molt throughout their life as with other species for example? Thanks! <All crabs molt throughout their lifetime. James (Salty Dog)>

Arrow crab   1/22/06 Hi, I just got an arrow crab in my 55 gal reef tank (he's fairly small, probably 3-3.5 inches flattened out), and so far he has been all right, he did eat a cleaner shrimp but I'm pretty sure that the AHC didn't kill it. Other than that he just scavenges around the tank, usually getting to the fish food before the coral beauty does! He is currently housed with a coral beauty (around 2.5 inches). There are no corals in the tank as of yet, but I was wondering if he would be OK with a false percula clown and a bubble-tipped anemone or will he even go after the coral beauty? I've heard that they can catch even healthy fish if the fish is small, but for a crab around the size that i have, how small is small?  Also, are there any inverts I can put into a 55 gal tank that would be able to clean up algae (none yet, yay!) that the AHC wouldn't harm?  Any info you could give me would be great! <Read FAQ's here on arrow crabs.  http://www.wetwebmedia.com/arrowcrabfaqs.htm> Thanks a bunch, <You're welcome.  In future queries please cap beginnings of sentences and all "I's".  Makes our editing job much easier.  James (Salty Dog)>

Arrow crab - 1/3/06 Hello and Happy New Year! <Happy New Year! John here with you this evening!> I have been reading all of your FAQs about arrow crabs and it has me worried. <Perhaps rightly so!> I have a 75 gallon tank that I am preparing to support corals.  I have one Mexican Turbo, seven scarlet hermits, two emerald crabs, and 16 Astrea snails.  Every one has ample room to roam and hide because i have 101 lbs of live rock in my tank.  I recently added an arrow crab because i notice a few bristle worms.  But now i am worried that the arrow crab will make a meal out of my other inverts rather than the bristle worms.  Should i bring the arrow crab back or will he leave his tank mates alone?  Thank you very much ! <He may leave them alone, but it is a gamble - he may damage other inverts, or, when larger, fish. Bristleworms are more often then not harmless (indeed, beneficial) - I would rather take the gamble with them than the arrow crab. Best regards, John>

Arrow crab question  11/21/05 I have a small 2 1/2g tank set up with live rock, Florida cr. coral substrate and Dictyota <Dictyota> with a small amount of Caulerpa... not sure which kind.  But anyway, I wanted to get a p. mantis shrimp just for interesting thing in here and a friend wanted to rid their tank of 2 arrow crabs so I took them. <This tank is too small...> I figured if nothing else they would be a tasty treat for my porc. puffer or Raccoon Butterfly in my main tank. But I like them and they are sooo odd looking. My question is will two survive in this tank being it is tiny? <No> Will they eat my macro algae? <No> I grow for my main tank's Foxface and do not want this eaten by the Arrows. That is what I question as well. Will two fight? <Yes> They have not, but....will they? I have noticed that they both have bright blue claws. Is this normal? They also have stripes on their heads/arrows unlike the photos on your site. I know their are many types. Thanks in advance. Sherry <Apply yourself Sherry... perhaps a library visit... Bob Fenner> 

Arrow Crab Question - Sabrina's go - 11/28/2005 <Sabrina here, in Bob's absence....> Apply myself? Perhaps a library visit? And exactly what did you mean by this? <Perhaps that some research was in order.... Even the ultimate size of the animal should've been some clue about tank size requirements....> I know the tank is tiny. It was not set up with animal life in mind. It was set up for plant life. I took these two crabs because they were going to be flushed by previous owners because LFS would not take them. <We're not mind readers; please understand we can't know what you don't tell us. Though being stuck in a closet is better than being dead, I would still urge you to re-home these animals.... urgently.... with someone else, a different LFS, or in a larger tank of your own - and separately.> They were hitchhikers on live rock of previous reef owner. I was not completely comfortable with the idea of putting them into my 130g and obviously a FOWLR due to the inhabitants I described, to fend for their lives; but which ever set up they go into I thought it much kinder than flushing them. <Again.... not psychic....> I read through your FAQs before writing and did not think my questions were too hard to answer compared to some of the ill informed questions ...like does a starfish need a heater? and you were rather 'un' rude to her. Perhaps I caught you on a off day. Clearly sorry for the intrusion. <Thank you for seeing to it the animals were not simply killed. Now, if possible, just try to find them adequate housing. Wishing you well, -Sabrina>

Arrow crab bucket 8/3/05 Hi... I was just reading a response about arrow crabs....one in which she spoke of her arrow crab having a hinged "bucket" that appeared to have eggs in it.   <Not unusual> Just for some input.... I have had my arrow for several months now and have seen it do the same, the first time I saw it doing this, it opened up its "bucket" and pulled out a little stick of some sort and started munching away at it.  On some days the bucket will be full...it went about a week where it could not shut it, it was completely full of something (it almost looked like little bits of brine shrimp).  It looks as though there are about 5 clear little shelves that hold "stuff" (for lack of better words :). My crab does this typically every day (at least once).  It does it before it molts, after it molts, at night, during the day.... I haven't seen it do it in the morning, but there doesn't seem to be any sort of connection I can make between the bucket and its natural doings. It is very odd. But  It is even more amusing....if any word is heard as to what this may be and what the purpose of it is, I will be looking forward to it. Have a good one :) <Is part of natural cleaning, and reproductive behavior... Bob Fenner>

Arrow Crab Spawn Hi folks, Martin here in the UK <cheers, mate :)> great site, it has been literally a life saver for me at times, well not  me personally but for some of my reef inhabitants. <thanks kindly... it is our hope/purpose indeed> Any way I wonder if you can shed any light on the happenings in my tank, last night my arrow crab opened up the bottom half of its abdomen and spewed out what seemed like thousands of tiny white pinheads with very short wriggling tails. <yep... you might research the "zoeae larval stage of crabs" for more info/insight> the fish ate most of them, the shrimps ate more but this morning there are still about 20 or so scattered around the aquarium clinging to the algae on the side walls. Some I know disappeared into the substrate as well. <a delightful occurrence! None will survive in the display proper, but you may choose to work with the breeders in a separate aquarium> the guy at my local fish shop says he has never heard of this happening before and a brief internet search revealed little about the habits of this crab other than to say don't keep it with fish, shrimps corals or anything it can eat. <the reproductive activity is rather common... rearing the larvae successfully is what's rare (hard work indeed)> I have had it in my reef tank for some time without incident. I guess you could say watch this space, but so far so good. Any idea as to what I can do with this spawn, can I rear it, does it have to be kept separate or under special conditions or should I just let my fish eat it. cheers martin <definitely must be done in a separate vessel... do look into purchasing the book on rearing Peppermint Shrimp (Lysmata) by April Kirkendall... quite similar techniques with rearing such Decapod crustaceans (although the Lysmata are hermaphrodites). I think Midland Reefs in the UK carries this titles. You can e-mail a chap named Tim Hayes at midlandreefs@inverts.demon.co.uk kind regards, Anthony>

Arrow crab killing shrimp? Very possibly I recently introduced a group of peppermint shrimp to my aquarium. They were fine for a while, but one of them died last night from what I assumed to be stress due to the fact that I am switching from a 30 gallon to a 90 gallon aquarium and had to move my existing 30 gallon to a different room to make space for the larger aquarium, which I am about to begin cycling.  I tried to keep the salinity level as close as possible, but it was a bit on the high end when I began because I hadn't topped off the water knowing that I would be relocating it so soon. I made no more than .001 change or less from 1.024 down to about 1.023. <Yikes... too much> I know shrimp are especially sensitive to changes, so when I found my peppermint shrimp on the bottom of the tank in the claws of my arrow crab I thought the crab just took advantage of the opportunity for a meal. The problem is, tonight I found a second dead peppermint shrimp that my crab was again feeding on. I have a tank full of bristle worms (which is why I got the arrow crab to begin with) and I feed my fish and inverts a variety of foods including Mysis shrimp, brine shrimp, emerald entree, squid, phytoplankton, zooplankton, enriched flakes, seaweed (the dry kind for my yellow tang and snails) and more-not at the same time of course. There should be no reason this crab is that hungry to have to go after his own dinner! <Perhaps they're "just tastier"> Do you think it is a coincidence that two shrimp just happened to die and the crab just happened to be there to consume them, or do you think this crab has developed a taste for peppermint shrimp. <Maybe... "third time/shrimp's the charm"> I am concerned about putting him into my new tank if he will go after my inverts! I have 4 more peppermints, 2 snails, about 16 small blue-legged hermit crabs, a flame scallop, a Condy anemone, a brittle star, a chocolate chip star (which I am also considering removing after it ate one of my Condys), an emerald crab (small-eats mostly seaweed and picks off live rock), a pencil urchin, a lettuce leaf sea slug, 4 conchs, a deep sea yellow gorgonian and a skunk cleaner shrimp. If the arrow crab is going to be a problem I'd rather take him out now than have to catch him in the 90 gallon tank later! <I would> What else can I use to control my worm population? <Posted on WWM... perhaps a beefy Dottyback...> The peppermint shrimp aren't interested in the worms at all! Any advice would be appreciated! Just to let you know-my fish population consists of 1 tomato clown, 1 blue devil, 1 yellow tang, 1 coral beauty and a scooter blenny. I hope to have my larger tank up and running within the next month or so, so fish suggestions would also be appreciated! Jen <Please read here: http://www.wetwebmedia.com/arrowcrabfaqs.htm re opportunistic omnivorous Stenorhynchus... and elsewhere on WWM re Polychaete control. Bob Fenner>

Green backed arrowhead crabs Hello! I was looking in my tank and noticed my arrowhead crab has green hair algae growing on his body....it is pretty long on pointed part of his head. I have also noticed that he is losing legs. Could he be sick and how can I help? <Doubt if the algae is a cause here, but your water quality, maybe nutrition are> Will he grow back his legs? <If the two above matters are corrected, yes> I also noticed that he hasn't molted in quite some time...is it normal for them to stop once they reach a certain size? <No... another good observation.> Thank you, Have an awesome day! Jessica Turner <Thanks. Do check for and correct your alkalinity, calcium... possibly magnesium and pH... And add a bit more food "with the shell" on it (shrimp, crab, shellfish...) that your Arrow crab will derive important molecules for its health, molting... and it should regenerate its legs in time. No worries re the algae. Bob Fenner>

Arrow Crabs and Bristle worms - part deux Hi Jim, I'm not sure how to measure or compare the size of the arrow crab. I guess if you were to put a half dollar under it, it would stand on it perfect? It's not that big. As for the Sixline wrasse, it was about 2 inches long. Sorry about calling the tang "Dory" I couldn't think of the name of the fish. How quick do the arrow crabs grow? My husband went to the LFS that we bought the crab from last night, they told him that he's got nothing to worry about and that he owns one and some gobies and never had a problem. Are there any other fish or anything that will eat the bristle worms and not harm anything else? As for my QT, it is just a 10 gallon with a accurate heater and an over flow filter. I don't have room for anything bigger. Every time I qt a fish, it will do fine for a few days and then die. I have no luck with fish. Also, what do I do if I had an ich outbreak in my main tank? The LFS sold my husband medicine called NO-ICH. It says it is safe with inverts. This stuff was quite expensive. If you could suggest anything I would greatly appreciate it.  Thank you, Kris >>>Hello again Kris, Call the fish what you want, just know some folks may now know what you mean. :) In the end, the Latin name is all that matters really. I forgot to tell you before, bristle worms are nothing to worry about! They are a natural and harmless part of the marine ecosystem. The consume detritus and uneaten bits of food - no need to try and eliminate them. Please do not put any medication in your main tank, I've heard too many horror stories to feel good recommending that to you. Also, these "reef safe" medications have a spotty record for effectiveness at best. Running hyposalinity (lowering the salinity to 1.009 over a week or so, and keeping it there for two weeks) in a hospital tank, or a more conventional ich medication in a hospital tank is your best bet. Do NOT use copper. Some people have had luck feeding garlic soaked foods in the display and getting rid of ich that way. I myself tried it some years ago, and it SEEMED to work, although I cannot be sure if the fish didn't just fight it off on their own. The best thing to do is just quarantine your fish properly, and you will not have to worry about C. irritans in your display. Myself, I use live rock in my QT tanks, and a bit of sand of the bottom. A small power head is used to circulate water, and of course there is a heater - that's it! They are pretty much another small reef tank, even with a few mushroom corals. The tank stays running all the time, and I've had EXCELLENT results with this method. I can't medicate in it, but I haven't had to medicate a fish in many years.  Jim<<<

Arrow Crabs and Bristle Worms.... Hi, I'm so sorry to bother you. I bought an arrow crab, a Sixline wrasse and a cleaner shrimp 4 days ago, and I have not seen the wrasse in 2 days. It was fine eating, but hiding . Now I can't find it. Could the arrow crab eat it? The arrow crab is small. I was told that the arrow crab and Sixline wrasse would help out with the bristle worms. After reading your facts on arrow crabs, I'm pretty ticked at my LFS, they're real good to us but I don't know why they sold me an arrow crab knowing what I have in my tank. Should I remove it? What else is good at removing bristle worms but not harming my inverts, corals, etc.? I just bought a little baby Dory tang, it's about an inch or so, will the arrow crab eat it? Another question, why do all the fish I buy and quarantine dye? The water is the same from my main tank, which all water conditions are good. I had an out break of ich in my main tank and quarantined my black clown and my orange clown. They were doing fine in my main tank but I wanted to treat them. Within 4 days, my black clown is dead. I have really bad luck with fish. Should I just save my money and stop buying them? I've had my tank for a year and I don't have not 1 original fish left. Please help! Kris >>>Hello Kris, Arrow crabs are carnivores, and therefore not completely safe for reef tanks with small fish. I can't say for sure what happened to your Sixline wrasse (did you look on the floor?) and I don't know the size relationship between the crab and fish, so I'm at a bit of a disadvantage here. However if large enough, or the fish small enough, it certainly COULD be the problem. Also, as much as I liked the Nemo film, I assume by "Dory" tang you mean a Hippo or Blue regal tang. "Dory" is not a commonly accepted common name for this critter. Anyway, they are a delicate fish, and a one inch individual certainly could fall victim to the crab you describe. Above all else, make sure you quarantine this fish for at least a month. As far as why your fish are dying in quarantine, without knowing anything about your setup, I can't begin to answer. It could be a problem with where your local store is resourcing their stock, or it could be your fault. Without further info however, I can't be of help there. Please feel free to drop me a line back with more info, and I'll be glad to guide you. :) Good luck! Jim<<<

Arrowhead crab Hi there, I purchased a small Arrowhead Crab which has now grown to bigger than a hand span. <Yikes> After reading all your questions/answers I was seriously considering getting rid but today I actually watched catch and eat a bristleworm which is why I brought him. He doesn't seem to be causing any damage to my tank and it's contents (although the B worms won't agree) however I have been told that the tip of his head is poisonous to both me and my fish. Is this true?? <Not as far as I've ever heard, read... I've handled MANY specimens... above and underwater... with no adverse effects. Bob Fenner> many thanks for your help Alison

Re: Arrow Crab Thanks for the info in regards to the Cleaner shrimp. I bought the arrow crab for the Bristleworms. I know that they also eat Featherdusters, but is there anything else I should be concerned about?  Craig <Just the overall crowding. Bob Fenner>

New to Saltwater + Coral How about an Arrow Crab with other species? Will kill off cleaner shrimp? Hermit crabs? >>>I'd steer clear of the arrow crabs. Jim

Arrow Crab Compatibility 10/03/04 Hello Crew, I was just wondering if I can add an arrow crab to my tank. The tank is a 30 gallon, with a Yellowtail damsel, Ocellaris Clownfish, and a Scooter Blenny. The dragonet eats vitamin enriched brine, Mysis and small pieces of krill. I also have some hermit crabs, snails, and an anemone crab.  No corals just yet but I'll be getting them soon. So will an arrow crab be compatible? Thanks for your time! <Arrow crabs, like most crabs are risky to snails and other smaller inverts.  Larger specimens can even be a risk to fish.  I would advise against it, but if you choose to try, do keep a careful eye on it. Best Regards.  AdamC.>

Crab Claw Hi, <Hi, MikeD here> My husband and I recently bought an arrow crab.<neat creatures and excellent scavengers> It has molted twice in the four weeks we have owned it, and its pinchers have increased four times the size of when we bought it, the rest of the legs look like they have only increased about twice the size. Is this normal? or are they swollen with some kind of illness I don't know about? <I strongly doubt that you have a crab illness, but you may have gotten a different type of arrow crab than the one commonly seen, and yes, there are many different kinds. You also may just have a male who is developing adult proportions.> Thank you, <You're very welcome> Alicia

Re: Arrowhead crab FAQ article hi,    I noticed this on your web site & may be able to  offer an insight into the crabs unusual behaviour  ................ Arrow Crab My fiancĂ© and I found quite a few fire-worms  in our salt-water tank during a recent cleaning.  After this, we  purchased an arrowhead crab to "thin-out the heard" of worms.  He  has been great, leaving the fish alone (aside from stealing Mysis and other  shrimp at feeding time) and earning his keep by eating the  worms.  More than once, we awoke to find him munching on a two inch  work.  He has grown very quickly (doubled in  size).  Yesterday, we noticed some unusual behavior.  He  appeared to open the bottom half of his shell like a hinge.  He  seemed to be sticking his claw in the opening and then eating what he  found.  He would do this on and off all day.  We  had never seen this before, despite hours of watching his antics. What would  he be doing, as I couldn't find anything on the net regarding this  hinged opening? Todd Aston <Interesting observation. Perhaps this  animal is "cleaning" itself... maybe easing a break for the purpose of ecdysis  (molting)... could this activity be part of reproduction? Bob Fenner> my arrowhead was displaying exactly the same actions as described  here, upon closer inspection I could clearly see tiny eggs contained within the  opening, the crab looked as though it was checking the eggs (perhaps removing  bad ones!) & then eating any removed ones ! That's how I found your site ! I was looking for reproduction  information on the arrowhead many thanks hope this helps mark <Thank you for this input, sharing. Bob Fenner>

Arrow Crab, headed for trouble? I've had trouble with my bristly worm population for a couple of months now. I've reduced fish food distribution amount, tried different traps (only caught a few) more water changes, and trying to keep the tank cleaner. <All good strategies.  Limiting food is the most important one, though.  This can be accomplished by limiting food and/or introducing other scavengers/detritivores.  FWIW, the worms pose no threat to living, healthy animals  (unless of course, you get a fist full of bristles!)> When I told my LFS this they advised me to buy an arrow crab. I told them that  I had a 29 gallon tank with lots of live rock, some coral, a few feather dusters, two snails, and one very territorial red clown fish (2 1/2"). They said that my clown and my other animals would be okay. But after reading about them, I am afraid that the crab will kill them all. <A valid, but perhaps exaggerated concern.  All crabs are problematic IMO and IME.  Your snails are certainly at risk as are the feather dusters.  Fish are probably pretty safe.  FWIW, some Pseudochromis sp. make excellent bristleworm predators and would probably pose less risk to your other inhabitants.> Not only do I feel stupid for trusting the LFS, but I am afraid I just bought my tank's doom.  Is there anything that I can do to not let the arrow crab eat my tank? Or should I take it out?  Justine <Nothing to feel stupid about!  Personally, I would evict the crab in favor of two things... aggressively trapping the bristle worms and introducing other detritivores like a sand sifting sea cucumber.  Predators will lower the population, but not fix the cause (excess food).  Removing the worms will remove the nutrients (unlike predators which will just cycle them back into the tank), and other detritivores will prevent them from coming back.  Try a baited piece/pair of old stockings spread out a bit on the bottom of the tank.  The worms will nestle into the folds of the material and can be easily removed.  Best Regards.  Adam>

Arrow crab, missing legs... a big Stenopus... Hey gang, just a quick question here! I Woke up this morning to find my arrow crab hiding in the corner...with only 3 legs remaining. This is a sad sight, even his pinchers have been eaten off, and his little body is just resting on the sand. Here is the question: Can the arrow crab regenerate his lost legs, or is there anything that can be done to help poor Mr. Crabs??? <Can, will regenerate legs if survives... might be best to move this animal... or the offendi> I am afraid that with only 3 legs, and no pinchers to defend himself, he will become a late-night snack...but for whom? I read on your website that my brittle star could be the culprit??? <Possible, but...> Never would have guessed that one. Also, we have a rather large coral banded shrimp (El Pincho) and an even bigger peppermint shrimp (Pepe'). Pepe seems to bother everything from the corals to the anemone, so he was my first suspect. <Mine as well> Any idea which of these might have developed an appetite for crab? On a side note: Had a cleaner shrimp for about a day, and haven't seen him (or any evidence of his body) since. Initially I thought he had found a great hiding spot, but after seeing Mr. Crabs, I am certain that our Cleaner shrimp came across a similar fate. Who is eating my invertebrate friends??? Thanks for the help! Jamie <Other invertebrate friends... I would move at least the larger CBS... if not both. Bob Fenner>

The Tank of Doom  >Hi Peeps!  >>Hello.  >My "Tank of doom" is slowly recovering now. However, I have another quick question:  >>Alright.  >Just before I began stripping the tank back, I lost a green Chromis. Then a few days later, I noticed that one of my cleaner shrimp was standing very still on the substrate with part of it's tail missing. It was so still, it could have been mistaken for dead. Which wouldn't have been a mistake. It was. Dead.  >>Oops.  >After referring to your WWM I have come to the conclusion that the likely culprit would be the Arrowhead crab that I introduced to help eradicate the Bristleworm problem.  >>That would be a very good guess, though it's not as common for them to get fish (unless they're big).  >The green Chromis was the clincher. In your opinion would the AHC also take down a cleaner shrimp?  >>Yes.  >Later, I found distributed around the tank the legs of the AHC, but no body. I thought maybe he had 'cast off' and was hiding.  >>Moltings don't normally go that way, they appear to be the actual carcass of the animal. This one sounds.. shredded.  >But no, he was gone. What possibly could have killed the AHC?  >>Mantis, pistol, predatory brittle star (which could also quite likely have taken the fish, too!).  >I have replaced the AHC with an even larger AHC (approx 8" span when flattened out), before I read your advice on their predatory nature. I also introduced a small cleaner shrimp the same day, who's lifespan could have been measured in hours!  >>Oops oops!  >I'm sure the AHC was eyeing up the shrimp on the journey home. I don't really want to remove it, so do you think feeding him a whole cockle a day would help contain his appetite?  >>I honestly couldn't say that would do it, and here's the thing. Let's say you did feed it very well, what would that do for your Bristleworm problem (which isn't as much of a problem in and of itself as some might believe, but quite the indicator of a more insidious underlying problem)? Well, you'd not only still have the Bristleworm problem, but on top of that you'd have the arrowhead crab problem. A well-fed crab at that. Who would become cranky with missed meals, and would prefer cockles to Bristleworms. See where I'm heading with this?>Thanks for your help, Deborah  >>You're welcome, Deborah. If you've still got plague proportions of Bristleworms, it means you've really got a problem with detritus (and the kind Head & Shoulders will help with). Take care of the root cause, and you won't be put in the situation of adding creatures with propensities to eat all they can catch. Then go looking for a brittle star, and when you find it, remove it. Marina

Mystery Fish Deaths--Arrow Crab Culprit? (3/2/04)   Hi, <Greeting. Steve Allen here.>   Previously I bought a small clownfish and a small regal tang, a month after that I purchased a Arrow crab. At first he just seemed timid and quite hiding away. Along with the arrow crab I have 2 cleaner shrimps. One morning I woke up and found my small clownfish dead, with one of its eyes gone; and the following morning I found my regal tang dead with a hole straight through where its eyes were positioned, something in my tank has a strange fetish for eyes. We have never experienced anything so horrible. <A sad occurrence indeed. :(> Please could you let us know whether you think it is the Arrow crab which always tries to grab the fish anyway or something of natural cause. I have heard that Arrow crabs eat small fish, is this true? <Yes, it is true. How big was your regal? Was the hole all the way through it? I'd wonder about a mantis shrimp in that case. Search on WWM for info. Do you have live rock in which one could have entered your tank? The arrow may be the culprit if these fish were only a couple of inches and it is on the large size. Then again, they may have died of something else. They eyes are often the first thing a scavenger goes after because they're soft and easily eaten. I'd suggest a check of your key water parameters.>   Thanks for your help, Aaron. <Sorry to hear of your loss. Hope this helps.>

Arrow Crab compatibility & anemone health 3/2/04 Greetings!  Once again I come to you seeking your seemingly infinite wisdom!<Semi-consciousness is more like it!> I feel like I am drowning in the sea of conflicting information and I am hoping you can help sort things out.  I have an arrow crab (Stenorhynchus seticornis) in my tank and I am wondering if it should remain there.  I have had no problems with it so far and the corner that he hangs out in (below a powerhead) is one of the cleanest in the tank.  He seems happy to snatch up some of the scraps drawn there that the fish miss and cleaning the algae that grows.  I  have read in some sources and told by the LFS that he should not harm any of the other tankmates as long as he has enough food--not a problem.  However, I have read from other sources that he will eat smaller fish and crabs if the opportunity arises.  ??????  How small is small? <The question is really how small and how slow.  All crabs are opportunistic omnivores at best, some are vicious predators.  Arrow crabs will prey on worms and other small animals, but are very likely to catch any but the smallest, slowest fish (very small gobies for example) or those that are sick.> Tank background:  55gal with 50#LR and about 15#Tufa base, live sand, overflow with skimmer at approx 150gph, 550gph powerhead, 170gph powerhead, whisper 3 filter, 220W PC lights (2-10,000K, 2-actinic).  I am building it up to be a primary fish tank with some coral accenting the fish (semi-reef you might call it).  Ammonia, nitrites, nitrates all zero, pH 8.3, SG 1.023 (slowly raising it to 1.024), dKH 10.  Using RO/DI water with 10% changes weekly. <All sounds good.> Current inhabitants:  3 Perculas, yellowtail damsel, yellow tang, coral beauty, Firefish goby and a small neon goby (about 1").  The tank is also complete with the cleanup crew of snails, hermits, serpent star, orange star (more decoration than cleanup) and a peppermint shrimp.  I also now have some anthelia polyps growing like mad (started with 3, now have almost 20 small polyps around them in last 10 days), and have added a green star polyp coral that seems to be doing fine.  I also have a sebae anemone that, unfortunately, arrived pure white (more on that in a sec).   <A 55gal will quickly become tight quarters for a yellow tang, and keep an eye on that star for any carnivorous activity.  You may want to feed a peanut size piece of meaty food to the star every couple of days to keep it sated.> So, will any of my inhabitants find their way to my arrow crabs plate?  The neon goby hangs out near the top of the tank on the other side (hangs out on the magnetic cleaner) and the damsel also is on the opposite side as well.  Crabs and snails go where they please.  Never seen the arrow on the other side of the tank. <The neon goby could be at risk if it wanders too close.  Snails may be at risk from both the arrow crab and hermits.  It is my policy to exclude ALL crabs from my reef tanks for their potentially destructive behavior and minimal benefit.> Now for the anemone.  Unfortunately, I learned about a day too late that they do not come in white.  It is still alive and I am attempting to nurse it back to health.  I ordered it from an online company that claims it is "the largest and most responsible supplier of aquatic life in the country" and who state that their "aquatic team takes extraordinary measures to ensure that all aquatic life receives the best possible care until they arrive safely to your home".  Well, it arrived pure white with the only color being some yellow splotches on the bottom of its base.  It has eaten a few small pieces of Formula one and has moved to a different spot and has been there for 2 days now.  It withdraws quickly when touched.  It also has been ejecting black stuff from its mouth that look like coffee grounds (poop?).   It has been 5 days now since I got it and no real change in condition.  Is there any hope for it?   Unfortunately being a medical student most of my time is spent reading about things like myasthenia gravis and periventricular leucomalacia rather than anemone health. <There are several anemones sold as "sebae anemones".  Some are more hardy than others.  My advice is to continue what you are doing.  Feed occasionally and allow it to roam until it finds a spot it likes.  It is very unlikely to recover, but it may.  The material it expelled is excrement.  Your best investment as someone on a budget (time and financial) is a couple of good books.  Bob Fenner's "The Conscientious Marine Aquarist", Anthony Calfo and Bob Fenner's "Natural Marine Aquarium" series (only vol 1 currently available.) and Delbeek and Sprung's "The Reef Aquarium" Vol 1&2 and Scott Michael's "Reef Fishes" pocket guide are all outstanding and will cost a fraction of what they will save you in livestock losses.> Anyway, my lesson has been learned....start my research at WWM! <This is an excellent place to start!  Please do make the most of the resource.>  Thanks again for all the help.  Hope to hear from you soon! -Ray <Glad to help!  Adam>

Arrow Crab Hi Bob, Thank you for your previous advice, and your current work to help us becoming a "conscientious marine aquarist" (loved your book!). I am currently in the process of ordering your new book---although an on-line form would be more convenient :-). <Thank you times three... Agreed... wish we did have the capability... but then, it would take away the fun and good I can do answering queries, maintaining and building the site... maybe some day soon, or better, hopeful that the pet-fish publishing world (it is small!) will "wake up, smell the java", and get more of my works (and several friends) into print, distribution, sales... In the meanwhile, nothing really wrong with snail-mail... Will get you the book in, um, maybe a week total transit time from you sending order to receiving it.> I wanted to have your expert opinion on the arrow crab (Stenorhynchus sp.). I just completed the initial phase (cycling) for a 90 gal. reef system. I envisioned to have mushrooms, star polyps, soft corals, cleaner shrimps, scarlet hermit crabs, and a few fish (orchid Dottyback, flame angel, and yellow or Kole tang). My questions are as follows: 1) Is it safe to add an arrow crab in my system I envisioned? <Maybe... depends on what you're going to put in with it... and thanks for prompting me... have my "crab" slides and reference works, old articles, on the tables here... re-did the shrimp, cleaner shrimp, fw shrimp... and lobster sections of WWM the last few days... and am on to the crabs today....!> 2) Does the arrow crab have a history of surviving/thrive in a captive environment? <Yes, but...> 3) What is a typical diet for an arrow crab? <Most anything it can get its claws on... which is considerable as this animal gets bigger. Have seen them eat Butterflyfishes in captivity...> 4) Where can I find more information regarding the arrow crab? <Take a look at the www.WetWebMedia.com site in a couple of days... for the references listed there... I'll be looking through the bibliographic tools at my disposal (online), and be adding the "pet-fish" references there as well> As always, any information will be greatly appreciated. Thanks, Dan <Chat with you soon my friend. Bob Fenner>

Can an arrow crab live without claws? Hello Mr. Fenner! First of all, thanks for offering your services! (I really appreciate your expertise.) <Glad to be here, help> I bought a large arrow crab a few months ago and it only had one claw. (Since I have a set-up that makes it easy for him/her to obtain food, I thought it wouldn't be a problem.) <Likely not> However, last night, he lost his other claw. I couldn't find it anywhere and was wondering if you could tell me if he'll a.) be able to survive? b.) How it might have happened? <b. a tussle of some sort, a. likely yes> I have a 60-gallon hex. with live rock and all I have are 5 small green Chromis and 1 yellow damsel. I also have 4 hermits and some Astreas. This past weekend, I purchased a Sally Lightfoot as it was recommended to me as a good and harmless algae grazer. Might it have been that? (Could a Sally Lightfoot take on a large one-clawed arrow crab? Would it?) <Yes...> Thanks for any insight you could provide me with. <Next molt your Stenorhynchus will likely have something in the way of claws back... and then next... a bit more> PS - Two weeks ago, I noticed one of his legs on the tank bottom. (I suspect he was molting since he still had 6 legs.) <Sounds like a miniature crustacean version of "Clash of the Titans"... I would separate these two if you have the facilities> Best, Declan Procaccini <Be chatting, Bob Fenner>

Arrow crabs Mr. Fenner I placed an arrow crab in my aquarium in hopes that it would rid my live rock of bristle worms. but recently, if been reading that these crabs could attack fish or even other crustaceans or invertebrates. is this possible? <Yes, unfortunately... for their size these oddities can/do become predaceous toward fishes, other invertebrates> the crab we have doesn't seem to bother anyone. should I remove it, or just keep a cautious eye on it. <The latter IMO. You will likely see signs (scraped fishes, stalking behavior) before outright missing livestock. Bob Fenner> thank you

Arrow crab, good lord! I'm in a state of confusion and disbelief. I recently got an arrow crab to ensure that I didn't have any bristle worms and I'm pretty sure that I now regret the purchase. I used to have quite a plump little lion on my last tank that died from a nasty fight with pseudomonas and I had two peppermint shrimp in there for him to much on at will, also to give it something to do, hunting and all. anyways they made it into the second generation tank. only one survived and I've had it for quite some time. I saw it last night in full health (about two inches, a "big un" by ornamental peppermint sizes) and now my arrow crab has its maws on its exoskeleton. I haven't seen it for about a day now... perhaps its molted, that would be fine. but is there a chance that the arrow crab nailed it? my arrow is insanely aggressive towards anything (I've got 8 blue fin damsels, I'm a bit nervous there too. always do my morning count...). can a 4in leg span arrow tackle a 2in peppermint? <Absolutely... and it will eventually eat those Damsels as well...> Jon Trowbridge I'm pretty sure it molted, but I'm always nervous about the arrow. kinda wish I never got it. <You will be more sure about that sentiment soon... see the section on "Marine Crabs", Stenorhynchus on the WWM site... Bob Fenner>

-Decorator arrow crab- Hello I have an interested in getting a Decorator Arrow Crab <Hmmm, I'll assume Camposcia retusa?>, however I have heard they can be troublesome to other animals. <Mainly with sessile inverts that easily become body ornaments.> I have the following in my fish only tanks. 1 Blue Moon Angel 1 Yellow Tang 1 Starcki damsel 4 young Clownfish 2 Emerald crabs 4 Urchins 2 Serpent Stars 1 Orange Starfish about 15 Small Hermits About 20 snails 2 Blood Red Shrimp My tank is a 210 Gallon with 180 lbs of rock. Do you see any problem adding this Crab? <These guys are opportunistic carnivores, but shouldn't seek out and kill fellow tankmates. If what you are referring to is Stenorhynchus seticornis, then there is an added threat to tube worms and it can pick on just about anything. This one is the true arrow crab, but not a decorator. I hope this helps! -Kevin> Thank you for your time. Kurt

Arrow crab hi all from Roanoke, VA- we just had a 4.5 earthquake here! anyway, I just saw this on your site, and wanted to report I saw the same thing with my arrow crab- but I have no idea what it means-mike <Thank you for sending your account along. Bob Fenner, in what he thought was earthquake country in S. California!> Arrow Crab

Arrow Crab My fiancĂ©© and I found quite a few fire-worms in our salt-water tank during a recent cleaning.  After this, we purchased an arrowhead crab to "thin-out the heard" of worms.  He has been great, leaving the fish alone (aside from stealing Mysis and other shrimp at feeding time) and earning his keep by eating the worms.  More than once, we awoke to find him munching on a two inch work.  He has grown very quickly (doubled in size).  Yesterday, we noticed some unusual behavior.  He appeared to open the bottom half of his shell like a hinge.  He seemed to be sticking his claw in the opening and then eating what he found.  He would do this on and off all day.  We had never seen this before, despite hours of watching his antics. What would he be doing, as I couldn't find anything on the net regarding this hinged opening? Todd Aston <Interesting observation. Perhaps this animal is "cleaning" itself... maybe easing a break for the purpose of ecdysis (molting)... could this activity be part of reproduction? Bob Fenner>

-Arrow crabs: Good or bad?- Dear WWM Arrow-Crab guru: <(Removes anemone guru hat in favor of arrow crab guru fez) Kevin here!> I've heard more conflicting advice about these marine spiders than just about any other invert out there. <K, I'll set you straight> The advice ranges from 'completely harmless and very beneficial' to 'devil-spawn that will kill everything in your tank.' <I don't see how they're of much benefit besides aesthetics. They generally do more harm than good.> Searching your site for FAQs also leaves me confused (very easy to do), and several books I have also give conflicting advice. Just looking at the creature at my LFS would lead one to believe they are dangerous and malevolent, but my Aunt Martha looked the same way and she was relatively harmless. <I just forwarded this email to her... ;) > Besides being rumored to munch on feather duster worms ... <Which they do, with much gusto> have any of you actually observed an Arrow Crab in action in a reef tank for any length of time? <Sure have. I have seen then take out green Chromis (as in grab a healthy one out of the water column!), yank hermits from their homes, and other assorted nasty stuff. I find them to be of little benefit besides the scavenging that they do. Just like any other crab, they're opportunistic feeders and are pretty well equipped to take out small critters. And to top it off, Bob's blurb from a crab article: 'Stenorhynchus seticornis (Herbst 1788), the Caribbean Arrow Crab. Not to be trusted with small to medium fish tank-mates (may spear with rostrum, otherwise consume). Safe with hardy native corals and anemones, larger fishes.' Hope this helps! -Kevin> Any advice would be 'swell.' Thanks, SLC

Arrow Crab Spawn Hi folks, Martin here in the UK <cheers, mate :)> great site, it has been literally a life saver for me at times, well not  me personally but for some of my reef inhabitants. <thanks kindly... it is our hope/purpose indeed> Any way I wonder if you can shed any light on the happenings in my tank, last night my arrow crab opened up the bottom half of its abdomen and spewed out what seemed like thousands of tiny white pinheads with very short wriggling tails. <yep... you might research the "zoeae larval stage of crabs" for more info/insight> the fish ate most of them, the shrimps ate more but this morning there are still about 20 or so scattered around the aquarium clinging to the algae on the side walls. Some I know disappeared into the substrate as well. <a delightful occurrence! None will survive in the display proper, but you may choose to work with the breeders in a separate aquarium> the guy at my local fish shop says he has never heard of this happening before and a brief internet search revealed little about the habits of this crab other than to say don't keep it with fish, shrimps corals or anything it can eat. <the reproductive activity is rather common... rearing the larvae successfully is what's rare (hard work indeed)> I have had it in my reef tank for some time without incident. I guess you could say watch this space, but so far so good. Any idea as to what I can do with this spawn, can I rear it, does it have to be kept separate or under special conditions or should I just let my fish eat it. cheers martin <definitely must be done in a separate vessel... do look into purchasing the book on rearing Peppermint Shrimp (Lysmata) by April Kirkendall... quite similar techniques with rearing such Decapod crustaceans (although the Lysmata are hermaphrodites). I think Midland Reefs in the UK carries this titles. You can e-mail a chap named Tim Hayes at midlandreefs@inverts.demon.co.uk  kind regards, Anthony>

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