Mystery Crab (actually an Anomuran) – Likely Porcelain Crab – 8/16/09
Hi.
<Hi Sarah, Lynn here today.>
I hope you can help me identify this little guy.
<Me too!>
I've had my tank up and running a year and a half now. The last time I bought any new critters for my tank was about 3 months ago.
<Okay>
This morning, this little guy showed up.
<Neat! It looks like a little porcelain crab, family Porcellanidae, of which there are many genera and species. They’re primarily filter feeders that rhythmically wave a pair of interesting feather/fan-like mouth parts through the water in order to gather organics, plankton, etc. Although these are referred to as crabs, they’re not actually “true” crabs (Brachyurans). They are instead Anomurans, a group that includes hermits, mole crabs, squat lobsters, etc. Both Brachyurans and Anomurans are decapod (ten-footed) crustaceans, however, crabs such as yours have 3 pairs of walking legs instead of 4. A much smaller fourth pair is folded and held either above or below the abdomen (not used for walking). They also have long antennae just distal to/behind the eyes, a mobile abdomen that can be used to propel the crab, and large claws that are used for territorial disputes and defense instead of feeding/predation. These crabs, although primarily filter feeders, are also opportunistic scavengers and detritivores. Luckily, they’re generally small and don’t pose anywhere near the risk that true crabs do (such as Xanthids – family Xanthidae). I would keep this little fellow around and enjoy! As a scavenger, it’ll likely take most foods – pellet, flake, bits of meaty foods (of marine origin). For more information, please see the following links: http://www.wetwebmedia.com/galatheids.htm
Similar species, listed under “A new porcellanid species from the Caribbean Sea”: http://www.uni-giessen.de/porcellanidae/ >
Thanks again,
<It was a pleasure.>
Sarah
<Take care, LynnZ>

Crab ID: Possible Xanthid or Pilumnid, Brachyuran Identification Issues – 7/20/09
Hello Incredibly Helpful WetWeb Crew,
<Hello incredibly nice Lianne, Lynn here today.>
If possible, is there someone who might identify this crab for me? (I checked your ID pages first.)
<I sure wish I could but for me to even begin to narrow things down, I’d need some additional photos such as close-up shots of the claws (showing the tips, and inside edges), a good shot from above showing the carapace detail and all legs, as well as a shot of the underneath/ventral surface. Unfortunately, even with excellent photos, we’d be doing well to narrow it to the family level. Identifying Brachyurans/true crabs can be quite a challenge. According to an excellent study last year (See this link: http://rmbr.nus.edu.sg/rbz/biblio/s17/s17rbz.pdf ), there are 38 superfamilies, 93 families, 1271 genera and subgenera, and a whopping 6,793 valid species! That’s a lot of crabs! Some are easy enough to narrow to family or genus because they have such obvious and unique features, but unfortunately there are way too many crabs left out there that look similar enough in available photos to be easily misidentified. If it were only as easy as “hairy crab = X” or “spatulate/spoon-tipped claws = Y”! Wouldn’t that be great? As it is, the most dependable way of getting a definite crab ID is to have the fellow right there in front of you, along with a really good identification key or better yet, preserved specimens of similar crabs. That way you're able see all the pertinent little details that separate one family, one genus, one species from another – and believe me, those details can be small. Barring that, we have to depend on photos and unfortunately not all species are available online or (at least within my) reference books. In a perfect world, we’d have online sites with up-to-date lists, dichotomous keys, and detailed photos of all the indigenous crabs from each ecosystem/locale around the world. I think we’re headed that way, but for right now, it’s just not out there.
By the way, please don’t construe what I wrote above as some sort of dressing-down because you asked for a crab ID. Believe me, that's not my intention at all. I love seeing all the neat little crabs that people find in their tanks and enjoy the challenge of ID work. The fact is, we get crab related ID queries fairly regularly and it’s frustrating to only be able to give general, instead of specific, information on most. I just wish I were an expert and could offer swift/solid ID’s, along with species summaries! Anyway, enough of that - let’s get back to your crab! From what I see in the photo, I’m guessing that it’s likely either some sort of Xanthid (Family: Xanthidae) or a Pilumnid (Family Pilumnidae). I’ve seen a number of Xanthids that have one or both claws spatulate/spoon-tipped (genera: Leptodius, Phymodius/Cyclodius, Chlorodiella, Etisus, Pilodius, etc). Unfortunately though, they tend to have hairy legs but not hairy carapaces. Pilumnus spp. crabs (Family: Pilumnidae) are notoriously “hairy” crabs (all over), but I’ve never seen one with spoon/spatulate claw tips. Also, they tend to have deeper bodies than what I’m seeing in your photo. That doesn’t necessarily rule this family out it’s just that I couldn’t find any like yours. Please see the links below for examples:
Leptodius : http://www.wildsingapore.com/wildfacts/crustacea/crab/xanthidae/leptodius.htm
Cyclodius: http://decapoda.free.fr/illustration.php?n=2&sp=628
Pilumnus hirtellus: http://www.klissurov.dir.bg/black_sea/crustaceans/big/PilumnusHirtellus04.jpg
Pilumnus spinosissimus: http://www.scielo.br/img/revistas/bn/v5n2/en_a03fig44.jpg >
He is dead, as you can see here,
<Well, based on the fact that the eyes look empty behind a thin shell/covering, I’m wondering if that isn’t just a shed from a recent molt? If you still have the “body”, flip it over and take a look underneath. Is the abdomen opened up? If so, can you see any of the crab's “innards” or is the shell empty? If it's empty, then the crab has recently molted and will be in hiding until his shell hardens.>
..but I still have a tiny one (1/3") in residence, who also hitchhiked into my reef tank on Vanuatu live rock. His carapace measures 5/8" across and his total width is about 1.25." His pincer claws are the forceps style of a Mithrax forceps crab, with only the tip of the pincer flattened, but he is hairy on all but the pincer itself and looks quite different to my eye.
<Heee! Don’t get me started again! The good news is that in lieu of us knowing which species you have, we can at least derive some information from the claws. It’s one of those “form follows function” issues. Flattened or spoon-like tips are excellent for scraping algae, etc., off hard surfaces, grasping filamentous algae, and for scooping up soft materials. These crabs tend to be a bit better risk in reef tanks, but they’re still omnivores and opportunists. They’re generally pretty well-behaved when small, but can be a problem later on as their bodies and appetites grow.>
I observed him shyly clipping and consuming the algae on my live rock over several months.
<Yep, that fits.>
I attempted to get him interested in meaty stuff to be able to trap him, but no go. Then found him dead this morning.
<He may be in hiding. If it was just a molt, he should be back out in a couple of days.>
His body appears to be completely unscathed, but perhaps he duked it out with my newly-out-of-quarantine Leopard Wrasse?
<Probably not. Any attack resulting in death would have been pretty obvious - dismemberment and/or nipped/missing appendages and parts(like the eyes)>
(The only other tank occupants: two young percula clowns, a young yellow-tail Damsel, two cleaner, two ghost, and one fire shrimp).
<Doubt any of those could have done it – unless the crab was in a molt and exposed/not hidden. If that had happened, there would have been obvious damage to the body as mentioned above. I’m thinking that unless you can actually see tissue inside that shell, there’s a good possibility that what you have is just a shed.>
Thank you for your thoughts
<You're welcome! Bet you weren’t thinking you’d get quite so many of my thoughts though, huh?>
..and your phenomenally helpful web column.
<On behalf of Bob and the crew past and present, you’re very welcome!>
Lianne
<Take care, LynnZ>
PS. He has 4 legs on each side in addition to his pincers.
<Thanks!>