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Sea Cucumbers, Class Holothuroidea: With body shapes ranging from spherical to long and worm-like, bizarre rings of tentacles circling a non-descript head-end, these slow-moving, drab to brightly colored and marked invertebrates are well-known at least by sight, by most aquarists.
Unfortunately they have a dark side. Like many other spiny-skinned animals, Sea Cucumbers should only be tried in captivity with knowledge, trepidation and utmost vigil. The reasons for my cautioning are offered here, as well as notes on general selection and care for the still curious.
Sea Cucumbers make up the Class Holothuroidea of the "Spiny-skinned-animal" phylum Echinodermata. Other living Classes comprise the familiar Sea Urchins, Sand Dollars, Sea- and Brittle Stars and the Crinoids, aka Sea Lilies and Feather Stars. Holothuroids are the odd-Class out in being secondarily non-radial appearing; often looking like strange ornamental sausages, some translucent, others opaque and warty. Cucumber-like!
There are some 900 described species, almost exclusively marine, distributed worldwide. Sea Cucumbers are a major component of the deep sea fauna. Most are black, brown or olive in color, but many brilliant colored and patterned species are encountered. They range in size to barely over an inch (@ 3 cm.) end to end to over a meter in length.
Genus Actinopyga: "Toothed Sea Cucumbers", named for the series of five teeth ringing their anus.
Some of the species used in marine aquariums.
Six living Orders distinguished on the basis of tentacular, podia and body wall characteristics.
Selection: General to Specific:
The body wall of holothuroids is made up of a non-ciliated epidermis covered by a variable thick/thin cuticle. The underlying dermis consists of a thick/thin layer of connective tissue surrounding microscopic ossicles ("little bones") beneath which there is a layer of circular muscles over five bands of longitudinal fibers. Hence the phylal designation of being pentaramous ("5" branching).
Check prospective purchases carefully for discontinuities; blemishes, tears and holes/sores in the body wall. They should be entire, with no broken cuticular or oddly colored areas.
Many species are known to tenuously "stick" to objects, tank walls, rock, you! Remove carefully with as little disturbance to the specimen as possible.
Don't discredit a specimen if it's tentacles are partially to completely retracted &/or it does not seem to be interested in feeding. This happens with healthy Cucumbers.
High and consistent water quality is a must.
Many species live in fine sand & mud. You must study diligently to determine the life-habits of the species in question. There are a few species that live on algae or exposed on the surface, like our Southern California Stichopus.
Made possible by the arrangement of longitudinal and radial muscle bands, holothuroids are masters at squeezing in and through small openings. If yours seems stuck, do not attempt pulling or breaking it out. The animal will come out when it wants.
An important re-warning concerning heaters and water intakes; mask or remote these in a fashion that makes them inaccessible to these sluggish creatures. You do not want to disturb them.
As mentioned above, keep water quality optimized and steady. Intertidal species are more tolerant of range in specific gravity, temperature, et al. Surprisingly intolerant of fouled water for messy eaters/defecators, sea cucumbers need clean water. As mentioned above, keep water quality optimized and steady. Intertidal species are more tolerant of range in specific gravity, temperature, et al. Surprisingly intolerant of fouled water for messy eaters/defecators, sea cucumbers need clean water.
Should consist of vigorous, complete circulation and copious chemical filtering capacity. I strongly suggest two-plus turns of the system's water every hour. Either a decent wet-dry filter or combination internal and canister filter, with (of a certainty) a protein skimmer, should be employed.
Is not an easy manner. All types of behaviors are offered by different species and individuals in this group. Don't get your heart settled on seeing Sea Cucumbers very much or very often. They are generally shy and retiring, if not dissolving!
Can be a problem. Some species do not get along with conspecifics. Lord help you and your other specimens should you have a critter that bugs you Sea Cucumber. This is trouble-city.
A major area/occasion for problems/disasters with the group. Under the "wrong" conditions/circumstances like rough handling, objectionable water quality, pheromonal cue-ing, pushy tankmates, pump/siphon intakes, sea cucumbers are known to eviscerate. Hey, one of my favorite terms... basically cast out their respiratory trees, gonads, gastro-intestinal and it's contents, most everything but the kitchen sink through their mouth and anus. This can be a very big bummer as they say in the sciences, with consequent poisoning/pollution/death & destruction to all other desirable tank-system-mates. Australasian "sea-apples" species are particularly notorious for this hobby-ending activity.
Some other species have a further refinement to this anti-predation mechanism, using what are called Cuvierian Organs (shades of my favorite French paleozoologist, Georges Cuvier!). These sticky fibers being discharged out their ends onto wannabe predators.
What To Do? #1, avoid these animals altogether; #2, if you can't/won't do #1, carefully (A) choose, (B) introduce new specimens and place in (C) suitable surroundings. #1, avoid these animals altogether; #2, if you can't/won't do #1, carefully (A) choose, (B) introduce new specimens and place in (C) suitable surroundings.
Do not mix or introduce the transport water into the home system! Mix the system water with the transport at your own risk and underwater slip the holothuroid into another container and move that into the new area. Mix the system water with the transport at your own risk and underwater slip the holothuroid into another container and move that into the new area.
Most sea cucumbers are filter feeders, however they will eat larger foodstuffs, like sessile fellow tank members. Only the very hungriest and naive fish will (re-)try chomping on a Sea Cucumber.
With members of Pearlfishes, Carapidae (Fierasferidae) living in and munching on their respiratory trees by day, emerging to feed at night, are known.
Most sea cucumbers are separate sexes (dioecious= "two houses") and contain a single gonad, unlike all other classes of echinoderms which have pairs. Most discharge sex cells into the water given environmental cues and stimulation from other's release. Some are brooding (that's not pouting) types that catch their eggs with their tentacles and transfer them from there to the sole (lower part of the body) or pockets in their dorsal surface for incubation. At least one species has coelomic development with the young rupturing through the mother's body wall (ouch!).
Skip the next paragraph unless you live for invertebrate developmental biology minutiae. Okay, I warned you.
Embryogeny is like asteroids (Sea Stars) up through gastrulation and typically on the third day larvae develop into an auricularia (similar to asteroids' bipinnaria, of course!) Next the larvae metamorphose into a barrel-shaped dololaria with five flagellated girdles. These planktonic larvae develop tentacles prior to functioning podia ("feet") and then settle to the bottom. Whew!
Oh, one more thing to worry about. Sea Cucumber eggs may be ingested by your fishes with fatal consequences. See Hopmann, 1977.
Sea cucumbers are noted for being capable of varying degrees of regeneration. If cut into numerous sections at least the terminal piece containing the cloaca grows back. Of course, evisceration is subsequently followed up with replacement of all blown out tissues.
Though they may not seem very active, a lot of what Sea Cucumbers do goes on inside them. They have highly developed hemal systems (like hem- in hemoglobin, yep, blood), much like other spiny-skinned animals' water-vascular systems that operate their tube feet. In Sea Cucumbers the hemal system serves primarily to distribute food materials.
In most Sea Cucumbers gas exchange is done in paired respiratory trees, a system of tubules located in the coelom ("sea-loam"= body cavity), one on each side of the digestive tract. They merge and by way of a common trunk pass into a cloaca which acts as a pump, filling and draining the respiratory trees. Most ammonia exits the body through these trees and circulation/ventilation.
Is basically like other echinoderms except the madreporite (canal pore) opens into the coelom instead of out into the environment, and therefore pumps coelomic fluid and not water.
The circumoral (around the mouth) nerve ring lies near the base of the tentacles and supplies nerves to the tentacles calcareous-ringed pharynx. Sensory cells are located in the epidermis and are most abundant at the ends of the animal.
Podia, "feet", as in podiatrist, are generally to entirely absent, occasionally scattered randomly over the body &/or arranged in rows. The ten to thirty modified podia making up the feeding tentacles surround the mouth. Sea Cucumbers live in or on the substrate, burrow into sand & mud, "swim" or float above the bottom, or hang out in the local flora; they're everywhere in marine environments. Forms with podia creep along like Sea Stars. Burrowing forms lack podia and burrow through alternate contractions of longitudinal and circular muscles like earthworms.
The ultra-strange deep water Order Elaspodida "walk" on extended tube feet and constrictions of the body. There are even swimming Sea Cucumbers! (sheesh).
The mouth of Sea Cucumbers is surrounded by ten to thirty mucusy tentacles. These are retractile and are used to wipe, swipe, sieve and glom onto food "stuff" pretty non-selectively, and periodically wiped across and into the oral cavity. As such, sea cucumbers are categorized as deposit or suspension feeders.
In captivity they accept most any fine foods. This can be tricky depending on the species' feeding habits. You should get and use timers for cycling off your filter and circulation pumps during regular feeding bouts, or otherwise engineer suitable food formats.
Solid wastes including some times large amounts of substrate in some species are termed casts. These should be periodically vacuumed from the system and tossed.
None that I found in my search. Another mention of conditions that seem to trigger evisceration; overcrowding, foul water, presence of certain chemicals. Throwing up one's guts even appears to be a normal seasonal phenomena in some species (no thanks).
Other Biology of Interest:
Maybe a note re human consumption. Trepang is an addition to soup I've had in Bangkok. This is a preparation of the (penta-) radial musculature of sea cucumbers that have been boiled (causing evisceration of the internal organs) and dried (Yum!).
Evisceration & regeneration has been mentioned enough. Here I want to especially emphasize the presence of an oft-alluded to toxic substance holothurin (a saponin to you chemistry freaks). This compound is thrown out with the genera noted for having and using Cuvierian Tubules (e.g. Holothuria & Actinopyga), but it is also found in the body of some (possibly all) species. Maybe a reason few living things tend to bug them, eh?
As a hint as to how the dissolution of a specimen might affect your system: It is a long known practice of South Pacific islanders to use the macerated (mashed up) bodies of certain Sea Cucumbers for catch tide-pool fish.
Many species are known to live 5-10 years.
Because of their beautiful color and patterns & "alien body shapes", alas, their will be a few hapless fools who hope to maintain these animals in captivity. If you do, either dedicate a special Cucumber-intensive-set-up, off-line with any central filtration system, or give them carte blanche for mal-affecting their environment; and when all heck breaks loose and you're F.O.L. (=Fish Out of Luck), think of this article and smile.
Post scriptum, almost: Yes, even your humble narrator has had (or did they have me?) & been had by Sea Cucumbers. They happily seem to be the last to die from poisoning their everyone else, quickly regenerating eviscerated organs in anticipation of the next tank full. Sigh.
Bibliography & Further Reading:
Barnes, Robert. Invertebrate Zoology. Saunders, Orlando. 1987. 5th ed.
Baugh, Thomas M. 1991. The Seaweed Cucumber. FAMA 4/91.
Fenner, Bob. 1995. Spiny-Skinned animals, phylum Echinodermata. FAMA 5/95.
Hopmann, J.A.M. 1977. A Bad Experience with Sea Cucumber Caviar. A.D.I. (Aquarium Digest International, Tetra) 4(77) #18.
Kloth, Tom. 1981. Basic Invertebrate Anatomy & Terminology V. Echinodermata. FAMA 2/81.
Michael, Scott. 1997. Sea Cucumbers. These are not your garden variety of reef tank inhabitant, and can cause real problems. AFM 3/97.
Shimek, Ron. 1996. Gerkins or dills- the Cucumbers of the sea. Aquarium Frontiers 3:3/96.
Toonen, Bob 2000. Invert insights. Holoturians. TFH 7/00.
Volkart, Bill. 1989. Sea Cucumbers- the ugly invertebrates. TFH 2/89
Wilkens, Peter. 1998. Death in a colorful package. Aquarium Frontiers Online. 5/98.