News / Feature Articles / Echinoderms For The Home Aquarium (12/03/18)

Echinoderms For The Home Aquarium

Echinoderms For The Home Aquarium
Echinoderms are one of the most successful groups of animal life in the ocean today, with a rich fossil history (estimated at 13,000+ species!) stretching back to the Cambrian, more than 500 million years ago. They are of enormous ecological importance and often can be found in significant abundance in just about every type of marine ecosystem imaginable, from intertidal mudflats to the abyssal plains of the deep sea, along with many that abound on coral reefs, where they are a prominent, diverse and often colorful component.

Yet, for all their success as a group, they are, as a whole, poorly known by most non-zoologists. Ask 10 people on the street what an echinoderm is, and its unlikely that youll get a single correct answer, but even a small child will be able to recognize at least one member of this group, the sea stars. Why do echinoderms have such poor recognition? Lets try to fix that, shall we?

First, we need to define what an echinoderm is. These 7000 or so species represent a unique phylum of animal life. This is the same level of classification used to distinguish mollusks from arthropods from chordates from cnidarians, and it indicates just how unique an echinoderm is, morphologically speaking.

Understanding the origin of this groups scientific name is helpful. Echinos is from the Ancient Greek for hedgehog, but is a name likewise applied to sea urchinsthe resemblance of these two spiny beasts should be obvious enough. Dermata, meanwhile, translates as skin. Its the internal skeleton common to all members of this phylum that is truly being referenced with this name, as only one subgroup, the sea urchins, actually have a notably spiny appearance. Unlike an arthropod, which has its skeleton external, or a chordate, which has its skeleton covered in musculature, the calcite skeleton of an echinoderm has only a thin veneer or skin protecting it from the outside world.

Another distinctive trait of this group is the water vascular system, which combines the roles of a respiratory systems and circulatory system in one, while also helping in locomotion and sensing the outside world. The tube feet on the underside of a sea star are the outward evidence of this unique organ system, each of which is capped with a small sucker used for gripping the substrate (and their prey, in carnivorous species). Internally, these tube feet connect to a canal running down the center of each arm, which meet in the center of the body to form a ring. In turn, this ring connects to the outside world via a pore, known as the madreporite, so that this canal is filled with the surrounding sea water.

Now, lets meet the five major subgroups of this phylum, each of which is recognized as a distinct class. Though only five are still around, its interesting to note that the fossil record is filled with many other classes, many of which are poorly understood.

Class Asteroidea Sea Stars
Sea stars (or starfish, as they are often known) are the most familiar of these subgroups, with somewhere around 1500 species. These vary enormously in size and shape, from minute forms just a few millimeters across to giants like the the Sunflower Sea Star (Pycnopodia) of the Pacific coast, which tops out at up to a meter across its arms. We tend to think of sea stars as having pentaradial symmetry (i.e. five arms), but there are many examples which have more than this (like Pycnopodia, with up to 24), as well as some without any arms (like the bizarre deep sea Xyloplax, though these do still contain an internal symmetry).

There are many species from this group collected for the aquarium trade. Protoreaster are some of the largest available, and includes the familiar Chocolate Chip Star and its red cousin, the African Red Tip Star. These can reach a foot across and are effective predators and scavengers, unsuitable for most reef aquariums, but a fine addition to a fish-only tank.

Astropecten polycanthus

A very different sort of sea star is exemplified by the burrowing Sand Stars (Astropecten). These belong to a different order, notable for lacking suckers on their tube feet, allowing them to effortlessly glide beneath the sand in search of their largely molluscan prey. There is a common misconception among aquarists that Astropecten are just another clean-up crew organism, which can be added to clean sand. This, however, couldnt be further from the truth; these stars are highly competent predators and will often starve in captivity (particularly in smaller or newer tanks) if not provided with something to eat, either in the form of chopped mollusks or crustaceans. Interestingly, unlike other asteroids that feed externally, everting their stomach onto prey items, the sand stars feed internally, actually ingesting their prey whole and later expelling any indigestible parts.

For those looking to add a reef safe star, suitable options include members of the genus Fromia, Iconaster, Echinaster, Nardoa, and Linckia. The latter includes the beautiful Blue Linckia, a widespread and common species across the Indo-Pacific. The diet of these reef-associated stars is, in general, very poorly understood. Its hypothesized that they feed on a diverse and generalized diet of benthic organisms, though what this might include is, again, unknown. For this reason, a large and well-established reef aquarium full of live rock is highly recommended, along with a low stocking density of these sea stars.

Class Ophiuroidea Brittle Stars
The sister group to the sea stars are the brittle stars, with around 2000 species, named for their tendency to willingly dispose of their arms when harassed. Its best to never grab a specimen by the arms, lest it deploy the appendage (though they are generally quick to grow back). Unlike the true sea stars, the tube feet in this group never have suckers. The body also tends to be small and circular, with thin arms that give these creatures a distinctive look. One of the most remarkable modifications of this body plan are the highly branched arms of the basket stars, specialized for plucking suspended particles from the water column. These are, however, rarely seen in captivity, owing to the difficulties of feeding them in an aquarium.

Unlike the asteroids, ophiuoids are often highly effective additions to a clean-up crew. The most commonly seen species in the aquarium trade is the Caribbeans Tiger or Serpent Sea Star

Ophioderma sp.

(Ophiolepis superba) recognized by the smooth arms and light/dark banded patterning. These will feed on larger bits of uneaten food, serving a role similar to Nassarius snails and hermit crabs, though, without a shell to hinder them, Ophiolepis is capable of reaching into more nooks and crannies. Their knobby cousins, Ophiomastix, are equally adept at this, as are the spiny Ophiocoma.

There are, however, a couple bad apples in this group to be on the lookout for. The most famous of these is Ophiarachna, a large green (or occasionally yellowish) species that is capable of trapping and eating small fishes and shrimps. They perform this feat by raising their bodies off the substrate at night, waiting for an unsuspecting creature to trespass between their legs. In an instant, they twirl these together, entrapping the prey, which is then brought into their (surprisingly flexible) body. This species is a fascinating addition to an aquarium, but is only suitable for those containing larger fish species. The rarely seen Red Serpent Star (Ophioderma) is also reputed to be an untrustworthy species, though this may be unwarranted.

There are many other brittle stars found on coral reefs, many of which live in close association with soft corals. These can on occasion find their way accidentally into aquariums, where they are an interesting and harmless addition. Others can hitchhike in on live rock or coral pieces, and these two make for a useful member of a diverse clean-up crew. Tiny, white or banded species can often be found buried in sand, reproducing via asexual fragmentation.

Class Echinoidea Sea Urchins
The name urchin comes from Middle English, by way of Latin, and refers to hedgehogs, and thus a sea urchin is, linguistically speaking, a marine hedgehog of sorts. Around 1000 species are known, and the group is well-represented in coral reef ecosystems.

We can break this group down into a few useful subgroups. From an evolutionary perspective, the biggest divide is found between the pencil urchins (Caidaroidea) and the remainder of this class (Euechinoidea). Pencil urchins are the most morphologically primitive of the urchins, having relatively few spines, which grow uncovered by epidermis, often resulting in an encrustation of various sponges, algae, etc. The rounded body (technically referred to as a test in this class) is also a bit different in the arrangement of its plates, and there are no gills surrounding the mouth. The cidaroids tend to be omnivorous and will gladly feed on pieces of clam and krill, along with scavenging other meaty foods, though they are generally safe to mix with corals.

Diadema setosum

Among the euechinoids, two informal subgroups can be recognized. There are forms that burrow into or beneath the substrate in search of food. These tend to be flattened to varying extent and are known varyingly as sand dollars, sea biscuits, and heart urchins, while the group as a whole are referred to as the irregular urchins. They mostly feed on organic material found in these sediments, but they are generally uncommonly kept in aquariums.

The rest of the euechinoids can be thought of as the regular urchins, tending to have a symmetrically rounded body (though not always so). This is where the bulk of the species found on coral reefs reside. They tend to spend their days grazing away on algae with a specialized mouthpart known whimsically as Aristotles lantern, so named as it was originally described by the Greek polymath as resembling a lantern. It contains rasp-like structures for grinding away at the bottom, making these one of the few invertebrates adept at feeding upon tougher algal species, like the coralline algae. Only the most robust of snails can compete with this efficient level of herbivory, making sea urchins one of the most useful additions to a reef aquarium.

Common aquarium species include the Longspine Urchins (Diadema), a genus that possesses enormous, delicate spines which can easily break off into the flesh of an unwary aquarist. Full-grown specimens can reach over a foot measured across the spines, making these most suitable for larger systems. For a smaller reef tank, good options include the Shortspine Urchin (Echinometra), the Pincushion Urchins (Lytechinus, Tripneustes, Salmacis), and the always dapper Tuxedo Urchins (Mespilia).

There are a couple genera that warrant special mention for their potent venoms. The more commonly seen of these in captivity is the Fire Urchin (Astropyga radiata), a large and colorful species which has needle-fine spines laced with an egregiously painful toxin. Woe be to the aquarist who foolishly handles this beast. Even worse is Toxopneustes, the Flower Urchin. This uncommon aquarium species looks very different from most urchins, being covered in peculiar, petal-like structures known as pedicellariae (the spines are small and hidden among these). These are defensive appendages that can deliver a medically significant venom, with reports of lethal reactions from the sting of this species.

Class Holothuroidea Sea Cucumbers
There are around 1700 species of sea cucumber, and together they form the sister group to the urchins. In essence, the morphology of a holothuroidean is like that of an urchin which has been tipped over onto its side and elongated, with the skeleton and spines greatly diminished. Similar to the irregular urchins, sea cucumbers mostly feed on organic-rich sediments (though a few remarkable forms are entirely pelagic and trap particulates from the water column). They also breathe through their butts, with respiratory trees residing within the anal cavity. Famously, this is where pearlfishes reside.

There are a number of major lineages that divide this class, though only a few occur around coral reefs. The most distinct of these are the medusa worms of the Order Apodida. The scientific name references the absence of tube feet, as these bizarre creature crawl about with peristaltic contractions of the body.

Holothuria atra

They feed on the bottom using numerous oral tentacles to mop up whatever edible bits might occur therethis is where their mythologically inspired common name comes from. Some species form a symbiotic relationship with large sponges and can be quite prominent on their host. In total, fewer than 300 species are known, but few are ever seen in aquariums, despite being common on coral reefs. They are, however, a useful addition to larger, well-established reef aquariums.

The more typical sea cucumber is embodied by the genus Holothuria, with dozens of species found all across the worlds tropical and temperate waters. This includes the Edible Sea Cucumber (H. edulis), also known poetically as the Bche-de-mer (literally, sea worm), a pinkish species often seen in the aquarium trade. Its cousin, the Black Sea Cucumber (H. atra) is also a common find, along with the beautifully patterned Tiger Tail Sea Cucumber (H. thomasi), an abundant Caribbean species. Less frequently seen are those from the distantly related family Stichopodidae, which tend to be a bit spikier and more cuboid in their shape. These tend to grow quite large and need a suitable expanse of substrate on which to feed. Less is more when it comes to stocking most sea cucumbers.

Lastly, there are the dendrochirotids, a group of cucumbers which has taken to a sedentary lifestyle, feeding upon suspended particles and plankton in the water column. On coral reefs, members of this group can vary from the small, yellow Colochirus robustus (which grows to just an inch or two in length and is known to reproduce asexually in aquariums via transverse fission) to the gorgeously mottled Sea Apples, Pseudocolochirus (which can grow to the size of a potato). Smaller species tend to fare fine in well-established aquariums, but the larger Sea Apples are notorious for their poor track record. Aquaculturists have maintained these with dense cultures of live phytoplankton, which likely explains why they fare poorly in most home aquariums. An additional concern is posed by the toxic chemical holothurin that these species can release when harmed. This might stem from an inquisitive fish bite, or it could be from their wandering too close to a pump intake, resulting in a macerated specimen. Like other echinoderms, sea cucumbers (even the sedentary ones) can slowly move about.

Class Crinoidea Feather Stars
The most primitive of the living echinoderms are the crinoids, with around 600 extant species (and many more in the fossil record). These are known varyingly as the feather stars and sea lilies, in reference to the two major body shapes found within this class. In the deep sea, we find stalked forms that look a bit like a lily, hence their common nameadditionally, the scientific name Crinoidea comes from the Greek for lily. More widespread are the unstalked varieties, the feather stars, which include the species seen on coral reefs.

Feather stars are quite common and diverse on reefs, but rarely are these found in aquariums. The group has two major issues as far as its captive husbandry is concerned. Firstly, we have little idea what these creatures actually eat. Crinoids, generally speaking, seem to selectively pluck minute particulates from the water column, but precisely what these are is hard to say. The feathery arms that give this group their name seem adapted to differing types of feeding strategies, with some species having densely pinnate (feathered) arms, while others have a few, sparse pinnules. The latter are reported to be somewhat more successful in captivity, but only slightly so. Successful husbandry requires heavy feeding of planktonic foods.

Another important concern is the ability of crinoids to swim via graceful thrusts of their arms. The effect is beautiful and almost hypnotic, but, within the confines of an aquarium, this can easily result in a rogue feather star being dismembered by a pump intake or powerhead. For these reasons, crinoids are one of the last frontiers in aquarium keeping, a challenge best left to the most dedicated aquarists.