Introducing  Echinoderms

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When it comes to bottom dwellers that earn their keep, you could do a lot worse than some of the sea stars, writes Tim Smith.

The echinoderms aren’t given as much credit as they deserve, at least not in proportion to their diversity. They’re this bizarre, unique group of creatures that are often inadequately thought of as simple marine invertebrates relegated to the ocean floor. But their numbers and roles in any given marine ecosystem can be staggering. 

In the marine aquarium world, too, their importance is often overlooked. In most instances they’re assigned some lesser role: clean-up crew, hitchhiker, nuisance. However, given a closer look and a new appreciation, these animals can become the stars of a marine tank, and challenges for the aspiring marine aquarist looking to broaden their horizons. 

Red knob sea stars are coral munchers.

Dazzling diversity

To put it very simply, the echinoderms are made up of the sea stars, brittle stars, sea cucumbers, sea urchins, and feather stars. These are neat categories, but it says little for the diversity represented by some 7000 species contained within. 

Even among these groups are a range of lifestyles and ecologies that reflect the absurd numbers of niches that these animals can fill. From tropical to temperate waters, reefs to deep sea, you’ll be hard pressed to find a marine environment without some form of echinoderm.

The group as a whole share a number of common traits, but the defining characteristic is their five-fold symmetry. While most animals (such as fishes and ourselves) can be split in two parts equally, echinoderms cannot; equal portions can only be achieved when splitting them five ways. It’s the reason that (most) sea stars are five-armed, or why urchin shells have five segments. Some very unique species are arranged in multiples of fives.

They also all have tube feet. These line the echinoderm exterior and facilitate all manner of important functions from feeding to locomotion to camouflage. These can be obviously seen in many species, but are a lot more subtle among others such as feather stars and some sea cucumbers. Other times they’re greatly modified, as in the huge feeding appendages of the sea cucumbers. 

Blue linckia sea star.

Classic clean-up crew?

When talking about marine clean-up crews (CUC), the echinoderms regularly make the roster. For good reason too: few species are problematic or predatory, and most can be classed as remarkably unobtrusive. 

Being benthic, they regularly find themselves in the right spot; which is to say, where excess foods and other organics settle out. Some echinoderms are particularly suited to sifting through the substrate, and are effective processors of all manner of organic waste and microorganisms nestled within the sea floor — in theory, making them great for the home aquarium.

The danger in regarding animals as a clean-up crew is that their specific needs are rarely met, relying on chance and fate that enough food makes its way into their realm. An animal in diminishing condition will lose size, and a warning sign of this is increased activity during daytime, as they desperately search for extra morsels.

Like other marine invertebrates they are sensitive to their environment, and their living conditions must be maintained at a high standard. Deteriorating water quality is the final straw for these animals; that echinoderms are so affected may only become apparent long after the fact, owing to their fairly inactive and inexpressive nature. 

Of particular importance are strict maintenance of low nitrates and stable, full-strength salinity, deviations from which can prove harmful even in the short-term.

There’s nothing wrong with allowing animals in your aquarium to take on the role of a CUC, but at the same time it is very important to monitor that they’re getting food, remaining in good health, and that particular requirements of each species is addressed, be that dietary or environmental

Sea stars

The popular kids among the echinoderms, the sea stars (note: we don’t call them starfishes anymore) are iconic marine animals that make the wish lists of aquarists the world over. Often sporting lovely colours, patterns, and textures, only a few species are really suited for the long-term in aquaria, but those that are make stunning additions.

The dietary requirements of sea stars are about as varied as their colour patterns. Usually carnivorous, each species may ‘hunt’ a particular type of prey that doesn’t move very fast: corals, sponges, molluscs, or even other echinoderms. A couple of species graze on algae and other surface-growing life. Generally, though, one should be wary of mixing sea stars with other sessile invertebrates.

Some species, although having dietary preferences, will scavenge on detritus and other sea floor goodies wherever available.

It’s this ability to shift to a detritivorous diet that has led to many sea stars being utilised as attractive options for clean-up. However, this is not always the case with many species being dietary specialists. Proper identification of a sea star and assertion that one can accommodate its particular dietary needs are important steps before bringing any star home. 

Of those that will happily scavenge, ensure that their diet is regularly supplemented with meaty fare, such as mussel or clam. They will ultimately deteriorate without a dedicated feeding schedule.

A common addition to the clean-up crew line-up are the sand-sifting stars, Archaster, who specialise in rummaging through the sediment and picking up all forms of organics. It’s actually a little too good at this job, and as such needs a deep substrate with a good overall surface area to satisfy its needs.

Some sea star species can become numerous and a nuisance, such as Asterina, which, despite forming an impressive army of a clean-up crew, can become an eyesore in such numbers. In other cases, your sea stars may consume other invertebrates that you’d rather they didn’t. Remember that corals form the natural diet of many stars, and as such several species are not strictly reef safe. Red knob sea stars, Protoreaster lincki, are among such beautiful yet harmful suspects.

Brittle stars

Also going by the more exciting moniker ‘serpent stars’, the brittle stars are arranged in body form much like the sea stars, but have considerably thinner legs and can be remarkable mobile. The ‘brittle’ in their title is no mistake: these animals will readily shed their arms at even the mildest provocation. While a useful escape tactic, such behaviour may be concerning for their caretakers, but in the rare event of this happening, do not panic — under good conditions, these arms grow back in no time.

Most commonly, brittle stars are found somewhere from a few weeks to a few months after the addition of live rock, as accidental additions. They’re quiet guests that rarely make an appearance during daytime hours, happy to sift through the gathering detritus at night.

Sand sifting  sea  star

Only suggested for tanks that have been running for several months, Sand sifting sea stars can be added at a rate of one per 100-litres tank volume, in which they’ll happily forage through the substrate without outcompeting each other. Sometimes gets confused with a similar looking but slightly smaller species, Astropecten polyacanthus (a species that contains highly toxic tetrodotoxin). Care requirements and feeding are pretty much identical for both species. 

  • Scientific name: Archaster typicus.
  • Size: To around 15cm span (males slightly smaller).
  • Origin: Indo-Pacific, usually shallower than 60m depth.
  • Habitat: Seagrass meadows and sand beds.
  • Feeding: Detritivore, but will appreciate regular targeted feeds of dry or frozen meaty foods.
  • Availability and cost: Readily available in marine stockists with prices from £15-£25 pending size.

Green  brittlestar

Ravenous but highly predatory sea stars that are a delight to interact with, Ophiarachna incrassata are large stars that will actively grab and eat their slower tankmates, so are suitable for housing with large fish only. They will feed in the daytime and will scuttle out to take directly from tongs once they smell food.

  • Scientific name: Ophiarachna incrassata.
  • Size: Body diameter about 5cm, legs can reach 20cm each.
  • Origin: Indo West Pacific from East Africa to the Philippines.
  • Habitat: Coral rubble and seagrass beds.
  • Feeding: Large meaty foods such as frozen molluscs and prawns.
  • Availability and cost: Makes a frequent enough appearance in marine stores, prices from around £14.

Of the species commonly found in aquaria, most are unfussy with regards to food types, readily picking up leftovers, detritus, or even grazing. A select few, however, are quite predatory and cannot be trusted around smaller fish and invertebrates.

Closely related to brittle stars are the basket stars. These are typically suspension feeders, capturing fairly large particulate matter from the water column. These do not always do so well in captivity, and are usually best avoided.

Sea apples are high risk additions. 

Sea cucumbers

The sea cucumbers may be divided roughly into two groups, each with a corresponding lifestyle that fits its diet: the sediment-sifting deposit feeders, and the filter feeders.

The first group may broadly be referred to as ‘sand moppers’ and are often thought of as part of the clean-up crew, slowly turning over sediment and processing organics trapped within. While they do an excellent job, they can rapidly deplete the food resources of a sand bed of insufficient size or depth. Holothuria spp. are the classic examples here, but many genera are representative of this lifestyle.

The remainder are those among the sea apples and others with similar ecologies, characterised by larger, tufty branches they stretch out into the water column in order to filter out suspended plankton and organic particles. These require specialised feeding regimes with absolutely minute food particles; live foods such as phytoplankton seem to be preferred. 

The sand moppers rarely need much attention aside from occasional dietary supplementation, whereas those filtering directly from the water will need regular doses of minuscule suspended matter to maintain a healthy condition — that’s bad news for your filter systems, and it’s labour intensive. 

The gut-spewing defensive manoeuvre that this group is famous for should be heeded in the confines of a marine aquarium. Some sea cucumber viscera are toxic; even amongst those that are not, the resultant bioload of mucus and guts can rapidly deteriorate the state of all but the largest aquariums. Rapid removal and dilution of these substances, facilitated by a good water change, may do the job. In a smaller tank, and not addressed immediately, gut spewing can easily be fatal for all inhabitants. 

Longspine rainbow fire urchin.

Sea urchins

Readily recognisable by their hardened exterior and adornment of spines, sea urchins can be both beautiful and dangerous additions to the aquarium. Perhaps their greatest contribution to aquarium life is their talent for consuming nuisance algae. Many species graze almost continuously but some prefer to cover the night shift only.

Outside of these chores, most sea urchins can be satisfied with regular offerings of macroalgae, with both freshly cultivated and dried seaweed (such as nori) being suitable options. Meatier food types make excellent supplements to their diets, too, especially among the more omnivorous pencil slate urchins,
for example.

Not all sea urchin spines are sharp, and not all sea urchin spines carry venom. Still, the aquarium environment is rich in bacteria and you’d do well to avoid puncturing your skin on an urchin. For those species that do carry venom — fire urchins for example are common enough in the hobby — treatment for a sting is straightforward.

Long spined sea urchins.

Long  spined  sea  urchin

This species has become invasive across the Mediterranean, but in the aquarium it makes for an excellent algae grazer. Interestingly, elsewhere in the world it is also ‘adopted’ by Banggai cardinalfish, that like to spawn amongst the safety of its long spines. Initially it can be nervous in a tank, only coming out at night, but will eventually learn to be day active once it feels safe.

  • Scientific name: Diadema setosum.
  • Size: The test (solid body) reaches around 10cm in diameter, with spines up to 30cm long.
  • Origin: From the Red Sea across to Australia, with invasive populations elsewhere.
  • Habitat: Coral reefs and seagrass beds mainly.
  • Feeding: Algae grazer that will benefit from some dedicated macroalgae growth in the tank.
  • Availability and cost: Makes a regular enough appearance, prices from £20 upwards.

Hot water, as hot as one can withstand, helps to denature the proteins within the toxin. Vinegar soaks thereafter can be useful in dissolving (and loosening) the splinter of spines still embedded in your skin, but won’t do much for larger pieces.

Be sure to get all fragments removed, too. The longer and more delicate spines readily break apart during the removal process, and leaving a spine embedded can lead to problems later down the line.

Urchins are reef safe, aside from two particulars: they’re not shy about pushing rocks around, threatening stability of unsecured hardscape; and secondly, some species may graze on coral surfaces, causing damage. Monitor closely for both of these behaviours. 

A feather star living up to its name.

Feather stars

Also called sea lilies, the crinoids are not seen as often, and for good reason — beyond being fragile, they’re also obligate filter feeders. Ensuring that they get sufficient food, and of appropriate nutritional quality, is the aquarist’s top priority. 

They won’t respond to all offered food equally. If you pay close attention as the food passes over their arms, you’ll notice the individual tubes twitch and fold inwards, indicating that it has found an acceptable food source. In the presence of an abundance of said food, the arms may be waved in the current. Less desirable foods will see no response whatsoever. 

High quality, small particulate foods are in order. Live foods, like juvenile Artemia, should be accepted readily; frozen foods will be your next best bet. Artificial diets are hit and miss, and careful attention should be paid if your animal is accepting this food at all.

Although they’ll typically remain in one spot, these animals are plenty capable of moving. Depending on how far they need to go, they’ll either slowly amble across the substrate with their ‘feet’ (called cirri) or they’ll swim away with alternating pulses of their feather-like arms. 

In an aquarium setting, they’ll usually find a spot with appropriate flow, and likely won’t move until either the flow is no longer appropriate or they’re not getting enough food - both good indicators of if you’re doing a good job of keeping your feather star happy.