Friend or Foe Part 11 Friend or Foe The Echinoderms Part 2 TRISTAN LOUGHER B.Sc. graduated from Manchester University in 1992 with a e began to look at the aquarist will follow this loose body plan it degree in Zoology. He has Echinoderms in the last issue of is worth mentioning some departures from worked at CheshireWaterlife WMarine World, concentrating on this. for five years. two of the Classes within this group that are In U.K. coastal areas dominated by referred to as "starfish" the Asteroidea and sandy beaches we can locate the aptly the Ophiuroidea (true starfish and brittle named "sea potato" (Echinocardium starfish). Of the remaining three Classes the caudatum). This yellowish urchin lives life aquarist is likely to experience only two of completely buried in sand, sometimes to a direction of a potential predator (or aquarist them, and these will be focused upon here. depth of several feet. That this species is trying to remove one from an aquarium). present is sometimes only obvious to the However, due to specialised tendons in the Class Echinoidea casual observer, as beaches will often have socket joint, the spine can be made totally their exoskeletons or tests washed up on rigid which is an essential defence One of the most unmistakable groups of them. Many visitors to Florida will be adaptation. Projecting from the test are also marine invertebrates the echinoids, or sea aware of the sand dollar test, which is often specialised structures called pedicellariae urchins, are a very successful group seen for sale in souvenir shops. This is and the tube feet both powered by the water numbering some 900 described species another sand dwelling Echinoid, which is vascular system unique to echinoderms. which have colonised all of the worlds specialised for an existence on top of sandy Pedicellaria are organs that resemble tube oceans from shallow waters to several miles substrates (see Figure 1). Due to their feet with pincer-like endings. They are used depth. The typical body plan is that of a flattened shape sand dollars and their primarily to clean the surface of the animal hard exterior, usually spherical or ovoid, relatives are referred to as irregular urchins. although they also have a secondary role in often covered in spines, which protects the The spines of regular sea urchins are protecting the urchin from small predators soft internal organs of the animal. Despite calcareous extensions of the test which are to whom the defences offered by the spines the fact that most of the individuals of this able to move with a simple ball and socket are no problem to breach. echinoderm class experienced by the joint at their base, thus they are able to act Given the relative abundance of sea as the main urchins in natural reefs it is unsurprising source of that specimens make their way into the Fig. 1 "rapid" hobby by accident. However, live rock is Sand dollar test locomotion unlikely to be the origin of these animals in in many our aquaria as most will be too sensitive to urchin survive the shipping and curing processes. species or Live rock from Vanuatu (Pacific storm wave around rock/South Pacific Rock) is particularly rich menacingly in the dead tests of urchins. Some species in the do make it as accidental arrivals on the base rock associated with soft corals and polyps. The tube feet of this urchin are obvious as they reach out beyond the spines 34 • MARINE WORLD – JUNE/JULY 2003 Friend or Foe The commonest of these imports seems to separate "teeth" (one for each sector of the the substrate and cannot be levered or be Echinometra spp., most of which that we body – remember, all echinoderms exhibit wafted free by triggerfish or other predators are likely to encounter are characterised by pentamerous symmetry) which are and the only exposed surface is extremely a white disc surrounding the base of each articulated and supported by small bone- prickly. spine. like structures and specialised muscles In reef terms the grazing of sea urchins called collectively is extremely important in the production of Echinometra sp. "Aristotle’s lantern". These sand. Estimates of up to 90% of all the teeth are extremely robust erosion of coral reefs caused by animals, and in many species can plants and bacteria in certain parts of the articulate in a number of world may be due to urchins. ways. Their ability to graze So, are the little Echinometra urchins algae from rock is incredible suitable for marine aquaria? Well, it and often the aquarist can depends on your feelings about calcareous determine the route an algae. If you like the latter forget about urchin has taken by the obtaining an urchin – they are just too clean rock left in its wake. destructive to this beautiful alga. In their This is great news for the defence, however, they make interesting person inundated with and hardy introductions to marine aquaria. nuisance algae, or so you Urchins are clumsy creatures too – they can would think if it wasn’t for knock over unsecured sessile invertebrates the fact that urchins are a and even "pick up" polyps and rubble that little more cosmopolitan in is an adaptation to concealment in some of their tastes than nuisance the urchins. These species are perhaps the only algae. A grazing urchin may remove large ones we may experience as an accidental amounts of calcareous algae that in most Symbionts introduction on what could be called a aquaria will not be able to recover quickly If you have an urchin you might want to "regular" basis. Superficially they resemble enough to withstand the grazing pressure. have a look and check for the presence of the long-spined sea urchins Diadema spp The desire of an urchin for calcium commensal or symbiotic creatures on the but the latter have much longer spines and a carbonate is such that you can supplement body and in between spines. There are prominent anal bladder (this is the large their uptake with pieces of cuttlebone, several small bivalve species that can be structure on top of the urchin which some which will be grazed to nothing in no time found around the underside of the urchin. aquarists believe to be its "eye"). In fact at all. Recently there have been some small, this structure surrounds the anus and is an Many Echinometra species elongated shrimp available in the trade that extension of the intestines. It is certainly demonstrate their ability to consume live on the spines themselves. It is always true however, that the majority of regular calcium carbonate substrates in their rock- possible to find the seemingly ubiquitous sea urchins have light sensitive organs on boring activities. Some will excavate a bristleworm family on urchins too, but they their aboral surface. depression into the rock that they occupy are unlikely to be harmful to their host in during the day or in times of most cases. low tide when they might be The anal bladder of this Zebra urchin is clearly visible in the centre of this image. It is perhaps less eye-like than those possessed in other species Echinometra presents no more vulnerable to problems to the aquarist than those predation. demonstrated by the vast majority of Thus the less ornamental urchin species that are readily well-protected imported for the aquarium hobby. In nature, oral surface of shallow water regular urchin species are the urchin is grazers of algae. The mouth, which is firmly in A . urchin utilising the calcium carbonate in the test of another located underneath the animal has five contact with Diadema sp species of urchin MARINE WORLD – JUNE/JULY 2003 • 35 Friend or Foe physiologically to the cucumber than tubule ejection. Here the gonads and digestive tract may be expelled and is usually the result of stress brought on by poor husbandry in a reef aquarium – e.g. Poor water quality etc. Such an incident could seriously jeopardise fish stocks. Fortunately, cucumbers have remarkable powers of regeneration and Figure 2 The so-called edible cucumber can often recover from such, (Holothuria edulis) seemingly suicidal, acts. The popular sea apples (Pseudocolochirus spp.) are another potential source of poisoning in the Class Holothuridea aquarium. They, too, have the capability of Another mobile Having rattled-on in the Echinoderm related holothurin release but this is not the suspension-feeding Friend or Foe articles about the phylum commonest threat. Every aquarist likes to sea cucumber exhibiting pentamerous symmetry we see natural reproduction occurring in their conclude this fascinating group with a class system but in the case of the sea apples this their feeding method. As owners of sea in which this characteristic is not at all is not the good news it first appears. apples will be aware, as the cucumber feeds obvious. The Holothuroidea are commonly Unfortunately the eggs of sea apples are it slowly and methodically pulls one of the known as sea cucumbers and many marine toxic to fish that, unfortunately, decide that ten feeding arms into the mouth where aquarists will be familiar with their more this is the best food they have ever tasted detritus and suspended food is removed. decorative or useful species. The former and gorge them. The result can be large- This process is repeated sequentially for all include the sea apple and candy cucumbers. scale fish deaths. tentacles and repeated throughout the In these species it is possible to discern five Accidental introduction of cucumbers feeding period. body sectors but species such as the edible to reef aquaria is commonplace although Other cucumbers are free-living and cucumber (Holothuria edulis) shown in perhaps not often noticed by the aquarist.