• Abstract and Figures
• Public Full-text

Friend or Foe Part 10 Friend or Foe The Echinoderms – Part 1 TRISTAN LOUGHER B.Sc. graduated from Manchester University in 1992 with a he phylum Echinodermata contains Another important feature of degree in Zoology. He has several species familiar to many echinoderms is their water-vascular system worked at CheshireWaterlife Tpeople – how many children don’t used in locomotion and food acquisition. for five years. know what a starfish looks like? However, We can see the tube feet of starfish on their there are many members of this assemblage underside and often all over the surface of that are not so well known and some may sea urchins – both are governed by the be encountered by marine aquarists. Before water-vascular system. Brittle stars lack we go on to look at individual species we tube feet but their highly mobile arms are expect to encounter in a live rock based will cover some of the unique attributes of powered by the same system as other reef aquarium although accidental echinoderms. echinoderms. Damage to this specialised introduction is not out of the question. system has been blamed for lack of success Echinoderm Classes with some starfish in captivity. Class Asteroidea Echinoderms are said to show Asteroidea – True starfish pentamerous symmetry – that is they are – True Starfish Ophiuroidea – Brittle starfish essentially made up of five similar True starfish have a variety of forms with Holothuridea – Sea cucumbers segments. Humans, insects, crustaceans, the average number of arms being five but Crinoidea – Feather starfish / Crinoids flatworms etc are bilaterally symmetrical. up to fifty are possessed by some species. Echinoidea – Sea urchins/ Sand dollars/ Sea That is, the left side of our body is almost In other species arms have become so potato etc. identical to the right. I say almost because shortened that the animal resembles a we are not totally symmetrical – there are pentagon shaped pillow. Correct Echinoderm means literally "spiny skin". several studies going on around the world identification of closely related species is This characteristic is due to the unique which hope to gain support for the theory not straightforward and the taxonomist may internal skeletal structure found in all that animals find symmetry attractive – the have to resort to dissection to be absolutely echinoderms, consisting of calcareous more symmetrical your appearance, sure of what they have. ossicles, which are articulated in sea stars whether it be wing length, facial One of the problems with starfish, and rigid in sea urchins and contains a type proportions or patterning, the more however small, is that prey doesn’t have to of tissue termed mutable connective tissue. attractive to the opposite sex you are. fit inside their mouths to be consumed. This can determine the rigidity of the Irrelevant here but interesting nonetheless! Starfish are capable of everting their echinoderms’body and is especially useful Echinoderms number for animals that utilise its versatility. For some 6000 described example some starfish need to be species divided into 5 Basket starfish manoeuvrable and flexible but then when further classes as listed predating on hard shelled animals they may above. Of these the brittle need a degree of rigidity when attempting stars have the most living to prise the bivalve open. As aquarists we species although the fossil can experience the incredible nature of record demonstrates that mutable connective tissue by observing sea crinoids were extremely cucumbers – a group of echinoderms. Sea numerous and diverse apples in particular can inflate themselves many millions of years up to relatively huge sizes (from tennis ball ago. Comatulid sea stars to football) in no time at all. It is reported or "feather starfish" are that vigorous rubbing of a sea cucumber sometimes purchased by causes the tissue to become almost liquid – aquarists due to their literally flowing through your fingers. Of beautiful colouration. course, this begs the question – why on However, they are not earth would you want to do this but it is an suitable for the vast interesting phenomenon nonetheless. majority of marine Brittle starfish also exploit the properties of aquaria and should be such tissue by shedding their arms when avoided. Crinoids are threatened – a phenomenon termed perhaps the only autotomy. echinoderms we might not 34 • MARINE WORLD –APRIL/MAY 2003 Friend or Foe stomachs and digesting whatever they are pest resting on. Thus large corals can be stripped population Linckia sp., Leiaster sp. and Echinaster sp.. Note the different colours but also two dark of tissue with relative ease. being purple individuals are the same species but Fortunately, few asteroid starfish make controlled have different numbers of legs. Five is the their way into reef aquaria as accidental rather than norm but the individual bottom centre has introductions. Those that do are likely to be eradicated – six. from the following genus. thus the pest species must Genus: Asterina always be present These small starfish are termed "cushion albeit in stars" due to their chunky appearance and small quite short arms. Colours vary and are not a numbers – reliable means of identification but the ok if you unifying feature of the species we don’t mind a encounter regularly in our aquaria is the few of your slightly "chewed" look – almost as if corals being something has been nibbling their legs! The scoffed numbers of legs vary although five appears rather than to be average. In the U.K. we have an all of them. Asterina species called the common We have cushion starfish. This species is unusual in seen this that it is not a broadcast spawner like most situation of the rest of the true starfish; instead eggs before with are laid under stones and rocks. If this is the nudibranch Chelidoneura varia that Thus counting legs is an unreliable true of the reef specimens then this could predates flatworms such as Convolutriloba identification technique. explain why this species seems to thrive in retrogemma. Once the pest population is Many aquarists experience problems aquarium situations although they are also drastically reduced the predator population with starfish within a relatively short time. said to reproduce through fission i.e. the starves to death. Then the risk is that the Many specimens seem to disintegrate, splitting of the body into two. pest population recovers or further pests are sometimes very rapidly, sometimes over a There are reported to be three species reintroduced. I believe that in this scenario period of weeks. Reasons for this are of Asterina that are commonly imported such a control technique is unethical but unclear. Many aquarists first believe that with live rock. The 0.5cm, whitish ones there could be scope for perhaps sharing this is the result of an attack by a appear to be pure algal feeders. This is the predators amongst aquarists (good luck in crustacean or bristleworm but this is species we are most likely to experience in catching the shrimp!). The other alternative extremely unlikely to be the cause, our reef aquaria. There are two other is to culture Asterina elsewhere and feed although such animals could scavenge species that I have only seen in reference your shrimp with them. partially damaged specimens. Some sources books. One is vivid green and the other Of course, it almost goes without state that it is due to poor handling and 1cm+ across the arms at maximum size. saying that there must be no other starfish shipping, others that it is due to diet. It is Both of these species are reported to species present as these are likely to be possible that both causes produce similar predate hard and soft corals. Thus only attacked also! symptoms but the experiences with these prolonged observation of any suspicious- Starfish have remarkable powers of animals lead many aquarists to believe that looking specimens will determine what you regeneration. Most species are capable of all starfish are sensitive. have. Indiscriminate removal will ease your regrowing an arm; some detached arms are Apart from the different Asterina fears but have you just dumped something capable of regrowing a complete animal. species another type almost guaranteed to quite useful? Sometimes we experience individuals from do well is a species of sand-shifting starfish a species that usually has only five arms – probably a member of the genus Control of harmful species but they have six, or four or more or less! Asteropecten. These animals have become Given the potential for reproduction that Asterina shows in reef aquaria it is unlikely that the aquarist will ever control nuisance or harmful species of starfish by manually removing them, although this may be a valid technique if you can identify a potential problem early enough. One technique of biological control has been suggested involving the use of the Harlequin shrimp (Hymenocerca elegans). These beautiful crustaceans are obligate starfish eaters and so will ignore other foods. Asterina spp. are on the list of suitable foodstuffs for this shrimp but simply adding this species to your reef and expecting problems to disappear is naive. Biological control can work very well in a Fromia nodosa. Maldives starfish showing tissue necrosis after seeming to "blow apart". This variety of situations but always relies on a phenomenon is very unlikely to be caused by predation. MARINE WORLD –APRIL/MAY 2003 • 35 Friend or Foe separation of legs in the aquarium. There’s not a lot you can from the body. As say about comments like that is there? The an escape response class, Ophiuroidea does contain a group this mirrors the which are the exceptions to the rule – the tactic used by basket starfish. These filter-feeding animals several lizard have arms which are branched to the extent species and the that they resemble fine lace when extended.

Load more

  4

Copy Link