Echinoderm means literally “spiny skin”. This characteristic
is due to the unique internal skeletal structure found in all echinoderms,
consisting of calcareous ossicles, which are articulated in sea stars
and rigid in sea urchins and contains a type of tissue termed mutable
connective tissue. This can determine the rigidity of the echinoderms’ body
and is especially useful for animals that utilise its versatility.
For example some starfish need to be manoeuvrable and flexible but
then when predating on hard shelled animals they may need a degree
of rigidity when attempting to prise the bivalve open. As aquarists
we can experience the incredible nature of mutable connective tissue
by observing sea cucumbers - a group of echinoderms. Sea apples in
particular can blow themselves up to relatively huge sizes (from
tennis ball to
football) in no time at all. It is reported that vigorous rubbing
of a sea cucumber causes the tissue to become almost liquid- literally
flowing through your fingers. Of course, this begs the question -
on earth would you want to do this but it is an interesting phenomenon
nonetheless. Brittle starfish also exploit the properties of such
tissue by shedding their arms when threatened - a phenomenon termed
Another important feature of echinoderms is their water-vascular system
used in locomotion and food acquisition. We can see the tube feed of
starfish on their underside and often all over the surface of sea urchins-
both are governed by the water-vascular system. Damage to this specialised
series of structures has been blamed for lack of success with some
starfish in captivity.
Echinoderms number some 6000 described species divided into 5 further
classes as listed above. Of these the brittle stars have the most living
species although the fossil record demonstrates that crinoids were
extremely numerous and diverse many millions of years ago. Comatulid sea stars or “feather starfish” are sometimes purchased
by aquarists due to their beautiful colouration. However, they are
not suitable for the vast majority of marine aquaria and should be
avoided. Crinoids are perhaps the only echinoderms we might not expect
to encounter in a live rock based reef aquarium.
Class Asteroidea – True Starfish
True starfish have a variety of forms with the average number of arms
being five but up to fifty are possessed by some species. In other
species arms have become so shortened that the animal resembles a pentagon
shaped pillow. Correct identification of closely related species is
not straightforward and the taxonomist may have to resort to dissection
to be absolutely sure of what they have. Fortunately, few asteroid
starfish make their way into reef aquaria as accidental introductions.
Those that do are likely to be from the following genus.
These small starfish are termed “cushion stars” due to
their chunky appearance and quite short arms. Colours vary and are
not a reliable means of identification but the unifying feature of
the species we encounter regularly in our aquaria is the slightly “chewed” look – almost
as if something has been nibbling their legs! The numbers of legs vary
although five appears to be average. In the U.K. we have an Asterina
species called the common cushion starfish. This species is unusual
in that it is not a broadcast spawner like most of the rest of the
true starfish, instead eggs are laid under stones and rocks. If this
is true of the reef specimens then this could explain why this species
seems to thrive in aquarium situations although they are also said
to reproduce through fission i.e. the splitting of the body into two.
There are reported to be three species of Asterina that are commonly
imported with live rock. The 0.5cm, whitish ones appear to be pure
algal feeders. This is the species we are most likely to experience
in our reef aquaria. There are two other species which I have only
seen in reference books. One is vivid green and the other 1cm+ across
the arms at maximum size. Both of these species are reported to predate
hard and soft corals. Thus only prolonged observation of any suspicious-looking
specimens will determine what you have. Indiscriminate removal will
ease your fears but have you just dumped something quite useful?
Control of harmful species.
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 Harlequin shrimp. 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 variety of situations
but always relies on a pest population being controlled rather than
eradicated- thus the pest species must always be present albeit in
small numbers - okay if you don’t mind a few of your corals being
scoffed rather than all of them. We have seen this situation before
with the nudibranch Chelidoneura varia which predates flatworms such
as Convolutriloba retrogemma. Once the pest population is drastically
reduced the predator population starves to death. Then the risk is
that the pest population recovers or further pests are reintroduced.
I believe that in this scenario such a control technique is unethical
but there could be scope for perhaps sharing predators amongst aquarists
(good luck in catching the shrimp!). The other alternative is to culture
Asterina elsewhere and feed your shrimp with them.
Of course, it almost goes without saying that there must be no other
starfish species present as these are likely to be attacked also!
Starfish have remarkable powers of regeneration. Most species are
capable of regrowing an arm, some detached arms are capable of regrowing
a complete animal. Sometimes we experience individuals from a species
that usually has only five arms – but they have six, or four
or more or less! Thus counting legs is an unreliable identification
Many aquarists experience problems with starfish within a relatively
short time. Many specimens seem to disintegrate, sometimes very rapidly,
sometimes of a period of weeks. Reasons for this are unclear. Many
aquarists first believe that this is the result of an attack by a crustacean
or bristleworm but this is extremely unlikely to be the cause, although
partially damaged specimens could be scavenged by such animals. Some
sources state that it is due to poor handling and shipping, others
that it is due to diet. It is possible that both causes produce similar
symptoms but the experiences with these animals lead many aquarists
to believe that all starfish are sensitive.
Apart from the different Asterina species another type almost guaranteed
to do well is a species of sand-shifting starfish probably a member
of the Genus Asteropecten. These animals have become increasingly
available over the past few years and many aquarists are using them
to control detritus build up in their sand which can , if untreated,
lead to cyanobacterial blooms (slime algae).
Class Ophiuroidea - The Brittlestars
Class of echinoderms get their common name from the fact that careless
handling or threat of predation causes the separation of legs
from the body. As an escape response this mirrors the tactic used by
several lizard species and the snail Stomatella varia which
leaves the potential predator with a twitching morsel while the victim
off rapidly and relatively unscathed. The brittlestar then regrows
a new arm. Brittlestars are perhaps the most abundant of all echinoderms
that arrive by accident in reef aquaria. The good news is that the
vast majority of such specimens are totally harmless and can be encouraged.
Some species, such as Amphioplus,
reproduce asexually and reach great numbers if conditions suit.
Most of the species we encounter are very small, however, we are often
tempted to purchase larger specimens as they have a useful role in
detritus consumption. I prefer the sand shifters for this as some species
of brittlestar are not as innocuous as they first appear. For example
the green brittlestar, is a commonly kept species that can breed
readily in home aquaria. It performs a useful role in many aquaria
as it will take uneaten food from the substrate and becomes very tame
with time. However, this species has a sinister side too. Each arm
is lined with elongated spines, similar to those in other species.
However, used in the right way the starfish is in possession of an
excellent trapping mechanism. Lifting its circular body clear from
the substrate the brittlestar forms a shaded area which seems to appeal
to fish which move innocently underneath. The brittlestar detects this
and springs its trap move quickly downwards and bringing the legs together.
The spines form a cage and the fish, if has not been alert enough,