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Echinoderms
Echinoderms are detrivores that feed on the
waste of other animals and may even sift through the sand or other substrate in search for it. Other echinoderms may sit under rocks waiting
for any morsel of food to come their way while others consume organic material found on live rock, keeping the rocks clean for corals to grow
on. Still others consume plankton and other material floating in the water column. Because of these attributes, echinoderms can be
considered the garbage collectors of the reef and provide an important service for the overall health of a reef system.
It should be noted that some of these animals have developed predatory habits and many possess toxins. Owners of these animals should
understand the potential risk of keeping them and design the aquarium accordingly. Restraint should be the watchword before adding any of
these animals since they can be difficult to remove once they have been released into your reef system.
There are five classes of echinoderms that are commonly kept in the aquarium hobby. These include members of the class Asteroidea (starfish),
Ophiuroidea (brittle and serpent starfish), Holothuroidea (sea cucumbers), Echinoidea (sea urchins), and Crinoidea (feather stars).
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Anatomical Characteristics:
All echinoderms share several common
characteristics despite very dissimilar outward appearances differentiating them from all other members of the animal kingdom. First,
they all possess five-part radial symmetry around a central disk. Second, they all possess a very unique water vascular system. These unique characteristics distinguish echinoderms from other animals in the animal kingdom. |
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Pincushion Urchin |
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Indian Starfish |
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Radial Symmetry:
This is the most obvious common trait
shared by members of this phylum. This symmetry is based on a five-part system where a central structure is surrounded by five equal
parts. This is especially apparent in starfish, brittle stars and serpent starfish where the five arms or a multiple thereof are
arranged around a central disk. This radial symmetry is not as apparent in sea urchins or sea cucumbers, but when these animals are
closely examined or dissected, it becomes readily apparent that their body is arranged based on this 5-part symmetry. |
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particular body arrangement, these animals do not possess what we would regard as a head. They do not possess anything close to a central
nervous system and lack eyes or other means of dealing with complex stimuli. Instead, they possess a very simple combination of chemo and
tactile receptors. Echinoderms are able to differentiate between light and dark and have a very keen sense of chemoreception as
indicated by their relatively rapid movement when food is introduced into an aquarium. |
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Vascular System:
Echinoderms are also characterized by a
unique water-based vascular system possessed by no other animal. This highly specialized system not only allows them to transport
food and water along the outside of their bodies, but it also allows for other nutrients and gases to be transported as well. Their
hydro-vascular system has evolved to the point that it provides a means of locomotion simply by changing the water pressure within
their tube feet. |
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Red Starfish |
This system
is also unique in that water within this vascular system is open to the outside, that is, it is not self-contained. This peculiar
characteristic may partly account for why these animals are so sensitive to rapid changes in water chemistry and may go into osmotic
shock when salinity or ions in their environment are changed rapidly. This water vascular system is controlled by a sieve plate
(the madreporite), the dark colored structure on the upper surface of most sea stars. |
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