The phylum Echinodermata consists of 5 main classes sea
stars, brittle stars, sea urchins, sea cucumbers and feather stars.
The Latin name echinoderm literally means spiny-skinned in reference
to the plate-like calcareous skeleton of these animals.
are characterized by a radial symmetry, which is very obvious in
most starfishes. Also common to all echinoderms is a remarkable
water vascular system, serving as locomotion. Water is drawn into
the system through a sieve plate on the upper surface and conveyed
to the arms by a series of canals.
Many echinoderms serve as hosts
to a variety of symbiotic or parasitic organisms.
Starfish are well known for their powers of regeneration. A complete
new animal can grow from a small fragment. In some species one of
the arms will virtually pull itself away from the body and
Sexual reproduction is also evident in this group. Sexes
are generally separate. Their eggs get fertilized externally. From
the huge amount of released eggs only relatively few of the free-swimming
large species which are about 40 cm in diameter.
coral polyps and can destroy entire reefs when occurring periodically
in big amounts.
It has sharp, toxic spines and wounds can
be very painful. Medical attention should be obtained.
among algae-covered coral debris and in sea grass beds from
shallow water down to 25 m.
This species prefers asexual reproduction
separating an arm from its body, which converts to a complete
sea star is sometimes inhabited by a parasitic transparent
fish that actually lives within its gut cavity.
Feather stars are one of the most visible members of the reef community.
They are abundant in areas with strong currents, which provides
an abundant supply of planktonic food.
They use special appendages
called cirri on the underside of the body to cling to the bottom. They are frequently perched on sponges and tall coral heads in order
to get maximum exposure to the passing current. They use the cirri
for crawling over the reef and can actually swim by flapping the
They have skin toxins which serve as a deterrent.
The bright colors displayed by many feather stars signal distasteful
frequently encountered feather star with many different colorations.
Primarily nocturnal in habit, it is not unusual to see them
They are opportunistic; feeding when currents
Sea urchins are generally nocturnal. They spend the day tightly
wedged under rocks or in crevices. However, the diadema sea urchin
sometimes forms aggregations in exposed situations. In spite of
their sharp spines, urchins are fair game for some fishes, particularly
trigger fishes and puffers.
Many sea urchins are algal grazers,
but others feed on a variety of organisms, especially sponges and
ascidians. The mouth is centrally located on the underside of the
body, equipped with well developed jaws and a set of horny teeth.
Sea urchins are basically hollow shells. Very little space is occupied
by the internal organs. During the breeding season, the body cavity
is crammed with eggs and sperm, what make them very attractive for
Although they should be respected as a potential
danger because of their venomous spines, sea urchins are beautiful,
found on hard substrates, feeding on algae and coral polyps.
Its black spines are long, thin, hollow and very fragile (harmful,
but not poisonous).
May be locally very abundant as they ar at the Similan Islands.
colorful member of the sea urchin family.
singly on sandy parts within coral reefs not deeper than 20
At first glance sea cucumbers appear to have little in common with
starfishes or sea urchins. Unlike other members of this phylum,
they do not have distinct radial symmetry, at least on the outside.
The more or less flexible calcareous skeleton of the echinoderms
is reduced to microscopic particles, embedded into the leathery
skin. The body is elongate with a head and a tail. This body form
evolved in response to their way of life.
Like living conveyor belts
they ingest large amounts of sand while slowly crawling over the
bottom. The edible, organic material is digested in a tube-like
digestive tract. The processed sand is then expelled from the anus,
leaving a characteristic trail on the bottom.