WHAT IS A STARFISH
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Starfish (also known as Sea Stars) belong to the phylum Echinodermata,
and are therefore related
to sea urchins and sea cucumbers. All echinoderms are characterised by their five-point
radial symmetry. There
are over 1,800 species of starfish and they vary considerably. Starfish and Sea Stars belong to the class Asteroidea,
whilst Brittle Stars or Basket Stars are ophiuroids.
They are all marine creatures, although some live in the intertidal zone,
some in deep water, some in tropical areas and some in cold water. They
typically have a central disc body and five arms (or multiples thereof). Some species have many more arms, such as the
40 arms of the sun star. They are often brightly coloured in shades of red or
orange, or blue/grey/brown.
The surface of their skin can be spiny or smooth, depending on the species. A starfish's skin may feel leathery, or slightly
prickly. Starfish have a tough covering on their upper side, which is made up
of plates of calcium carbonate with tiny spines on their surface. A starfish's
spines are used for protection from predators.
While they can't see as well as we do, starfish have an eye spot at the
end of each arm. This is a very simple eye that looks like a red spot. The eye
doesn't see much detail, but can sense light and dark.
Instead of blood, starfish have a water vascular system, in which the
sea star pumps sea water through its sieve plate. Starfish have
hundreds of little feet along their arms called tube-feet. These tubes are
connected by small canals and allow them to operate a water vascular system
which enables their respiration, locomotion, food and waste transpiration. They move across both land and sea by
alternatively contracting muscles that allow water into the tube feet, causing them
to extend and contract in order to push them against the ground.
Their mouth and gut is located at the bottom of their central disc. They
have a unique way of eating their prey. They eat bivalves like mussels and
clams, as well as small fish, snails, and barnacles. Starfish have
two stomachs and they can push their first stomach out through their mouth and
into its preys shell. This means they can eat something like a clam by pushing
their stomach into its shell, rather than trying to get it into their mouth. It
starts to digest the animal, then pulls its stomach back in and passes it to
the second stomach. This allows the sea star to eat larger prey than it would
otherwise be able to fit into its tiny mouth.
Starfish can reproduce both sexually and asexually, although they commonly reproduce by spawning. To increase their chances of
fertilization, starfish probably gather in groups when they are ready to spawn,
use environmental signals to coordinate timing (day length to indicate the
correct time of the year, dawn or dusk to indicate the correct time of day),
and may use chemical signals to indicate their readiness to each other. Fertilized
eggs live as plankton, suspended in the water and swimming by using beating cilia. Unlike adults, the larvae are bilaterally symmetrical and have a distinct left
and right side. Eventually, they undergo a complete metamorphosis, settle to
the bottom, and grow into adults.
Some, but not all, species of starfish can regrow a new limb given time. Some
species can even regrow a new central disc from a single arm, therefore
creating a whole new starfish. However, often part of the central body is required
to be joined to the arm. This fragmentation is often used to evade predators or
as an escape response. It often takes a starfish several months or even years
to carry out this regeneration, and they are vulnerable to infections during
this stage.
Starfish are important ecologically,
and are keystone species, playing a critical role in maintaining the structure
of an ecological community. Its impact on the community is greater than would
be expected based on its relative abundance or total biomass. The carnivorous starfish Pisaster ochracceus feeds on mussels and plays a key role
in maintaining the balance of all other species in the community. Without the Pisaster
ochracceus the populations of mussels within the community would soon grow unchecked and greatly reduce the community's diversity.
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