Brittle
stars
Class Ophiuroidea
| On Chek Jawa, brittle
stars are found everywhere but are rarely seen as they shun
the light and are more active at night. Often, all that can
be seen of a brittle star are its arms! |
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'Ophiuroidea'
means 'snake-like'. There are about 2,100 species of
brittle stars, making them the largest class of echinoderms.
Most brittle stars have tiny central disks of 1-3cm,
but many have very long arms.
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Ones with longer arms may be seen in the coral rubble and seagrass lagoon,
hiding among rubble or under the sand. Tiny ones shelter under rocks,
beneath the sand and in sponges.
Brittle stars
are related to sea stars but belong to a different class and have
somewhat different features and habits. Brittlestars have thin, long
and highly flexible arms. The arms are attached to tiny central disks.
However, like other echinoderms, brittle stars are symmetrical along
five axes, have spiny skin and tube feet.
An armful: A brittle star is almost
all arms. Its central disk is usually only a few centimeters wide
while its arms can be very long. The arms are made up of large, well
developed ossicles (plates made mostly of calcium carbonate). The
ossicles are connected together like a bicycle chain. A brittle star
lengthens its arms by adding ossicles where the arm joins the central
disk.
Speedy stars: Brittle stars are the fastest-moving echinoderms!
While sea stars use their tube feet to move slowly, brittle stars
use their highly flexible, spiny arms instead. Their arms move in
a snaky manner, hence their Class name, which means 'snake-like'.
To move, a brittle star generally gets a grip on something with one
or two spiny arms. These then pull while the remaining arms push or
trail behind. Some brittle star may also 'swim' by vigorously rowing
its highly flexible arms, almost as if it was doing the breast-stroke!
Falling apart: As its name suggests,
a brittle star has a tendency to fall apart. It may purposely throw
off an arm when threatened. The dropped arm may continue to wriggle
to distract the predator while the brittle star escapes. The brittle
star is able to do this because the ossicles in its arms are connected
by mutable connective tissue. The brittle star can rapidly change
the consistency of this tissue from rock hard to almost liquid. The
arm eventually re-grows, but it can take months before it is fully
restored.
Stars come out at night: Brittle stars are plentiful but
seldom seen. They have many predators, so brittle stars usually only
come out at night. Creatures that snack on brittle stars include fish,
crabs, hermit crabs, mantis shrimp and even sea stars and other brittle
stars.
Star-spangled sponges: Tiny brittle
stars (1-2cm with arms) often live inside sponges.
Look closely at the holes of a sponge and you might see their little
arms sticking out.
Brittle star food: Many brittle
stars feed on detritus, using their arms to gather this from the surface
or to filter these out of the water. Unlike sea stars, a brittle star
doesn't have a groove on the underside of its arms. Tiny tube feet
emerge from holes between the ossicles in the arms. These may 'wipe
off' food particles stuck on the hooked or mucous-coated spines, or
collect particles off the surface, and pass these on to the central
mouth. Other brittle stars are carnivores that use their arms to sweep
tiny creatures to their mouths. Yet others are scavengers, nibbling
on their food with their jaws or using the tube feet near their mouth.
Some brittle stars use their tube feet to sense chemicals released
by their food.
Brittle stars have only one opening on their underside that functions
as both a mouth and anus! Unlike sea stars, the digestive system of
brittle stars doesn't extend into their arms. A brittle star's mouth
is surrounded by jaws made up of a circle of five large toothed plates
that meet in the middle. Unlike sea urchins, the jaws cannot be extended
outwards.
Brittle star babies: Most brittle
stars have separate genders and are usually either male or female.
Some spawning brittle stars assume a push-up posture then release
sperm or eggs simultaneously into the water. This usually happens
at night. Brittle stars undergo metamorphosis and their larvae look
nothing like their adults. The form that first hatches from the eggs
are bilaterally symmetrical and free-swimming, drifting with the plankton.
They eventually settle down and develop into tiny brittle stars. Some
brittle stars brood their eggs. Here is a fascinating photo
of a brittle star larva on Image
Quest 3-D Marine Library
Human uses: Brittle stars do not
appear to be widely eaten or used for other purposes.
Status and threats: Like many
other sea creatures, brittle stars are threatened mainly by human
activities that degrade or destroy their habitat. |
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Large
brittle star
with very long arms!
Close-up of
central disk
Close-up of
spiny arm
Night Stars
Brittle stars are more active at night
Tiny star
Tiny brittle stars
live in sponges
Reaching Out
Arms of tiny brittle
stars stick out
holes in a sponge |
