Brittle stars are largest of the major
groups of echinoderms in numbers of
species, and they are probably the
most abundant also. They abound in
all types of benthic marine habitats,
even carpeting the abyssal sea bottom
in many areas.
Form and Function
Apart from the typical possession of
five arms, brittle stars are surprisingly
different from asteroids. The arms of
brittle stars are slender and sharply set
off from the central disc (Figure 23-11).
They have no pedicellariae or papulae,
and their ambulacral grooves are
closed and covered with arm ossicles.
Their tube feet are without suckers;
they aid in feeding but are of limited
use in locomotion. In contrast to asteroids,
the madreporite of ophiuroids is
located on the oral surface, on one of
the oral shield ossicles (Figure 23-12).
Ampullae on the podia are absent, and
force for protrusion of the podium is
generated by a proximal muscular portion
of the podium.
A, Brittle star Ophiura lutkeni (class Ophiuroidea).
stars do not use their tube feet for
locomotion but can
move rapidly (for an
echinoderm) by means of their arms.
star Astrophyton muricatum (class
Basket stars extend their many-branched
to filter feed, usually at night.
Oral view of spiny brittle star Ophiothrix.
A, This brittle star Ophiopholis
aculeata has its
with eggs, which it is ready to
expel. The arms have been
broken and are
Oral view of a basket star
Each jointed arm consists of a
column of articulated ossicles (the socalled vertebrae
), connected by muscles
and covered by plates. Locomotion
is by arm movement. Arms are
moved forward in pairs and are placed
against the substratum, while one (any
one) is extended forward or trailed
behind, and the animal is pulled or
pushed along in a jerky fashion.
Ophiuroid with aboral disc wall cut away
show principal internal structures. The
are fluid-filled sacs in which water
circulates for respiration. They
also serve as
brood chambers. Only bases
of arms are
Five movable plates that serve as
jaws surround the mouth (Figure 23-12).
There is no anus. The skin is leathery,
with dermal plates and spines arranged
in characteristic patterns. Surface cilia
are mostly lacking.
The visceral organs are confined to
the central disc, since the rays are too
slender to contain them (Figure 23-13).
The stomach is saclike, and there is no
intestine. Indigestible material is cast
out of the mouth.
Five pairs of invaginations called bursae
open toward the oral surface by
genital slits at the bases of the arms.
Water circulates in and out of these sacs
for exchange of gases. On the coelomic
wall of each bursa are small gonads that
discharge into the bursa their ripe sex
cells, which pass through the genital
slits into the water for fertilization (Figure
23-14A). Sexes are usually separate;
a few ophiuroids are hermaphroditic.
Some brood their young in the bursae;
the young escape through the genital
slits or by rupturing the aboral disc. The
larva is called an ophiopluteus, and its
ciliated bands extend onto delicate,
beautiful larval arms (Figure 23-10C).
During metamorphosis to a juvenile,
there is no temporarily attached phase,
as there is in asteroids.
Water-vascular, nervous, and
hemal systems are similar to those of
sea stars. Each arm contains a small
coelom, a radial nerve, and a radial
canal of the water-vascular system.
Brittle stars tend to be secretive, living
on hard bottoms where little or no
light penetrates. They are generally negatively phototropic and insinuate
themselves into small crevices between
rocks, becoming more active at night.
They are commonly fully exposed on
the bottom in the permanent darkness
of the deep sea. Ophiuroids feed on a
variety of small particles, either browsing
food from the bottom or suspension feeding. Podia are important in
transferring food to the mouth. Some
brittle stars extend arms into the water
and catch suspended particles in
mucous strands between arm spines.
Regeneration and autotomy are
even more pronounced in brittle stars
than in sea stars. Many seem very fragile,
releasing an arm or even part of
the disc at the slightest provocation.
Some can reproduce asexually by
cleaving the disc; each progeny then
regenerates the missing parts.
Some common ophiuroids along
the coast of the United States are Amphipholis
, both sides of,
, horny scale) (viviparous and
hermaphroditic), Ophioderma (Gr. ophis
snake, + dermatos
, skin), Ophiothrix
(Gr. ophis, snake, + thrix, hair), and Ophiura
, snake, + oura
tail) (Figure 23-11). The basket stars Gorgonocephalus
, name of
a female monster of terrible aspect,
, a head) (Figure 23-14B)
, creature, animal) (Figure
23-11B) have arms that branch repeatedly.
Most ophiuroids are drab, but
some are attractive, with bright color
patterns (Figure 23-14A).