Class Ophiuroidea
Class Ophiuroidea
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.
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.
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.
Biology: 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 (Gr. amphi, both sides of, + pholis, horny scale) (viviparous and hermaphroditic), Ophioderma (Gr. ophis, snake, + dermatos, skin), Ophiothrix (Gr. ophis, snake, + thrix, hair), and Ophiura (Gr. ophis, snake, + oura, tail) (Figure 23-11). The basket stars Gorgonocephalus (Gr. Gorgo, name of a female monster of terrible aspect, + kephale, a head) (Figure 23-14B) and Astrophyton (Gr. asteros, star, + phyton, 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).
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.
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| Figure 23-11 A, Brittle star Ophiura lutkeni (class Ophiuroidea). Brittle stars do not use their tube feet for locomotion but can move rapidly (for an echinoderm) by means of their arms. B, Basket star Astrophyton muricatum (class Ophiuroidea). Basket stars extend their many-branched arms to filter feed, usually at night. |
Figure 23-12 Oral view of spiny brittle star Ophiothrix. |
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| Figure 23-14 A, This brittle star Ophiopholis aculeata has its bursae swollen with eggs, which it is ready to expel. The arms have been broken and are regenerating. B, Oral view of a basket star Gorgonocephalus eucnemis, showing pentaradial symmetry. |
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| Figure 23-13 Ophiuroid with aboral disc wall cut away to show principal internal structures. The bursae are fluid-filled sacs in which water constantly circulates for respiration. They also serve as brood chambers. Only bases of arms are shown. |
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.
Biology: 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 (Gr. amphi, both sides of, + pholis, horny scale) (viviparous and hermaphroditic), Ophioderma (Gr. ophis, snake, + dermatos, skin), Ophiothrix (Gr. ophis, snake, + thrix, hair), and Ophiura (Gr. ophis, snake, + oura, tail) (Figure 23-11). The basket stars Gorgonocephalus (Gr. Gorgo, name of a female monster of terrible aspect, + kephale, a head) (Figure 23-14B) and Astrophyton (Gr. asteros, star, + phyton, 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).
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