Common brittle star

Ophiothrix fragilis

ORDER

Ophiurida

FAMILY

Ophiothricidae

TAXONOMY

Ophiothrix fragilis Abildgaard, 1789, Denmark. OTHER COMMON NAMES

Dutch: Gewone brokkelster; German: Zerbrechlicher; Schlangenstern; Norwegian: Hastjerne.

PHYSICAL CHARACTERISTICS

A large brittle star with a disk up to 0.78 in (20 mm) in diameter. The arms are long, colorfully banded and very bristly with seven glassy spines on each segment. Its color varies; it may be patterned in red, yellow, orange, violet, gray or brown.

DISTRIBUTION

Widely distributed in the eastern Atlantic from northern Norway to the Cape of Good Hope; also common in the Mediterranean.

HABITAT

Occurs from the lower shore to about 1,640 ft (500 m) on hard sea bottoms swept by currents; or beneath boulders, in crevices, or on substrates among sessile organisms. It may also form dense groups or aggregations (brittle star beds) on bare sand or shell sediments, with as many as 2000 individuals per square meter in habitats with few predators.

BEHAVIOR

Predation pressure controls whether Ophiothrix fragilis hides in crevices or under rocks; that is, whether it is cryptic or lives in the open on the substrate. The formation of groups or aggregations on bare substrates helps individuals to maintain their position in strong currents. Under these conditions individual brittle stars interlock their supporting arms while lowering their disks onto the substrate. Lowering frictional resistance to the current helps to keep them stable. Juveniles seem to prefer to settle on or seek out adults; they can be seen clinging to the spines and bodies of adult individuals. They also settle on such other suspension feeders as sponges.

FEEDING ECOLOGY AND DIET

Ophiothrix fragilis is mainly a passive suspension feeder; like Amphiura filiformis, it collects organic particles on long and sticky tube feet covered with numerous papillae. It raises its arms about 3 in (7-8 cm) above the substrate; the collected particles are formed into a bolus and carried along the arm to the mouth.

REPRODUCTIVE BIOLOGY

The sexes are separate, with a seasonal developmental pattern. The ophiopluteus larva can be found in the northwestern Mediterranean from March to October, with the peak settlement period in June. The time from fertilization to metamorphosis is about 26 days. The ophiopluteus has unusually long arms, indicating that it is capable of dispersing over long distances.

CONSERVATION STATUS

Not listed by the IUCN.

SIGNIFICANCE TO HUMANS None known. ♦

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