Snake star

Astrobrachion constrictum

ORDER Euryalida

FAMILY

Asteroschematidae TAXONOMY

Astrobrachion constrictum Farquhar, 1900.

OTHER COMMON NAMES None known.

PHYSICAL CHARACTERISTICS

A large brittle star with a disk as much as 0.9 in (23 mm) in diameter. The arms are long, about 13.7 in (350 mm) and broken near the tip. A soft, smooth skin covers both the disk and the arms. The spines are shorter than the arm width; the distal spines are flattened, turning into compound hooks with 2-4 lateral teeth. Colors range from uniform bright yellow or deep red to almost black. Dark red or black individuals may have white spots or stripes.

DISTRIBUTION

The species is found throughout New Zealand, southeastern Australia, and New Caledonia.

HABITAT

Astrobrachion constrictum lives in a mutualistic relationship with the black coral Anthipates spp. These species are normally found together in relatively deep water (164-590 ft or 50-180 m) along the continental shelf but also in shallower water (up to 33 ft or 10 m deep) in the fjords of southwestern New Zealand. The black corals attach themselves to hard substrates and the snake stars coil around the branches of the coral.

BEHAVIOR

Astrobrachion constrictum is a nocturnal feeder. During the day it remains tightly coiled around its host. The relationship with the black coral is probably mutualistic; in the process of feeding by gathering mucus from the coral, the snake star also protects its host from organisms and detritus that would otherwise smother the colony. Snake stars show a strong preference for black corals; if they are removed, they move relatively quickly toward a black coral colony, probably by sensing chemical signals from the coral.

FEEDING ECOLOGY AND DIET

Snake stars feed during the night by stretching 2-3 arms out in the water column to snare drifting plankton with their tube feet and arm spines. They also wipe the branches of the coral in order to feed on the plankton and detritus collected on the coral.

Plankton And Detritus

REPRODUCTIVE BIOLOGY

A distinctive feature of this family is the position of the gonads within the arms. One shallow-water study of the fjords of southern New Zealand reported that these brittle stars have a prolonged reproductive period, possibly even continuous, as found in many deep-sea ophiuroids. The oocytes are relatively large (0.015-0.019 in or 400-500 pm), indicating that development is lecitotrophic (larvae do not feed), either direct or abbreviated. A larval stage, however, is not known.

CONSERVATION STATUS Not listed by the IUCN.

SIGNIFICANCE TO HUMANS None known. ♦

hooks on the arms assist the basket star in capturing food by sticking to the prey.

REPRODUCTIVE BIOLOGY

Not much is known about the reproduction of gorgono-cephalids. The genus has a large number of gonads, as many as 1000 per individual. The closely related Gorgonocephalus caput-medusae lays its eggs free in water; however, a larval form is not known. Juveniles of Gorgonocephalus spp. are also known to live on the soft coral Gersemia rubiformis.

CONSERVATION STATUS

Not listed by the IUCN.

SIGNIFICANCE TO HUMANS None known. ♦

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