User:SydneyEGrace/Astrobrachion adhaerens

Astrobrachion adhaerens (A. adhaerens) is a species of basket star in the order Euryalida. Euryalidan order encompasses brittle stars, ranging from basket to snake stars. A. adhaerens is one of two species, the other being Astrobrachion constrictum, in the genus Astrobrachion. As a whole, There is currently very little is known about these two species.

 Anatomy and Morphology 

Basket stars have extensively branched arms made of a complex system of tissues and muscles. These arms are capable of regeneration, which is a characteristic that likely evolved as a response to frequent breaks of long, thin, and fragile arms.

Genera of Euryalida typically have thick skin and possess lateral arm plates but rarely have ventral arm plates or dorsal arm plates. The genus Astrobrachion is set apart by having small ventral plates separated from the lateral arm by a supportive dermal tissue layer.

The length of arm spines on Astrobrachion is the same length as the corresponding arm segment, like its sister genus Squamophis. These two genus hence have the shortest spines in the Asteroschematidae family.

A. adhaerens has longitudinal stripes while its sister species A. constrictum has transverse color banding on the arms. Both species vary from red to yellowish-white in color.

Little is known of the reproductive system of A. adhaerens, but some sexual characteristics are likely shared with closely related ''A. constrictum. A. constrictum'' are dioecious and exhibit no sexual dimorphism. They have a fused bursae and long, tubular gonads in the proximal half of the arms. Somewhat unique to A. constrictum reproduction within the Asteroschematidid family is the structure of the germinal epithelium: folding over the testes and concentric tapering of the germinal epithelium in the ovaries.

 Distribution and Habitat 

The genus Astrobrachion was first identified in New Zealand. Sister genera have been identified in the Caribbean and Philippines.

A. adhaerens is found in the tropical regions of Australia, from the western and northern coasts to New South Wales, and has additionally been observed in the Lord Howe Islands and the Kermadec Islands. The species is allopatric except at Lord Howe Island. Basket stars are found to reside primarily on black corals, but juvenile basket stars have been found in great abundance on Nephtheidae soft coral.

 Behavior 

Very little of Astrobrachion behavior has been observed due to nocturnal activities and a deep water habitat inhibiting observation and making it difficult to successfully collect specimens. A. adhaerens are suspension feeders that unfold their long, thin arms to feed at night.

 Taxonomy 

The taxonomy and phylogeny of species in the order Euryalida and class Ophiuroidea has been ambiguous and continues to be investigated.

Euryalid fossils date back to strata as young as Cretaceous and Neogene. In the past, the ophiuroids were divided into euryalids and non-euryalids due to stark morphological differences.

Increased study in the areas of ophiuroid and euryalid historical morphology have revealed that Euryalids diverged from Ophiurids during the Triassic period and thus ancestors shared some of the morphologies of the class Ophiurids.