Euretaster insignis

Lore & Background

This starfish was first described in 1882 by marine biologist Percy Sladen as Retaster insignis, Retaster being a small segregate genus of Pteraster. In 1940, the genus had to change its name after Walter Kenrick Fisher noted that the type species of Retaster he had himself designated, Pteraster capensis, was in fact a sound species of Pteraster, which automatically made the former genus a synonym of the latter. The genus known as Retaster required a new name, and the one Fisher coined was Euretaster, with R. insignis as its type species.

The order Velatida are presumed to be the earliest lineage of living sea stars to diverge from the ancestral line, their closest living relatives being the abyssal sea daisies in the order Peripodida. The three members of the genus Euretaster are the only tropical, shallow-water species in the order Velatida, the remaining members living at abyssal depths. It might be presumed that the order has its origin in deep water, however there are numerous shallow-water fossils of velatidans dating from the Cretaceous period and it seems that these ancient forms have largely been driven out of shallow water habitats by competition from recent, more-advanced taxa.

This small starfish is star-shaped and has five stubby arms. The aboral surface has an inflated appearance due to it being covered by a supradorsal membrane; this is supported by bundles of spinelets borne on the flat plates that are arranged in a mosaic pattern on the cuticle below. This membrane encloses a supradorsal cavity filled with sea water, which is connected to the water column by a central osculum, which opens and closes periodically. The membrane has a reticulated pattern of ridges and small conical protrusions. It is patterned in some combination of red, white and brown.

Reader's Guide

Euretaster insignis holds significance as one of only three species in the order Velatida that inhabit shallow tropical waters, the rest of the order being found at abyssal depths. This suggests a relict distribution, as numerous shallow-water velatidan fossils from the Cretaceous period indicate that ancient forms were once common in shallow habitats but have largely been displaced by more advanced taxa. The species also illustrates a unique reproductive strategy: the developing embryos are brooded in the supradorsal cavity and receive nourishment from a mucosal secretion produced by the mother's cuticle, a process described as 'cannibalistic ectoparasitism.' The larvae are brooded until they reach about 1 cm in diameter, at which point they are released as juvenile starfish. Its taxonomic history reflects the careful work of early marine biologists: originally placed in the genus Retaster by Percy Sladen in 1882, the genus was renamed Euretaster by Walter Kenrick Fisher in 1940 after a nomenclatural issue with the type species. This starfish is found on coral reefs, rocky coasts, sand and muddy rubble in the central west Pacific, from the intertidal zone down to about 132 m.

Did You Know?

• The genus name was changed from Retaster to Euretaster in 1940 by Walter Kenrick Fisher.
• The supradorsal membrane encloses a cavity filled with sea water, connected to the water column by a central osculum.