Acaudina

Acaudina is a genus of sea cucumbers belonging to the family Caudinidae. These animals are small, smooth, and spindle-shaped, with a thin body wall.

Their most notable feature is the lack of a tail-like appendage, which sets them apart from similar genera. They live in tropical and subtropical waters, occupying habitats like intertidal zones, estuaries, reefs, sandbars, seagrass meadows, and soft-bottom or anoxic mudflats where salinity is slightly lower. As benthic detritivores, they feed by swallowing sediment to digest organic matter, and in doing so, they help churn up the seafloor as bioturbators.

Etymology

The genus was first described in 1907 by Hubert Lyman Clarke, who created it for a species that differed markedly from other members of the order Molpadida. The name comes from Latin, combining "a" (without) and "caudina" (tail), referring to the missing caudal appendage found in related genera like Caudina. This difference in the rear body region was key to recognizing it as a separate group. The name Aphelodactyla (H. L. Clark, 1908) has been used as a synonym, but it is not accepted.

Taxonomy

Taxonomically, Acaudina belongs to the order Molpadida, sea cucumbers that lack feeding feet except for their tentacles. They are considered apodous holothurians—slim, wormlike, with thin body walls. The type species is Molpadia demissa (Sluiter, 1901), later recognized as Acaudina demissa, originally described from a preserved specimen collected in the Madura Strait, Java.

According to WoRMS, the valid species include Acaudina bacilla, A. demissa, A. leucoprocta, A. molpadioides (with the unaccepted synonym Aphelodactyla delicata), A. pellucida, A. punctata, A. rosettis, A. spinifera, and A. suspecta. Many species have been synonymized over time; for example, there is debate over whether A. australis is distinct from A. molpadioides and whether A. hualoeides is a valid species. WoRMS currently lists both A. australis and A. hualoeides as valid, despite conflicting assessments by different authors.

Morphology

In terms of morphology, Acaudina has a cylindrical, stout body that does not taper toward the rear. Unlike Caudina, the posterior end is blunt and rounded, not set off as a tail. Colors range from dark grayish to semi-translucent, with some species showing orange, brown, or purple pigment.