Loss of arm spines in the basketstar Astrophyton muricatum (Echinodermata: Ophiuroidea: Gorgonocephalidae), with a key to West Indian basketstars

摘要

Adult specimens of the West Indian basketstar Astrophyton muricatum lack spines on basal branches of the arms. This character has been used in a number of taxonomic treatments to distinguish several genera of basketstars from others that bear arm spines on basal branches. Examination of a growth series revealed that juvenile A. muricatum bear arm spines on basal regions and lose them progressively farther out on the arm over time. At all sizes, spines on more basal spine-bearing branches lose the morphology that is typical of most spine-bearing segments. Individual spines seem to be resorbed from the imperforate claw-like tip to the bulbous perforated base. Spine bosses on spineless segments lose the characteristic morphology that supports the attachment and operation of spines. Of four possible hypotheses to explain the absence of spines from basal arm segments, we reject the hypothesis that juveniles fail to form spines on those segments. Data predominately support the hypothesis that basal spines are eventually resorbed, but the hypothesis that spines are released by histolysis at some point in life does not conflict with the evidence on spine loss. There is no evidence that spines are abraded and not regenerated. A taxonomic key to basketstars of the West Indian Province is provided that does not rely on presence or absence of arm spines.