A comparative study of selected skeletal structures in the seastars Asterias forbesi Desor, A vulgaris Verrill, and A rubens L, with a discussion of possible relationships
Worley, E.K.; Franz, D.R.
Proceedings of the Biological Society of Washington 96(3): 524-547
ISSN/ISBN: 0006-324X Accession: 004558928
Morphological structures from the congeneric north Atlantic seastars, A. forbesi (Desor), A. vulgaris Verrill and A. rubens L., were measured and compared. Significant differences in size and/or shape were found between A. forbesi and A. vulgaris in the following structures: ventral pedicellariae, adambulacral spines, oral spines, the madreporite, skeletal ossicles and the shape of the arms. Slight but insignificant differences in size, but not shape of these structures was shown between A. vulgaris and the few samples of A. rubens examined. The firmer, more rounded, less tapering shape of the arms in A. forbesi was attributed to the shape of the ossicles and their long processes which form junctions directly, or by 1 plate, with adjacent processes throughout the length of the arm. In the more flaccid, tapering arms of A. vulgaris (and A. rubens) short, blunt ossicle processes connect with adjacent processes by several plates in the proximal dorsolateral region forming a more open meshwork and more flaccid skeleton. The uniformly small size of these plates throughout the length of the arms in young A. vulgaris may account for the less tapered conditions sometimes found in small specimens causing them to resemble the shape of the A. forbesi arms. Variation in size and number of these plates is suggested to be associated with the production of morphs in A. vulgaris and A. rubens. The A. forbesi-like animals for the Maine [USA] population were therefore diagnosed as local morphological variations of the variable species A. vulgaris and not hybrids. Earlier ideas and hypotheses concerning the relationships and origins of the Asterias spp. are summarized and evaluated. A hypothesis is formulatd to account for the origin of all Asterias spp. from a common north Pacific ancestor.