RECOGNIZING SIBLING SPECIES AMONG REEF-DWELLING COMATULID CRINOIDS: IMPLICATIONS FOR ECOLOGICAL PARTITIONING AND TAXONOMY

Over the last decade, there has been a marked decline in abundance of the most common reef-dwelling comatulid crinoids in the Netherlands Antilles, namely Davidaster rubiginosa and D. discoidea. However, one morphotype of Davidaster, distinguished by orange arms with yellow-tipped pinnules, has maintained relatively consistent numbers throughout this period. Numerous lines of evidence indicate that this "yellow-tip" morphotype represents a sibling species closely related to D. rubiginosa. Besides the difference in coloration, recent field observations from Curaçao and Bonaire show "yellow-tips" to vary regularly from other comatulids in habitat, depth range, posture, and skeletal morphology. Preliminary morphometric and electrophoretic work by Liddell (1980) indicated that "yellow-tips" differ from D. rubiginosa, D. discoidea, and Nemaster grandis. Further multivariate morphometric and DNA sequence analyses have been undertaken to explore these differences. Recognition of "yellow-tip" as a potential sibling species of D. rubiginosa has several implications. First, the sustained presence of "yellow-tip" on Curaçao and Bonaire reefs suggests that this is a distinct species, responding differently than its sister taxa to yet unknown stresses. Second, the existence of sibling species among reef crinoids provides further support to the recent recognition, from corals and other invertebrates, that reef species diversity may be higher than previously appreciated. Third, such taxonomic revisions are helping to redefine ecological zonation in reefs; niches are recognized as more narrow and pronounced as species with tighter environmental constraints are defined. Finally, careful morphometric and ecological studies on modern fauna will help to determine whether sibling species complexes, like that present in Davidaster, may be recognizable in the fossil record.