Worth a second look: gill structure in Hemipecten forbesianus (Adams & Reeve, 1849) and taxonomic implications for the Pectinidae

摘要

The gill of the reef-dwelling Hemipecten forbesianus was examined using histology and scanning electron microscopy, in order to verify its reported homorhabdic state, and to provide the first detailed information on tropical pectinid gill structure. Due to the difficulty of obtaining live individuals, museum reference specimens were used for this study. The gill is heterorhabdic, and presents several notable differences compared to the larger, temperate species studied to date: (1) the small number of ordinary filaments per plica (probably associated with the very small size of these specimens), (2) the outer demibranch is approximately 15% shorter than the inner demibranch, (3) several characteristics which confer gill cohesion in other pectinid gills are absent and (4) the principal filaments are devoid of mucocytes, such that particle transport of positively-selected particles must be entirely effected in water. The apparent universality of the heterorhabdic condition within the Pectinidae leads us to recommend its use as a reliable soft-body taxonomic character at the family level. The anatomical differences between the gill of H. forbesianus and that of the larger, temperate species call for further comparison with both large and small, temperate and tropical species, in order to elucidate the effects of size and habitat on gill structure in this family.