CHAETAL ARRANGEMENT AND HOMOLOGY IN SPIONIDS (POLYCHAETA: SPIONIDAE)
Radashevsky, V. I. 1 & Fauchald, K. 2
1 Institute of Marine Biology, Vladivostok, 690041, Russia
2 National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560, USA
Chaetal arrangement in spionids is analyzed to provide data for phylogenetic analysis of the family. In anterior segments notochaetae are generally capillaries arranged in three distinct groups or rows including an anterior and a posterior row and a dorsal superior tuft; neurochaetae are also capillaries arranged in anterior and posterior rows plus a ventral inferior tuft. In succeeding segments, the notopodial capillaries are reduced in number with the groups becoming indistinct. The neuropodial capillaries are usually replaced by hooded hooks in differing patterns. These patterns of replacement include those with replacement only in the posterior row, in both anterior and posterior rows, or of all neuropodial capillaries. A pattern of alternating neuropodial capillaries alternating with the neuropodial hooded hooks is described for the first time. In most polydorid taxa the chaetal arrangement on segment five is changed from a vertical to a diagonal or horizontal position, presumably associated with the elongation of the segment. In these taxa the superior notochaetae are positioned anterior to the neurochaetae. The modified chaetae are usually arranged in two curved diagonal or horizontal rows corresponding to the anterior and posterior rows of notochaetae in other spionids. Based on positional criteria different kinds of spionid chaetae can be shown to be homologous making it possible to code them consistently for a cladistic analysis.