Meeting Abstract

P3.68  Wednesday, Jan. 6  Organization of the musculature in species of Seison (Rotifera), ectosymbionts of the marine crustacean Nebalia pugettensis HOCHBERG, Rick*; MALISKA, Max; LEASI, Francesca; Univ. of Massachusetts Lowell; Univ. of Washington; Univ. of Milan, Italy rick_hochberg@uml.edu

Species of Seison are sexually dimorphic, ectosymbiotic rotifers on marine leptostracans of the genus Nebalia. Many studies of rotifer evolution hypothesize an ancestral position for Seison within Rotifera, but several details on the anatomy of the inclusive species remain unknown, hindering an understanding of rotifer evolution. In this study, we examine the muscular system of two unidentified species of Seison to determine if muscle patterns show a plesiomorphic organization relative to species in more derived clades. Both male and female rotifers of Seison sp. were collected from Nebalia pugettensis on the coast of San Juan Island, Washington, stained with phalloidin, and examined with CLSM. Results reveal that the musculature of Seison is segmentally arranged, with separate sets of longitudinal muscles restricted to the head, neck, trunk and foot. Complete somatic circular muscles are absent from both species and both sexes; however, somatic, semicircular muscles (incomplete circular muscles) are present in the head of one species and the head and trunk of a second species. Splanchnic circular muscles are complete and line the entire digestive tract. Males and females of both species possess similar somatic muscle patterns, with minor differences generally present in the number and organization of muscles that supply the reproductive systems. A comparison of seisonoids with rotifers from the other major clades is presented.