Morphological variation in glochidia shells of six species of Elliptio from Gulf of Mexico and Atlantic Coast drainages in the southeastern United States

The genus Elliptio, with 36 currently recognized species, is the largest genus in the family Unionidae in North America. The genus is represented by two species, Elliptio crassidens and E. dilatata, in the Interior Basin and 34 species in drainages of the eastern Gulf of Mexico and Atlantic Coast. The paucity and variation of conchological characters in the genus Elliptio makes it extremely difficult to define species and determine relationships. We examined glochidia from six species of Elliptio in an effort to determine if there are useful characteristics for species level identification and/or characters for identification of species groups. Elliptio species were selected to represent different morphological groups from four drainages in the southeastern United States. The glochidia from E. crassidens, E. dariensis, E. hopetonensis, E. icterina, E. shepardiana, and E. mcmichaeli were qualitatively compared, using scanning electron microscopy, with each other and with descriptions of these and other Elliptio glochidia described in the literature. Two groups were identified. The crassidens group, including E. crassidens, E. dariensis, and E. mcmichaeli, had subtriangular glochidia with a triangular styliform hook extending from the ventral margin of the valve and rough exterior valve sculpturing. Adults of this group had wrinkled or corrugated sculpturing on the posterior slope of the shell. The complanata group, including E. hopetonensis, E. icterina, and E. shepardiana, had subelliptical glochidia with a broad flange extending the entire ventral margin and loose-looped exterior valve sculpturing. Adults of this group lack sculpturing on the posterior slope of the shell. Differences in glochidial morphometrics were found, however, additional work is needed to determine if they are reliable for species level identification.