Subgeneric differences in head shield and ephippia ultrastructure within the genus Bosmina Baird (Crustacea, Cladocera)

Using S.E.M. techniques, we compared head pore andephippia ultrastructures in 115 different populationsof the four subgenera: Bosmina, Eubosmina,Neobosmina and Sinobosmina. We differentiatedthree types of head pores: median, lateral and frontalpores. The pattern of lateral pores conforms topreviously published descriptions. The frontal porewithin the subgenus Eubosmina ishorseshoe-shaped, except in B. longispina andB. oriens, in which they are oval. In thesubgenus Bosmina, the frontal pore is oval withthe longer axis perpendicular to the animal plane ofsymmetry. Within the subgenera Neobosmina andSinobosmina, the frontal pores are eithercircular or slightly oval. Ephippial surface sculpturedoes not differ noticeably among species within thegenus. In species with pronounced reticulation of thecarapace, the same pattern appears on the ephippialsurface. The function of the median and frontal poresare unknown. The pores seem to be connected with thesupraoesophageal ganglion. The lateral pores areconnected to a tissue of glandiform cells, and anarrow band or rope-like protrusion from the pore issometimes visible in preserved specimens.