Calceoli: Antennal Sensory Organs of Amphipods (Crustacea, Amphipoda, Gammaridea) from Lake Baikal

The list of endemic calcioliferous (carrying calceoli, the antennal sensory organs) amphipods is presented for Lake Baikal. It includes 47 species and subspecies (13% of the total amphipod fauna) belonging to six genera and three families. Most of the calceoliferous amphipods belong to the family Gammaridae. All species of the genera Corophiomorphus and Odontogammarus, as well as one-third of the species of the genus Eulimnogammarus, carry calceoli. In the family Micruropodidae, only the genus Micruropus is represented by calceoliferous species. In the family Pallaseidae, two species of the genus Ceratogammarus and Polyacanthisca calceolata (representing a monotypic genus that formally belongs to this family) are calceoliferous. The Baikal fauna of calceoliferous amphipods includes shallow-water and abyssal species, giant and dwarf forms, inhabiting various environments and differing in life style. The common features of these amphipods are a smooth body and a high mobility. Most of the calceoliferous species are benthic or nectobenthic organisms; one species is benthic-pelagic. Burrowing forms of the genus Micruropus are characterized by horizontal and vertical migrations. Most of the Baikal amphipods carry the calceoli only on the flagellum of the second antenna; sometimes, these are males only, and sometimes, both sexes. Only three species carry calceoli on both pairs of antennae and in both sexes: the shallow-water species Micruropus wohlii wohlii and M. wohlii platycercus, and the abyssal benthic-pelagic species Polyacanthisca calceolata. They migrate actively, and their mature males and females ascend from the bottom to the water column. In some abundant species taken as examples, the presence of calceoli on antennae seems to correlate with the life style. The data obtained confirm the view that calceoli play an important role in mate search and/or mating in the water column.