DISTRIBUTION PATTERN, GILL AREA RELATIONSHIPS, AND FINE STRUCTURAL CHARACTERISTICS OF THE GILL EPITHELIA IN SUPRALITTORAL AND TERRESTRIAL TALITRID AMPHIPODS

Abstract

The distribution patterns, gill size, gill distribution, and gill ultrastructure were compared among 6 talitrid amphipods that occur in a range of habitats near Mt. Hakodate, southern Hokkaido, Japan. Two are supralittoral beachfleas, one is a supralittoral and coastal terrestrial beachflea, one is a sandhopper, and the rest are landhoppers. In all species, the posterior gills are the largest. Reduced gill size of the supralittoral species compared with terrestrial species supports the previous suggestion that the supralittoral species inhabit extremely dry environments. The gill sizes of 2 terrestrial species, smaller than those of most of Bousfield's (1984) group IVa landhoppers, confirm the belief that Japanese landhoppers invaded land via the dry supralittoral zone. In all species, the gills are covered by ion-permeable, thin cuticle layers and are composed of thick typical transporting-type epithelia characterized by both apical (AIS) and basal (BIS) infolding systems of the cell membranes. The AIS are better developed in terrestrial species than in supralittoral species. On the other hand, the BIS are associated with a large number of elongated mitochondria and are well developed in all species, resulting in the involvement of the greater part of the cell surface and the intercellular space. These facts strongly indicate that the gill epithelia should be regarded as ion-transporting, as well as respiratory, organs. Moreover, the gills may play important roles in the active transport of electrolytes to regulate osmotic pressures of body fluids in terrestrial or semiterrestrial environments in which the animals are frequently exposed to extremely hypotonic conditions.