Structural properties of bimodal chemo- and mechanosensitive setae on the pereiopod chelae of the crayfish, Austropotamobius torrentium.

The setae on the inner edges of the pereiopod chelae of Austropotamobius torrentium contain eight receptor-cell endings. Two units are mechanosensitive, four react only to amino acids, to amines, or to pyridines. The remaining two units are most probably also chemoreceptors. All sensory cells possess long dendritic outer segments that extend to the tip of the seta, where a small pore is found. Structurally, two units differ from the other six by having (1) densely packed microtubules in their dendritic outer segments, (2) dense A-tubules with arms in their ciliary segments, (3) a well-developed ciliary rootlet in their dendritic inner segments, and (4) desmosomal junctions between the dendritic inner segment and the inner enveloping cell, which contains a scolopale. These features are probably general characteristics of crustacean mechanoreceptors. The mechanoreceptors respond only to strong mechanical stimuli. This corresponds to the structural features of the setae, which lack specialized socket structures. Deflection of the setae may lead to longitudinal stress to the dendrites; the latter seem to be attached proximally to the inner enveloping cell and distally to the cuticle. Thus, the mechanoreceptor structure suggests a function analogous to scolopidial receptors. The chemoreceptors are accessible to chemical stimuli via the subterminal pore. The walls of the setae, however, may be permeable as well. It seems probable that action potentials were recorded also from the dendrites of the sensory cells.