Unimodal sensilla

Ache 1982 Derby 1982 Hallberg and Chaigneau 2004). Both bimodal and unimodal sensilla contain the following structural elements (1) a cuticular hair-like structure, (2) a number of sensory cells (either chemosensory cells only or combined with mechanosensory cells), and (3) enveloping cells which surround the sensory cells (Fig. 6.4). These three structures will now be described further, first in general and then specifically for the two different types of sensilla, clarifying the differences between them. 

Fig. 6.1 Artist s drawing of a crayfish showing some locations for important chemosensory sensilla. Above the crayfish the antennule is where the unimodal (olfactory) chemosensory aesthetascs are situated. To the left of the crayfish on the tips of the walking legs, bimodal chemo- and mechanosensory sensilla may be present e.g., as hedgehog hairs (above left) and smooth, squamose setae (below left). Bimodal sensilla show a considerable structural variety and can also be found e.g., on the mouthparts of the animal (not shown). Drawing by artist Jorge A. Varela Ramos...

Fig. 6.2 Bimodal sensilla. (a) Hedgehog sensilla from the walking leg of Homarus gammarus. Scale bar 100 gm. (b) Antennules of Pacifastacus leniusculus with both unimodal chemosensory aesthetascs (arrows) and slender bimodal chemo- and mechanosensory sensilla (arrowheads ). Scale bar 50 pm. (c) Transverse section through the dendritic outer segments (dos) of a bimodal sensillum from the antennule of Lophogaster typicus. There are three gustatory cells (gu) and one mechanosensory cell (me). The latter has a dense array of microtubules. Scale bar 500 nm...

Fig. 6.3 Decapod antennules and sensilla. (a) Antennule of the crab Carcinus maenas. The aesthetascs (arrow) form a dense array on the lateral filament. Scale bar 500 pm. (b) Part of antennule of the European lobster Homarus gammarus. Unimodal aesthetascs (ae) are surrounded by bimodal guard hairs (arrows) and companion hairs (arrowheads). Scale bar 500 pm. (c) The aesthetascs on the antennule of H. gammarus are arranged in two rows (black arrows) on each annulus. The sockets of the guard and companion hairs surround the aesthetascs (white arrows). Scale bar 200 pm. (d) Part of antennule of the crayfish Pacifastacus leniusculus. There is one distal group (dg) and one proximal group (pg) of aesthetascs on each annulus. Scale bar 100 pm...

A morphologically distinct type of unimodal olfactory chemosensors is the male-specific sensilla (Johansson and Hallberg 1992) (Fig. 6.6) found on the base of the first antenna in (male) mysids and some other peracarid crustaceans. Whereas aesthetasc hairs are ubiquitous, male-specific sensilla are found only in a few groups and, as the name implies, only in the males. In mysids, female antennae are very similar to those of the males except for the absence of male-specific... 

The chemosensory organs of crustaceans are divided into bimodal chemo- and mechanosensory sensilla (found e.g., on mouthparts and legs) and unimodal olfactory sensilla (usually found on the first antenna). Although the general morphology of these structures has often been thoroughly described, less is known about the function of the sensilla in an ecological and behavioral context. 

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