Chemosensory Sensilla in Crustaceans
Crustaceans, like most other animals, have two types of chemosensory organs. In crustaceans these organs consist of sensilla that differ structurally as well as functionally. Unimodal olfactory sensilla are usually considered as more long-range and bimodal chemo- and mechanosensory sensilla as short-range or contact chemosensory sensilla. All chemosensory sensilla are characterized by the presence of ciliated bipolar sensory cells. The bimodal sensilla are unevenly distributed over the entire body, with dense arrays on the mouthparts and walking legs. These sensilla contain both chemosensory and mechanosensory cells. The chemosensory cells contain one transformed cilium (dendritic outer segment) each. Bimodal sensilla have a relatively densely arranged cuticle and also feature an apical pore. The most common unimodal olfactory sensilla in crustaceans are the aesthetascs, which vary in structure, number, and distribution. Another type of unimodal olfactory organ is the male-specific sensilla, found basally on the first antenna in some isopods, mysids, and amphipods. Excluding these groups, sexual dimorphism of the olfactory organs is not as pronounced in crustaceans as in some insects. The chemosensory cells of both aesthetascs and male-specific sensilla have two transformed cilia each, thus differing from gustatory chemosensory cells which have only one. Further, the cuticle of aesthetascs and male-specific sensilla is “spongy,” possibly functioning as a molecular sieve. Crustaceans molt in order to grow, and there are two known patterns of molting of the chemosensory sensilla: the first retains the chemosensory ability through maintained contact between the old and the new sensillum (the “mysid-type” of molt) and the second where there is no such contact and thus possibly no chemosensory ability (the “isopod-type”). Previous studies have mostly described the general morphology of the chemosensory sensilla, which is well-known in crustaceans. Future studies should focus on the function of these sensilla in an ecological and behavioral context.
KeywordsSexual Dimorphism Sensory Cell Chemosensory Cell Olfactory Sensilla Apical Pore
We would like to thank NFR, Formas and the Royal Physiographic Society in Lund for supporting this research. Rolf Elofsson and Ali Steinbrecht have been inspiring and most helpful during the years. The long and fruitful cooperation with Rita Wallén, a most competent and enthusiastic technician in the field of electron microscopy, is deeply acknowledged.
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