File:American malacological bulletin (1988) (18152829542).jpg

Title: American malacological bulletin
Identifier: americanmal6719881990amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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FISCHER ETAL: GIRDLE SENSE ORGANS IN CHITON OLIVACEUS 137 construction. They are composed of a varying number of secretory cells that surround one spicule cell and one ciliary cell. The ciliary cell invaginates into the spicule cell which is highly specialized in this zone. The spicule cell is connected with the organic cup of a calcareous structure. Both these components vary considerably in size. The same pattern is also found in the primitive polyplacophoran Lepidopleurus ca- jetanus Poli (Fischer, unpublished) as well as in Lepidochitona cinereus (Haas and Kriesten, 1975). In Acanthochitona fascicularis a similar appearance has been found (Fischer ef a/., 1980) with three major differences: the secretory cells are more prominent; photoreceptor cells are present in many papillae; a stalked nodule protrudes from many papillae into the cuticle. This nodule resembles the swelling of the hair papilla in Chiton olivaceus. It looks like a distal part of a papilla that has lost its spicule. In young Mopalia muscosa a pattern similar to Chiton olivaceus is found (Leise, 1986) (Fig. 6). It seems that the type described here is the basic structure of the girdle sense organs in the polyplacophora. In Acanthochitona fascicularis, another type of spine has also been described, in addition to this general type. These spines are not connected with a papilla. Each is based on top of a large cup-like cell in the epidermal layer and grows basally as the animal gets larger. In contrast, the "normal" type of spine does not grow after it is produced. Behavioral observations and the fine structure of the cup-like cell sug- gest that this spine type in A. fascicularis is merely defensive (Fischer, 1979). Adult mopaliid chitons have elaborate sensory hairs in the girdle (Leise and Cloney, 1982; Leise, 1986). Leise (1986) has demonstrated that these hairs are formed by the growth of several spines (very similar to the hairs of Chiton) close to one another. As they grow, the whole bundle is surround- ed by an organic cortex. Thus, the complex hair in Mopalia is an elaboration of the "normal" type. In Acanthochitona fascicularis the spicule cell forms a neurite (Fischer et a/., 1980). Nerves have also been demonstrated in the girdle sense organs of Mopalia muscosa (Leise and Cloney, 1982; Leise, 1986). In Chiton olivaceus, structures resembling neurons are present in the papillae of every type of girdle formation. However, the presence of such structures seen in the electron microscope is only an indica- tion of a sensory function, for two reasons. Cells that are not sensory, such as the secretory cells in the aesthetes, can form fiber-like extensions that are very similar in structure to neurons. However, they certainly have another function, as they are not connected with the nervous system (Knorre, 1925). If the fibers observed in the papillae are nerves, they could have other functions such as stimulating the secretory cells. To establish a sensory function, appropriate neurophysiological or behavioral experiments must be car- ried out. Neurophysiology in chitons is very difficult, as single nerve fibers are thin and the amplitude of potential changes is quite low (Fischer ef a/., unpub. data). The results of the stimulation experiments show that the girdle sense organs are mechanoreceptors. Due to the fact that the basic structure is the same in all species and in all areas in Chiton, we suggest that, apart from the func- tion of the secretory cells, mechanoreception is the basic func- tion of the girdle papillae. A possible function of the secretory cells could be to produce or impregnate the cuticle. The chemical composition of the secretory granules is unknown. The presence of mechanoreceptors is certainly of great importance for a relatively small animal which lives in the tidal region and moves actively, but slowly, on exposed substrata for feeding. Individuals of Chiton olivaceus which have been detached from their stone have great difficulty settling again in turbulent water (pers. obs.). Under normal conditions, the girdle is pressed onto the substratum. There is no gap and the animals are not vulnerable to strong water movement. The ventral scales could provide feedback information about the pressure of the girdle on the substratum. The reactions to stimulation of the marginal spines show that these structures can detect an obstacle or movements of other animals. Most chitons including Chiton olivaceus do not possess eyes. The photoreceptor cells in the aesthetes (Fischer, 1978) are involved in the photonegative behavior (Arey and Crozier, 1919; Boyle, 1972; Bergmann, 1984) and in the shadow
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