Formation of the Epicuticle in a Marine Isopod, Idotea Baltica (Pallas)

Abstract

The development of the distinctive epicuticle of the marine littoral isopod Idotea baltica is described. Synthesis of the outer epicuticle is conventional. However, formation of the outer layer of the inner epicuticle is unusual among arthropods in that this layer appears to be formed from materials released simultaneously from both epidermal microvilli and Golgiderived secretory vesicles. During central zone formation electron-dense fibres join the tips of apical plasma membrane microvilli to the developing inner epicuticle. Vesicles, which are found in the epicuticle, are observed only when the latter is completely formed. Aspects of the formation of certain structures within the epicuticle are correlated with specific molt cycle events, and their significance is discussed. Although the fine structure of the epicuticle of insects has been studied in detail (Noble-Nesbitt, 1963; Rinterknecht and Levi, 1966; Filshie, 1970a, b; Weis-Fogh, 1970; Wigglesworth, 1973; Altner, 1975; Hepburn, 1978), comparatively little attention has been given to that of crustaceans. The crustacean epicuticle appears simple in organization, composed of a single layer in the cyprid cuticle (Walker and Lee, 1976) and Stage I and II cirripede nauplii of Balanus (Klepal and Barnes, 1978), the gill cuticle of a crayfish (Filshie and Smith, 1980), and the antennal sensilla of an ostracod (Heimann, 1979) and a mysid (Guse, 1980), two layers in the woodlouse (Price and Holdich, 1980a, b) and a crayfish (Kummel et al., 1970), three layers in Daphnia (Halcrow, 1976; Schultz and Kennedy, 1977), four layers in Porcellidium (Gharagozlou-van Ginneken and Bouligand, 1975), and six subdivisions in the fiddler crab (Green and Neff, 1972). A striking exception to the simple epicuticle plan is the remarkable complexity of the entire epicuticle of the marine isopod Idotea baltica. A brief account of the fine structure of epicuticle of I. baltica has been given by Halcrow (1980). The ultrastructure of the crustacean epicuticle is relatively constant, conforming to a basic pattern comprising a trilaminar outer epicuticle and an inner epicuticle. The outer epicuticle is reported by Neville (1975) to be a consistent feature of all arthropod cuticles. The inner epicuticle, by custom, is the area of the epicuticle lying adjacent to the outermost lamella of the exocuticle. In the present study we have kept to the conventional nomenclature. Two layers can be identified in the epicuticle of I. baltica: an outer epicuticle (=superficial layers of Halcrow, 1980) and an inner epicuticle (=3 major regions of Halcrow, 1980). The outer epicuticle is a trilaminar structure consisting of two electron-dense laminae separated by an electron-lucent lamina. Beneath the outer epicuticle lies the inner epicuticle consisting of three regions: an outermost moderately electron-dense layer, a central zone, and an innermost layer. The extensive central zone is characterized by closely packed elongate cylindrical cavities arranged perpendicular to the surface. Numerous vesicles occupy all layers of the epicuticle and are often seen communicating with the exterior (Halcrow, 1980; Powell, 1981). In view of these unusual features of the epicuticle of I. baltica, a full electron microscopical study of epicuticle formation was undertaken in an attempt to determine how the central zone is formed and to determine the origin of the