Formation of the hinge in the podocopan ostracode Loxoconcha pulchra.

The hinge structure in the podocopan ostracode Loxconcha pulchra was examined throughout its molt cycle using ultrastructural and histological procedures. The structure consists of ligament and hingement, and develops along the attached margin of the right and left valves. In Stage C the hingement of both valves interdigitates beneath the ligament, and a series of outer epidermal cells (dorsal epidermal cells), exhibiting abundant granules, underlie the hinge structure. Apolysis occurs at Stage D1, and electron-dense granular materials of variable diameter are seen within the ecdysial space. Epicuticle formation begins at Stage D2 and is complete before Stage D4. In Stage D2 the new epicuticle appears as a dotted line consisting of numerous grain-like materials. The dorsal epidermal cells, which actively secrete the numerous granules during molting, increase their size and reveal the electron-dense substances in the cytoplasm from Stage D2. At early Stage D3 the procuticle deposition of ligament commences inside the epicuticle, and is completed in Stage D4. In Stage D4 the uncalcified procuticle is secreted under the whole area of carapace, and the new carapace is then ready for ecdysis. After ecdysis, calcification of the carapace commences from the dorsal and ventral marginal areas towards the central area. During Stage A there is no further cuticle deposition in the ligament, although the dorsal epidermal cells secrete as actively in the postmolt stage as in premolt. The dorsal epidermal cells begin to form the hingement just after ecdysis. Cuticle deposition of the hingement proceeds asynchronously in the two valves: the hingement of the right valve is formed prior to that of left one in L. pulchra. The right hingement functions as a mold for the left hingement to form the precise interdigitated structure in L. pulchra. These observations suggest that the ostracode ligament is a unique cuticle, which should not be confused with the cuticles of other arthropods. The work establishes, for the first time, a description of the formation of the hingement in podocopan ostracodes.