Saldanha, João Pedro1, Hegna, Thomas Arthur2r>
(1) Programa de Pós Graduação em Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil, (2) Department of Geology and Environmental Sciences, SUNY Fredonia, Fredonia, NY, USA,
Abstract:
As a rule, arthropods grow by molting their exoskeleton. Clam shrimp break this rule with their carapace, which grows by marginal accretion at the outer edge of the carapace. Previous hypotheses to explain this postulated that during each molt, they grew an entire new carapace beneath — giving an arthropod flavor to molluscan style marginal accretion. To evaluate this, we examined both histological slides and SEM mounts of clam shrimp carapaces from multiple genera. These examinations revealed a bilayered carapace with an internal cuticular membrane and a thicker external cuticle — both containing the expected endo-, exo-, and epicuticle. In cross-section, the external epicuticle exhibits staggered interruptions and elevations that mark the growth lines, whereas the overlying endocuticle and exocuticle are composed of continuous lamellae. These findings are incompatible with the previous model, which require cuticle superposition. Our model indicates that only the epicuticle is retained throughout ontogeny, while growth is accommodated through progressive remodeling of the external endo- and exocuticle.