Ultrastructural comparison of Aplysia and Dolabrifera ink glands suggests cellular sites of anti-predator protein production and algal pigment processing

Abstract

Ultrastructural comparison between the ink gland of a sea hare species that produces copious purple ink (Aplysia californica) and one that produces none (Dolabrifera dolabrifera), suggests that the rough endoplasmic reticulum rich cell and not the ink vesicle cell is the site for synthesis of A. californica's anti-predator ink protein, escapin. Dolabrifera dolabrifera were found to have vestigial ink glands incapable of producing ink or its associated anti-predator proteins regardless of diet. This study also suggests that the granulate cells serve only as a storage site for excess ink pigment acquired during periods of luxury feeding on red algae. Slit dimensions in sieve areas of granulate cells are also significantly different between the two species. These slit sizes are larger than those of rhogocytes, a related cell type commonly found in connective tissue of gastropod molluscs. Several traits of granulate cells suggest that they are distinct from rhogocytes and are a special cell type in the ink gland of sea hares.