Apoptotic‐like morphology is associated with annual synchronized death in kleptoplastic sea slugs (Elysia chlorotica)

Certain digestive cells of the opisthobranch mollusc, Elysia chlorotica, contain functional chloroplasts which they steal from the filamentous alga, Vaucheria litorea. The adult portion of the life cycle of the slug lasts for ∼10 months and is completely synchronized among individuals. All the adults die each year within a few weeks of each other. We have examined the microscopic morphology of the slugs near the end of the life cycle. Light microscopy demonstrated an absence of chloroplasts in most of the digestive epithelial cells, the appearance of many crypt cells containing residual bodies and an invasion of the blood sinuses by neoplastic morula‐like cells as the animals die. Electron microscopy revealed a degeneration of the digestive diverticulum which had several morphological characteristics in common with apoptosis: expansion of the endoplasmic reticulum and Golgi apparatus, DNA fragmentation, formation of primary lysosomes, and condensation of chromatin. These are followed by fragmentation of the nucleus and cytoplasm into autosomes merging to form a large central autolysosome. In addition in the aging slugs, the plastids begin to degenerate until none were left in the digestive epithelial cells and the central autolysosome contained numerous viral particles.