Life cycles of nemerteans that are symbiotic egg predators of decapod Crustacea: adaptations to host life histories

Abstract

Life cycles of five species of nemerteans that are symbiotic egg predators of decapod crustaceans are reviewed and compared. These life cycles range from the relatively simple life cycle of Carcinonemertes regicides from the red king crab, Paralithodes camtschatica, to the complex life cycle of C. errans on the Dungeness crab, Cancer magister. Increased complexity of these life cycles is evaluated as a series of adaptations to cope with life history features of their respective hosts. These host attributes include seasonal brooding, rapid embryogenesis and brooding in alternate years. Lengthy host brood periods permit autoinfection of the respective nemertean symbionts with resultant loss of many host eggs. Adaptations associated with short or intermittent brooding periods include migrations of regressing adult worms to the gills and their return to the egg mass at the next oviposition, transfer of worms from old to new cuticles at ecdysis, use of male and juvenile female crabs as hosts, and sexual transmission at host copulation. Future research needs are briefly discussed.