Anatomy and Biology of Mysella pedroana (Mollusca: Bivalvia: Galeommatoidea), and its Commensal Relationship with Blepharipoda occidentalis (Crustacea: Anomura: Albuneidae)

Abstract The distribution, prevalence and abundance of the galeommatoidean bivalve, Mysella pedroana , on the spiny sand crab, Blepharipoda occidentalis , were studied based on newly collected material and museum specimens. Although sometimes free-living, M. pedroana occurs most often inside the branchial chamber of B. occidentalis , and appears to be a commensal associate. It is not known from outside the range of the host along the Pacific coast of California and Mexico. Approximately 60% of hosts examined contained typically 1—2 M . pedroana in their branchial chambers, but many more (maximum 22) were sometimes found. M . pedroana showed no preference for host sex, size, or branchial chamber (left vs. right) and were not more abundant in any part of the distributional range. No M . pedroana were found on specimens of the five taxa mostly closely related to B . occidentalis . M . pedroana is re-described and its systematic position discussed with special reference to difficulties of generic placement. Although the presence of the bivalves causes physical deterioration to the hosts' gill filaments, available evidence indicates that the relationship is commensal, rather than parasitic.