THE PHYSIOLOGICAL CONSEQUENCES OF HARBORING A SYMBIONT: THE EFFECT OF PEA CRABS (PINNOTHERES MACULATUS) ON MUSSELS (MYTILUS EDULIS) (COMMENSAL, PINNOTHERID, PARASITE)

Pea crabs can adversely affect mussel physiology on three time scales: immediate, seasonal, and long-term. Most researchers classify hosts only by presence/absence of the symbiont, but evidence for mobility of small pea crabs (all males and immature females) suggests the effect of these transients is intermittent and reversible. Adult female pea crabs (>6mm) are physically trapped within a host and represent a continuous drain on host resources. The presence of any size pea crab reduces respiration by about 60%, while there is a gradual decrease of feeding rate with increasing size of pea crab. Mussels with a long-term resident have lower glycogen reserves in gonadal tissue and lower total gonad weight whether measured at quiescent times or at peak gametogenesis. For these animals, although a constant percent of the gonad is filled with gametes, lower tissue weight/body weight results in decreased reproductive output. Under a normal ration, male mussels display marked reductions in follicle size; while few females with large pea crabs could be classified as fully ripe, those meeting the criteria showed no difference in follicle size from uninhabited females. Under some treatments, maximum egg size was less for mussels with large pea crabs. Over the short-term, mussels respond to pea crab infestation by decreasing metabolic rate. Over the course of a season, effects become cumulative and are manifested in decreased energy reserves and reduction of gonadal material. Over several years, the cumulative effects of a reduced metabolism and energy reserves become irreversible; continual decreased growth rates become reflected in mussel shell shape.

When the drain of supporting a pea crab is exacerbated by additional stresses, (e.g., low salinity or poor nutrient conditions), negative effects are more pronounced or appear sooner.