A Pycnogonid Infestation of Mytilus californianus

Chivers, D; Benson, P

Thirty-two specimens of Mytilus californianus Conrad were collected by the authors at Duxbury Reef (37 53'30" N., 122 42' W.), Marin County, California, on February 2O, 1960. Upon examination, 16 of the 32 mussels exhibited infestation by the pycnogonid Achelia chelata (Hilton, 1939) (=Ammothea euchelata Hedgpeth, 1940). A total of 89 pycnogonids were counted, the incidence ranged from one to 2l parasites per host. The latter specimen is illustrated in plate 3, fig. 1. Immature and apparently mature parasites of both sexes were represented. Obvious destruction to the host's ctnidial and gonadal tissue was evident (see plate 3, fig. Z). Damage to the visceral mass, foot and palps was also apparent in several mussels. In the specimen exhibiting the greatest amount of damage, there was a complete loss of ctenidia and the major portion of mantle and gonadal tissue, accompanied by apparent atrophy of the entire animal. It is of interest to note that a group from Chico State College under the direction of Dr. Rod g e r s examined over 50 specimens of Mytilus californianus from Tomales Point, Marin County, on May: 1, 1960, and found no pycnogonids. They did, however, encounter a high incidence of the pea crab Fabia subquadrata Dana within the bivalves. This commensal was not encountered in the mussels examined by us. Another commensal pea crab, Pinnotheres latissimus Btirger, was encountered by Ohshima (1935) in Japan.

A Pycnogonid Infestation of Mytilus californianus

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