Measurements. — Carapace: male, width, 1.4 to 

 4.6 mm.; female, width, 1.3 to 2.7 mm. (Christen- 

 sen and McDermott, 1958) . 



Color. — Dark or medium-dark brown with two 

 large, distinct, almost circular, pale white spots on 

 both carapace and sternum; dorsal spots on bran- 

 chial regions, ventral spots flanking abdomen and 

 medial to first pair of legs; color and spots per- 

 sistent in alcohol (various authors) . 



Habitat. — Parasitic [or commensal] chiefly in 

 the oyster, Crassostrea virginica, also in Pecten 

 spp. and Anomia simplex (Christensen and Mc- 

 Dermott, 1958), and in Mytilus edulis (McDer- 

 mott, 1961). Also occasionally found in Chaetop- 

 terus tubes (Gray, 1961). 



Type locality. — Given as — "inhabiting the com- 

 mon oyster." 



Known range. — Salem, Mass., to State of Santa 

 Catarina, Brazil. 



Remarks. — Say's P. depressus appears almost 

 certainly to be the hard-stage male as described 

 above and discussed below. 



The works of Hyman (1924a), Stauber (1945), 

 Sandoz and Hopkins (1947), and Christensen and 

 McDermott (1958) together have made knowledge 

 of the biology and life history of P. ostreum the 

 most complete for any species of Pinnotheres in 

 the world. The serious student should refer to 

 these thorough works, for they can be summarized 

 only in barest outline here. The complex life 

 cycle of this species encompasses many develop 

 mental stages, as well as a striking sexual dimor- 

 phism in the mature animals, which, together with 

 the structural specializations and mode of exist- 

 ence, demonstrate a beautiful accommodation to 

 an unusual habitat. 



The larval stages include four zoeae followed 

 by one megalops. The first two zoeal stages were 

 described by Hyman (1924a), and a description 

 of all these stages (partial for fourth zoeal stage) 

 was given by Sandoz and Hopkins (1947). In 

 general, the zoeae and megalops have no dorsal or 

 lateral spines on the carapace. Time of develop- 

 ment from hatching to molting of the megalops to 

 first crab stage is 25 days. 



From the first crab stage on, development is 

 summarized by Christensen and McDermott 

 (1958, p. 154). The first crab stage, actually the 

 stage which invades oysters, is called the invasive 

 stage by these authors. It was described by Sandoz 



and Hopkins (1947) and in many respects is simi- 

 lar to the later hard stage in its flattened shape, 

 legs adapted for swimming, and characteristic 

 color markings (carapace width, 0.59-0.73 mm.). 



In Delaware Bay, few invasions take place be- 

 fore August 1. The peak of oyster setting there is 

 in July; spat will have grown to size sufficient to 

 harbor one or more crabs by the peak of the crab 

 invasions in September. Though invasive stages 

 in oysters are found all winter, growth and devel- 

 opment stop about the first of November when 

 water temperatures begin to drop below 15° C. 

 Surprisingly small spat may be invaded. Two 

 crabs were found in an oyster 4.2 mm. long, and 

 in larger spat up to seven crabs were found in a 

 single specimen. The crabs prefer to invade spat 

 or yearling oysters rather than older ones (76.7, 

 54.6, and 21.5, being respective infestation per- 

 centages for a given year class of crabs), but 

 survival rate of crabs is better in yearlings and 

 older oysters. 



Following the invasive stage are two ill-defined 

 stages designated as prehard. These stages, de- 

 scribed by Christensen and McDermott (1958), 

 are soft and resemble later posthard stages of the 

 females (carapace width: male, 1.4-4.6; female, 

 0.75-2.7 mm.). The legs are rounded and not 

 adapted for swimming. These stages are found in 

 all parts of the water-conducting system of in- 

 fested oysters. In the region of Delaware Bay, 

 most young crabs reach the prehard stages before 

 growth ceases in fall and they overwinter in these 

 stages. Development resumes when temperatures 

 rise above 15° C. 



The hard stage, formerly regarded as the inva- 

 sive stage, is characterized above. On the average, 

 males are larger than females, as they are in the 

 preceding stages. The form of this stage resem- 

 bles that of the invasive stage, and males of this 

 stage swim freely. This is the copulatory stage, 

 and the males normally die in this stage. 



The succeeding female stages, described by 

 Stauber (1945), resemble the adult female, and 

 are found only in the host on the gills. Stage II 

 (the hard stage was designated as stage I by 

 Stauber) has a thin flexible carapace but a narrow 

 abdomen contained wholly in the sternal groove 

 (carapace width, 1.3-3.1 mm.). Stage III has an 

 abdomen extending beyond the depression in the 

 sternum (carapace width, 2.6-4.4 mm.). Stage IV 



MARINE DECAPOD CRUSTACEANS OF THE CAROLINAS 



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