Reproductive biology ofArthritica crassiformisandA. bifurca, two commensal bivalve molluscs (Leptonacea)

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Abstract
Arthritica crassiformis Powell and A. bifurca (Webster) are functional hermaphrodites. Specimens of both species were isolated and maintained in individual plastic containers until they produced two broods of larvae. A. crassiformis produced an average of 7.75 larvae per day, with brood sizes of 245–3120 larvae and 11–21 days between broods. A. bifurca produced an average of 8.56 larvae per day, with brood sizes of 5–5016 larvae and 8i to 20 days between broods. Larger clams of both species produced more larvae than did smaller clams. Larvae were incubated for about 1 week and were usually released at one time although releases continued for as much as 3 days. A. crassiformis larvae averaged 150 )im in length at release. The hinge increased from 80 to 105 μm in length and was without discernible hinge teeth. A low, rounded umbo appeared at lengths of 175–200 μm and larvae set at 235–270 μm. A. bifurca larvae averaged 124 μm in length at release. The hinge line increased from 76 to 89 μm and was without teeth. A low, rounded umbo appeared at 150–169 μm and larvae set at 243–275 μm.