PINNOTHERES OSTREUM, PARASITIC ON THE AMERICAN OYSTER, OSTREA (GRYPHAEA) VIRGINICA

¹ Oyster Research Laboratory, N. J. Agricultural Experiment Station, Bivalve, N. J.

1. All the parasitic stages of Pinnotheres ostreum are described, many for the first time. Besides the invasive first stage male and female crabs, one other male stage and four other female stages are reported.

2. The first crab stage (male or female) is hard-shelled, relatively flat and hairy with a distinctive brown color and white markings.

3. Subsequent stages are quite different with membranaceous, yielding carapace, slender legs, and more rounded body form.

4. The later stages of the females are distinguished from one another by size, color, width and shape of abdomen with relation to the rest of the body and size and differentiation of the pleopods. Maturity is usually reached in the fifth stage female and ovigerous crabs have been seen.

5. All stages of crabs have been found in oysters either singly or in various combinations except that no more than one specimen of a Stage III, IV, or V female has ever been observed within a single oyster.

6. Multiple infestations have been observed which involve chiefly first stage crabs. One oyster contained 262 of these crabs.

7. The oyster crab feeds chiefly on the particulate food material strained from the water by the oyster's food sorting mechanism.

8. In so feeding the crab injures the demibranchs of the oyster causing erosions which interfere with normal feeding of the oyster. These erosions are sharply localized in oysters with first stage crabs.

9. Damage to the oyster gills involves a reduction in the amount of area available for food collecting and in the efficiency of both the straining and the collecting mechanisms. The more serious lesions may also show leakages of the water pumping or conduction system which is such an important feature of food sorting.

10. Rapid healing and regeneration of gill tissue almost keeps pace with destruction and probably saves many oysters from death.

11. The hard shelled, almost tick-like, invasive first stage crabs are less adapted to life within the oyster than the modified later stages. The latter are more resistant to their host's exposure to unfavorable environmental conditions such as storage out of water.

12. Experimental invasion of oysters by crabs has been accomplished but artificial duplication of typical, extensive gill lesions has not yet been obtained.

13. The unprecedented invasion of oysters in 1941 was followed by a lesser invasion in 1942 and probably will follow the usual course of population cycles in future years.

14. The parasitic nature of the crab's relations with the oyster is outlined and reasons suggested why this previously was not found out.