US Fish & Wildlife Service


Date of this Version



Published by the U.S. Department of the Interior in Fishery Bulletin 201 (1962) Volume 62, 20 p.


Events in the attack on the southern oyster drill, Thais haemastoma, by miracidia of the digenetic trematode, Parorchis acanthus, are described.

Infection rates in wild drill populations of various Gulf Coast localities from Florida to Texas were low, but intensity of infection in individual drills was high and caused castration. Infection rates in laboratory experiments were high, but intensities were low.

Pathology of natural infections in drills is described. The infection lasts at least two years, possibly even for life.

Natural infection rates of juvenile herring gulls (Larus argentatus) , ring-billed gulls (L. delawarensis), and juvenile laughing gulls (L. atricilla) are reported. Juvenile herring and ring-billed gulls were readily infected experimentally with P. acanthus, juvenile laughing gulls were less susceptible, and nestling least terns (Sterna albifrons) appeared to be resistant. Intensity of infection was generally low.

P. acanthus offers little hope of being useful as a biological control of the drill, Thais, because of difficulties in spreading the parasite or assuring a significant rise in wild drill infection rates.