SOUTHERN OYSTER DRILL AS A PREDATOR 



85 



Stunkard and his coworkers (1931, 1932). This 

 finding has been confimied in the present study. 

 Under these conditions, the cercaria encysted al- 

 most immediately upon contacting the substratum. 

 The encystment process was described briefly by 

 Stunkard and Cable (1932), more fully by Rees 

 (1940). My own observations amply confirm 

 their findings. However, the following observa- 

 tions should be added to their descriptions. 

 About 1 minute after extrusion of the cystogenous 

 material, the body rapidly shrinks, withdraws into 

 the center of the cyst and becomes immobile. 

 The shrinkage and withdrawal results in decauda- 

 tion. The detached tail, now attached by its base 

 to the outer cyst membrane, lashes about for sev- 

 eral hours. Aft«r some hours, the body begins 

 to move about within the cyst membranes and 

 ultimately assumes the characteristic folded po- 

 sition figured by Stunkard and Cable, and by 

 Kees. 



NATURAL INFECTIONS IN THAIS 

 Incidence 



The incidence of natural infections of P. acan- 

 thus in T. haemastoma examined between July 

 1956 and September 1959 is summarized in table 5. 

 Although the parasite is widely distributed along 

 the Gulf coast, the infection rate in any sampled 

 locality was low. The apparent absence of in- 

 fections in snails from Apalachicola Bay, Florida, 

 and Port Aransas, Texas, and the low rates in 

 drills from Dauphin Island Bay, Alabama, and 

 Barataria Bay, Louisiana, may be due to small 

 sample-size or, possibly, to more resistant snail 

 populations or fewer infected birds in those 

 localities. 



The following data suggest that the drill in- 

 fection-rate in a given locality may be higher on 

 the feeding grounds of the local gull population 

 than at a site away from them. Between June 

 1958 and June 1959, 5.37 percent (57 of 1064) of 

 the drills collected at a site on the north shore of 

 Pensacola Bay, Florida, had P. acanthus infec- 

 tions, but only 0.47 percent (4 of 859) of those 

 from a site on the south shore were infected. Al- 

 though these two sites are only 3.5 miles apart, 

 similar marked differences in incidence of infec- 

 tion have been observed in all collections made 

 there since the summer of 1956 and appear to be 

 correlated with the high concentration of gulls at 

 feeding grounds on the north side of the bay. 



Table .5. — Incidence of natural infections of P. acanthus 

 in T. haemastoma, 19S6~59 



Pathology in the Snail 



Stimkard and Shaw (1931) and Stimkard and 

 Cable (1932) noted that infected Urosalpinx ci- 

 nerea and Thais [ = Pv,rpura^ lapillus in Massa- 

 chusetts harbored P. acanthus larvae in the 

 interlobular spaces of both digestive gland and 

 gonad. The picture was similar in both species : 

 the uninfected snail had a plump visceral mass, 

 yellow digestive gland, and cream-colored gonad, 

 whereas the parasitized snail had shrunken organs 

 and lighter colored body, and its gonad might 

 have been destroyed. 



Eees (1937) noted the parasite in the same 

 organs in Thais [ = Purpura] lapiUus in Wales 

 and reported a similar picture, except for finding 

 the visceral mass of the parasitized snail much 

 swollen. 



Menzel and Hopkins ' noted that many old 

 Thais haemastoTna from Barataria Bay, Louisi- 

 ana, had heavy infections of P. acanthus larvae, 

 which destroyed the gonads and caused complete 

 sterility. Hopkins's (1957) report appeai-s to be 

 the earliest published record of Pararchis-mduceA 

 castration of T. haetnastoma. The following 

 pathology study, part of which confirms their ob- 

 servation, had already been completed before the 

 writer learned of their findings. 



» Menzel, R. W., and S. H. Hopkins, 1954. Studies on oyster 

 predators in Terrebonne Parish, La. Mimeographed report, 

 Texas Agricultural and Mechanical Research Foundation, College 

 Station, Texas. (Released, 1959). 



