STUDIES ON Parorchis acanthus (TREMATODA: DIGENEA) AS A BIO- 

 LOGICAL CONTROL FOR THE SOUTHERN OYSTER DRILL, Thais 

 haemastoma 



By Nelson R. Cooley, Fixhert/ Research Biologist, Bureau of Commercial Fisheries 



Althougli the biology and control of the chief 

 Atlantic coast oyster drill, U ro-sal pinx cinerea. 

 has been the subject of considerable study (re- 

 viewed by Carriker, 1955), the southern drill, 

 Thais haemastoma Linne,^ long known as an oys- 

 ter predator, has received scant attention. 



The southern drill is widely distributed in oys- 

 ter-producing waters along the northern coast of 

 the Gulf of Mexico, exists in incredible numbers 

 in coastal bays and estuaries, and is extremely 

 prolific. Its reproductive cycle has been studied 

 in some detail by Butler (1953). Females deposit 

 egg capsules regidarly from March to August. 

 Each capsule contains from several hundred to 

 4,000 eggs, and there is almost no mortality within 

 the capsule. In a growth experiment performed 

 by Butler, 250 females deposited an estimated 100 

 million eggs during 1 month. Hatchuig occurs 

 in 12 to 16 days at 25° C, and after a planktonic 

 life that may be as much as 7 weeks, the veliger 

 larvae metamorphose into tiny snails. Sexual 

 maturity is usually attained during the second 

 summer, i.e., at 1 year of age, but normal egg 

 and capsule deposition by snails not more than 

 8 weeks old has been observed. 



Thais is probably the most important oyster 

 predator in this area (Butler, 1953). In 1956 

 drill predation was so severe on depleted reefs in 

 Mississippi waters (about one-half of the reef 

 bottoms of the State) that much of the annual 

 spatfall in the area was destroyed. In addition, 

 drills destroyed a half of the oysters on the pro- 

 ducing natural reefs in Mississipjji that year 

 (Chapman, 1958). 



If such severe losses in the Gulf oyster industry 

 are to be reduced, control measures must be in- 

 stituted acainst the drill. 



' Clench (lt)47) considprs this mnrine snail to be two sub- 

 «pecl*'s. T. haemnstomn floridana Conrad, 18.^7. and 7'. h. haysae 

 Clench, 1027, but because of ecological similarities, thoy are 

 treated here as the same animal. 



Note,— Approved for publication May 25, 19G1. Fishery Bul- 

 letin 201. 



Broadly speaking, predation may be controlled 

 by trapping (physical control), by poisoning 

 (chemical control), or by parasites or predators 

 (biological control). Physical control of Thais 

 is impractical, except in restricted areas, because 

 of expense and inefficiency of available methods. 

 Chemical control is superficially feasible, but lack 

 of specificity of most available chemicals raises 

 fear of damage to oysters and other economically 

 important species. Biological control by means 

 of predators or parasites specific for the drill ap- 

 pears to he least likely to damage other species 

 and, for that reason, was selected for investiga- 

 tion. 



Few natural enemies of Thais are known. But- 

 ler's (1953) laboratory- observations that hermit 

 crabs attack drills to gain possession of their 

 shells and that stone crabs, if sufficiently hungry, 

 crack and eat drills are the only known proved 

 reports of predatoi-s. The drill larva, a free- 

 swimming veliger, may be eaten by pelagic fishes. 

 Thus, although normal predation on larval stages 

 may, probably does, cause enormous losses to drill 

 populations, there appears to be no immediate 

 prospect of further population reduction by this 

 means. 



Only a few parasites are known from Thais. 

 The commensal polyclad, Iloploplana inquih'na, 

 and larvae of Parorchis acanthus, a digenetic 

 trematode, were reported by Shechter (1913). 

 Larval stages of at least two unidentified trema- 

 tode species which caused considerable gonad 

 damage were noted by Butler (1953). 



During this study, examination of about 7,600 

 snails since the summer of 1956 re\ealed : one, 

 possibly two, unidentified protozoans tiiat invade 

 and slightly damage digestive gland follicular 

 cells; isolated instances of larval nematode infec- 

 tions and a few cestode infections (almost cer- 

 tainly lar\al tetraphyllideans) which ap])i'ared to 

 be encysted single individuals, latlier timu repro- 

 ducing populations; a single infection by sporo- 



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