FISHERY BULLETIN: VOL. 86, NO. 3 



to return to normal metabolic condition after the 

 stress is removed (Portner et al. 1979; Pickering et 

 al. 1982; Kendall et al. 1984). This ability, termed 

 recovery, is an important adaptation to changes in 

 the natural environment. Recovery is not always 

 complete nor does it occur over relatively short time 

 scales in all cases. The time required for recovery 

 varies depending on the parameter being studied, 

 the stress that was applied, and the species involved 

 (Kendall et al. 1984; Haux et al. 1985; Neff et al. 

 1985). Apparently deleterious effects occurring after 

 exposure, during the so-called recovery period, are 

 well described (e.g. Kendall et al. 1984; Powell et 

 al. 1984). White et al. (1984) showed that C. 

 virginica could attain normal growth rates within 

 one week after B. impressa were removed. 

 However, growth rates frequently return to normal 

 more rapidly than other metabolic parameters (Ken- 

 dall et al. 1984). Boonea impressa are extremely 

 mobile and are more contagiously distributed than 

 their hosts, so that some oysters are highly para- 

 sitized while others remain parasite free (Powell et 

 al. 1987). Individual B. impressa change hosts often 

 but typically move between existing aggregates 

 (Wilson et al. in press). Therefore, refugia from 

 parasitism may exist and recovery may be impor- 

 tant in assessing the overall impact of B. impressa 

 on oysters. 



Stress, produced by laboratory conditions, fre- 

 quently accompanies laboratory experimentation 

 (e.g., Koenig et al. 1981; Powell et al. 1984; Kukal 

 and Kevan 1987). The effect of B. impressa on C. 

 virginica has been assessed primarily through 

 laboratory experimentation. Consequently, we ex- 

 amined the effect of B. impressa on C. virginica 

 under field conditions and assessed the ability of 

 oysters to recover normal growth rates and bio- 

 chemical composition once snail parasitism ceased. 



MATERIALS AND METHODS 



Field Study 



Oysters and snails used in this study were col- 

 lected at Goose Island State Recreation Area near 

 Rockport, TX. The oysters were weighed on a 

 Mettler balance using the underwater m.ethod of 

 Andrews (1961). Fifteen oysters (precontrols) were 

 sacrificed to define the biochemical composition of 

 the oysters and Perkinsus marinus levels that ex- 

 isted naturally at the collection site. The rest of the 

 oysters were placed in semi-enclosed plexiglass 

 domes (see figure 2 in Kendall et al. 1984 for descrip- 

 tion) in a tidal creek near the Aransas Pass 



Lighthouse on Lydia Ann Channel, near Port Aran- 

 sas, TX. The domes allowed water to circulate 

 over the oysters, while excluding such large 

 predators as oyster drills and crabs. Boonea im- 

 pressa, however, could readily move into or out of 

 the domes. 



Two of the four domes (20 oysters per dome) con- 

 tained oysters exposed to B. impressa at a concen- 

 tration of 10 snails per oyster, a level of parasitism 

 commonly observed on reefs in the collection area 

 (White et al. 1984). The domes were positioned so 

 that the probability of snails moving from the ex- 

 posure domes with parasitized oysters to the con- 

 trol domes was minimized (Fig. 1). Snails on each 

 experimental oyster were counted twice weekly for 

 4 weeks. No B. impressa were ever found on con- 

 trol oysters. Boonea impressa did emigrate from the 

 exposure domes, however, so snails were added as 

 needed to maintain the 10:1, snaihoyster ratio. Con- 

 trol oysters were handled the same as experimentals 

 each week in an effort to minimize differential ef- 

 fects caused by handling stress (see Pickering et al. 

 1982; Andrews and Hewatt [1957] were unable to 

 find any effect of handling on P. marinus infection 

 in oysters). 



At the end of 4 weeks, the oysters were reweighed 

 and half from each dome were sacrificed. The re- 

 maining oysters were replaced in the field for a 4-wk 

 recovery period without B. impressa. All B. im- 

 pressa were removed from the previously para- 

 sitized oysters by hand prior to replacement. The 

 domes were visited twice weekly and each oyster 

 handled as before. No B. impressa were found on 

 the oysters during the recovery period. 



Laboratory Analysis 



Perkinsus marinus infection was measured in 

 each oyster by incubating a small piece of mantle 

 tissue in thioglycollate medium by the method of Ray 

 (1966b). Intensity of infection was based on a semi- 

 quantitative to 5 rating assigned during micro- 

 scopic inspection of the tissue after treatment with 

 Lugol's solution (Mackin 1962). Small pieces of 

 gonadal tissue were preserved in Bouin's fixative, 

 sectioned and stained in 0.5% toluidine blue (Preece 

 1972) for gonadal analysis. Mantle cavity volume 

 was determined by filling the shells with silicon 

 caulking. Oyster growth, as expressed by shell de- 

 position, was measured using the underwater weigh- 

 ing method. Condition index was derived by divid- 

 ing the total lyophilized dry weight by the mantle 

 cavity volume (Lawrence and Scott 1982). 



The mantle and adductor muscle from each oyster 



554 



