WILSON ET AL.: EFFECT OF SNAIL ON OYSTERS 



Parasitized 



100 200 300 



Snail Scale 



400 



500 



100 200 300 



Snail Scale 



400 



500 



Figure 2.— Left: Weight gain (g) per g initial weight of individual snail-parasitized oysters after the 4-wk experimental period as a func- 

 tion of the level of Boonea impressa parasitism, or snail scale. Right: Weight gain (g) per g initial weight of individual oysters after 

 the 4-wk recovery period as a function of the level of B. impressa (snail scale) during the 4-wk treatment period. 



groups than in the treatment and precontrol groups 

 (Table 2). 



During the recovery period, when all oysters were 

 parasite-free, the previously parasitized oysters 

 (those with snails during the treatment period) 

 gained significantly more weight than the recovery 

 controls (Table 1). Oysters which were previously 

 parasitized by fewer B. impressa during the treat- 

 ment period gained more weight during the 

 recovery period than those that were previously 

 parasitized by more snails (Spearman's Rank corre- 

 lation, P = 0.07, Fig. 2), but the relationship was 

 considerably weaker than during the treatment 

 period. 



The intensity of infection by P. marinus increased 

 throughout the experiment in parasitized oysters so 

 that, after recovery, previously parasitized oysters 



had higher prevalences and intensities of infection 

 than they did after 4 weeks of parasitism; these 

 values in turn were higher than the precontrol 

 values at the experiment's inception (Table 3). 

 Exactly the opposite trend occurred in control 

 oysters. Recovery controls had the lowest values of 

 prevalence and intensity. Consequently, after the 

 recovery period, previously parasitized oysters had 

 significantly higher intensities of infection than 

 recovery controls. The proportion of infected in- 

 dividuals (33% of the controls, 93% of the previously 

 parasitized oysters) was significantly higher as well 

 (x^, P < 0.05). Within the parasitized oysters, the 

 intensity of parasitism (snail scale) did not correlate 

 with the increase in intensity of P. marinus infec- 

 tion during either the treatment or recovery period 

 (Fig. 3, Spearman's Rank, P > 0.05; the interaction 



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