Proterometra macrostoma Cercaria — Rosen et al. 



91 



0.9 

 0.8 



Figure 2. Average number of Proterometra macrostoma 

 cercariae/snail shed daily over 21 days for 36 snails. 



12 hr periods for 21 days. Snails were trans- 

 ferred into containers with new dechlorinated 

 water at the end of each 12 hr period. At the 

 termination of the experiment, snails were dis- 

 sected to assess the number of rediae in each. 



RESULTS 



Decalcification of the snail shell revealed 

 rediae in the bottom whorl of the host (Figure 

 la). Rediae usually contained only 5-6 cercar- 

 iae at markedly different stages of develop- 

 ment (Figure lb). Histological sections of the 

 whorl region revealed numerous rediae within 

 the mantle cavity in close association with the 

 gills (Figure Ic). Cercariae in rediae were oc- 

 casionally observed adhering to one another 

 with their oral suckers (Figure Id). 



The majority of cercariae (365/369; 98.9%) 

 were released during the 12 hr dark cycle of 

 each 24 hr period. An oscillating pattern of 

 continuous, low-level cercarial release from 

 our pooled snail population was demonstrated 

 over 3 weeks, with averages ranging between 

 0.250 and 0.833 cercariae/snail/day (Figure 2). 

 Both infrequent (2-6 days; Figures 3a and 3b) 

 and moderately frequent (7-12 days; Figures 

 3c and 3d) cercarial shedding were noted from 

 individual snails over 21 days, and no consis- 

 tent pattern was apparent. Individual snails 

 shed cercariae on average ± SE = 7.3 ± 0.4 

 days of the 21-day experiment (range 2-12 

 days; Figure 4). Snails shedding cercariae only 

 2-6 days (e.g.. Figures 3a and 3b) of the 21 

 day experiment contained an average ± SE of 

 11.9 ± 2.9 rediae, while snails shedding be- 

 tween 7-12 days (e.g.. Figures 3c and 3d) pos- 

 sessed 35.2 ± 8.5 rediae. These values were 



significantly different when assessed with a 

 Student's f-test (t = 2.218; df = 32; P = 

 0.034). 



DISCUSSION 



Our histological obsei"vations established 

 that P. macrostoma rediae and their develop- 

 ing cercariae inhabit the mantle cavity of E. 

 semicarinata. Once cercariae are released into 

 this space by rediae, the route for emergence 

 is clear as the mantle cavity communicates di- 

 rectly with the outside environment. One pos- 

 sible consequence associated with this host lo- 

 cale is that P. macrostoma cercariae might not 

 encounter sudden osmotic stress when re- 

 leased from the snail host as previously re- 

 ported (Braham et al. 1996). The ciliated gills 

 in prosobranchs like E. semicarinata establish 

 a unidirectional circulation of freshwater 

 through this cavity for respiration (Voltzow 

 1994). This circulation would likely expose P. 

 macrostoma cercariae within rediae to a hy- 

 potonic environment prior to emergence, but 

 samples of fluid from this space will be re- 

 quired for confirmation of its hypotonic na- 

 ture. How such an environment might impact 

 cercarial development in rediae remains un- 

 known. 



Theron (1981), in a long-term study of 

 snails experimentally infected with Schistoso- 

 ma mansoni, linked peaks and valleys in the 

 shedding of cercariae over 3 months to a suc- 

 cession of cercarial generations, the valleys be- 

 ing intervals of lower production. No such 

 peaks and valleys were obvious in our 3-week 

 study of cercarial shedding from snails infect- 

 ed with P. macrostoma. By contrast, a contin- 

 uous and low-level release of P. macrostoma 

 cercariae from our sample population of snails 

 over 21 days was observed and was similar to 

 that previously reported by Lewis et al. (1989). 

 Unlike Theron's (1981) infections, our snails 

 were not synchronized as to time of initial in- 

 fection. 



One snail shed cercariae 12/21 days in this 

 study, while the remaining snails shed only be- 

 tween 2 and 11/21 days. This rather infre- 

 quent daily emergence was likely associated 

 with the markedly small number of large cer- 

 cariae produced within rediae of P. macrosto- 

 ma. Cercariae within a redia were at notice- 

 ably different stages of development as pre- 

 viously reported by Horsfall (1934), and sev- 



