J. Ky. Acad. Sci. 66(2):89-93. 2005. 



Location of Rediae of Proterometra macrostoma 

 (Trematoda: Azygiidae) in the Snail Elimia semicarinata 

 (Gastropoda: Pleuroceridae), and Daily 

 Emergence of its Cercaria 



Ronald Rosen, Jonathan Fleming, Bojana Jovanovic, Aishe Sarshad, Emilie Throop, 



Fady Zaki, and Andy Amnions 

 Department of Biology, Berea College, Berea, Kentucky 40404 



ABSTRACT 



The objectives of this study were to describe the (1) location of Proterometra macrostoma rediae in the 

 snail intermediate host, Elimia semicarinata, and (2) daily emergence of cercariae from individual snails over 

 3 weeks. Decalcification and histological sections of infected snails revealed rediae restricted to the bottom 

 whorl within the mantle cavity in close association with the host gills. A continuous, low-level emergence of 

 cercariae (0.25-0.833 larvae/snail/day) was observed over 21 days. Both moderately frequent (7-12 days) and 

 infrequent (2-6 days) shedding of cercariae were observed (i.e., mean ± SE = 7.3 ± 0.4 days; range = 2- 

 12 days), but no consistent pattern was apparent. The average ± SE number of rediae was significantly 

 smaller in snails shedding only 2-6 days (11.9 ± 2.9) than snails shedding between 7-12 days (35.2 ± 8.5) 

 at the termination of the experiment. 



INTRODUCTION 



The original work on morphology and gen- 

 eral biology of the cercaria of Proterometra 

 macrostoma was completed by Horsfall (1934) 

 and Dickerman (1945). Horsfall (1934) noted 

 the presence of larval forms of P. macrostoma 

 in the snail body cavity. Dickerman (1945), 

 based on his study of crushed snails, indicated 

 that the size and number of emerging cercar- 

 iae may cause the body cavity wall to rupture, 

 thus introducing cercariae into the mantle cav- 

 ity from where they can exit into the outside 

 environment. However, Hyman (1967) and 

 Voltzow (1994), in extensive reviews of pros- 

 obranch anatomy, did not mention the pres- 

 ence of a body cavity associated with these 

 snails. Thus, these gross observations should 

 be reassessed by histological techniques aimed 

 at describing the correct location of the P. 

 macrostoma rediae and cercariae within their 

 snail host. 



Lewis (1988) determined that an average of 

 only 0.35 and 0.29 P. macrostoma cercariae/ 

 snail were released daily in the field and lab- 

 oratory from sample populations of snails, re- 

 spectively. This low production of P. macro- 

 stoma cercariae was also documented in a 3- 

 week study of daily emergence (Lewis et al. 

 1989). According to Lewis et al. (1989), this is 

 100-1000-fold less than most other digeneans, 



representing an adaptation for the creation of 

 a few, large, conspicuous cercariae. However, 

 no hypothesis was offered for this low cercarial 

 output. Evaluation of cercarial emergence pat- 

 terns from individual snails and a reexamina- 

 tion of P. macrostoma redial stages may fur- 

 ther our understanding of this phenomenon. 



The objectives of this study were (1) to de- 

 termine the location of P. macrostoma rediae 

 in the snail intermediate host and (2) to de- 

 scribe the daily emergence of cercariae during 

 3 weeks from individual snails naturally in- 

 fected with this worm. 



MATERIALS AND METHODS 



Snails of the species Elimia semicarinata 

 were collected from North Elkhorn Creek in 

 Scott County, Kentucky (lat 38°ir00" N, long 

 84°29'19" W), during summer 2004. They 

 were then screened for patent infections (i.e., 

 shedding cercariae) as described by Rosen et 

 al. (2000). Thirty-six infected snails were si- 

 multaneously fixed and decalcified in Cal-Ex 

 II (Fisher) for routine paraffin sectioning to 

 describe redial location in the snail host. Serial 

 sections (5-10 |jl) were stained with hematox- 

 ylin and Gomori's trichrome. Emergence of 

 cercariae was assessed for 36 additional in- 

 fected snails that were individually isolated, 

 held at 20°C under a 12 hr light : 12 hr dark 

 cycle, and checked at the end of each of these 



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