Effect of Light Wavelength — Rosen et al. 



95 



trolHng for light intensity) on this swimming 

 behavior and associated electrical burst activ- 

 ity in the cercarial tail. 



The objectives of our study were to deter- 

 mine (1) the range of osmolalities tolerated by 

 the cercarial tail as demonstrated through 

 swimming longevity and the subsequent 

 emergence of the cercarial body from its tail 

 and (2) if a graded swimming response and 

 associated electrical activity in the cercarial 

 tail result from exposure of cercariae to se- 

 lected wavelengths of the visible spectrum. 



METHODS 



Snails, Elimia semicarinata, were collected 

 from North Elkhorn Creek in Scott County, 

 KY (38° 11' 00", 84° 29' 19" W), during sum- 

 mers 2000 and 2004. They were then screened 

 for patent infections (i.e., shedding cercariae) 

 as described by Rosen et al. (2000). To assess 

 the effect of osmolality on cercarial swim- 

 ming, freshly released cercariae (i.e., less than 

 1 hr PEm) were placed into beakers contain- 

 ing each of the following hypotonic or hyper- 

 tonic solutions; distilled water (0 mOsm; hy- 

 potonic), artificial pond water (APW; 15 

 mOsm; 0.5 mM NaCl, 0.05 mM KCl, 0.4 mM 

 CaClo, and 0.025 mM MgCK; hypotonic), ar- 

 tificial snail water (ASW; 102 mOsm; 15.0 

 mM NaCl, 1.0 mM KCl, 5.0 mM CaCl„ 0.5 

 mM MgCU; hypotonic), and ASW with 100 

 and 200 mM mannitol, resulting in 180 and 

 267 mOsm hypertonic solutions, respectively. 

 A crude extract of snail fluid was made by 

 removing the snail shell in ASW and blotting 

 the body for 5 sec on filter paper. Next, snail 

 bodies were placed in a tissue grinder, the re- 

 sulting fluid centrifuged with an Eppendorf mi- 

 crocentrifuge for 1 min, and the supernatant 

 osmolality determined. This value served as 

 the isotonic osmolality baseline for the exper- 

 iment. Final osmolalities of the test solutions. 

 North Elkhorn Creek water, and the snail fluid 

 were determined using a Wescor Vapro vapor 

 pressure osmometer. Individual cercariae were 

 placed into separate 2-liter graduated cylin- 

 ders filled with the test solutions to assess 

 swimming. Vertical swimming distances were 

 obtained by dividing the distance traveled by 

 a cercaria (according to the horizontal ml 

 marks on the cylinder) by five based on the 

 actual distances between these graded marks 

 (e.g., 1.0 ml = 0.2 mm). These distances were 



measured in five replicate experiments on an 

 hourly basis for 12 hr. A one-way ANOVA 

 was used to compare mean swimming dis- 

 tances at 6 and 12 hr PE (post-exposure). The 

 following technique was used to assess the ef- 

 fect of long-term exposure of cercariae to dif- 

 ferent osmolalities on the subsequent emer- 

 gence/infectivity of the cercarial body from its 

 tail. Additional cercariae (6-8/osmolality) 

 from each of the test solutions were placed in 

 10 ml beakers filled with a solution adjusted 

 to pH 2.0 at 6 and 12 hr PE. for 1 hr to sim- 

 ulate conditions found in the host fish stomach 

 following ingestion of cercariae. The numbers 

 of completely emerged cercarial bodies were 

 recorded at the termination of the 1-hr 

 incubation. 



A preliminary experiment demonstrated 

 that there was no decrease in cercarial swim- 

 ming during the initial 9 hr PEm at 15°, 20°, 

 and 25°C. Thus, cercariae, less than 6 hr PEm, 

 were individually isolated in 2-liter graduated 

 cylinders filled with aerated and filtered North 

 Elkhorn water to assess the effect of light 

 wavelength on vertical swimming distance. 

 Broad spectrum, 25-watt fluorescent lights 

 double-wrapped in 2-mm red, green, or blue 

 filters (Edmond Scientific F35235-F35137) or 

 black fiberglass window screen were placed 

 horizontally on stands above the cylinders and 

 adjusted to achieve a light intensity of 11.5 

 foot candles (fc; Extech Light Meter) at the 2- 

 liter mark The light intensity selected approx- 

 imated daylight field values (14.5 fc) obtained 

 at North Elkhorn Creek at 0.6 m depth. Twen- 

 ty-three cercariae were exposed in 7-min in- 

 tervals to a sequence of red-blue-red-white- 

 red-green-red light conditions. Each 7-min 

 time span consisted of a 2-min acclimation pe- 

 riod followed by a 5-min recording period. A 

 single swimming burst distance was recorded 

 at the beginning of each minute of the 5-min 

 recording interval for each light treatment, and 

 an average distance, using the aforementioned 

 conversion factor, was calculated from these 

 five observations. 



Cercarial electrical activity was subsequent- 

 ly recorded under the same light condition se- 

 quence and timing using the technique of Ug- 

 lem and Prior (1983). Electrical recordings 

 were made by placing cercariae in fingerbowls 

 with APW. The large size of the P. macros- 

 toma cercaria made it possible to pin the up- 



