The effects of host size and temperature on the emergence ofEchinoparyphium recurvatumcercariae fromLymnaea peregraunder natural light conditions

Abstract

The production of cercariae from their snail host is a fundamental component of transmission success in trematodes. The emergence of Echinoparyphium recurvatum (Trematoda: Echinostomatidae) cercariae from Lymnaea peregra was studied under natural sunlight conditions, using naturally infected snails of different sizes (10-17 mm) within a temperature range of 10-29 degrees C. There was a single photoperiodic circadian cycle of emergence with one peak, which correlated with the maximum diffuse sunlight irradiation. At 21 degrees C the daily number of emerging cercariae increased with increasing host snail size, but variations in cercarial emergence did occur between both individual snails and different days. There was only limited evidence of cyclic emergence patterns over a 3-week period, probably due to extensive snail mortality, particularly those in the larger size classes. Very few cercariae emerged in all snail size classes at the lowest temperature studied (10 degrees C), but at increasingly higher temperatures elevated numbers of cercariae emerged, reaching an optimum between 17 and 25 degrees C. Above this range emergence was reduced. At all temperatures more cercariae emerged from larger snails. Analysis of emergence using the Q10 value, a measure of physiological processes over temperature ranges, showed that between 10 and 21 degrees C (approximately 15 degrees C) Q10 values exceeded 100 for all snail size classes, indicating a substantially greater emergence than would be expected for normal physiological rates. From 14 to 25 degrees C (approximately 20 degrees C) cercarial emergence in most snail size classes showed little change in Q10, although in the smallest size class emergence was still substantially greater than the typical Q10 increase expected over this temperature range. At the highest range of 21-29 degrees C (approximately 25 degrees C), Q10 was much reduced. The importance of these results for cercarial emergence under global climate change is discussed.

Effect of temperature changes on the cercarial-shedding rate of two trematodes

“…The developmental state of trematodes in mollusks is influenced by temperature since an increase leads to a reduction in the cercarial developmental time (Morley et al, 2010). Temperature -either directly or indirectly -also triggers cercarial release (Koprivnikar & Poulin, 2009b).…”

“…II): emergence per h (Schmidt & Fried, 1996;Tab. II [2]) or "daily emergence was subsequently observed on 3 variable days over a 6-day period at each temperature" (Morley et al, 2010;Tab. II [5]).…”

“…II [3]; Terhune et al, 2002;Tab. II [4]; Morley et al, 2007Morley et al, , 2010, while others exposed the host individuals abruptly to different temperatures (Schmidt & Fried, 1996;Fried et al, 2002;Koprivnikar & Poulin, 2009a;Vignoles et al, 2014;Paull et al, 2015). Despite differences in the expression of results or the protocol for evaluating the effect of temperature, the examples listed in Tab.…”

“…The bioassays were made with naturally infected H. parchappei because the latter are more suitable for the investigation of the levels of cercarial emergence (Morley et al, 2010). Approximately 1,300 snail individuals were collected from the Martín Stream (Buenos Aires province, Argentina) during December 2014.…”

“…According to several authors, cercarial emergence from the snail is stimulated by light by means of a larval positive phototropism occurring at the time when the snail tissues emerge -a timing that is, in turn, governed by the circadian light-dark cycle along with temperature differences as Zeitgebers (Morley et al, 2010).…”