Reference: Biol. Bui/. 205: I 10-120. (October 2003) O 2003 Marine Biological Laboraton Patterns and Processes of Larval Emergence in an Estuarine Parasite System JONATHAN T. FINGERUT, 1 '* CHERYL ANN ZIMMER. 1 t AND RICHARD K. ZIMMER 1 2 ' 1 Department of Biology, 2 Neurosciences Program and Brain Research Institute, University of California, Los Angeles. California 90095-1606 Abstract. Trematode parasites in intertidal estuaries ex-perience constantly varying conditions, with the presence or absence of water potentially limiting larval transport be-tween hosts. Given the short life spans (<24 h) of cercariae, emergence timing should be optimized to enhance the prob-ability of successful transmission. In the present study, field measurements and laboratory experiments identified pro-cesses that regulate the emergence of cercariae from their first intermediate snail hosts in an intertidal marsh. Larvae emerged over species-specific temperature ranges, exclu-sively during daylight hours, and only when snails were submerged. The three factors operate over different tempo-ral scales: temperature monthly, light diurnally (24-h pe-riod), and water depth tidally (12-h period). Each stimulus creates a necessary condition for the next, forming a hier-archy of environmental cues. Emergence as the tide floods would favor transport within the estuary, and light may trigger direct (downward or upward) swimming toward host habitats. Abbreviated dispersal would retain asexually re-produced cercariae within the marsh, and local mixing would diversify the gene pool of larvae encysting on sub-sequent hosts. In contrast to the timing of cercarial release, emergence duration was under endogenous control. Dura-tion of emergence decreased from sunrise to sunset, perhaps in response to the diminishing lighted interval as the day progresses. Circadian rhythms that control cercarial emer-gence of freshwater species (including schistosomes) are often set by the activity patterns of subsequent hosts. In this estuary, however, the synchronizing agent is the tides. To-Received 12 February 2003; accepted 16 June 2003. * Current address: Patrick Center for Environmental Research, Academy of Natural Sciences. 1900 Benjamin Franklin Parkway, Philadelphia. PA 19103-1 19? t Formerl) Cheryl Ann Butman. Correspond i,"j author. E-mail:
[email protected] gether. exogenous and endogenous factors control emer-gence of trematode cercariae, mitigating the vagaries of an intertidal environment. Introduction Parasite larvae typically disperse prior to finding and infecting a host. As with propagules of free-living organ-isms, such as crabs (Forward ct ai. 1986; Morgan, 1996), sponges (Amano. 1988), and plants (Horn ct al., 2001; Sehauber ct al., 2002), external cues may direct emergence of parasite larvae under favorable conditions. Because dis-persal stages of digenetic trematodes have life spans of 24 h or less (McCarthy, 1999; Toledo et al.. 1999), timing emer-gence to correspond with host availability would be espe-cially advantageous (Combes et ai, 1994; Pechenik and Fried, 1995). Moreover, the widespread distribution of trematodes in fresh water (Pages and Theron, 1990; Gerard, 2001) and saltwater (Martin, 1972; Bartoli and Combes, 1986; Jonsson and Andre. 1992; Curtis, 1997) environments allows for cross-habitat comparisons of emergence charac-teristics. Trematode emergence has been studied largely in fresh-water systems, with much of this research addressing med-ical and agricultural concerns (Bergquist, 2002; McKerrow and Salter. 2002). In common parasites, such as schisto-somes, larval emergence from intermediate host snails var-ies on a circadian cycle and is synchronized with definitive host availability (Pages and Theron, 1990; N'Goran et al., 1997). Circadian rhythms are usually entrained by photope-riod or thermoperiod (Theron, 1984; Mouchet ct al.. 1992; Combes ct al.. 1994). Freshwater parasite larvae moving from aquatic to terrestrial vertebrate hosts time their emer-gence to coincide with waterfront activities of the hosts, on scales of hours (Theron, 1989; Raymond and Probert, 1991). no
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