Encystment site affects the reproductive strategy of a progenetic trematode in its fish intermediate host: is host spawning an exit for parasite eggs?

Abstract

Each transmission event in complex, multi-host life cycles create obstacles selecting for adaptations by trematodes. One such adaptation is life cycle abbreviation through progenesis, in which the trematode precociously matures and reproduces within the second intermediate host. Progenesis eliminates the need for the definitive host and increases the chance of life cycle completion. However, progenetic individuals face egg-dispersal challenges associated with reproducing within metacercarial cysts in the tissues or body cavity of the second intermediate host. Most progenetic species await host death for their eggs to be released into the environment. The present study investigated temporal variation of progenesis in Stegodexamene anguillae in one of its second intermediate fish hosts and the effect of the fish's reproductive cycle on progenesis. The study involved monthly sampling over 13 months at one locality. A greater proportion of individuals became progenetic in the gonads of female fish hosts. Additionally, progenesis of worms in the gonads was correlated with seasonal daylight and temperature changes, major factors controlling fish reproduction. Host spawning events are likely to be an avenue of egg dispersal for this progenetic species, with the adoption of progenesis being conditional on whether or not the parasite can benefit from fish spawning.

The missing host hypothesis: do chemical cues from predators induce life cycle truncation of trematodes within their fish host?

“…The frequency of progenesis in this experiment was similar to levels found in nature at the same time of year (Herrmann & Poulin, 2011 a ). Additionally, abundance of the anguillid definitive host was not found to affect progenesis in a field survey (Herrmann & Poulin, 2012).…”

“…Alternatively, the correlation between variation in egg size and mean egg size may be spurious, because one is derived from the other, although there is no solid statistical reason to dismiss a biological explanation on that basis alone (Prairie & Bird, 1989). Once outliers in egg size and variation in egg size were removed, these results were similar to previous studies on S. anguillae (Herrmann & Poulin, 2011 a,b , 2012).…”

“…Anguillid abundance may be more stable than hypothesized and thus may not be an important selection force for progenesis in S. anguillae . Other selective factors may be the drivers of facultative progenesis in S. anguillae , such as encystment site and G. cotidianus reproductive cycle (Poulin & Lefebvre, 2006; Herrmann & Poulin, 2011 a ) and G. cotidianus host longevity (Herrmann & Poulin, 2011 b ), if S. anguillae is perceiving transmission opportunities via host reproductive hormones and hormones associated with longevity. To assess the role of alarm cues and plasma cortisol levels, it would be interesting to repeat this experiment with cues from predator odour alone, alarm cues alone (from the skin of conspecifics) and cues from a host predator fed conspecifics, using a shared filtration system so that G. cotidianus are constantly exposed to cue treatment.…”

“…The first intermediate snail host, Potamopyrgus antipodarum, both anguillid species, the definitive hosts for S. anguillae, and perch Perca fluviatilis L. 1758, a non-host predator, are also found in Lake Waihola. Natural infections in G. cotidianus of this size and from this site range between four and >100 S. anguillae per fish (Poulin & Lefebvre, 2006;Herrmann & Poulin, 2011a, 2012. Prior to the acclimation period and again during allocation to experimental treatments, fish were treated for fungal skin infection using Profurin (1 g 100 l −1 ) for 30 min and given a 5 min saltwater bath to eliminate monogenean ectoparasites due to the problematic nature of these infections while housing G. cotidianus.…”

The scaling of parasite biomass with host biomass in lake ecosystems: are parasites limited by host resources?

“…In the case of the fish host, the parasites accounting for most of the biomass are four trematode species occurring as encysted metacercariae in various host tissues. An earlier study of one of these species found that the size achieved by an individual parasite is totally unaffected by the number of con‐ or heterospecific metacercariae in the same host (Herrmann and Poulin ). Typically, after a brief initial growth phase immediately following infection of the fish host, metacercariae encyst and probably derive very little further energy from the host.…”