Reproductive success in a natural population of male three-spined stickleback Gasterosteus aculeatus: effects of nuptial colour, parasites and body size.

The effects of nuptial colour, parasites and body size on reproductive success were examined in a natural population of three-spined stickleback Gasterosteus aculeatus. Reproductive males were collected, with the contents of their nests, during the embryo-guarding stage from Lynne Lake (Cook Inlet, Alaska, U.S.A.), and nuptial colour, infection status and body size were recorded. Regression analysis revealed that male body size was the only predictor, of those measured, of reproductive success in nature.

Egg dispersal in the acanthocephalan Acanthocephalus dirus: field data.

Among acanthocephalans, eggs are typically dispersed in the feces of definitive hosts. A recent laboratory-based study provided support for the hypothesis that some female acanthocephalans (Acanthocephalus dirus) carry eggs into the environment prior to dispersal. Here, we examined the potential occurrence of this relationship under natural conditions. Using 6 field surveys, we searched the sediment of a local stream to determine whether the bodies of A. dirus females could be located. We recovered the bodies of 24 intact A. dirus individuals from the stream sediment, of which 5 were mature females. All 5 of the mature females contained mature eggs, with 1 female carrying approximately 10,000. These results are consistent with the interpretation that eggs can be dispersed from the bodies of female A. dirus in nature. We also found that there was significant variation in the number of mature eggs present in the females, with 4 of the 5 females carrying fewer than 400 mature eggs. In addition, we recovered approximately 20,000 mature eggs from a fecal pellet that had been expelled from a fish. We propose that eggs may be dispersed both in the feces of definitive hosts and from the bodies of expelled female A. dirus under natural conditions.

Intra-specific host sharing in the manipulative parasite Acanthocephalus dirus: does conflict occur over host modification?

The acanthocephalan parasite Acanthocephalus dirus induces a colour change in the intermediate host, the aquatic isopod Caecidotea intermedius, which increases transmission to definitive hosts (creek chub, sunfish). We examined the potential for conflict to occur between infective (cystacanth) and non-infective (acanthor, acanthella) stages of A. dirus over the level of colour modification that should be induced when these stages share a host. Using a field survey, we showed that host sharing by infective and non-infective stages was relatively common and that infective and non-infective stages differed in their effects on colour modification. Non-infective stages induced a colour change over 40% of the body, whereas infective stages induced a colour change over 80%. Thus, conflict could occur between stages over the level of modification that should be induced. We then showed that mixed-stage infections induced a colour change in the host that was consistent with the level of modification induced by the infective stage. We discuss the potential significance of these results to patterns of host modification and their effects on stage-related survival in nature.