Life history of Mermithid nematode parasites of the migratory mayfly, Ephemerella maculata

Background/Question/Methods

Parasites shape ecological communities by changing host behaviors, altering host sex, and changing host distribution and abundance. We investigated the relationship of Mermithid nematode parasites and their migratory aquatic host, the mayfly, Ephemerella maculata. While most aquatic insects complete their life cycle within the same segment of the river, E. maculata mayflies seasonally migrate between the mainstem and its tributaries. Eggs of E. maculata hatch in tributaries, then the aquatic nymphs drift downstream to mature and rear in the mainstem. Aerial adult mayflies emerge synchronously from the mainstem and return to the tributaries to lay eggs. Nematodes exit from adult mayflies after the mayflies have oviposited and drowned in the tributary. We asked: 1) how does Mermithid nematode infection influence E. maculata host dynamics? and 2) how is the nematode life cycle adjusted to the migration of host mayflies? To address the first question, we collected 304 adult female mayflies to measure the infection rate and the reduction in host fecundity. For the second question, we surveyed the spatial distribution and interaction of E. maculata and Mermithid nematodes across seasons.

Results/Conclusions

Forty-six percent of adult mayflies were infected (140 out of 304). None of the infected mayflies produced eggs, indicating that Mermithid nematodes sterilize the host mayflies. Infected mayflies were shorter (average 5.40 mm+/-0.15 mm SE) than their non-infected counterparts (average 6.41 mm+/- 0.13SE), suggesting that nematodes decrease both individual survival and reproductive ability of the host mayflies (p<0.001, t-test).

Monthly sampling showed that immature nematodes emerged from host mayflies in summer when the adult mayflies oviposited.  These juvenile nematodes matured outside of the host, and oviposited in tributary sediments near host egg masses. E. maculata eggs hatched and the nymph mayflies drifted downstream in December/January before the nematode eggs hatched. Nematode egg masses probably wash down to mainstems when winter storms flush tributaries.  We are investigating how and where juvenile nematodes infect their nymphal hosts.