Around the Isles in 80 Days: A Snail and Crab Adventure

Parasites have an important role in the ecosystem. Trematodes are a parasite species that have a three-host life cycle. The first host in the trematode life cycle are snails, which trematodes will asexually reproduce into a larval stage that will encyst the second host. The cycle starts off with trematode eggs that are passed through the feces of the gulls and then are released into marine environments. The first hosts, the snails, are castrated as the trematode asexually reproduces in the gonad region, generating copious cercariae that escape from the snail. The second hosts are encysted with the cercariae, with the cercariae going dormant until the definitive host consumes the second host. Three snail species (Littorina littorea, Littorina obtusata, Littorina saxatilis) were hand collected from nine different locations throughout the Isles of Shoals. The European Green Crab, Carcinus maenas, was collected by hand and with baited traps. During each trip out into the field, birds were counted by eye. Data showed that parasites were more likely to be found in larger snails, and that there was a high likelihood of difference based on the sites collected from. There did not show a strong relationship of sea bird abundance although it could be possible that as long as there are any sea birds present, then the other species in the trematode life cycle have the potential to be infected.

Hello! My name is Liam and I am a senior in Environmental Science. For the GMU OSCAR celebration, I am presenting my project on snail and crab parasite abundance.

When I first got interested in parasites, it’s because I really like the weirder creatures in the world since they are just funky little dudes, then I learned how cool and important parasites are. So, what is so important about parasites? Surprisingly, a lot is. Parasite populations are indicators of a stable environment. The parasites I have been looking at in particular are known as trematodes, which I will touch on more in just a moment. The Isles of Shoals has been used as an important study system for parasites over the past 20 years, and what has been found is that gulls are the driving forces to trematode life cycles.

Speaking of life cycles… I’d like to go over one of the life cycles of the trematode using a graph that shows the life cycle of the trematode microphallus similis. This is just a general overview of these lifecycles, which can be complex and include multiple different hosts. There is a three-stage life cycle which includes both asexual and sexual reproduction. Starting off, the eggs are passed through the feces of the gulls and then are released into marine environments. For Microphallus similis, the first intermediate hosts are either Littorina obtusata or Littorina saxatilis, however the most commonly infected snail is Littorina littorea with the trematode Cryptocotyle lingua.

That leads us to our first hosts, which in this case are snails, which are castrated as the trematode asexually reproduces in the gonad region, generating copious cercariae that escape from the snail. The castration process can lead to not only decreased reproductive function, but also can cause tissue damage as well as organ dysfunction.

Then we go to the second intermediate host, in this case it’s Carcinus maenas but with the aforementioned trematode Cryptocotyle lingua, the second intermediate host would be a fish of some kind. Basically, they go dormant in the secondary host until that host is ideally eaten by the gull, at least ideally for the parasite.

The final host, which are gulls, are where trematodes sexually reproduce and the cycle continues.

I collected 240 snails and 30 crabs from a total of nine different locations by hand and by trap. In the lab, I dissected the specimens and assessed them for trematode presence.

What I found is that parasites were more likely to be found in larger snails

I looked at the prevalence of parasite rate by location and it showed that there is a high likelihood of there being a difference in parasite rate based on location. This was shown to be the same in Littorina obtusata and Littorina saxatilis, although I only show Littorina littorea and Littorina obtusata.

There didn’t show to be strong evidence showing a difference between the numbers of birds present and the parasite percentage for any of the species of snails or for the crabs.

Overall, I found that there was a pretty significant difference between parasite rate and the size of the snail as well as the location the snails were found at. While there doesn’t seem to be a strong relation with the amount of sea birds that are present, it could be possible that as long as there are any seabirds present, then snails and crabs have the potential to be parasitized.