Temperature and Indicators of Parasite Infection in Marine Snail Hosts

Author(s): Grace Loonam

Mentor(s): Amy Fowler, Environmental Science and Policy

Abstract

Changes in temperatures of marine habitats caused by anthropogenic climate change have the potential to influence ecosystem dynamics within these landscapes. The impact that these changes could have on parasitic relationships with their hosts is a significant topic of study that has critical implications for ecosystem function. Specifically, I chose to examine the effect of temperature on cercarial release (the emergence of clonal, free-swimming parasite larvae from the intermediate host into the surrounding water column) in two species of marine snails (Littorina littorea and Littorina obtusata). It was hypothesized that increased water temperatures would lead to increased numbers of cercarial emergence of trematode parasites due to evidence from previous research suggesting this trend for trematodes. The snails, collected in June 2021 from Appledore Island, Maine, were desiccated for 12 hours to cue cercarial release, then submerged in vials with 50 mL of saltwater and placed in one of three test temperatures for six hours. Two of the test temperatures, 31°C and 33°C, were maintained by keeping the snail vials in incubators, while the third temperature (24°C) had the vials sitting out on the lab bench as a control. While only four out of the 159 L. obtusata analyzed were infected (thus making the species unsuitable for verifiable analysis), 57 out of the 171 L. littorea examined were infected. The results of the L. littorea analysis suggest that there was a negative relationship between the number of cercariae release and increasing temperature. These findings add to the broader scientific theme of how organismal fitness may change due to climate fluctuations and offer significant justification for further research examining the impacts of temperature on parasite-host relationships.

Video Transcript

Hi everyone! My name is Grace Loonam, and today I’m going to be giving a presentation discussing the indicators of parasite infections in marine snail hosts, and the impact that temperature has on this relationship. I worked with two different species of marine snails this summer. The first, Littorina littorea, or the common periwinkle snail, is pictured on the left, while the other, Littorina obtusata, the smooth periwinkle snail, is pictured on the right. Both of these snails are parasitized by organisms known as trematodes, which are parasitic flatworms that infect snails in an intermediate stage in their life cycle. This relationship is shown in the figure on the right, and in the bottom right portion of the figure, you can see the miracidia that the snails consume to get infected. The parasites then asexually reproduce in the snails, resulting in the production of the redia and cercariae shown in the bottom left portion of the figure. This is the free-swimming larval stage of the parasite’s life cycle, and it emerges from the snail to go infect another intermediate host, such as fish. These second intermediate hosts ultimately get consumed by the final host, in this case the gull shown on the top of the figure, thereby infecting the bird with the trematode. The bird then passes on the infection to the snails again via the miracidia contained in the bird’s guano, or feces, which the snails then consume. Within the snails, we know even more about the nature of this parasitic relationship. The trematodes mainly infect the gonads of the snails, circled in the figure on the right, which is the reproductive area of the snail. The trematodes inhibit reproduction here and are also thought to cause a change in the foot color of the snail, as illustrated in the pictures here, although the reliability of this indicator has been debated in later studies. On the left is a littorea with a comparatively lighter foot color, which is thought to indicate that the snail is not infected with any parasites. However, the comparatively more orange foot of the snail on the right suggests that that individual is infected. With the existing knowledge and discussion about this parasitic relationship, I wanted to learn a bit more about the alleged correlation between foot color and infection status. Additionally, I was curious about how temperature could impact the relationship, especially with the potential temperature changes in the future due to climate change. This led to the formation of my research questions, which were “Is foot color associated with infection status in L. littorea and L. obtusata,” and “Does increased water temperature lead to increased cercarial emergence in L. littorea and L. obtusata.” Also, on the right, the pictures show more variation in the foot colors of one of the snails I worked with, L. littorea, and the other to pictures show examples of some of the parasites I saw throughout my work. The picture on top is of Cryptocotyle lingua in a littorea host, and the picture below is of Microphallis similis in an obtusata host. In the bottom picture, the yellow arrows point to some of the individual cercariae in the sample, while the blue arrows point to the large white sporocysts that these cercariae emerge from. In order to evaluate these research questions, I went to Shoals Marine Laboratory on Appledore Island in the Gulf of Maine to collect the snails I worked with. The snails were brought back to George Mason’s Potomac Science Center n Woodbridge, Virginia, and held in tanks with aerators until they were used in the experiment. The night before the individual snails were to be used, they were left out to dry for 12 hours with algae, and then submerged in vials with 50 mL of saltwater. This mimics the rise and fall of the tide on the shore where these snails are found, and often induces the parasite release into the surrounding water. The vials were put into incubators at temperatures of 31 degrees celsius and 34 degrees celsius, as well as left out in the lab as a control temperature of about 24 degrees celsius. The snails were left in these conditions for 6 hours. At the end of the six hours in the incubator or on the bench, the snails were removed from the vial and placed in separate labelled boxes, while the water from the vial was emptied into the trays shown in the above picture. Snails were then dissected to determine infection status, and then the water trays affiliated with the infected snails were examined for cercariae. The number of cercariae in the tray was then counted. The next steps in my project are to analyze some of the data we have collected thus far, and see what the results reveal about the relationships between foot color and the number of cercariae released, foot color and infection status, and the number of cercariae released and the temperature that infected snails were kept at. I also plan on conducting a few more trials of the experiment in order to add to the number of replicates at each temperature. I want to acknowledge several people and organizations who made my work this summer possible, specifically Shoals Marine Laboratory, George Mason’s URSP program, and the Potomac Environmental Research and Education Center at the Potomac Science Center. I also want to thank my mentors, Dr. Amy Fowler, Dr. April Blakeslee, and Dr. Carolyn Keogh, and my fellow intern at Shoals, Amanda Wolf. And these are the resources I used throughout my research and for this presentation, thank you so much for watching!!

6 replies on “Temperature and Indicators of Parasite Infection in Marine Snail Hosts”

Hi Grace!

Awesome project, and I loved your video! That is fascinating that trematode infection may change foot color in these snails. How do you record foot color? Is it a binary white/orange type of thing or is there a scale (like 1-10, 1 = white 10 = darkest orange) you use? Are you primarily interested in if snails are infected or not, or are you interested in the intensity of infection as well?

Lulu

Hi Lulu, thank you for the great questions! There was a scale of 0-5 (0 being the lightest foot color, white, while 5 represented a very dark orange), and we made this scale using the excel color gradient function. And we recorded primarily if the snails were infected (with lots of fully formed redia and cercariae) or not, but we also recorded cysts when they were found.

How cool! Do you think the change in color would be similar in other snail species infected with this virus as well?
I’m so impressed (and a little jealous) that you got to go to Maine and collect snails. That sounds like so much fun. : )

Hi! It was pretty fun haha, and yes, I started to do some work on that while I was still up in Maine. We haven’t finished the data analysis for the other snail species we looked at yet, but they also appear to have the same coloration impact.

Nice presentation. So jealous you got to go to Shoals! 34 C is pretty warm. Was it meant to mimic higher temperatures during exposure and/or during future warming events? Look forward to hearing about your results. Best, Dr. Lee

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