Can Endoparasites Affect the Feeding Habit of Their Crab Hosts?

Though often overlooked in the field of ecological studies, parasites can alter trophic food web structure by affecting the behavior of their hosts to improve their own individual fitness. While some parasites manipulate host behaviors to make the host more available to potential predators (e.g., trophically transmitted parasites), other parasites do not want their host to be eaten and instead cause hosts to reduce movement. This reduction in movement could possibly negatively affect foraging capabilities. Here, we tested how the presence of two different endoparasites could affect the ability of the estuarine mud-crab species Rhithropanopeus harrisii to feed. The endoparasites that this research focuses on includes the invasive parasitic barnacle, Loxothylacus panopaei, and the novel, putatively native entoniscid isopod, Cancrion sp. We hand-collected these crabs from various sites around Virginia and Maryland. Whilst their height, sex, and infection status were noted, the crabs were individually placed into an area with a pre-measured prey item (5 mm of a bristled worm) and were allowed to forage for 45 minutes whilst being videotaped. Each individual was tested three times and starved for 48 hours between trials. The analysis of the trial videos is still occurring, and all crabs are being dissected to confirm infection status. However, we have observed that these mud crabs can host both parasites at the same time. This was shown by an individual crab that acquired L. panopaei externa as well as released Cancrion sp. larvae. Another observation was that, like gravid crabs, crab hosts infected with L. panopaei in the externa phase will not molt, as molting would remove the parasitic externa. This research is important because it will provide a greater understanding of the natural history of the new Cancrion species, but it may also represent the importance of parasites in food-web dynamics.

Hello, my name is Mounia Hammadi, and with the help of my colleagues, Dr. Amy Fowler, Darby Pochtar, Sarah Greenberg, and Victoria Wang, this summer I set out to answer the question, do endoparasites affect the feeding of their crab hosts? So, what are endoparasites? Well, the simplest definition shows that they are part of a symbiotic relationship known as parasitism and in order for endoparasites to parasitize their hosts they must internally inhabit them. The most common is the adult roundworm, which is commonly found in household pets, such as dogs. However, the endoparasites that are being studied in this research project are more unknown to the general public, this includes the Loxothylacus panopaei and the Cancrion sp. The L. panopaei, or “loxo” for short, is a parasitic barnacle in which it parasitizes its host by castrating it. Castration is the inhibition of reproduction, and it does so by taking the energy that the host requires in order to reproduce and uses it for its own survival. Parasitism by this parasite is confirmed by a virgin externa which is a sac of eggs that has yet to be fertilized, or a much larger mature externa which holds the baby larvaes of the parasite that has yet to be released. The Cancrion sp. is a lesser-known parasitic isopod and is thought to be native to the East Coast. Other parasitic isopods that scientists have been found has shown to castrate their hosts, thus it is possible that this species would do so as well. But unlike loxo where the infection status of a host is visibly apparent, the confirmation for this new species either requires the host to release the parasitic larvae into the water in which the host resides or by dissection, which would determine whether or not the parasitic mother can be found lying near the reproductive tissue of the host. The designated model that will show this parasitic-host relationship for this experiment is known as the white-tipped mud crab, or more formally known as the Rhithropanopeus harrisii. With that, the question that I have been trying to answer all summer is “Can endoparasites affect the feeding habits of their hosts?”. I hypothesize, that they do, and here is my reasoning why. Previous experimentation with another crab known as Eurypanopeus depressus, has shown that when infected with loxo, they tend to remain inactive much longer before they attack their prey, which may have contributed to their reduced consumption. As explained before, since loxo achieves castration by an energy drain, which is causing the host to become more lethargic, it is possible that it is taking a heavy toll, which may be driving the host’s behavior. However, in order to test this hypothesis, we need samples, so we collected our samples of R. harrisii in various parts of Virginia and Maryland. These samples were then collected and brough to the Potomac Science and Research Center where they were housed in a common salinity of 10 ppt so that no other variables were in play during the experimentation. The initial status of the 45 crabs that were designated to be run in the first part of the experiment all differed greatly. 25 were considered healthy, while 9 of them actually were infected with Cancrion sp., as shown with their release of the parasitic larvae and 11 had the externa which is caused by the Loxothylacus panopaei. After the trials, all 25 of the healthy crabs were then frozen in order to be dissected to ensure that the status that was previously given is actually accurate. Come to find out that it wasn’t accurate for three of these crabs. One actually was infected with the Loxothylacus panopaei, the other two actually were infected with the Cancrion sp. However, the work is far from over. I still have many crabs that need to be dissected, especially those that haven’t even been experimented on in order to obtain a proper infection-healthy ratio between each of the sites. However, none of this would have been possible if it wasn’t for the help of my co-workers, the Potomac Science and Research Center for providing me with the space in order to do my experiments, Dr. Amy E. Fowler for allowing me access to her lab in order to collect all my samples, run my trials, as well as the OSCAR Undergraduate Research Scholars Program for providing me with support and everything that I need for the summer to make this experiment happen. I hope to get back to y’all with some more concrete information in the near future, thank you for your time.