Newsroom: Crouching Crab, Hidden Parasite: How habitat structure affects predator-prey dynamics of parasitized crabs

Parasites put a large strain on their unwilling hosts, and some host-parasite relationships are known to have crazy side effects. But sometimes the surrounding habitat can make that take-and-take relationship even more complicated, especially when that parasite is from out of town.

Researchers at East Carolina University recently published a study that investigated those complications, using an invasive parasite of a small mud crab as their model system. The parasite, Loxothylacus panopaeii, (or Loxo, for short), is a parasitic barnacle native to the Gulf of Mexico but is now widespread in populations of mud crabs all along the east coast. Loxo is capable of parasitizing several species of crabs across its range, but along the Atlantic coast it is only known to parasitize three: Eurypanopeus depressus (flatback mud crab), Rhithropanopeus harrisii (white-fingered mud crab), and Dyspanopeus sayi (Say’s mud crab). Loxo is known to castrate its crab hosts (making them unable to reproduce), and it is also known to “feminize” its male hosts, making them act in ways that normally only the females of those species would, such as tending to parasite eggs as if they were their own. The researchers studied this host-parasite system focusing on the host Eurypanopeus depressus, from here on referred to as E. depressus.

An inflated Loxo egg sac is carried on the underside of a black-fingered mud crab infected with Loxo. Photo by Will Parson, Chesapeake Bay Program [Source].

To test the effects of both predator presence and habitat complexity, the researchers recreated artificial habitats with different complexities. They used two kinds of aquaria for their set up: simple habitat with only gravel and complex habitat with both gravel and live oyster clumps. In addition to the healthy and unhealthy host crabs, two crab species that are known predators were also introduced, Menippe mercenaria (stone crab) and Panopeus herbstii (Atlantic mud crab). Multiple predators were used to simulate natural predator-prey dynamics.

A stone crab (Menippe mercenaria; left) and an Atlantic mud crab (Panopeus herbstii; right).
Researcher Christofer Brothers collecting mud crabs from “crab condos” along the Pamlico River in North Carolina.

The experiment consisted of three 7-day trials using 8-10 matched pairs of healthy and infected E. depressus crabs and one of each predatory crab in each of the two aquaria. Daily checks of each aquaria were performed, and the number of healthy and infected crabs remaining in each habitat type was recorded, in addition to the hiding preference of crabs in the complex habitat (in the oysters or in the gravel). Since individuals were not marked and the predatory crabs consume the majority of their prey, a reduced number of crabs was indicative of predation. In the simple habitat (only aquarium gravel), the infected crabs were all eaten quickly, while the healthy crabs lasted several days longer. However, in the complex habitat, there was no significant difference in how long the healthy and infected crabs were able to avoid predation. Additionally, the infected crabs were more often found using the oysters as hiding spots as opposed to the gravel. 

Experimental setup: This graphic depicts the two aquaria used in this study. The right tank shows the “complex” habitat with both aquarium gravel and three oyster clumps. The left tank shows the “simple” habitat with just aquarium gravel. Both tanks contained three species of crabs: two different predators, Menippe mercenaria (top left) and Panopeus herbstii (top right), as well as twenty host crabs, Eurypanopeus depressus (middle). Half of the host crabs were infected with Loxo, for a total of 8-10 each of infected and uninfected crabs in each trial of the experiment.

This experiment has helped further the understanding of how hosts of this invasive parasite behave and interact with other organisms in their surrounding environment. The parasite puts a great strain on its host and reduces its ability to avoid predators in simple habitat conditions. However, in complex habitats, this vulnerability to predators is made up for with the addition of plenty of hiding spots. This suggests that Loxo may be more successful at parasitizing its hosts and reproducing in more complex environments where its hosts can survive while infected. Looking at the habitat structure of the locations where Loxo is more prevalent may help answer this question on habitat complexity. Additionally, it is unclear if the hiding preference of infected crabs to hide in oysters is a result of the parasite directly or a general response to illness.

This figure shows the average survival rate of infected and uninfected crabs in the simple (top) and complex (bottom) habitats. Nearly all the infected crabs were preyed upon quickly in the simple habitat while the uninfected crabs lasted longer. However, in the complex habitat, there was no significant difference in how well the different crabs avoided their predators.

For more information:

Brothers, C. A., & Blakeslee, A. M. (2021). Alien vs predator play hide and seek: How habitat complexity alters parasite mediated host survival. Journal of Experimental Marine Biology and Ecology535, 151488. https://doi.org/10.1016/j.jembe.2020.151488

[All images provided by Christofer Brothers; Edited by Lindsey Broadus]

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