Introduced Crab Parasites Hijack Mud Crab Reproduction in Chesapeake Bay
Published August 2015
The introduced parasite Loxothylacus panopaei (Loxo for short) is affecting native mud crabs in Chesapeake Bay. Loxo is native to the Gulf of Mexico, the Caribbean, and parts of Florida and parasitizes several species of mud crabs throughout this range. The first discovery of the parasite in Chesapeake Bay was in 1964 on the native White-fingered Mud Crab (Rhithropanopeus harrisii) in the York River. The parasite is now common in much of the Bay, but its population and abundance varies greatly between years. Mud crabs are easily transported with oysters, and Loxo was likely transported to the Chesapeake with mud crabs caught up in shipments of oysters from the Gulf of Mexico.
The parasite has a highly evolved life cycle tailored for its life as a crab parasite. As a larva, the female Loxo is free-swimming and looks like a typical nauplius larva. Following the nauplius stage, Loxo molts into a cyprid larva. At this stage she has a specialized spear-like stylet used to inject herself into a soft-shelled crab's carapace. The part of her body that is injected into the crab is called a vermigon. Once inside she undergoes a series of physiological and morphological changes and assumes control over the host crab, controlling major functions such as molting and reproduction. After a time Loxo grows into a reproducing adult and forms small sacs (reproductive bodies or externa) that emerge through the abdomen of the crab, usually after the crab molts. As soon as the sac emerges, a free-swimming male Loxo attaches to the sac to fertilize the eggs. The sac is filled with of thousands of larvae that are released into the water every couple weeks to seek out a new crab host. This process eliminates the crab’s ability to reproduce and results in the crab caring for the developing larvae of the parasite. Occasionally multiple reproductive sacs can be seen on a single crab. Early genetic work indicates that these are multiple parasites rather than a single parasite producing more than one reproductive sac.
Loxo is an equal opportunity parasite, infecting both female and male crabs, but it is in the male crabs that her power to transform her host is most striking. She changes the crab’s behavior, especially males, in order to trick them into mothering her young. Female crabs have the physical attributes and behaviors need for mothering, such as a wide apron for holding and aerating eggs, so taking care of Loxo’s eggs rather than her own isn’t a major behavioral shift. But to teach a male crab to mother Loxo larvae is a major feat. In the male crabs, Loxo causes the apron of the male crabs to widen, making it easier for him to hold, aerate and protect her young. Loxo then causes a big change in behavior, giving the male crab the mothering instinct to care for her young, in essence “feminizing” the males. This level of host manipulation speaks to the power of Loxo and other parasites that assume complete control over their hosts.
Researchers at the Smithsonian Environmental Research Center (SERC) having been studying the prevalence of Loxo in populations of native White-fingered Mud Crab, R. harrisii, since the 1990s in hopes of understanding how this introduced parasite affects the mud crab population and how genetics and the uneven distribution of the crabs influences infection rates. In 2003, Dr. Ruiz began a large scale survey with Dr. Mark Torchin, now at the Smithsonian Tropical Research Laboratory, to look at the effect and prevalence of Loxo on mud-crab populations over time. Each summer for the last 12 years, crabs and their parasites have been collected from 10 sites in Maryland including sites near Queenstown, Centerville, Oxford, Aqualand (near the Governor Harry W. Nice Memorial Bridge), Combs Creek, Broomes Island, Harrington Harbor, Deale, Corn Island and SERC. At each site, crabs are collected in habitat collectors, or crab condos, which are small plastic crates filled with dead oyster shells that sit on the surface of the sediment. These crates stay in the water for two months, during which time mud crabs take up residence between the oyster shells. After two months, each crate is pulled up and the crabs are hand-collected from the oyster shell habitat. Back in the laboratory these crabs are measured, sexed, and examined for outward signs of the Loxo parasite.
Since 2013 a large term of citizen scientists have been helping with the survey. In 2014, 50 people participated in the June and August events. Fine more information here about this citizen science effort and to register.