Parasites Hijack Sexual Function of Crabs

An alien parasite transforms males to look like females, and hijacks their reproductive
systems to pump out thousands of larval parasites.

This sounds like a science fiction plotline. But it is actually happening to a species of crab in the Chesapeake Bay: pea-sized, brown and olive mud crabs (Rhithropanopeus harrisii).

Scientists at the Smithsonian Environmental Research Center discovered that many mud crabs are infected with an invasive species of parasitic barnacle called Loxothylacus panopaei that are multiplying through this gender-twisting, reproductive system take-over, said Dr. Amy Fowler, a researcher at the Smithsonian.

“The parasite itself is from the Gulf of Mexico,” Dr. Fowler said, during a recent visit to one of her study sites on the Rhode River in Southern Maryland.  “Researchers believe that it was brought over in the 1960s as part of the oyster aquaculture trade here in the Chesapeake Bay. When they were moving the oysters, they inadvertently moved a lot of other organisms, including this particular (parasite).”

Dr. Fowler (left) stood on docks on Corn Island and lifted out of the water a milk crate full of oyster shells.  The shells were crawling with white-fingered mud crabs, which live on oyster reefs.

“See how small he is?  He’s cute,” Dr. Fowlers said, holding a fly-sized crab.   “If you look at its white claws, the points – the tips of them are white.”

More than half of the mud crabs –- male and female --  at one of Dr. Fowler’s study sites
had pouches hanging from their abdomens that looked like normal egg sacs. But these
were actually fake egg sacs brimming with thousands of larval parasites.

Dr. Fowler explained how the microscopic barnacles enter the crabs and change their sexual
form and function.

“The (parasites) are swimming through the ocean. And they can smell crabs.  And so the female will come down and smell crab and attach to the surface of that crab and bury through the crab’s carapace. And then find its way through the crab to the reproductive system,” said Dr. Fowler, who is also a post-doctoral fellow in the biology department at Villanova University   “So the barnacle takes hold of that, and transforms it into a barnacle making machine.”

Loxothylacus are not the only parasites that can hijack their hosts.

 “It’s probably more common than we know it, because we haven’t studied them all,”
said Dr. Ann Barse, an associate professor of biology at Salisbury University. “But there are many examples of parasites that change the behavior of their hosts.”

Dr. Barse gave the example of a worm called the lancet fluke. It can force infected ants to march to the top of grass blades and wait there so they can be more easily eaten by grazing sheep, transferring the parasite.

Why should we care?  Well, the Loxothylacus panopaei only infects mud crabs –- which are not commercially harvested or a species that many people care about. But there is a similar species of parasite that also lives in the Gulf of Mexico called Loxothylacus texanus which castrates blue crabs, which are key to the Chesapeake region’s economy.

Smithsonian scientists warn that this other, blue crab parasite could also be transferred
to the Chesapeake Bay if people continue to be careless about moving oysters and other aquatic life from one body of water to another.

Dr. Mario Tamburri, director of the University of Maryland Center for Environmental
Science’s Maritime Environmental Resource Center, said that aquaculture, international shipping, the
pet trade, and the sales of fishing bait all spread invasive species when precautions are not taken.

“The important message really is that there is lots of ways that invasive species, aquatic invasions, can be moved around the world,” Dr. Tamburri said. “We often think of big ships –- their ballast water, and organisms growing on their sides. But there are also other vectors…. It’s not uncommon to see with aquaculture that organisms are being moved around the world and parasites are
following along."

It is this unconscious spreading of invasive species that is the real horror story.

By Tom Pelton

Chesapeake Bay Foundation

(Photo of mud crab with parasite at top from Smithsonian Environmental Research Center)

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This evasive parasite is most concerning, especially when it is known that there is a parasite in the Gulf of Mexico that castrates male Blue Crabs. You add in the problems that the Chesapeake Bay has regarding polution and it seems to be no end to the dangers that the Bay is facing.

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