The world is full of parasites that can force their hosts to do strange, even suicidal, things. But very few can functionally switch an animal’s gender. One such parasite lurks in Chesapeake Bay: an invasive barnacle that hijacks a mud crab’s reproductive system and impregnates it with parasite larvae—even if the crab is male
The invasive parasite Loxothylacus panopaei (Loxo for short) is a type of barnacle, but looks and acts nothing like the typical barnacles growing on rocks along the shoreline. Loxo has a highly evolved life cycle, essentially custom-made for acting as a crab parasite. As a free-swimming larva, Loxo resembles a typical barnacle larva. A female larva infects a recently molted crab by burying into its shell. Once inside, she undergoes a series of changes and assumes control over the host crab, dictating major functions such as molting and reproduction.
After a time Loxo grows into adulthood and forms small sacs (reproductive bodies) that emerge through the abdomen of the crab, usually after the crab molts. Once a free-swimming male Loxo comes along and fertilizes them, the sacs are filled with of thousands of larvae that are released into the water to seek out a new crab host. This process eliminates the crab’s ability to reproduce, resulting in the crab instead caring for the developing larvae of the parasite. If this weren’t bad enough, Loxo also compromises the crab’s immune system. Infection endangers both present and future generations of the mud crab.
Loxo are native to the Gulf of Mexico, the Caribbean and parts of Florida, and parasitize several species of mud crabs throughout the area. Scientists first discovered Loxo in Chesapeake Bay in 1964 on the native White-fingered Mud Crab (Rhithropanopeus harrisii), a tiny brown crab barely the size of a quarter. Mud crabs are easily transported with oysters, and Loxo likely entered the Chesapeake with mud crabs caught up in oyster shipments from the Gulf of Mexico. The parasite is now common throughout much of the Bay.
https://insider.si.edu/2012/09/undersea-parasite-turns-male-mud-crabs-female/