The disc shaped suction cup mouth is an excellent tool for tightly latching onto the skin of its host where the lamprey then scrapes through the flesh with a rasping tongue-like. Some species of lampreys have tongues and mouths specialized to gauge out pieces of flesh from fish, while other species have mouths specialized for blood feeding. The species of parasitic lamprey that eventually live in the ocean begin their life as larvae in rivers, only turning into flesh-eaters once they’ve reached adulthood.

There are fifty different species of gall crabs, all small in size. To avoid predators, they form depressions in living coral, a small pit or cave that the crab then lives in. The home also provides easy access to food. The crabs feed on mucus that the coral produces and also other algae bits that they can reach from inside their small coral caves.

For the most part, insects live life on land. However, the Echinophthiriidae, a family of lice that infect seals, sea lions, walruses, and otters, have invaded the sea like no other group of insects. While whales and manatees have lost much of their hair, these other marine mammals retained their furry bodies as they evolved from life on land to life at sea, meaning they have the perfect environment to harbor lice. Like their hosts, these lice have evolved amazing capabilities that allow them to survive the hardships of life deep under sea, a place no other insects go

Many sharks have unwanted passengers, called copepods, clinging to their skin and gills. They feed on their host in a similar way that a flea feeds on a dog. This means the shark parasites, not the sharks, are actually at the top of the food chain. There is even a copepod called Ommatokoita elongata that permanently attaches itself to the cornea in the eyes of Greenland sharks and sleeper sharks.

There are ectoparasites (living on the outside of their host) and endoparasites (those that live inside their hosts). Take for example, the largest roundworm in the world, Placentanema gigantisma. It lives in the uterus and on the placenta of sperm whales, where it can grow to be 24 feet long. As for the longest parasite in the world, that is likely another parasite of sperm whales—the tapeworm Tetragonoporus calyptocephalus—can grow an alarming 90 feet long.

Sacculina is a species of barnacle that infects crabs and then manipulates their behavior to benefit itself. They do so by growing a rootlike system throughout the crab’s entire body, which the parasite uses to feed on the crab. As the parasite develops, part of its body grows outside of the crab on its lower abdomen, right where a crab would carry its eggs. The parasitic barnacle then manipulates the crab by tricking it into treating the parasite like it would treat its own eggs – carefully caring for and nurturing the parasite by protecting it, grooming it, and flushing it with oxygenated water.

The cymothoid isopod are crustaceans sever the host fish’s tongue, latch themselves onto the remaining nub with hooked appendages, and then nestle down to live in the space vacated by the missing tongue. The isopods then allow the fish to carry on its normal activity, all the while it feeds on the host’s blood. This is the only known instance in the whole animal kingdom where a parasite replaces a host’s organ.