menhaden isopod

Sea Grant photo

This menhaden’s tongue has been replaced by a parasitic isopod.

The oceans are full of all sorts of life. Some creatures are beautiful and majestic. Some are not. Parasites rarely make anyone’s “favorite species” list. But while they may be weird and even a little gross, they’re fascinating anyway.

Did you know that parasitic isopods (they look like a roly-poly or pillbug, but they’re aquatic) frequently take up shop in the mouths of menhaden fish? Yes, these creepy little isopods, attach to the base of the tongue and suck blood. Eventually the menhaden’s tongue falls off and it begins using the isopod as its tongue. The isopod will stay attached as long as the fish is alive getting its blood meal from the fish.

Word to the wise, once their host dies, the isopod detaches and looks for a new host … and they bite! Isopods are known to parasitize other fish as well. Menhaden just seem to be at the top of their favorite list.

Parasitic isopod species are quite small, but not all isopods are. Giant isopods are found throughout the world’s oceans. The first giant isopod was discovered by a French researcher in the Gulf of Mexico in 1879. There are at least 20 different species of giant and super-giant isopods, and the super-giants can get as big at 30 inches in length.

Another weird and creepy critter is called a rhizocephalan (rye-zo-SEFF-uh-lun). Rhizocephalans are a special kind of barnacle. You see most barnacles happily live their lives stuck to rocks and pilings. They glue their heads to a hard surface, build a shell around themselves, and then use their feet to catch drifting food particles and stuff them in their bellies.

But rhizocephalans are rude and crude. They hitchhike on other animals, mostly crabs. Once a rhizocephalan finds a host, it secretes a cement to anchor itself, just as a typical barnacle would. And — are you ready for this? — then the rhizocephalan drills into the crab and injects its own cells into the crab’s hemolymph (the equivalent of crab blood). Then it grows roots all through the crab’s body to steal nutrients from its host. Even as the crab grows and sheds its shell, the rhizocephalan remains.

Not only does the rhizocephalan take advantage of its host’s body, it also takes advantage of its brain, brainwashing it to carry and care for its eggs. And through this brainwashing, it somehow convinces its host to travel and release the eggs in areas suitable for rhizocephalan survival — guaranteeing the next generation of crustaceans have a new supply of parasitic barnacles to invade them.

Finishing out my creepy critter collection are the most primitive fishes, the Agnatha. Fossil records indicate that this group of fishes appeared at least 500 million years ago, although the living Agnatha date to about 350 million years ago. Agnatha includes hagfish and lampreys, which are collectively known as jawless fish. They are essentially slimy, scaleless eel-looking fish with a suction mouth for a head.

Agnatha have skeletons made of cartilage instead of bone, and as such are very flexible. In fact, hagfish can actually twist themselves up into a knot, which they do to rid themselves of the noxious slime they produce to deter predators from eating them. Most are temperate species, where they are parasites on other fishes much like the way ticks attach to dogs.

Agnatha lack fins, so when they are not parasitizing other fishes they live on the bottom, and occur in both fresh and salt water. Juvenile lampreys feed by sucking up muddy bottom sediments and feeding on organic material and micro-organisms found in these sediments. Hagfish have no eyes, but lampreys actually have good vision.

Our freshwater and saltwater environments are full of cool and creepy critters. These are just three of my favorites — there are lots more out there.

Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. Contact her at or 941-764-4346.

Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. Contact her at or 941-764-4346.


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