Top 10 Most Diabolical Macroparasites In Nature

When we hear the word “parasite,” most of us picture critters like worms, ticks, and mosquitoes. They suck (literally), sure, but at least they’re just bugs. But then you find out there are macroparasites: full-sized animals that latch onto other living things and treat them like a snack bar, a free ride home, or a long-term vacation suite.

Here are ten of the most unsettling examples that might change how you view the world.

1. Castrator Barnacle

At first glance, barnacles seem harmless, but that’s just the majority of them. There’s Sacculina, a parasitic barnacle that basically hijacks crabs and turns them into chitin and meat puppets. A microscopic larva slips into a crab through a joint and grows into root-like tissue inside the body, eventually forming an external “brood sac” under the crab. It can shut down the crab’s reproduction (hence the pet name “castrator”) and even mess with hormones so badly that the crab starts behaving like it’s caring for the parasite’s young. Nature is wild.

2. Pea Crab

Continuing the crab theme, let’s take a look at pea crabs. These little guys like to enter oysters and other shellfish while they’re still growing and then squat inside the shell. Instead of going outside hunting for food, these freeloaders steal everything they need from the host. The host usually survives, but it’s basically stuck feeding an unwanted roommate that won’t and can’t move out. Stuck together forever, the two are forced to figure it out.

3. White-winged Vampire Bat

Bloodthirsty vampire bats are not a secret to anyone, but the white-winged vampire is extra weird because it specializes in birds. It waits for a bird to settle, then swoops in, makes a small cut, pulls out a bendy straw, and starts drinking the life juice like it’s a frat party. Birds can’t exactly reach around and flick a bat off like a cow can swat a fly, which gives the bat a big advantage. If you’re a slumbering parrot, it’s advisable to keep your eyes open.

4. Sharp-beaked Ground Finch

This little songbird looks like it should be chirping in a postcard, but it’s one of those “if it looks harmless—start running” types of horror tropes. On some islands, sharp-beaked ground finches approach resting seabirds and other animals and use their beaks to open up tiny wounds to drink the blood. You’ll definitely survive the “feast,” but you’ll never trust a cute songbird ever again.