Crab Leech

A leech is a soft, worm-like animal that drinks blood. This one is tiny — only about 26 millimeters long, about an inch, like the width of a crayon. It is called the "crab leech," but here is the big surprise: it does not drink crab blood at all! It is a fish parasite — it clings to a fish and sips the fish's blood. It has no teeth and no jaws. Instead it feeds through a tiny built-in straw. In Spanish, a leech is called a sanguijuela.

The crab leech (Myzobdella lugubris) is a small, jawless leech, only up to about 26 millimeters (roughly an inch) long. It belongs to the fish-leech family, and despite its name it is a parasite of fish, not crabs: it attaches to a fish's skin, fins, or mouth and feeds on its blood through a tube-like proboscis instead of jaws. Leeches like this one have been recorded on more than 40 kinds of fish. Good news for swimmers — it has never been known to bite a person.

So why is it called the crab leech? Because it uses crabs in a clever way. When it is time to lay eggs, the leech glues little egg cases, called cocoons, onto the hard shell of a crab — especially the blue crab. The crab swims around carrying the eggs like a tiny moving stroller, so when the babies hatch they are already in a brand-new place. Scientists once counted more than 100 leech cocoons stuck to a single crab! The crab is not the leech's food — it is more like the leech's babysitter and taxi.

The name "crab leech" comes from a clever trick, not a meal. When the leech is ready to reproduce, it cements its egg-filled cocoons onto the shell of a crab — usually the blue crab (Callinectes sapidus). The crab then carries the cocoons as it travels, spreading the leech's young to new water before they even hatch; a single crab may carry over a hundred cocoons. Scientists proved the leech feeds on fish and not crabs by checking the blood inside leeches found riding on crabs — it was fish blood. So the crab is not a victim here; it is more like an unpaid babysitter and delivery service. (A different, look-alike leech really does bite crabs — but not this one.)

Here is an honest fact: the crab leech does not live up in El Yunque's cool mountain streams. It likes warm, low-down water near the coast, where rivers meet the sea and the water turns a little salty. It is not originally from Puerto Rico, either — people probably brought it here by accident along with farm-raised fish. But it still belongs in the rainforest's story, because it depends on the same kinds of animals — fish and crabs — that fill Puerto Rico's rivers, including the streams that tumble down from El Yunque. Every creature, even a blood-sipping leech, is one link in a giant food web.

To be honest about where it fits: the crab leech is not a mountain-rainforest animal. It lives in lowland, brackish water — estuaries, lagoons, and the slow lower reaches of rivers where fresh water mixes with the sea — not in El Yunque's cold headwater streams. It is also introduced, not native: scientists think it reached Puerto Rico with farm-raised fish and then spread around the island. So why include it? Because it connects to the forest's food web from below. The fish and crabs it depends on are the same kinds of animals that live throughout Puerto Rico's rivers, including the lower streams fed by El Yunque's rain. Following a parasite like this is a reminder that a rainforest doesn't stop at the mountain's edge — its rivers carry life, and hitchhikers, all the way down to the sea.