This evening, I walked into my girlfriend’s bathroom and suddenly noticed something on the floor that completely caught me off guard.

Learning that it appeared to be a leech changed the emotional reaction once again. The creature was no longer simply an unidentified dark object. Leeches have a reputation that immediately creates strong reactions in many people. Their association with blood-feeding is widely recognized, in part because certain leech species have been used historically and in modern medical contexts. However, leeches are biologically diverse, and not every species feeds on blood. Some feed on other invertebrates rather than vertebrate blood.

Leeches belong to a group of segmented worms related to earthworms. Their bodies are usually flexible, and many species use suckers to attach themselves to surfaces or hosts. They can contract and extend their bodies as they move, which helps explain why a leech can look surprisingly different depending on its position. Some may appear short and thick at one moment and longer and thinner when stretched. This changing shape can make identification more difficult for someone who has never encountered one before. It can also contribute to the impression that the creature resembles wet rubber.

Many people associate leeches primarily with ponds, streams, marshes and other wet environments. Numerous species are aquatic or closely connected with freshwater habitats, while some species can live in moist terrestrial environments. Moisture is therefore an important part of the ecology of many leeches. That makes a damp-looking creature in a bathroom seem superficially plausible. But again, habitat preference does not tell us precisely how one individual reached a particular bathroom. The mystery of its arrival remains separate from its identification.

The Smithsonian has documented the remarkable diversity of leeches and the different ways they feed. Blood-feeding species can attach to a host and use specialized mouth structures to obtain blood. Their saliva can contain anticoagulant substances that help prevent blood from clotting while they feed. Other leeches are predators rather than blood feeders. This diversity is one reason broad statements about all leeches should be avoided. The animal group is more varied than its popular reputation suggests.