As an avid hitchhiker earlier in my life, meeting loads of cool people and getting to a lot of places for free, the term hitchhiker describing an ecological interaction has immediate appeal. In contrast, the movie the hitchhiker with Rutger Hauer certainly has left no one seeing it untouched! So, what is a hitchhiker in ecological terms.
In ecology, hitchhikers are animals, or plants, that by some means get a free ride with another animal in order to move from one spot to another. It will often be focused on parasites, and how their rides deliver them to the correct host.
In marine ecosystems, hitchhikers are far from the Rutger Hauer version. Hitchhikers are found on jellyfish, boats, turtles, whales and a whole lot of other creatures that provide free transport without incurring any harm. The most well known hitchhikers for divers are the emperor shrimp, Periclemenes imperator. These vividly colored shrimp are often found in pairs on sea cucumbers, where they spend all their life. More rarely one can be lucky to find one, or very rarely two of those beautiful shrimp on nudibranchs.
What do the emperor shrimp do on their rides? Well, in contrast to many examples of symbiosis, these hitchhikers do not harm their host. The hosts are simply used as transport, and also give the brightly colored shrimp protection from predators. The shrimp find a good position on the side of its ride, and picks up detritus from the sandy bottom, that gently moves by as the ride moves forward. Thus, the shrimp get access to a never ending stream of food being presented to it, while its transport is not harmed and will have no reason to try to get rid of its hitchhiker.