Everywhere in Nature the small, weak and apparently helpless manage to survive by parasitism — sponging off hosts who may in their turn protect and help these hangers-on. Worms, ticks, fleas and various kinds of bacteria are common examples. But there are more spectacular cases among fish and other sea creatures.
On riverbeds, a species of fresh water clam tosses her young at passing fish to attach themselves with hooks. The host carries them about, nourishing them until they are adult enough to let go, settle as the bottom, and start another lifecycle. This form of parasitism disperses the clams more widely.
Many water creatures who benefit from spongers and hangers-on will go to great lengths to protect them. Deep down in the depths of oceans where light never penetrates, fish identify each other by emitting light. The Photoblehuron and the Anamalopa have space below their eyes for rearing bacteria which are luminous. Opaque screens protect the surrounding tissue from these organisms. If these fish are threatened by larger ones and want to “disappear” they can switch off their light! The microbes in their turn get good from the fish from supplies of oxygenated blood.
Indeed, the manner in which some sea creatures take help from each other makes them seem like mutual aid societies. Shrimps maintain “cleaning stations” on some coral reefs. They wave their feelers at huger creatures swimming past who would otherwise gobble them. These bigger fish suffer from itchy skin, festering wounds, dead tissue or parasites. So they are happy to let the shrimps nibble it all away. In the process the cleaners earn themselves a meal.
Cleaner-fish render the same service. They work steadily and swiftly, going over their customers quite thoroughly. One cleaner-fish was observed servicing 300 customers in six hours! When experimenters removed all the cleaner fish, their clients moved off to other reefs where “service stations” were available.
These larger hosts are often very protective of the cleaners. If a small goby is busy cleaning the teeth of a giant grouper and it sees danger approaching, the grouper partially closes its mouth to hide the goby. Or if the goby is cleaning its grills it will compress them until the predator fish has passed. All large fish who need servicing present themselves to the cleaners, remaining still in the water and opening their mouths wide just as we do in a dentist’s chair.
The remora or sucker-fish is a life-long sponger. It has a modified dorsal fin to attach itself to giant sharks and turtles. When its host makes a kill it detaches itself only long enough to snap up morsels of food floating about. It gets free board, lodging, and free passage to travel long distances in the sea without exerting itself. In return for these benefits it cleans its host every now and then, giving the larger fish a thorough treatment.
Sea anemones are creatures of prey, their colourful tentacles spread wide to capture passing creatures. But clown fish flick about among their deadly petals without harm. Anemones leave these guests alone, however, since they nibble off dead or diseased tentacles to keep these lethal blooms in good health. But if an anemone should turn nasty and sting a clown fish it wouldn’t have any effect. The clown fish has a special mucous coating to protect it from being poisoned.