Crustaceans form a very large group which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill, …. The 67,000 described species range in size from 0.1 mm, to the Japanese spider crab with a leg span of up to 3.8 m; and a mass of 20 kg. Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. For UW macro photographers they are interesting because they are literally to be found everywhere and are often very colorful. They are also a challenge because some of them are very shy, hiding in crevasses or coral heads, some of them are almost translucent, some of them are masters of camouflage and almost completely disappear into the background of the habitat they’re living in or on. They are on the menu of lots of sea dwelling creatures (and men) but some of them have made themselves so useful that no fish would ever even consider eating them or they use some fish as allies to protect them from predators.
Cleaner shrimp are one example. It’s a common name for any swimming crustacean that cleans other organisms of parasites. This is a widely cited example of cleaning symbiosis: a relationship in which both parties benefit. The fish benefit by having parasites removed from them, and the shrimp gain the nutritional value of the parasites. In many coral reefs, cleaner shrimp congregate at cleaning stations.
The other example is partner shrimps. They are known for burrowing into sand, mud, and gravel with their front claws. However, they prefer to burrow in lagoons and reef edges - areas with little to no cover from predators. This precarious housing location, combined with their relatively poor eyesight, requires partner shrimp to solicit the help of certain gobies. As the goby uses the shrimp's burrow for protection from predators, it also acts as "eyes" for the shrimp. During the day, the goby hovers above the burrow, feeding and interacting with other gobies. Meanwhile, the shrimp uses its antennae to stay in constant contact with the goby's tail while searching for food (detritus, tiny crustaceans and worms) and maintaining the burrow opening. If a predatory fish approaches, the goby flicks its tail several times, alerting the shrimp to retreat into the burrow. If the predator comes within striking distance, the goby will dart headfirst into the burrow. During the night, the two simply rest together in the burrow.