Gobies belong to one of the most speciose fish families consisting of around 2000 members. Most of the gobies are rather small and have very little direct defence against predators. Thus, most gobies in reef areas live in close association with animals such as wire corals, branching corals and sea pens that the gobies can hide in, around or between.
However, that leaves major and rather productive parts of the reef area such as sand or rubble patches or silty slopes uninhabitable for gobies, as there is no way for the small fish, with the defensive skills of an overboiled macaroni to evade predators in such areas. But, as it so clearly is stated in the classic movie “Jurassic park”, nature finds its way! Some gobies have in an amazing evolutionary process acquired the help of a group of shrimps in order to create shelter in the barren open areas. The shrimps belong to the pistol shrimps of the genus Alpheus, shrimps that often dig burrows.
Some species of these shrimp have far from 20/20 eyesight, which obviously is a problem being diurnal and living in open areas abounding with predators. The gobies, in contrast, have excellent vision, and, furthermore, have their pelvic fins extended as a pedestal. In the relationship between the shrimp and the goby, the Alpheus shrimp digs a burrow, which is used as shelter by both the shrimp and the goby. In turn, the goby spends its day outside the opening of the tunnel, resting on its extended pelvis fins, and keeping carefully watch over the immediate area, alerting the shrimp when danger comes to close, resulting in the shrimps retreat into the burrow.
The goby-shrimp relationship is an example of what is called an obligate mutualism. These gobies are never found without their shrimp partners, and, conversely, the partner shrimp are never found without their goby partners. As far as I know, coral reef areas and their immediate surroundings offer by far the most examples of such interspecies symbiotic relationships essential for both species survival. The real cool thing about the shrimp-goby symbiosis is that the shrimp and the goby go one step further in their coevolution than most other species pairs. The goby is capable of communicating levels of danger to the shrimp. Thus, the shrimp sometimes respond to signals from the goby by working closer to the burrow opening, sometimes by working in the actual burrow opening, and sometimes by totally retreating into the burrow itself. This quite detailed interspecific communication is very rare in nature, at least when invertebrates such as shrimp are parts of the interaction.
The actual method of the communication between the pair is performed by contact of one of the very long antennas of the shrimp to the posterior dorsal fin of the goby. When the shrimp wants to get out of the burrow the shrimp first extends one of the antennae out of the opening, contacting the fin of the goby. If the coast is clear, the goby wiggles its fin in a certain way, telling the shrimp that it can come out. As long as the shrimp is outside the burrow, its antenna will be touching the gobies fin. The goby communicates danger levels corresponding to the proximity of a predator to the shrimp by using different “wiggle” patterns of the fin, differing in frequency and intensity.
How about nighttime, then? During the dark hours, he goby cannot see much. The burrow then turns into more of a trap than a refuge, as many of the small eels hunting on the sand can also penetrate the burrow, thus capturing both the goby and the shrimp. Luckily the shrimp and the goby have a solution for that. Outside the burrow entry there are always small pebbles scattered. When dusk falls, after the goby has retreated down into the burrow, the shrimp uses these pebbles to close the burrow, Thus, with the burrow entry securely closed, the couple can spend the rest of the night in safety.