THE OCEAN DEEP

The gobies form the family Gobiidae, which is one of the largest families of fish, with more than 2,000 species in more than 200 genera. Most are relatively small, typically less than 10 cm (4 in) in length. Gobies include some of the smallest vertebrates in the world, like species of the genera Trimmaton and Pandaka, which are under 1 cm (3/8 in) long when fully grown. There are some large gobies, such as some species of the genera Gobioides or Periophthalmodon, that can reach over 30 cm (1 ft) in length, but that is exceptional. Although few are important as food for humans, they are of great significance as prey species for commercially important fish like cod, haddock, sea bass, and flatfish. Several gobies are also of interest as aquarium fish, such as the bumblebee gobies of the genus Brachygobius.
The most distinctive aspect of goby morphology are the fused pelvic fins that form a disc-shaped sucker. This sucker is functionally analogous to the dorsal fin sucker possessed by the remoras or the pelvic fin sucker of the lumpsuckers, but is anatomically distinct: these similarities are the product of convergent evolution. Gobies can often be seen using the sucker to adhere to rocks and corals, and in aquariums they will happily stick to glass walls of the tank as well.
Triplefin goby on a wire coral from East Timor
Gobies are primarily fish of shallow marine habitats including tide pools, coral reefs, and seagrass meadows; they are also very numerous in brackish water and estuarine habitats including the lower reaches of rivers, mangrove swamps, and salt marshes. A small number of gobies (unknown exactly, but in the low hundreds) are also fully adapted to freshwater environments. These include the Asian river gobies (Rhinogobius spp.), the Australian desert goby (Chlamydogobius eremius), and the European freshwater goby Padogobius bonelli. Most gobies feed on small invertebrates, although some of the larger species eat other fish, and a few eat planktonic algae.
Reproduction
Gobies attach their eggs to a substrate, such as vegetation, coral, or a rock surface. They can lay anything from five to a few hundred eggs, depending on species. After fertilising the eggs, the male remains to guard them predators and keep them free from detritus. The eggs hatch after a few days. The larvae are born transparent, developing their colouration after dispersing to find a suitable habitat. The larvae of many freshwater species are carried downstream to the brackish waters of estuaries, or even to the sea, and only return to fresh water weeks or months later.
Gobies in warmer waters reach adulthood in a matter of months, while those in cooler environments may take up to two years. The total lifespan of gobies varies from a single year to up to ten years, again with the temperate species generally living longer. A few species of goby are known to be able to change sex from female to male, although most do not do this. In such species, most individuals are born female, and the male must expend considerable effort in guarding the eggs of the multiple females with which he breeds.
Symbiosis
Gobies sometimes form symbiotic relationships with other species. Some goby species live in symbiosis with burrowing shrimps. The shrimp maintains a burrow in the sand in which both the shrimp and the goby fish live. The shrimp has poor eyesight compared to the goby, but if it sees or feels the goby suddenly swim into the burrow, it will follow. The goby and shrimp keep in contact with each other, the shrimp using its antennae, and the goby flicking the shrimp with its tail when alarmed. These gobies are thus sometimes known as watchmen or prawn gobies. Each party gains from this relationship: the shrimp gets a warning of approaching danger, and the goby gets a safe home and a place to lay its eggs in. Only the alpha male and female reproduce, other fish in colony eat sparingly to resist being eaten by alpha male or female. This way only the largest and fittest are able to reproduce.
Another example of symbiosis is demonstrated by the neon gobies (Elacatinus spp.). These gobies are known as "cleaner gobies", and remove parasites from the skin, fins, mouth, and gills of a wide variety of large fish. The most remarkable aspect of this symbiosis is that many of the fish that visit the gobies' cleaning station would otherwise treat such small fish as food (for example, groupers and snappers). Again, this is a relationship where both parties gain: the gobies get a continual supply of food as bigger fish visit their cleaning stations, and the bigger fish leave the cleaning stations healthier than they were when they arrived.