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, such as species of the genera Trimmatom nanus and Pandaka pygmaea, which are under 1 cm (3/8 in) long when fully grown. Some large gobies, such as some species of the genera Gobioides or Periophthalmodon, 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 such as 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 aspects 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 stick to glass walls of the tank, as well.
Gobiidae contains six subfamilies:
Members of the subfamily Amblyopinae are elongated mud-dwelling gobies commonly known as the eel gobies or worm gobies. Their two dorsal fins are connected by a membranous structure and their eyes are highly reduced. They are usually pink, red, or purple in coloration. Amblyopinae contain 12 genera and about 23 species.
Members of Benthophilinae are endemic to the Ponto-Caspian region (including the Marmara, Black, Azov, Caspian, and Aral Seas). The representatives of the subfamily have fused pelvic fins and elongated dorsal and anal fins. They are distinguished from the closely related subfamily Gobiinae by the absence of a swimbladder in adults and location of the uppermost rays of the pectoral fins within the fin membrane. Its members include tadpole gobies, monkey gobies, and bighead gobies.
Members of Gobionellinae mostly inhabit estuarine habitats, though some are freshwater. They are found in tropical and temperate regions around the world with the exception of the northeastern Atlantic Ocean, the Mediterranean Sea, and the Ponto-Caspian region. It includes around 370 species and 55 genera.
Members of Oxudercinae are commonly known as mudskippers. They are highly specialized members of the family. They are able to survive for extended periods on land through a combination of behavioral and physiological adaptations, including pectoral fins that act as simple legs; the ability to breathe through their skins (like frogs); and the digging of damp burrows to avoid drying out. Mudskippers live in tidal areas, particularly on mudflats and in mangrove forests, and are only found in tropical and subtropical regions.
Ecology and biology
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 planktic algae.
Gobies attach their eggs to a substrate, such as vegetation, coral, or a rock surface. They can lay from five to a few hundred eggs, depending on species. After fertilizing the eggs, the male remains to guard them from predators and keep them free from detritus. The eggs hatch after a few days. The larvae are born transparent, developing their coloration 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 lifespans of gobies vary from one to ten years, again with the temperate species generally living longer.
A few species of goby, such as the blackeye goby, are known to be able to change sex from female to male. 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.
Gobies sometimes form symbiotic relationships with other species, such as with burrowing shrimps. The shrimp maintains a burrow in the sand in which both the shrimp and the goby 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. Only the alpha male and female reproduce, other fish in the colony eat sparingly to resist being eaten by the 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, known as "cleaner gobies", remove parasites from the skin, fins, mouth, and gills of a wide variety of large fish. The most remarkable aspect of this symbiosis is many of the fish that visit the gobies' cleaning stations would otherwise treat such small fish as food (for example, groupers and snapper. 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.
Gobies have commercial importance in Ukraine. They are fished in the Sea of Azov, northwestern Black Sea. Most important species are round goby, monkey goby, toad goby, and grass goby. The grass goby is also a commercial fish in Italy.
Several species of gobies are kept in aquaria.  Most captive gobies are species from saltwater, and make excellent additions to healthy reef or fish-only aquariums. Perhaps the most popular is the small but colorful neon goby. Most gobies stay toward the lower portion of the aquarium, hiding in the rockwork, but some species (most notably the shrimp gobies) prefer to dig themselves little burrows. Potential keepers of these striking fish should provide them with a fine-grained substrate to prevent damage to their delicate undersides.
Commonly kept saltwater species include the Randall's shrimp goby, and watchman goby. The bumblebee gobies from the genus Brachygobius are perhaps the most widely traded freshwater species, being small, colorful, and easy to care for. They need tropical, hard and alkaline freshwater or slightly brackish conditions to do well.
Gobies are generally peaceful towards their tankmates, though territorial among themselves. Since most are small and few are predatory toward other fishes, they usually make good community fishes. Typically, the main problem with gobies is feeding them; with a few exceptions, the small species kept in aquariums prefer live or frozen foods rather than flake, and they are not very good at competing with active species such as cichlids. Another problem is they commonly jump out of the tank, so having a tight-fitting lid is a must. They are recommended to be kept on their own or with peaceful surface dwelling species, such as halfbeaks and guppies.
- Sleeper gobies are a closely related family (Eleotridae) that lack the fused pelvic fin sucker typical of most gobies, but are otherwise very similar in size, shape, and ecology.
- Blennies are a group of shallow-water marine fish often confused with gobies.
- Dragonets are superficially similar to gobies and sometimes confused with them.
- Pholidichthys leucotaenia is commonly called the engineer goby or convict goby, but is not a goby.
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