Gobiesocidae


Clingfishes are fishes of the family Gobiesocidae, the only family in the order Gobiesociformes. These fairly small to very small fishes are widespread in tropical and temperate regions, mostly near the coast, but a few species in deeper seas or fresh water. Most species shelter in shallow reefs or seagrass beds, clinging to rocks, algae and seagrass leaves with their sucking disc, a structure on their chest.
They are generally too small to be of interest to fisheries, although the relatively large Sicyases sanguineus regularly is caught as a food fish, and some of the other species occasionally appear in the marine aquarium trade.

Distribution and habitat

Clingfishes are primarily found near the shore in the Atlantic, Indian and Pacific Oceans, including marginal seas such as the Mediterranean, Gulf of Mexico, Caribbean and Gulf of California. The greatest species richness is in tropical and warm temperate regions, but the range of a few extends into colder waters, like Diplecogaster bimaculata, Apletodon dentatus, Lepadogaster candolii and L. purpurea, Gobiesox maeandricus, Gobiesox marmoratus and Sicyases sanguineus, and Gastrocymba quadriradiata.
Clingfishes mainly inhabit shallow rocky reefs and shores, coral reefs, seagrass meadows and algae beds. They often live in places exposed to strong currents and wave action, and some are amphibious. As long as the strongly amphibious, intertidal-living species are kept moist by splashing waves, they can survive for up to three–four days on land, gaining oxygen from the air by the branchial surfaces, skin and perhaps the mouth. At least a few species even tolerate a relatively high degree of water loss when on land.
A relatively small number of species shelter in sea urchins or crinoids. Whether this relationship is obligate or facultative varies with species. In some, only young clingfish are obligate and gradually move away as they become adult. Three clingfish species, the Australian Cochleoceps bicolor and C. orientalis, and the warm East Atlantic Diplecogaster tonstricula, are cleaner fish that will cling onto the bodies of larger fish.
Although several species can occur in brackish water, only seven from warmer parts of the Americas are freshwater fish that live in fast-flowing rivers and streams.
Most known clingfish species are from relatively shallow coastal waters, but several inhabit the mesophotic zone and a few even deeper, with Alabes bathys, Gobiesox lanceolatus, Gymnoscyphus ascitus, Kopua kuiteri, K. nuimata and Protogobiesox asymmetricus reported from depths of. Because of their small size and typical habitat, it is however suspected that still-undiscovered deep-water species remain. Even in shallow coastal waters many clingfish are highly cryptic and easily overlooked, mostly staying under cover, although there are species that are active and will swim in the open. As a consequence their abundance is often not well known. Several species are only known from a single or a few specimens. Species that appear uncommon or rare based on standard methods can actually be common if using methods that are more suitable for detecting them. Studies of better-known species have shown that they can be locally abundant. As many as 23 individuals of Lepadogaster lepadogaster have been documented from a single square metre., the IUCN has evaluated the conservation status of 84 clingfish species. The majority of these are considered least concern, 17 are considered data deficient, 8 considered vulnerable and a single endangered. The vulnerable and endangered species all have small distributions, restricted to islands or a single bay. Three Gobiesox species that are restricted to fresh water in Mexico have not been rated by the IUCN, but are considered threatened by Mexican authorities.

Description

Clingfishes are typically small fish, with most species less than in length, and the smallest no more than. Only a few species can surpass in length and the largest, Chorisochismus dentex and Sicyases sanguineus, both reach up to. Males typically grow larger than females.
Most clingfish species have tapering bodies and flattened heads, appearing somewhat tadpole-like in their overall shape. They lack a swim bladder. The lateral line of clingfish is well developed, but may not extend to the posterior parts of the body. The skin of clingfishes is smooth and scaleless, with a thick layer of protective mucus. In at least Diademichthys lineatus and Lepadichthys frenatus, the mucus production increases if the fish is disturbed. The taste of their mucus is highly bitter to humans and it can kill other fish. This is due to their skin and mucus containing a grammistin-like toxin. Whether any other clingfish has toxins in its skin or mucus is currently unknown. Another defense appears to be present in a couple of Acyrtus and Arcos species. They have a spine at their gill cover and it appears to be connected to a venom gland. Although the evidence presently is circumstantial, this strongly suggests that the world's smallest venomous fish is Acyrtus artius, which is less than long.

Sucking disc

Clingfish are named for their ability to firmly attach themselves to various surfaces, even in strong water currents or when battered by waves. This ability is aided by their sucking disc, which is located on the underside at the chest and is formed primarily by modified pelvic fins and adjacent tissue. In some species it is divided in two, resulting in a larger front and a smaller rear sucking disc. The sucking disc is covered in tiny hexagons and each of these consists of many microscopic hair-like structures. This is similar to the structures that allow geckos to cling to walls. The sucking disc can be remarkably strong, in some species able to lift as much as 300 times the weight of the clingfish. Gobies can have a similar sucking disc, but unlike that family the single dorsal fin in clingfish is not spiny. In a few clingfish species the disc is reduced or even absent, notably Alabes, which are quite eel-like in their shape and aptly named shore-eels. The sucking disc is also reduced in some deep-water clingfish species.

Colours

Most clingfish species have a cryptic colouration, often brown, grey, whitish, black, reddish or green shades, and in some cases they can rapidly change colour to match their background.
Species of deep water are often orange-red. Diademichthys lineatus, Discotrema species, Lepadichthys caritus and L. lineatus are strongly banded, which may function as a disruptive pattern when among sea urchin spines or crinoid arms, but may also be warning colours, as some members of these genera have poisonous skin and mucus. There are species with colours or patterns that are unsuitable for camouflage. Although Lepadogaster purpurea overall is cryptic, it has a pair of distinct large eyespots on the top of its head. Cochleoceps bicolor, C. orientalis and Diplecogaster tonstricula are yellow to red with fine bluish lines. These three are cleaner fish.

Feeding

Feeding varies depending on exact clingfish species. Most primarily feed on tiny crustaceans or gastropods. Other small animals that have been recorded in their diet include chitons, bivalves, medium-small crustacean like crabs and barnacles, sea urchins, worms, insect larvae, fish and fish eggs. In some species, cannibalism where a large clingfish eats a smaller clingfish is not uncommon.
Limpets and other shelled invertebrates are well-protected and often strongly attached to the rock surface. Clingfish species that feed extensively on them have developed specialized teeth and techniques to dislodge them. This includes rapidly inserting their relatively large, fang-like front teeth under the edge of the prey to flip it, or jamming the teeth on or under the shell's edge to make a small break. However, the teeth of clingfish vary extensively depending on species. In the opposite extreme of the species with relatively few large teeth is Nettorhamphos radula. This species has 1,800–2,300 microscopic teeth, but its feeding behavior is unknown.
Three clingfish species, Cochleoceps bicolor, C. orientalis and Diplecogaster tonstricula, have become cleaner fish. Large fish approach them and allow the small clingfish onto their body where the clingfish eats tiny parasites. In contrast to this mutualistic relationship, certain clingfish species that live among the spines of sea urchins appear to be part of a more varied relationship. It can be either commensal or parasitic.
No clingfish species is known to be exclusively herbivorous, but some are omnivorous and will feed extensively on a range of algae, while other, more strictly carnivorous species may ingest plant material incidentally.

Classification and taxonomy

The classification of the clingfishes varies. FishBase places Gobiesocidae as the only family in the order Gobiesociformes, under the superorder Paracanthopterygii; whereas ITIS place them in the suborder Gobiesocoidei of the order Perciformes, under superorder Acanthopterygii. ITIS lists Gobiesociformes as invalid. The 5th edition of Fishes of the World places the Gobiesociiformes in the clade Percomorpha as part of the series Ovalentaria.
Mostly being very small and often cryptic, new species are regularly discovered and described. A major authoritative work on the family is a monograph that was published in 1955 by J.C. Briggs, but in the half century after its publication, up until 2006, fifty-six new clingfish species were described, or on average more than one per year. This pattern with regular descriptions of new species—and even new genera—has continued since then., there are more than 170 recognized clingfish species.

Subfamilies and genera

Subfamilies and genera. The delimination of the subfamilies, and to some extent the genera, is not fully resolved. The 5th edition of Fishes of the World recognises only two subfamilies, Cheilobranchinae and Gobiesocinae. Fishbase does list a third subfamily, the monotypic Protogobiesocinae which contains a single species Protogobiesox asymmetricus, this species having been described in 2016.
Subfamily Cheilobranchinae
Subfamily Gobiesocinae