The suborder Gobioidei contains about 2,121 species in about 268 genera, representing about 9% of living teleostean species. Several different groups of acanthomorphs (spiny-rayed teleosts) have been suggested as the group to which gobioids are most closely related, but the proposed relationships are usually based on relatively little evidence. A morphological study in 1993 suggested that gobioid fishes might be related to one of the following: the gobiesocids (cling-fishes) with the callionymoids (dragonets); the trachionoids (weeverfishes and their relatives); or the hoplichthyids (ghost flatheads).
The taxonomy and phylogenetic (evolutionary) relationships within the Gobioidei are also poorly known. It is unsurprising, therefore, that the classification of gobioid fishes has been described as "chaotic." Since 1973 they have been classified in as few as two families, Rhyacichthyidae and Go-biidae; or as many as nine families: Rhyacichthyidae, Odon-tobutidae, Eleotridae (called Eleotrididae by some authors), Xenisthmidae, Microdesmidae, Ptereleotridae, Kraemeriidae, Schindleriidae, and Gobiidae (including the subfamilies Go-biinae, Amblyopinae, Oxudercinae, Sicydiinae, and Gob-ionellinae).
Gobioid fishes are characterized by a lack of several bones in the head and axial skeleton, the presence of free sensory neuromasts borne on raised papillae on the head, and sperm-duct glands associated with the testes. The Rhyacichthyidae, or loach gobies, comprising just two species, is accepted as the most primitive gobioid group. The remaining gobioid groups share several features not seen in the Rhyacichthyi-dae; for example, the modification of the bones in the head, the reduction of the lateral line canal on parts of the head, and the complete loss of the lateral line canal on the body.
Several features of the skeleton and scales unite the seven families of higher gobioids (Eleotridae, Xenisthmidae, Microdesmidae, Ptereleotridae, Kraemeriidae, Schindleriidae, and Gobiidae) into one large group. Some authors apply the name Gobiidae to this whole group, which does not include the Odontobutidae. However, molecular analyses of the cytochrome b genes in gobioid fishes (made in 2000), and morphological analyses (made in 2002) indicate that the Odontobutidae, as distinct from the other higher gobioids, is an artificial assemblage (i.e., the characters that apparently unite species in the Odontobutidae are not genuinely comparable across all the species, or the characters are not restricted only to those species but may also be found in other, unrelated species).
Since 1998 two new gobioid genera, Protogobius and Ter-ateleotris, have been described with lateral line canals extending onto the body (a primitive feature that is otherwise restricted to the Rhyacichthyidae). The affinities of these two genera to other gobioid fishes are problematic, and neither has been formally assigned to a family. Morphological evidence suggests similarities between Protogobius and Rhyaci-chthys, but this is based mainly on shared primitive features. Analysis of mitochondrial cytochrome b genes also showed that Protogobius and Rhyacichthys appear to be closely related. Terateleotris lacks several of the derived features that are shared by the seven families of higher gobioids, so this group may be related to either of the more basal groups, Rhyci-chthyidae or Odontobutidae.
A late Cretaceous or early Tertiary origin of gobioid groups has been proposed. Fossil otoliths that might have come from gobioid fishes have been found in the Harudi Formation of Katchchh, India (Lutetian age; 43.6-52 million years old) and the Naggulan Formation of Java (Bartonian
age; 40-43.6 million years ago). The oldest reliably identified skeletal remains are of Pomatoschistus (?) cf. bleicheri (family Gobiidae), from the Headon Hill Formation, Isle of Wight, Great Britain, and date to the Priabonian age (36.6-40 million years old). The oldest fossil material of any eleotrid gobies are otoliths from Faluns de Gaas, Landes, France, and date to the Rupelian age (30-36.6 million years old).
Gobioids play a particularly important role in the freshwater fish communities of oceanic islands, as well as long-isolated fragments of Gondwana such as Madagascar and Australia.
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