Gobies are tiny bottom fish that diversified through an extinction
Gobioidei packs about two thousand two hundred eleven species across seven families—one of the richest ray-finned assemblages. Mostly small, big-headed bottom dwellers without a gas bladder, they hug the seafloor. Oddly, the huge families Gobiidae and Oxudercidae radiated amid the Eocene–Oligocene extinction.
Once parked under Perciformes, the group is now a suborder inside an expanded Gobiiformes that also holds cardinalfish and nurseryfish relatives. Roughly one tenth of species live in fresh water; most are benthic or burrowing and must stay near the bottom because they cannot regulate buoyancy. "Goby" is a loose label: true gobies sit in Gobiidae, mudskippers and allies in Oxudercidae, and some unrelated fish still wear the name.
Isolated otoliths from India's Cambay Formation slightly pre-date the earliest skeletal member, Carlomonnius from Early Eocene Monte Bolca in Italy, now read as a stem butid. Molecular clocks place the split of Gobiidae and Oxudercidae in the Late Eocene or middle Oligocene—young for such species-rich families, and timed to a mass die-off rather than a quiet recovery. Taxonomy still shifts: Eschmeyer's Catalog restores Gobioidei as a suborder and reshuffles sleepers, collared wrigglers, and mudskipper allies.
Primitive loach-gobies (Rhyacichthyidae) hold just three species in Oceania and the western Pacific. Freshwater sleepers of eastern Asia form a sister clade with them. Cave gudgeons span Western Australia and Madagascar. Eleotridae sleepers reach from the eastern United States to Stewart Island, New Zealand. Oxudercidae holds about six hundred species, including mudskippers that endure long spells out of water. Within Gobiidae, some one hundred twenty species live with invertebrates—Amblyeleotris and Cryptocentrus share burrows with alpheid shrimp—while fused pelvic fins let island climbers such as Lentipes concolor scale waterfall rock faces.
Source: Gobioidei
