No clingfishes and singleslits have been identified from the fossil record, and the evolutionary history remains to be examined with molecular methods. The examination of morphological relationships has led to the formulation of competing hypotheses to describe the history and systematic position of clingfishes and singleslits, and these questions are far from resolved. Some workers place these fishes in the order Gobiesociformes along with the dragonets (Callionymi-dae) and draconetts (Draconettidae). Others place the Gobiesocidae in the suborder Gobiesocoidei within the Per-ciformes, next to the suborder Callionymoidei (Callionymi-dae and Draconettidae); such is the treatment in this chapter.
The family Gobiesocidae is partitioned into two subfamilies, the Gobiesocinae (clingfishes) and the Cheilobranchinae (singleslits) within the Gobiesocoidei. Both subfamilies are united by the presence of a joint that connects the supraclei-thrum and cleithrum bones within the pelvic girdle. Two other specializations link these two subfamilies: a joint connecting the interopercle and epihyal and a heart with a structure unique to both subfamilies. The singleslits are sometimes recognized as a separate family within the suborder Go-biesocoidei; the family consists of a single genus, Alabes, with four species. Briggs recognized the following subfamilies (denoted here as tribes), which now are placed under the Go-biesocinae: the primitive Trachelochismini, the monotypic Haplocylicini, the Lepadogastrini, the monotypic Choriso-chismini, the Diplocrepini, the Gobiesocini, and the highly specialized Diademichthyini, which are mostly commensal with sea urchins and crinoids. Briggs concluded that more specialized taxa are found in the tropical Indo-West Pacific region; intermediate taxa are found in the eastern Pacific, western Atlantic, and eastern Atlantic; and relict Southern Hemisphere taxa are confined to the cool temperate waters of southern Africa, Australia, and New Zealand. There are at least 120 species of clingfishes (two with two subspecies each and one with three subspecies) in 44 genera plus undescribed genera and species.
The known genera include Acyrtops (two species), Acyrtus (two species), Alabes (four species), Apletodon (four species, one with two subspecies), Arcos (five species), Aspasma (one species), Aspasmichthys (one species), Aspasmodes (one species), Aspasmogaster (four species), Chorisochismus (one species), Cochleoceps (four species), Conidens (two species), Creocele (one species), Dellichthys (one species), Derilissus (two species), Diademichthys (one species), Diplecogaster (three species, one with three subspecies), Discotrema (one species), Eckloniaichthys (one species), Gastrocyathus (one species), Gas-trocymba (one species), Gastroscyphus (one species), Gobiesox (26 species), Gouania (one species), Haplocylix (one species), Kopua (one species), Lecanogaster (one species), Lepadichthys (nine species), Lepadogaster (three species, one with two subspecies), Liobranchia (one species), Lissonanchus (one species), Modicus (two species), Opeatogenys (two species), Parvicrepis (one species), Pherallodichthys (one species), Pherallodiscus (one species), Pherallodus (two species), Posidonichthys (one species), Propherallodus (one species), Rimicola (five species), Sicyases (two species), Tomicodon (13 species), and Trachelochismus (two species).
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