Gobiodon Bleeker, 1856

Genus Gobiodon Bleeker, 1856 View in CoL

Gobiodon ( Bleeker, 1856) View in CoL (separation from Gobius Linnaeus, 1758 View in CoL , as type species Gobiodon heterospilus )

Ellerya ( Castelnau, 1873b) (as a new genus, type species Ellerya unicolor View in CoL , by monotypy)

Pseudogobiodon ( Bleeker, 1874) (as a new genus, type species being Gobius citrinus View in CoL )

Diagnosis. Laterally compressed deep body with thickened epidermal mucus layer. Head scaleless. Body generally naked with some squamation on caudal peduncle in some species. Dorsal-fin rays VI, I,9–13; pectoral-fin rays 16–21; anal-fin rays I,8–11; pelvic-fin rays I,5. Upper procurrent caudal-fin rays 4–6; lower procurrent caudal-fin rays 4–6; principal caudal-fin rays 7–8 + 9–10 (all segmented rays branched). Vertebra 25–27 including hypural plate. First dorsal fin varied in length. Second dorsal fin similar in length to first. Pelvic fins connected ventrally by membrane. Dentary bone triangular or elongate. Uniform or varying sized teeth. Post-symphysial canine teeth may be present.

Description. Dorsal-fin rays VI, I,9–11; pectoral-fin rays 16–21, typically 19–20. Anal-fin rays I,8–10. Pelvic-fin rays I,5. Upper unsegmented caudal fin rays 4–6. Lower unsegmented caudal fin rays 4–6. Upper segmented branched caudal fin rays 8–9. Lower segmented branched caudal fin rays 7–9. Vertebrae 25–27 including hypural plate.

Body laterally compressed, deeper than wide or somewhat fusiform. Ground colour during life variable, ranging from green/blue, yellow, brown, red, orange, black or grey. Ground colour once preserved often brown, beige, or yellow, with variation depending on preservation method. Live colouration markings when present usually as spots, lines, or ripples, occasionally restricted to the head as facial pigment bars. Small black spot on upper opercular margin in some species. Body of most species naked. When present, squamation is minimal and restricted to the caudal peduncle, e.g., Gobiodon aoyagii . Body covered in thick epidermal mucus layer.

Anterior head profile steep and blunt. Groove between isthmus and interopercle typically either deep or absent. Interopercle shallow and lance-like, deep sub-elliptical or anteriorly elongated and expanded. Metapterygoid deep or shallow in form. Lacrimal elongate and narrow, broadly spatulate or ventrally expanded, triangular or quadrilateral in shape. Interhyal basally expanded and triangular or dorsal/ ventral extremities equal width.

Dentary bone triangular or elongate and recurved. Jaw teeth uniform in appearance or varying size with rows. Post symphysial canine teeth present in some species. Variation in protractor hyoidei muscle attachment process on ventral lower jaw.

Cephalic sensory system composed of anterior naris, posterior naris, six anterior oculoscapular canal pores, and three preopercular canal pores. Facial papillae equal width/ length or longer than wide. Facial papillae reduced compared to related genera, e.g., Paragobiodon . Pit organs on head adjacent to lips, nostril, orbits and opercular margin.

Dorsal fin with varied configuration, some species having signature configurations, including varied length of rays to form a ‘sail’ or ‘squared’ shape. First and second dorsal fins attached by membrane. May appear with clear separation, with sixth spine of first dorsal shorter than first spine of second. In a small number of species first dorsal sixth spine length may be equal to second dorsal first spine, appearing more fused. Pelvic fin connected by a membrane, producing a fin capable of generating suction. All other fins share common shape throughout genus, with some variation of caudal and pectoral fins.

Species known to undergo bi-directional sex change ( Nakashima et al., 1996; Munday et al., 1998; Munday et al., 2010). Difference between male and female Gobiodon present in the genitalia shape, with no known sexual dimorphism ( Nakashima et al., 1996; Munday et al., 1998; Shibukawa et al., 2013).

Etymology. The commonly accepted source for Gobiodon comes from the Latin ‘gobius’ meaning gudgeon and the Greek ‘odous’ meaning teeth.

Habitat and biology. Gobiodon are small (20–70 mm SL) cryptobenthic coral-associated reef fish from family Gobiidae . Typically, they are observed deep within a coral colony, living between the branches. Basic body plan seen in Fig. 2. Their diet is debated, but is typically thought to include plankton, small invertebrates, algae, and occasionally tissue from the coral host ( Brooker et al., 2010). Members can be observed living solitarily, in pairs, or in groups, but most frequently as pairs ( Hing et al., 2019). They are known to be highly specialised for life amongst the branches of their mutualist partner cnidarians, typically from the genus Acropora , but have also been seen living in Echinopora , Hydnophora , Stylophora , and Pocillopora , and are typically found at depths of 1– 10 m. Their distribution is widespread, occurring where their host coral species can be found, throughout the Indo-Pacific from the Pacific Islands, GBR and Japan, through the Indian Ocean into the Red Sea.