The Callionymidae are relatively small, sometimes diminutive, usually elongate fishes, having both a depressed trunk and broad or depressed head. Most of these fishes are sexually dimorphic. Males tend to have larger body sizes and dorsal fins, greater fin ornamentation, and more distinctive color patterns. Dragonets usually have 2 dorsal fins, the first containing 1-4 spines, the second has only 6-11 rays; the last ray is divided at the base, as in the anal fin. Dorsal fins of males may also be large or high, and also have elongated rays. Dragonets also have relatively large pelvic and pectoral fins, with the former positioned forward of the latter. The anal fin has 4-10 rays. The shape of the caudal fin varies, but may be elongate or spade-like and include elongated rays. The pre-operculum has well-developed spines and bars along the margin, but no opercular or subopercular spines are present. Dragonets are further distinguished by the presence of small gill openings, generally pore-like, positioned at the top of the head or along the upper flanks, with gill membranes that are united at the isthmus that separates the gills. Their bodies lack scales, but their lateral lines are quite well developed. Many species have color patterns well suited as camouflage, but others are brilliantly, if not spectacularly, colored. Some species, such as members of the genus Foetorepus, produce an acidic, bitter-tasting slime that covers their bodies and may serve as a toxic deterrent to predators.
Dragonet larvae develop quite rapidly, and most species are able to settle with a full compliment of fins at a small size. The larvae are denoted mainly by having a relatively large head, a short snout that lengthens with growth, a small, pro-trusible mouth placed terminally on the head, the presence of teeth only after settlement, a large eye that migrates dorsally with growth, and a gas bladder located anterior to the gut, which disappears in adults. The gills are free from the isthmus at first but become attached after the larvae undergo flexion. The bodies of dragonet larvae are heavily pigmented, with most of the pigment concentrated on the ventral, rather than dorsal, surface of the body.
The Draconettidae have small, elongated, and rounded bodies, pointed snouts, relatively large pectoral fins, and elongated pelvic fins. The gill openings are relatively broad. There are two nostrils on each side of the head. Scales are absent, but the body is distinguished by the presence of a grooved or vestigial lateral line that is well developed on the head. These fishes are also distinguished by having large eyes. There are two dorsal fins, with three spines on the first and 12-15 rays on the second. There are 12-13 soft rays on the anal fin. The operculum and suboperculum have single strong spines but these are absent on the preopercle. Their larvae are very poorly known. The body is elongate and compressed, the head is also compressed and is of moderate size, the snout is pointed, and the mouth is small and reaches as far as the anterior margins of the large, round eyes. The gas bladder is absent, and the fin elements are present in larvae at least 0.31 in (0.8 cm) long.
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