Linophrynids are spectacular even among the most bizarre of their anglerfish relatives in having the largest mouths, with the largest and longest dagger-like teeth of any fish and perhaps of any vertebrate. In addition to an elaborately adorned esca that harbors symbiotic bioluminescent bacteria, most members of the family are uniquely equipped with a complex bioluminescent chin barbel. These features, coupled with low dorsal and anal fin-ray counts and a sinistral anus easily differentiate female linophrynids from those of all other lophiiforms.
The body of metamorphosed females is short, more-or-less oval in shape to globular. The length of the head, measured from the tip of the snout to the base of the pectoral fin, is 50–60% SL. The snout is relatively short, usually less than 20% SL. The mouth is large, the opening horizontal or somewhat oblique, the cleft extending past the eye. The oral valve is well developed. The nostrils are set on rounded papillae. The dentition is highly variable among genera: the teeth of Acentrophryne, Borophryne, and Linophryne are long and few, arranged in several oblique longitudinal series; those of Haplophryne and Photocorynus are considerably shorter and more numerous, arranged in several series (for details, see the generic accounts below). The vomer is well toothed in Acentrophryne, Borophryne, and Linophryne, but toothless in Photocorynus and Haplophryne. The first epibranchial is free, not bound to the wall of the pharynx by connective tissue. All four epibranchials are closely bound together by connective tissue. The fourth epibranchial and ceratobranchial are bound to the wall of the pharynx, leaving no opening behind the fourth arch. The proximal one-half of the first ceratobranchial is bound to the wall of the pharynx, but its distal one-half is free, not bound by connective tissue to the adjacent second ceratobranchial. The proximal one-quarter of the second ceratobranchial is bound to the third ceratobranchial and the proximal one-half of third ceratobranchial is bound to the fourth ceratobranchial. Gill filaments are present on the full length of second ceratobranchial, the distal three-quarters of the third ceratobranchial, and the distal one-half of the fourth ceratobranchial. A pseudobranch is absent. The illicium is relatively short, the stem shorter than the diameter of the escal bulb in Haplophryne, Photocorynus, Borophryne, and several species of Linophryne, but reaching 35–40% SL in other species of Linophryne and about 70% SL in Acentrophryne. The pterygiophore of the illicium is short, its anterior end hidden beneath the skin of the head or slightly protruding on the snout. A bioluminescent hyoid barbel is present in Linophryne, but absent in all other genera. The neuromasts of the acoustico-lateralis system are located at the tips of low cutaneous papillae, the pattern of placement as described for other ceratioids.
Free-living and parasitic males are known for all genera, except Acentrophryne. The eyes of metamorphosed free-living males are relatively large (their diameter 6–9% SL), more-or-less tubular, and directed anteriorly, without an aphakic space. The olfactory organs are inflated and moderately to strongly enlarged. The anterior nostrils are directed anteriorly. There are 3–13 olfactory lamellae, but the number is variable among genera. Jaw teeth are present in Photocorynus and Haplophryne, but absent in Borophryne and Linophryne. The upper denticular bone is simple or bears a short dorsal prolongation, not reaching the tip of the illicial pterygiophore. The lower denticular bone is simple or (in Haplophryne) divided into a bilateral pair. Each denticular bears a single series of 3–7 upper and 2–13 lower denticular teeth. Sphenotic spines are prominent in the males of Borophryne and in Linophryne subgenus Linophryne, but absent in all other taxa. A hyoid barbel is absent in the males of all genera. The skin is smooth and naked, without dermal spinules. The denticular teeth, eyes, and olfactory organs of parasitic males appear somewhat degenerated; the belly is greatly inflated in mature specimens.
The larvae are more elongate than those of most other ceratioids, the length of the head generally about 45% SL. The skin of the head and body is moderately inflated. The pectoral fins are relatively short. A dorsal group of subdermal melanophores is absent. A small rudiment of a hyoid barbel is present in the largest known female larvae of Linophryne. Pointed sphenotic spines are present in Borophryne and the Linophryne subgenus Linophryne, in contrast to the larvae of all other ceratioids.
The color of metamorphosed females is usually dark brown to black over the entire surface of the body, except for the escal appendages, the distal parts of the escal bulb, the barbel of Linophryne, and fin rays. The skin is everywhere unpigmented in Haplophryne (in contrast to all other metamorphosed ceratioid females). The free-living males of Linophryne are dark brown to black, but unpigmented in all other linophrynid genera. The parasitic males of Borophryne and Linophryne are dark brown to black, but those of Photocorynus and Haplophryne are unpigmented.
The largest known female of the family is a 275-mm specimen of Linophryne lucifer (ZMUC P922443, with a 29-mm parasitic male); several additional specimens of Linophryne are larger than 180 mm. The largest recorded females of the other four genera of the family range from 50–159 mm: 69 mm in Photocorynus (SIO 53-356), 159 mm in Haplophryne (NMNZ P.21248), 105 mm in Acentrophryne (HUMZ 175257), and 101 mm in Borophryne (LACM 30053-10). The largest known free-living male of Photocorynus spiniceps is 8.6 mm (ZMUC P921727), 16–21 mm maximum known lengths in the other genera. The largest known parasitic males are 7.3 mm in Photocorynus (ZMUC P92134), 15 mm in Haplophryne (AMS I.21365-8), 22 mm in Borophryne (LACM 30053-10), and 30 mm in Linophryne (IMB).
The family Linophrynidae differs from all other ceratioid families in having 3 (rarely 4) dorsal-fin rays, 3 (rarely 2 or 4) anal-fin rays, 5 (rarely 4) branchiostegal rays, and a sinistral anus.
Metamorphosed females are further differentiated by having the following combination of character states: The supraethmoid is present. The frontals lack ventromedial extensions, and are separated from each other in most genera, but meet on the midline in Photocorynus. The parietals are present. The sphenotics are overlapped by the anterolateral margin of the pterotic. Sphenotic spines are well developed. The pterosphenoid and mesopterygoid are absent. The metapterygoid is present. The hyomandibular has a single head. There are 2 hypohyals and 5 (1 + 4), rarely 4 (0 + 4), branchiostegal rays. The opercle is bifurcated, its posterior margin moderately concave. The subopercle is long and extremely slender, without a spine or projection on the anterior margin. Quadrate and articular spines are absent. The angular is produced to form a spine in some taxa. The preopercle usually bears one or more spines (preopercle spines are absent in Acentrophryne). The jaws are more-or-less equal anteriorly, the lower extending slightly beyond the upper in same taxa. The lower jaw usually has a symphysial spine (absent in the Linophryne subgenus Stephanophryne). The postmaxillary process of the premaxilla is absent. The anterior-maxillomandibular ligament is weak to absent. The first pharyngobranchial is usually absent (present but rudimentary in Photocorynus). The second and third pharyngobranchials are well developed and toothed. The fourth pharyngobranchial is absent. The first epibranchial I is well developed in Photocorynus and Haplophryne, but somewhat reduced in Borophryne and Linophryne. The fifth ceratobranchial is usually absent (a small rudiment is present in Photocorynus). Ossified hypobranchials are usually absent (a single ossified hypobranchial is present in Photocorynus). An ossified basibranchial is present or absent. Epibranchial and ceratobranchial teeth are absent. Epurals are usually absent (a single small element is apparently present in some specimens of Photocorynus). The posterior margin of the hypural plate is usually deeply notched (entire in Photocorynus). The pterygiophore of the illicium lacks a remnant of the second cephalic spine. The escal bulb has a central lumen and a pore. The esca lacks large tooth-like denticles. The posteroventral process of the coracoid is absent. There are 3 pectoral radials. Pelvic bones are absent. There are 13–19 pectoral-fin rays and 9 (2 simple + 4 bifurcated + 3 simple ) caudal-fin rays, the ninth very short in Photocorynus, about one-half the length of the eighth caudal-fin ray in all other genera. The skin is everywhere naked, dermal spinules are absent. The ovaries are paired. Pyloric caecae are absent.
Metamorphosed males of the family Linophrynidae are further differentiated by having the following combination of character states: The eyes are prominent, tubular, and directed anteriorly. The olfactory organs are relatively large, the anterior nostrils well separated, but directed more-or-less anteriorly. The denticular bones are short and stout, the upper denticular bearing 3–7 teeth, the lower denticular with 2–13 teeth. There is no connection between the pterygiophore of the illicium and the upper denticular bone. Fin-ray counts are as given for metamorphosed females. The skin is naked, without dermal spinules. The eyes and olfactory organs of parasitic stages are somewhat degenerated. Sexual parasitism is apparently obligatory.
The larvae of the Linophrynidae are further differentiated by having the following combination of characters states: The body is slender, more-or-less elongate, the skin highly inflated. Subdermal pigmentation of the body is either absent or distributed along the lateral margins, never on the dorsal part of the body. The pectoral fins are small, not reaching beyond the dorsal and anal fins. Pelvic fins are absent. Fin-ray counts are as given for metamorphosed females. Sexual dimorphism is well developed, the females bearing a distinct illicial rudiment (females of Linophryne also have a rudiment of the hyoid barbel, present as an opaque, wart-like thickening of the skin in specimens greater than about 10 mm). Metamorphosis is delayed in at least some taxa, the larvae attaining lengths of 17.5–22 mm. Specimens in metamorphosis measure 15–32 mm.
