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The body of metamorphosed females is relatively short and oval to globular, the length and depth of the head about 50% SL. The sphenotics, epiotics, posttemporals, preopercles, anterodorsal margins of the frontals, heads of the palatines, symphysis of the lower jaw, all with well-developed spines. The posteroventral margin of the articulars is notched to form a pair of short spines. The positions, numbers, and relative length of the spines of the head is somewhat variable: the left posttemporal bears only a single spine in one known specimen; in all specimens examined, the 5 or 6 preopercular spines include a dorsal and ventral pair, while the position of one or two median spines varies in relation to that of the pairs.
The nostrils are set on low papilla. The lateral-line organs are stalked and unpigmented. The escal bulb is unpigmented and globular or slightly oval in shape, nearly sessile on the blunt tip of the pterygiophore of the illicium. The diameter of the escal bulb is 3.5–4.5% SL. Escal filaments and appendages are absent. The inner wall of the spherical photophore is black except for a relatively narrow distal transparent field.
The teeth are slender, recurved, depressible, and relatively short. The longest teeth are 2–3% SL and placed in the anterior part of the lower jaw. There are about 30 teeth on each premaxilla in a 24 mm specimen and about 80 in a 45 mm specimen; there are about 45 teeth on each dentary in a 24 mm specimen and about 110 in a 45 mm specimen. Teeth in the lower jaw are placed in a very regular pattern of oblique diagonal series: 5–10 overlapping longitudinal series each with teeth increasing in length posteriorly; more numerous transverse series each with teeth increasing in length medially. The pattern of placement of the premaxillary teeth is similar but less regular. There are 3 or 4 dorsal- and anal-fin rays, and 15–17 pectoral-fin rays .
The color of metamorphosed females in preservation is light brown, the skin semitransparent in smaller specimens (16–26 mm), but more densely pigmented in larger specimens (45–69 mm). Subdermal pigmentation is absent except on the peritoneum.
The body of free-living and parasitic males is elongate, the posterior part of the head strongly arched. Sphenotic spines are absent. The dorsal body contour is concave. There are 8 or 9 short recurved teeth on each side of the upper and lower jaws, arranged in 1 or 2 series, the largest teeth about one-half the length of the denticular teeth. The slightly tubular eyes are directed anteriorly, their diameter about 8% SL. The anterior nostrils are directed anteriorly, but are well separated and oval in shape, their vertical diameter about two-thirds that of the posterior nostrils. The posterior nostrils are directed anterolaterally and are well separated from the eyes. The olfactory organs and to some extent the eyes are degenerated in parasitic males. There are 3 or 4 dorsal- and anal-fin rays, and 15–17 pectoral-fin rays . The skin is unpigmented in free-living males, but faintly pigmented in parasitic males. Subdermal pigment is restricted to the peritoneum.
Females attain a maximum known length of 69 mm (SIO 53-356); the largest parasitic male measures 7.3 mm (ZMUC P92134) and the largest free-living male, 8.6 mm (ZMUC P921727).
Metamorphosed females of the genus Photocorynus are distinguished from those of all other ceratioids by having a spine on the epiotic, a pair of spines on the posttemporal, and 5 or 6 spines on the preopercle. They differ further from those of all other genera of the family in having the following combination of character states: The frontals meet on the midline, each with a well-developed anterodorsal spine. The maxillae are moderately strong, the jaws with numerous short teeth arranged in several series. Vomerine teeth are absent. The first pharyngobranchial is present. The ceratohyal lacks an anterodorsal process. The posterior margin of the hypural plate is entire. The ninth caudal-fin ray is extremely short, the length of the illicium less than 10% SL. The esca is nearly sessile on the snout, without appendages. A hyoid barbel is absent. The second and third pectoral radials are subequal. The skin is darkly pigmented.
Free-living juvenile males of Photocorynus are unique in lacking cranial and preopercular spines. The preopercle is not strongly curved. The epiotic region of the skull is highly elevated. There are 8 or 9 teeth on each side of the upper and lower jaws, all shorter than the denticular teeth. The denticular bones are small and placed slightly behind the tip of the jaws. The denticular teeth are slender, 3 on the upper denticular and 3 or 4 on lower denticular. The eyes are slightly tubular, their diameter 8–9% SL. The olfactory organs are moderately enlarged and inflated, nearly as large as the eyes. Ther are 3 olfactory lamellae. The skin is unpigmented.
Adult males become parasitic. Their skin is very faintly pigmented. The olfactory organs and to a lesser extent the eyes are degenerated. Jaw teeth are lost except in the lower jaw of one known specimen.
The larvae are unknown, but extrapolating from the features displayed by recently metamorphosed males, they most probably lack sphenotic spines and subdermal pigmentation (except on the peritoneum), and have relatively small pectoral fins.
Until recently, Photocorynus spiniceps was known only from the Gulf of Panama and off Peru, but the available material now includes records from Hawaii, the Western North Pacific, the Indian Ocean, and both sides of the Atlantic between latitudes of 32°N and 13°S. Included is a new record from off Japan (SIO 53-356) that represents both the largest known individual (69 mm) as well as the northernmost record (35°N) for the species.
Meso- to bathypelagic. It appears to be a relatively deep-living species: four specimens were captured in closing nets at depths of 990 m and 1420 m; and of those caught in open nets, none were taken in maximum fishing depths of less than 1000 m.
Pietsch TW. 2009. Oceanic Anglerfishes: Extraordinary Diversity in the Deep Sea. Berkley: University of California Press. 638 p.
The 6.2- and 7.4-mm parasitic males of P. spiniceps are the smallest known within the suborder and, if regarded as adults, which histological evidence seems to indicate in the case of Photocorynus, they are among the world’s smallest known sexually mature vertebrates as defined in terms of length, volume, and weight.
Known from thirty-one metamorphosed females at 16–69 mm SL, three free-living metamorphosed males at 6.5–8.6 mm SL, and four parasitic males at 6.2–7.3 mm SL.
DANA station 1209(3), Gulf of Panama, 7°15'N, 78°54'W, 2500 m of wire, 17 January 1929.
Holotype of Photocorynus spiniceps: ZMUC P92134, female, 46 mm, with 7.3-mm parasitic male.
