View data on Catalog of Fishes here.
Barbel length increases from 11% SL in the smallest specimen (59 mm) to 35%–47% SL in those longer than about 100 mm. The axis of the stem is lightly peppered with melanophores in small specimens, more densely peppered or streaked with pigment in larger specimens (many of the paratypes are badly faded with little pigment remaining). The axis between the bulbs has no pigment in small specimens, but becomes more darkly pigmented with growth; the largest specimen (164 mm) has the darkest axis.
The external chevron-shaped or roundish striated areas on the stem are unpigmented.
Both proximal and distal bulbs vary in shape from ovoid to long ovoid, as much as three times longer than wide. In one specimen (126 mm), a minute ovoid bulblet about 1/4 distal-bulb length is situated between the two terminal bulbs. The proximal bulb is 0.5%–1.2% SL, the distal bulb 0.5%–1.5% SL, and neither appears to change relative to SL with growth, although the relatively larger bulbs are in specimens ~80 mm SL. The distal bulb varies from 0.8 to 1.6 times as long as the proximal bulb, in most specimens being equal to or longer than the proximal bulb. The distance between proximal and distal bulbs is 1.2%–3.7% SL, apparently not changing with growth, and is 1.5 to 4.2 times distal–bulb length.
There are no terminal filaments. One or two small, hair-like, filamentous projections (often difficult to see even under magnification) arise laterally from the distal bulb in some specimens, rarely from the proximal bulb. These projections never exceed bulb length.
The five largest males, 105–126 mm SL, have postorbital organs 1.6%–1.9% SL, 50%–65% of fleshy orbit length.
There are eight paired dorsal spots (seven in one specimen) between occiput and dorsal-fin origin, one beneath dorsal fin, and one or two on caudal peduncle.
This species has been taken in the northern Gulf of Mexico and the Straits of Florida; one specimen is known from off the Guianas.
Gomon JR, Gibbs RH, Jr. 1985. Taxonomy and distribution of the stomioid fish genus Eustomias (Melanostomiidae), II: Biradiostomias, new subgenus. Smithsonian Contributions to Zoology No. 409:1–58.
Eustomias hypopsilus is a member of the subgenus Biradiostomias Gomon and Gibbs (1985). Biradiostomias differs from all other subgenera of Eustomias (see Gibbs et al., 1983) in the possession of two long, separate pectoral rays. Dinematochirus, when pectoral fins are present, also has two pectoral rays, but these are closely bound together in black membrane; furthermore, the species of Dinematochirus have a well-developed ventral body groove that extends beyond the pectoral–fin bases, and the barbel is short, usually with a pigmented stem, and usually with branches arising from the stem well before the terminal bulb. Biradiostomias is most similar to the subgenera Nominostomias, Haploclonus, and Eustomias in possessing a relatively long, slender barbel that has little or no external pigment and in lacking a well-developed ventral groove behind the pectoral bases. These three subgenera have three long, separate pectoral rays. Biradiostomias generally is intermediate in photophore, vertebral, anal-ray, and tooth numbers between the higher counts in Nominostomias and Eustomias and the lower counts of Haploclonus. The subgenus Eustomias is unique in having paired photophores in the lateral series. Gibbs et al. (1983, table 1) compare counts of Haploclonus, Biradiostomias (as "2-pectoral-rays"), and Nominostomias.
The following are characters that apply to all species of the subgenus. Two well-developed, free pectoral rays. Seven pelvic rays. Barbel with slender stem having little or no external pigment (axis often pigmented), no row of dark spots, and no branches proximal to terminal bulbs. One to three, rarely four, relatively small terminal bulbs, with or without terminal filaments or projections. No wide ventral body groove posterior to pectoral-fin base. Photophores in ventral series (IC) 69–77 (mostly 71–75), in lateral series (OC) 64– 71 (rarely more than 68, species modes mostly 66–68), VAV and VAL 13–19 (seldom more than 17, species modes 15–17 and 16–17, respectively). Usually 4–6 (rarely 7) VAV photophores located over anal–fin base. No paired photophores in lateral series. Vertebrae in continuous series 63–68 (seldom more than 66, species modes mostly 65–66). Anal rays 31–40, usually 33–38. Premaxillary teeth 7–15; mandibular teeth 9–18 (fewer of each in many specimens less than 100 mm SL).
Gibbs RH, Jr, Clarke TA, Gomon JR. 1983. Taxonomy and distribution of the stomioid fish genus Eustomias (Melanostomiidae), I: Subgenus Nominostomias. Smithsonian Contributions to Zoology 380:1–139.
Gomon JR, Gibbs RH, Jr. 1985. Taxonomy and distribution of the stomioid fish genus Eustomias (Melanostomiidae), II: Biradiostomias, new subgenus. Smithsonian Contributions to Zoology No. 409:1–58.
To at least 164 mm SL.
Northwestern Atlantic, 27°00'N, 86°00'W, depth 0-200 meters.
Holotype: USNM 223639.
