Comprehensive DescriptionRead full entry
Diagnosis.—Species of Malacosteus differs from species of all other genera of the family Stomiidae (sensu Fink, 1985) in having one round nostril on each side of head anterior to eye; palatine bones unossified and lacking teeth; margin of anterodorsal portion of neurocranium convex; a median toothplate associated with fourth basibranchial, and a single large (4.2–7.6% SL) tear-shaped accessory light organ (AO) along anteroventral margin of eye (Fig. 1). It differs further in having the following combination of character states: hyoid barbel and skin between mandibular rami absent, with only protractor hyoideus muscle linking mandibular symphysis to hyoid; jaws enormous (UJL 23.4–32.7 %SL, LJL 22.3–30.0% SL), extending far posterior to fleshy orbit; all jaw teeth fixed and not depressible (Type 1 attachment; Fink, 1981); vomerine teeth absent; maxillary teeth minute, closely and regularly set; lateral ethmoid, parietal, mesopterygoid, and posttemporal absent; eyes directed slightly anteriorly; lateral and ventral photophore series clustered, reduced in size, those of IP series in two distinct rows.
Description.—Body elongate, depth tapering slightly from cleithrum (9.6–18.0% SL) to anal-fin origin (5.9–10.7% SL), and more abruptly toward caudal peduncle (1.6–3.0% SL). Head small (7.8–12.3% SL). Eye large (EW 4.3–6.9% SL). Snout blunt and short. Opercular flap long and sloping posteroventrally to posterior tip of jaw. A single round nostril on snout facing anterolaterally. Tip of basihyal with a white, fleshy, finger-like tab. Branchiostegal rays 7–10.
Dentary teeth on left side 18–53, variable in size and decreasing in number with SL; anteriormost tooth minute, next to mandibular symphysis, followed by a large (7.5–19.2 %UJL), barbed, sharply curved, fang-like tooth emanating from dorsolateral margin of mandible; fang-like tooth followed by several large (2.1–7.5% SL), slightly curved, barbed, knife-shaped teeth; middle third of jaw populated by small barbed teeth of subequal size, some directed anterodorsally. Premaxillary teeth less numerous (12–53), small to moderate, slightly curved, often produced in two rows, a ventral row positioned along the ventral margin of the premaxilla and a dorsolateral row positioned along the anterolateral margin of the premaxilla; anteriormost tooth of the dorsolateral row barbed and usually isolated; posterior teeth of the ventral row often directed anteroventrally. Maxillary teeth minute, numerous. Palatine teeth absent. Basibranchial tooth patches placed dorsally in four groups, anteriormost three groups as bilateral patches associated with basibranchials 1, 2, and 3; posteriormost group associated with basibranchial 4, produced either as a pair of patches or as a singular, often laterally offset patch; 1–6 teeth per patch. Pharyngobranchial teeth in two groups on each side.
Pectoral fin moderately elongate (10.3–23.8% SL); pectoral-fin rays 3–4[5–6]; pelvic fin moderately elongate (14.5–25.7% SL), placed at about mid-body, prepelvic length 54.0–62.2 % SL; pelvic-fin rays 6. Dorsal and anal fins placed far back on body and nearly opposite, predorsal length 76.2–84.1 % SL, preanal length 74.1–83.2 % SL. Dorsal- and anal-fin bases subequal (11.7–17.4 and 12.2–18.5% SL, respectively). Dorsal-fin rays [16–17]18–20, anal-fin rays 19–22[23–24]. Caudal fin small and emarginate, lower lobe longer than upper. Total vertebrae 45–51.
Three cephalic photophores near orbit: AO, PO, and SO (Fig. 1). AO large (15.3–27.2% UJL, 4.2–7.6% SL), subequal to orbit in specimens larger than 50 mm (79.1–126.8%), relatively larger than orbit in smaller specimens (110.6–144.7%); teardrop-shaped and positioned at anteroventral edge of fleshy orbit. PO small to moderate (2.5–11.8 % UJL, 0.6–3.1% SL), an ovoid body posterior to fleshy orbit, ventral edge aligned with ventral margin of fleshy orbit, center aligned about one orbit-width posterior to posterior margin of eye; covered by a transparent membrane; size varying with sex and between species, larger in males, smaller in females. SO minute, round, situated on posteroventral margin in an elongate pocket with a dorsomedial opening. Small photophores and unorganized areas of accessory white luminous tissue scattered over head and body, often arranged around lateral photophores or along dark vertical lines composed of minute photophore-like structures. Ventrolateral and opercular photophores glowing blue in fresh specimens.
Lateral and ventral photophore series present, arranged in discrete groups (e.g., IP, PV, VAV, etc.); each group often containing one to several clusters of photophores. Lateral series low on body extending posteriorly from a position just anterior to level of posterior angle of jaw (when closed) to just anterior to origin of anal fin; ventral series extending posteriorly from anterior tip of isthmus along ventral midline to insertion of anal fin. IPp 4–7, IPc 3–7 clusters, arranged in off set rows; PVp 4–7[8–9], PVc 2–6 ; VAVp [2–3]4– 6, VAVc 2–4. OVp [3–4]5–9[10–11], OVc 3–6, arched anteroventrally; VALp 2–6, VALc 1–4; ACp 1–3, ACc 1–4, rarely absent, with several subcutaneous photophores anterior to visible group or clusters. A single small photophore placed approximately at mid-point of opercular flap on posterior margin of operculum.
Skin thin, scaleless; color of fresh specimens black, fading to dark brown after lengthy preservation. Size moderate, up to 253 mm.