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Dorsal spines (total): 0-0; Dorsal soft rays (total): 10-10; Anal spines: 0-0; Anal soft rays: 14-14; Vertebrae: 39-40. Diagnosis: This species differs from all other alestid and all other characiform species in having supposed adults with greatly enlarged conical teeth in two rows on the premaxillary and on the dentary (Ref. 123194). Mouth slightly upturned; jaws short, entirely anterior to eyes; teeth with large round base and sharp slightly recurved tip, evidently specialized for puncturing (Ref. 123194). Head and body laterally compressed, slender; circumorbital bones absent; scales absent; postcephalic laterosensory system absent (Ref. 123194).
Description: Lower jaw relatively massive but short, lying entirely anterior to eye; branchiostegal rays four, as in most characiforms; first gill arch with about 27 gill rakers, including 16 lower gill rakers, 1 gill raker in arch and 10 upper gill rakers; upper and lower pharyngeal toothplates with sharp slender conical teeth on trailing edges (Ref. 123194). Dorsal fin entirely anterior to anal fin, its base above vertebrae 19-24; anal fin entirely posterior to dorsal fin, its base below vertebrae 28-34; anal fin margin gently rounded in smaller specimens, but distinctly indented or falcate in largest specimen, possibly due to sexual dimorphism; pelvic fins entirely in advance of a vertical line through origin of dorsal fin, its origin below vertebra 16, with a very thin splint and 7 thin but fully formed rays, none ending in actinotrichia; adipose fin present, small, overlying vertebra 35; dorsal fin rays 10, last ray divided to base; anal fin rays 14, last ray divided to base; pectoral fin with fleshy peduncular base, two uppermost rays very thin, divided into well separated ray halves, rest of pectoral fin apparently entirely actinotrichous, accurate count not possible (Ref. 123194). Total vertebrae 39-40; vertebrae 1-4 involved with Weberian apparatus, presumably cartilaginous; vertebra 5 with first normally formed pleural ribs; pleural ribs present on vertebrae 5-19; median hemal spine with hemal arch extending continuously from vertebra 19 to last vertebra, last hemal arch passing through base of parhypural; lower part of hypural fan with a parhypural and two hypurals, upper part with four hypurals, total hypurals six (Ref. 123194).
Colouration: The specimens of Alestion rapax were recognized as distinctive when they were collected; the only remark about their appearance recorded as 'largely translucent' (Ref. 123194). No indication of sexual dichromatism (Ref. 123194). Overall colouration of head and body uniformly light cream colour; the most distinctive feature of colouration is an elongate rectangular area with numerous melanophores of variable size on middle of anterior half of body; immediately in front of this rectangle of melanophores, and between it and the head, is a small part of the lateral body wall overlying the Weberian apparatus, where the pseudotympanum is located when it is present, that is entirely devoid of melanophores; dorsal surface of head and body partially covered with numerous close-set melanophores, especially large and numerous on anterior and especially posterior parts of dorsal cranial fontanel; melanophores on dorsum of body forming a broad dorsomedian stripe of uniform thickness, somewhat more pronounced along the bases of dorsal and adipose fins, ending abruptly just before end of caudal peduncle; on either side of dorsomedian stripe dorsal surface of body devoid of melanophores, so that stripe stands out starkly; mentum and jaws and occipital region of head with numerous large melanophores; large suborbital area devoid of melanophores; gill cover with a few small melanophores; dorsolateral part of body without melanophores, uniformly light cream colour in preserved specimens; side of body at horizontal septum with a faint thin midlateral stripe expanding into an elongate triangle on caudal peduncle just before insertion of middle caudal fin rays (Ref. 123194). Pectoral, pelvic, and adipose fins and ventromedian fin fold without melanophores; dorsal, anal and caudal fins with melanophores mainly concentrated along the margins of the fin rays, effectively outlining the rays so that they stand out; ventral surface of head posterior to mentum and of body almost devoid of melanophores, thus pale cream overall in preserved specimens (Ref. 123194). |