ABSTRACT
The Family Idiacanthidae comprises one genus (Idiacanthus) with one Atlantic species I. fasciola and 3 additional species elsewhere. Their larvae are the most distinctive in the sea and were first described by Brauer (1906, 1908) and fully described by Beebe (1934). The body is long and slender at all stages, not greatly compressed, lacking hexagonal pigment areas and scales; pectoral fins present in young stages, absent in adults; pelvics in females in advance of mid-body, absent in males; dorsal fin long in adults, but opposite anal in young and preceded by long finfold; both dorsal and anal fins end just in advance of caudal; in adults a pair of short, bony projections anterior to and flanking each dorsal and anal ray (Gibbs (1964b). The unique feature of the larvae is the greatly extended, pencil-like eyestalks which make them the easiest of any larval form to identify. In addition, larvae have trailing guts (deflected to left of anal fin), flat heads, and unique pigment pattern of lateral melanophores and melanophores on the trailing gut (Kawaguchi & Moser 1984). Beebe (1934) provides a great deal of detail on the development. Novikova (1967) and Moser (1996g) also provide information on young stages. Meristics are given in Table Stomioidea 1 and photophore data in Table Stomioidea 2. Suffice it to say these are the easiest to identify in early stages by the long eye stalks in larvae and meristics in juveniles and adults. In our waters only Bathylagus has long eye stalks (a few Myctophum sp. have very short eye stalks), but they are short in comparison to Idiacanthus. They are also unique in the sexual dimorphism of late larvae, juveniles and adults due to the absence of pelvic fins in males. IDIACANTHIDAE Idiacanthus fasciola Peters 1876
MERISTICS Vertebrae:
LIFE HISTORY
Range: Throughout area. Habitat: Meso-& bathypelagic. ELH Pattern: Oviparous, planktonic eggs & larvae.
