Molecular phylogeny and larval morphological diversity of the lanternfish genus Hygophum (Teleostei : Myctophidae)

Larvae of the deep-sea lanternfish genus Hygophum (Myctophidae) exhibit a r emarkable morphological diversity that is quite unexpected, considering the ir homogeneous adult morphology. In an attempt to elucidate the evolutionar y patterns of such larval morphological diversity, nucleotide sequences of a portion of the mitochondrially encoded 16S ribosomal RNA gene were determ ined for seven Hygophum species and three outgroup taxa. Secondary structur e-based alignment resulted in a character matrix consisting of 1172 bp of u nambiguously aligned sequences, which were subjected to phylogenetic analys es using maximum-parsimony, maximum-likelihood, and neighbor-joining method s. The resultant tree topologies from the three methods were congruent, wit h most nodes, including that of the genus Hygophum, being strongly supporte d by various tree statistics. The most parsimonious reconstruction of the t hree previously recognized, distinct larval morphs onto the molecular phylo geny revealed that one of the morphs had originated as the common ancestor of the genus, the other two having diversified separately in two subsequent major clades. The patterns of such diversification are discussed in terms of the unusual larval eye morphology and geographic distribution, (C) 2000 Academic Press.