The morphological development of four trachichthyoid larvae, Anomalops katoptron in the Anomalopidae, Hoplostethus sp., Gephyroberyx japonicus and Aulotrachichthys sp. in the Trachichthyidae, are described and illustrated based on larval specimens collected
from the western North Pacific. The striking characters shared by these four trachichthyoid larvae are well developed head ornamentation with bony ridges and spines, spinous scales, and spinules on the fins and branchiostegal rays. Head spination is different in arrangement and configuration
among larvae of the four species. Other features, such as body shape, anus position, presence of scutes on the abdominal keel, the incipient luminous organs beneath the eye or on the ventrolateral side of the trunk, and pigmentation on the body and pelvic fin, are useful for identification
of the postflexion larvae of the four species. Cladistic analysis of relationships among the Anoplogastridae, Diretmidae, Anomalopidae, Monocentridae, and Trachichthyidae using larval and some adult characters corroborates hypotheses based on adult characters that these five families are a
monophyletic assemblage, anoplogastrids and diretmids are sister groups, and the Trachichthyoidei (anomalopids + monocentrids + trachichthyids) are monophyletic. Our analysis also suggests that anomalopids and monocentrids are sister taxa, and the Trachichthyidae are paraphyletic. To clarify
the interrelationships among trachichthyids, more complete ontogenetic series of their larvae are needed.
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