Cusk-eel confusion: revisions of larval Luciobrotula and Pycnocraspedum and re-descriptions of two bythitid larvae (Ophidiiformes).

Since 2006, an ophidiiform larva with an ovoid body, elongate anterior dorsal-fin ray, and long trailing fleshy filament has been identified as Pycnocraspedum squamipinne. Similarly, the larvae of the ophidiid genus Luciobrotula have been tentatively identified since 1988, with posteriorly displaced dorsal fins and bulging or exterilium guts. However, neither of these larval forms morphologically agree with their adult counterparts. Recently, blackwater divers captured and photographed specimens of larval Luciobrotula and Pycnocraspedum off the coast of Hawaiʻi and Florida, making them available for both morphological and molecular sampling. After examining these larvae and analyzing DNA barcode sequences, as well as a newly captured and sequenced adult of Pycnocraspedum phyllosoma, we revise the previously identified "Pycnocraspedum" larvae to species of Luciobrotula. We describe the larvae of Luciobrotula bartschi and Luciobrotula corethromycter for the first time, highlighting an extraordinary loss of multiple anterior dorsal-fin elements in their ontogeny. We also generate the first DNA sequences for L. corethromycter and P. phyllosoma, adding to the depauperate number of sequences available for ophidiiforms. For the previously identified "Luciobrotula" larvae, neither morphological nor molecular characters provide definitive identification other than recovering them among the Bythitidae. We provide new morphological observations, revised descriptions, and generate a phylogeny of ophidiiform fishes based on COI to place these larvae in a phylogenetic context, prompting further investigation into the relationships of the Ophidiiformes using additional genetic markers. Our study emphasizes the importance of blackwater diving to improving our understanding of marine larval fishes and the need for additional molecular sampling of the diverse order of brotulas, cusk-eels, pearlfishes, and their allies. [ABSTRACT FROM AUTHOR]