Resolving deep-sea pelagic saccopharyngiform eel mysteries: Identification of <i>Neocyema</i> and Monognathidae leptocephali and establishment of a new fish family "Neocyematidae" based on larvae, adults and mitogenomic gene orders

<div><p>Deep-sea midwater “saccopharyngiform” eels of the families Cyematidae, Monognathidae, Eurypharyngidae and Saccopharyngidae (order Anguilliformes) are extraordinary fishes having major skeletal reductions and modifications compared to the general anguilliform body structure. Little is known about most aspects of the systematics, phylogeny, and ecology of these families, and few of the approximately 30 species described from adult specimens have been matched with their leptotocephalus larvae. Based on mitogenomic sequence data from rare new specimens, we show that the long-speculated-about larval form referred to as “<i>Leptocephalus holti</i>”, which was thought to possibly be the larva of the rare orange-colored eels of <i>Neocyema</i> (5 known specimens; speculated to belong to the Cyematidae) are actually the larvae of the one-jaw eels of the family Monognathidae. One of the 5 types of <i>L</i>. <i>holti</i> larvae that were collected in the Pacific is genetically matched with <i>Monognathus jesperseni</i>, but multiple species exist based on larval sequence data and the morphology of adult specimens. A rare leptocephalus from the Sargasso Sea, with unique morphological characteristics including many small orange spots on the gut, was found to be the larva of <i>Neocyema</i>, which is presently only known from the Atlantic Ocean. We demonstrate that <i>Neocyema</i> constitutes a separate family being most closely related to Eurypharyngidae and Saccopharyngidae based on mitogenomic DNA sequences and unique mitochondrial gene orders.</p>