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

Abstract

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 “Leptocephalus holti”, which was thought to possibly be the larva of the rare orange-colored eels of Neocyema (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 L. holti larvae that were collected in the Pacific is genetically matched with Monognathus jesperseni, 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 Neocyema, which is presently only known from the Atlantic Ocean. We demonstrate that Neocyema constitutes a separate family being most closely related to Eurypharyngidae and Saccopharyngidae based on mitogenomic DNA sequences and unique mitochondrial gene orders.

Hiding in Plain Sight: Elopomorph Larvae Are Important Contributors to Fish Biodiversity in a Low-Latitude Oceanic Ecosystem

“…Leptocephalus giganteus was only collected in the MOCNESS from nets fished between 1000-1200, 600-1000, and 200-600 m depth. Leptocephali remaining at depth may help explain the absence in this study of larvae of Saccopharynx and Monognathus, and the absence or rarity of other bathypelagic species (Poulsen et al, 2018).…”

“…Elopomorph fishes are basal teleosts, comprising the Elopiformes (tarpon and ladyfishes), Albuliformes (bonefishes), Notacanthiformes (spiny eels and halosaurs), and Anguilliformes (true eels, including the formerly separate Saccopharyngiformes) (Forey, 1973;Arratia, 1999;Dornburg et al, 2015;Poulsen et al, 2018). One unique characteristic of elopomorph fishes is a larval stage known as the leptocephalus (Greenwood et al, 1966;Pfeiler, 1986;Wiley and Johnson, 2010).…”

“…This rapidity of metamorphosis means there are even fewer specimens of individuals caught in the midst of this transformation, making assembling a series of specimens difficult. These methods are even more likely to fail for rare or deep-sea species, where their numbers may be greatly reduced and their duration as epipelagic plankton may be brief or nonexistent (some deep-sea eel larva may rarely rise to the epipelagic, for example, Saccopharynx spp., Monognathus spp., or cyematids; Poulsen et al, 2018).…”

Phylogenetic relationship of the Anguilliformes (Teleostei: Elopomorpha) with an emphasis in cephalic morphology

“…Poulsen et al (2018), based on mitogenomic sequence data, performed molecular and morphological analysis over "Leptocephalus holti," the incertis sedis larvae, however, the authors pointed out these specimens as larval stage of one-jaw eels of the Monognathidae. Poulsen et al (2018) also generated a cladogram in which Neocyematidae is more closely related to Eurypharyngidae and Saccopharyngidae. In that study, Protanguilla is not a basal group among Anguilliformes, but rather sister group of Synaphobranchidae.…”

“…Hypothesis of the relationships of Anguilliformes inferred from morphological characters according toPoulsen et al (2018). Mitogenomic phylogenetic tree of 79 taxa of the…”