Australyichthys gen. nov. (Teleostei, Syngnathidae), a lost mesophotic lineage from southern Australia, with description of A. aegis sp. nov. and reclassification of Hippocampus minotaur and H. paradoxus

Abstract

Australyichthys gen. nov. is described as a new genus of Syngnathidae (seahorses, pipefishes, seadragons, pipehorses and pygmy pipehorses) endemic to temperate mesophotic habitats of southern Australia and erected to include A. minotaur comb. nov. and A. paradoxus comb. nov., together with A. aegis sp. nov. The genus occurs at depths of 50–124 m within a mesophotic zone characterized by reduced light penetration and structurally complex benthic habitats. The included taxa possess hippocampiform features including a ventrally angled head, a fully enclosed brood pouch, and a prehensile tail, but are distinguished by a consistent osteological combination. Micro-computed tomography reveals an elevated, anteriorly inclined supraoccipital, a reduced anterior nuchal plate not forming a coronet, and unfused inferior trunk plates lacking a continuous ventral ridge. The external morphology is comparatively unsegmented, with subtle dermal lobulation contributing to a fleshy habitus. A derived ventral trunk complex is present, composed of anterior and posterior ventral elements flanking a median anal pterygiophore, homologous to the anal-fin pterygiophore complex of Hippocampus but reorganized into a novel ventral support architecture not observed elsewhere in Syngnathidae. Phylogenomic analyses of ultraconserved elements recover Australyichthys within Haliichthyini, the sister clade to Hippocampini, indicating that the hippocampiform body plan arose independently of Hippocampus and reflects convergence rather than shared ancestry. The genus is represented by eight specimens collected incidentally over nearly eighty years, with no collections within the last two decades and no verified in situ observations, and therefore meets the criteria of a lost taxon. Its rarity, restricted depth range, and predominant occurrence in mesophotic habitats underscore the need for targeted surveys and formal conservation assessment.