We report the discovery of an enigmatic, small eel-like fish from a 35 m-deep fringing-reef cave in the western Pacific Ocean Republic of Palau that exhibits an unusual suite of morphological characters. Many of these uniquely characterize the Recent members of the 19 families comprising the elopomorph order Anguilliformes, the true eels. Others are found among anguilliforms only in the Cretaceous fossils, and still others are primitive with respect to both Recent and fossil eels. Thus, morphological evidence explicitly places it as the most basal lineage (i.e. the sister group of extant anguilliforms). Phylogenetic analysis and divergence time estimation based on whole mitogenome sequences from various actinopterygians, including representatives of all eel families, demonstrate that this fish represents one of the most basal, independent lineages of the true eels, with a long evolutionary history comparable to that of the entire Anguilliformes (approx. 200 Myr). Such a long, independent evolutionary history dating back to the early Mesozoic and a retention of primitive morphological features (e.g. the presence of a premaxilla, metapterygoid, free symplectic, gill rakers, pseudobranch and distinct caudal fin rays) warrant recognition of this species as a ‘living fossil’ of the true eels, herein described as Protanguilla palau genus et species nov. in the new family Protanguillidae.

A new epigonid fi sh, Epigonus cavaticus, is described on the basis of eight specimens (59.2-69.5 in standard length: SL) collected from a cave at depth 20 m, southern fringing reef of Ngemelis Island, Palau. The species differs from other congeners by having minute teeth on both jaws, no opercular spine, pyloric caeca 7-8, gill rakers 25-27, total pored lateral line scales 48-50, dorsal fi n rays VII-I, 10-11 (mode VII-I, 10), pectoral fi n rays 16, vertebrae 10 + 15, body depth 21.4-25.0% SL, pectoral fi n length 22.7-24.6% SL, eye diameter 44.4-47.5% head length: HL, upper jaw length 40.2-42.5% HL. Four paratypes (63.7-66.8 mm SL) of the new species are female with mature gonads, it is the smallest in size at sexual maturity among the congeners.

Graphical Abstract

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