Parabrotula (Zugmayer 1911a), included in the family Bythitidae (Ophidiiformes), is a genus of small viviparous mesopelagic fishes with two valid species. Parabrotula plagiophthalmus is known from collections made worldwide except in the eastern Indian, eastern Pacific, and western South Atlantic Oceans; most specimens are from the North Atlantic Ocean. Most anatomical data for P. plagiophthalmus are limited to aspects of external anatomy, the reproductive system, and meristic data that can be gathered through radiographs; virtually no data on other aspects of the osteology of P. plagiophthalmus are available. In this study, we describe the skeleton of P. plagiophthalmus from cleared and stained specimens and computed tomography data. Many aspects of the skeleton of P. plagiophthalmus are reduced (subopercle; ceratobranchial 5; gill rakers; gill filaments; distal radials of median pterygiophores; caudal skeleton; posttemporal; supracleithrum; scapula; distal pectoral radials; pelvic girdle and fins; scales) or absent (mesethmoid; nasal; pterosphenoid; intercalar; infraorbital bones; endopterygoid; coronomeckelian; supramaxilla; pharyngeal toothplates). All bones of the skull are extremely thin dermal or perichondral ossifications, with little to no endochondral components. The vomer supports a variable number of teeth (0–3). The pars autopalatina and its ossification are separated from the pars metapterygoidea and pars quadrata and their ossifications; the thin ectopterygoid bridges these elements of the suspensorium. The lower jaw is very large compared with all other elements of the viscerocranium. The pectoral radial plate was never observed to be divided into separate radial elements. We discuss the skeleton of Parabrotula in the context of other taxa but call attention to the need for further comparative studies of the anatomy of ophidiiform fishes.
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Vol. 569 • No. 1