Scripps Institution of Oceanography

The salmoniform fish family Platytroctidae (Searsidae) was last revised by Parr (1960) and then included 17 species in 12 genera. Since Parr's revision, 20 nominal species and 2 genera have been introduced. This rapid increase in new taxa, a number of which are poorly diagnosed and described, has left the taxonomy in an unsatisfactory condition. As a result of our study, 5 species and 2 genera are synonymized. We recognize 37 species (5 new) in 13 genera (1 new). A key to the family is presented and each genus is diagnosed and described, with a synopsis for each species. Besides the sac beneath the cleithrum that contains luminous fluid (shoulder organ), platytroctids are clearly set apart from other alepocephaloids by a number of synapomorphies including the presence of a subcutaneous canal system, a unique arrangement of the anterior predorsal spines, and the configuration of the caudal skeletal complex. Intrafamilial relationships were determined by use of a number of characters, including nature of photophores, presence or absence of a cleithral spine, fin position, shape and configuration of supraorbital and infraorbitals, path of cephalic lateral line canals, and dentition and jaw structure. Subfamilies are not recognized, but lines of relationship between the genera may be discerned. The deeper-living genera (bathypelagic) form a natural group and are regarded as the more advanced. They differ from the shallower-living (mesopelagic) genera in having a spinous cleithral symphysis, and are more compressed, with considerable nonmuscular tissue along the dorsal margin of the body. The platytroctids and alepocephalids are viewed as having had a common ancestor, which perhaps was closest to the Bathyprionidae among living forms. The course of platytroctid evolution is viewed as involving movement from mesopelagic to bathypelagic depths, with many of the morphological changes being reductional. Persparsia is regarded as the most primitive and generalized genus. Most platytroctid distributions are in the form of relatively narrow bands along the highly productive equatorial waters and the western side of continents. More oceanic records are primarily in areas of high-relief bottom, such as ridges and fracture zones, and near oceanic islands. Records of the generalized genera Persparsia and Paraholtbyrnia are from areas where temperatures at mesopelagic depths are relatively high -- up to 12 deg C. However, the remaining mesopelagic genera are mostly distributed meridionally in colder waters of the eastern boundaries of the Atlantic and Pacific. They tend to be rare toward the tropics. The more advanced genera predominate at low latitudes. The generalized eastern boundary forms are typically provincial in distribution. In contrast, four of the more advanced equatorial forms are circumglobal. However, none of them extends south of 24 deg S, and two, Platytroctes apus and Searsia koefoedi, have never been taken in the South Atlantic. Both species show geographic variation, with the eastern Pacific and Atlantic populations more similar to each other than to the Indo-Pacific population. From this and other evidence, we suggest that the former Panama seaway may have provided a passage for deep-water as well as shallow-water fishes.