Evolution and convergence
The greatest development of a pelagic fish fauna is in the ocean. However, most major lakes have an open water fauna that consists partly of members typically associated with open waters as well as species whose ancestors were obviously inhabitants of nearshore regions. These limnetic fishes include osteoglossomorphs (Goldeye, Mooneye), clupeids (shads), characins, cyprinids (Golden Shiner, Rudd), salmonids (whitefishes, trouts, chars), smelts, silversides, moronid temperate basses, and cichlids. Many of these fishes live at the air–water interface and show specializations that are apparently influenced by this habitat, including upturned mouths, ventrally positioned lateral lines, and convergent fin placement and body proportions. These surface-dwelling traits occur in both marine and freshwater families, including characins, minnows, silversides, marine and freshwater flyingfishes (exocoetids and gasteropelicids), halfbeaks, and killifishes (Marshall 1971). Regardless of ancestry, the same anatomical and behavioral themes that are seen in the ocean recur in freshwater limnetic species, including silvery color, compressed bodies, forked tails, schooling, high lipid content, and planktivorous feeding adaptations. Analogously, Pleuragramma antarcticum, a pelagic nototheniid in Antarctic waters, shows many traits characteristic of epipelagic fishes worldwide. Although derived from stocky, dark-colored, benthic ancestors, Pleuragramma has deciduous scales, a silvery body, forked tail, high lipid contents for buoyancy, and is compressed in cross-section. The pelagic larvae of many benthic Antarctic fishes are also silvery, compressed, and have forked tails (Eastman 1993; see below, Antarctic fishes). These examples of convergence suggest that fairly uniform and continuous selection pressures characterize the open water habitat.
With the exception of the clupeoids, most successful taxa of adult marine pelagic fishes are acanthopterygians. Missing among otherwise successful marine groups are elopiforms and paracanthopterygians, although both groups have done well in deepsea mesopelagic and bathypelagic regions. These two groups may be phylogenetically constrained from inhabiting shallow open water regions, not the least because of their tendency to be nocturnal in habit. Other strongly nocturnal taxa are also missing from pelagic and limnetic habitats, including the otherwise successful catfishes, seabasses, croakers, grunts, and snappers, to name a few. Which is not to say that pelagic waters are devoid of life at night. The diel vertical migrations of many mesopelagic fishes bring them near the surface after sunset, where they can forage comfortably in the dark.
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