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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