Convergence in the deep sea
The deep sea offers numerous striking examples of the Principle of Convergence. Benthopelagic fishes from at least 12 different families have evolved an eel-like body that tapers to a pointed tail, often involving fusion of elongated dorsal and anal fins with the tail fin (Gage & Tyler 1991). Another aspect of convergence exemplified in the deep sea is that selection pressures can override phylogenetic patterns, producing closely related fishes that are biologically very different because they live in different habitats (Marshall 1971). Gonostoma denudatum and G. bathyphilum are Atlantic bristlemouths in the stenopterygian family Gonostomatidae. G. denudatum is a mesopelagic fish, whereas G. bathyphilum, as its name implies, is a bathypelagic species. G. denudatum is silvery in color and has prominent photophores, well-developed olfactory and optic organs and body musculature, a well-ossified skeleton, a large gas bladder, large gill surface per unit weight, large kidneys, and well-developed brain regions associated with these various structures. G. bathyphilum, in contrast, is black, has small photophores and small eyes, small olfactory organs (except in males), weak lateral muscles, a poorly ossified skeleton, no gas bladder, small gills and kidneys, and smaller brain regions. Only the jaws of G. bathyphilum are larger than its mesopelagic congener. Similar comparisons can be drawn between other mesopelagic and bathypelagic gonostomatids, and between mesopelagic and bathypelagic fishes in general. Even bathypelagic forms derived from benthopelagic lineages, such as the macrourids and brotulids, have converged on bathypelagic traits (Marshall 1971).
The extreme demands of the deepsea habitat have also led to convergence in non-teleostean lineages. The mesopelagic cookie cutter sharks, Isistius spp., have a high squalene content in their livers that increases buoyancy. They also
possess photophores and migrate vertically with the biota of the deep scattering layer (the widespread nature of bioluminescence, some fish producing their own light and others using symbiotic bacteria, is in itself a remarkable convergence). Deepsea sharks and holocephalans also possess visual pigments that absorb light maximally at the wavelengths that penetrate to mesopelagic depths, as is also the case for another mesopelagic non-teleost, the Coelacanth, Latimeria chalumnae. Deepsea crustaceans and mollusks have also evolved anatomical and physiological traits similar to those of fishes, including the emission of luminous ink (e.g., platytroctids, ceratioids, squids) (Marshall 1980; Hochachka & Somero 1984).