ABSTRACT
Epipelagic fishes The open ocean beyond the continental shelf covers nearly two-thirds of the surface of the Earth, and some 2500 species are found there, distributed vertically from the uppermost waters to the greatest depths, about half being benthic, half pelagic. Near the surface is the euphotic zone, where light drives (phytoplankton) photosynthesis year-round in the tropics and sub-tropics, and for the warmer part of the year in cold and temperate waters (Figure 2.2). This zone of primary production (down to 200 m in the clearest waters), supports an epipelagic fish fauna of some 250 species, as well as many larvae of fish from deeper levels (Figure 2.3). Sharks, flying fish, scombroids such as tunas and billfish, halfbeaks, garfish, the large sunfish Mola, and stromateoids are typical of this zone, and are usually countershaded, colored dark blue above and lighter below so that they match their background when viewed from any angle. Countershading, which is more fully discussed in Chapter 10 (p. 326), is found in fishes in almost every habitat, but it is most strongly developed in the epipelagic zone where there is little else to hide a fish from
predators. Floating objects attract both smaller epipelagic fishes such as the stromateoid driftfish (Nomeus) and medusafish (Schedophilus), which hide under medusae for protection, as well as larger scombroids preying on the smaller fishes. Since medusae are found at many levels of the ocean it should come as no surprise that even deep-water medusae have their commensal fish associates (Drazen and Robison, 2004), although, in the case of Stygiomedusa gigantea, the fish most likely will be a deep-water species such as the Merlangius merlangus or the ophidiform, Thalassobathia pelagica. The distinctive Sargassum community consists of a variety of fishes and invertebrates that associates with the pelagic brown alga Sargassum. Some, like the Sargassum fish (Histrio histrio) are found nowhere else, and have their closest living relatives among the benthic community, but others associate as juveniles with the seaweed, presumably for protection or food (Figure 2.4). The perciform wreckfish (Polyprion) is named from its habit of living under floating wreckage, old teacases seemingly being favorite lairs. The epipelagic fauna is richest in warmer regions, but some species, such as the “warm” isurid sharks and blue-fin tuna Thunnus thynnus (see p. 82) migrate to colder waters in the productive season.
