<P> Some deep - sea pelagic groups, such as the lanternfish, ridgehead, marine hatchetfish, and lightfish families are sometimes termed pseudoceanic because, rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably seamounts and over continental slopes . The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures . </P> <P> The fish in the different pelagic and deep water benthic zones are physically structured, and behave in ways, that differ markedly from each other . Groups of coexisting species within each zone all seem to operate in similar ways, such as the small mesopelagic vertically migrating plankton - feeders, the bathypelagic anglerfishes, and the deep water benthic rattails . "</P> <P> Ray finned species, with spiny fins, are rare among deep sea fishes, which suggests that deep sea fish are ancient and so well adapted to their environment that invasions by more modern fishes have been unsuccessful . The few ray fins that do exist are mainly in the Beryciformes and Lampriformes, which are also ancient forms . Most deep sea pelagic fishes belong to their own orders, suggesting a long evolution in deep sea environments . In contrast, deep water benthic species, are in orders that include many related shallow water fishes . </P> <Table> <Tr> <Th_colspan="2"> (show) Species by pelagic zone </Th> </Tr> <Tr> <Td_colspan="2"> Many species move daily between zones in vertical migrations . In this table they are listed in the middle or deeper zone where they are regularly found . </Td> </Tr> <Tr> <Th> Zone </Th> <Th> Species and species groups include: </Th> </Tr> <Tr> <Td> Epipelagic </Td> <Td> <Ul> <Li> mackerel, requiem and whale sharks </Li> <Li> clupeiforms--herring, anchovy </Li> <Li> Salmonidae--salmon </Li> <Li> atheriniforms--flyingfishes, halfbeaks, sauries </Li> <Li> perciforms--jacks, dolphinfish, pomfrets, barracudas, tunas, billfish . </Li> </Ul> </Td> </Tr> <Tr> <Td> Mesopelagic </Td> <Td> Lanternfish, opah, longnose lancetfish, barreleye, ridgehead, sabretooth, stoplight loosejaw, marine hatchetfish </Td> </Tr> <Tr> <Td> Bathypelagic </Td> <Td> Principally bristlemouth and anglerfish . Also fangtooth, viperfish, black swallower, telescopefish, hammerjaw, daggertooth, barracudina, black scabbardfish, bobtail snipe eel, unicorn crestfish, gulper eel, flabby whalefish . </Td> </Tr> <Tr> <Td> Benthopelagic </Td> <Td> Rattail and brotula are particularly abundant . </Td> </Tr> <Tr> <Td> Benthic </Td> <Td> Flatfish, hagfish, eelpout, greeneye eel, stingray, lumpfish, and batfish </Td> </Tr> </Table>

Why are some fish at the bottom of the ocean