All cephalopods are mobile throughout their lives, except for the egg stages that in many species are attached to various substrates. As far as we know, all cephalopods are either carnivorous (the neocoleoids) or scavengers (the nautilids) throughout their life cycles after hatching. Some researchers have proposed that some cephalopod paralarvae feed on phy-toplankton, but the evidence in support of this hypothesis currently is not very strong.

The life cycles of most neocoleoid cephalopods are very different from that of Nautilus. Whereas the latter is long-lived—it may live for 20 years or more—the lifestyles of other cephalopods have been characterized as "live fast and die young." Their life spans seem to range from a few months for small species to a few years for larger species. Many of the generalizations that have been made for neocoleoid cepha-lopods, however, have been based on observations of a few coastal species whose habitats are convenient for research.

Some cephalopod species, especially squids, can be very abundant; they are occasionally among the dominant organisms in their ecosystems. Because they are important food for larger animals, in addition to being voracious predators, such species are key members of some marine food webs. There are, however, significant gaps in current information about the life history and ecology of these organisms. Furthermore, many generalizations about cephalopods based on one or a few species are turning out to be either questionable or wrong. For example, because the common European octopus, Octopus vulgaris, and the California market squid, Loligo opalescens, spawn once and then die, their reproductive cycle has been widely regarded as the general pattern for cephalopods. Researchers are accumulating evidence, however, that many species of squids and octopods spawn many times and continue to live and feed after laying their eggs. Squids have been thought not to care for their eggs because the coastal species that have been most closely observed, as well as a few oceanic species, seem to release their egg masses and then depart either by moving away or by dying. Recently, though, some gonatid squids have been found to carry their egg masses around after releasing them. We know so little about most cephalopod species, especially oceanic and deep-sea species, that many such surprises likely await discovery. In short, many generalizations about the group may need to be modified.

Cephalopods live in a range of water depths from intertidal levels to over 16,400 ft (5,000 m). Different marine ecosystems have very different cephalopod faunas. For exam ple, no cuttlefishes are found in American waters. Many oc-topod species have recently been described from Antarctic Ocean waters, whereas the Arctic appears to have few octo-pod species. The deep sea is home to both primitive vampire squids and morphologically similar finned octopods as well as some strange oegopsid squids. Oegopsid squids tend to be dominant in epipelagic and mesopelagic oceanic waters (the upper layer of water that admits enough light for photosynthesis to occur, and the "twilight zone" just below it); whereas myopsid squids share the continental shelves with incirrate octopods and cuttlefishes. Some incirrate octopods are found in deep-sea habitats on the ocean bottom whereas others are entirely pelagic. Because cephalopods are so widespread throughout marine habitats, their patterns of habitat utilization also vary widely.

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