CephalopodaEdit

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Cephalopoda is a class of marine invertebrates within the phylum Mollusca that includes some of the ocean’s most recognizable and behaviorally sophisticated animals. Members such as octopuses, squids, cuttlefishes, and nautiluses inhabit oceans worldwide and occupy a range of ecological niches from coastal shallows to the deep sea. They are notable for a prominent head, a belt of arms or tentacles, a highly developed nervous system, complex eyes, and remarkable control over coloration and texture. The group is divided into two major lineages, the Coleoidea (which includes octopuses, squids, and cuttlefishes) and the Nautiloidea (nautiluses), with the former containing most living species. See also Mollusca and Coleoidea for broader context.

Cephalopods have several distinctive anatomical and physiological features. A mantle houses the visceral organs, while a siphon or funnel enables locomotion by jet propulsion. Most cephalopods have a beak-like jaw and a radula, used for feeding in conjunction with a diet that ranges from crustaceans to fishes. In contrast to many other invertebrates, they possess a closed circulatory system and a highly centralized brain, which supports a wide repertoire of behaviors. Their eyes are often described as camera-type and highly capable, enabling acute vision in dim ocean depths. Chromatophores, iridophores, and leucophores allow rapid changes in color, pattern, and even texture, facilitating camouflage, signaling, and predation. Internalized or external shells vary among lineages: nautiluses retain external shells, while most coleoid cephalopods have either a reduced shell or none at all. See Mantle (anatomy); Beak; Siphon (anatomy); Chromatophore; Nautilus; Cuttlefish; Squid; Octopus.

Taxonomy and evolution - Classification places Cephalopoda within the Mollusca, a diverse phylum within the supergroup Lophotrochozoa. For living cephalopods, two major subgroups are the Nautiloidea (nautiluses) and the Coleoidea (octopuses, squids, and cuttlefishes). See Mollusca and Nautiloidea; Coleoidea. - The fossil record shows a long, rich history. Early cephalopods possessed external shells and complex chambered buoyancy systems, with nautiloid-looking forms persisting into modern times, while many later cephalopods evolved towards internalized shells or loss of the shell altogether. See Fossil and Cambrian for the deep-time context. - Molecular phylogenetics and ongoing paleontological discoveries continue to refine relationships among living cephalopods, but the major split between Nautiloidea and Coleoidea is well supported. See Molecular phylogenetics.

Anatomy and physiology - Body plan: The cephalopod body has a distinct mantle, a head with a mouth and armlike appendages, and, in many species, a pair of specialized pupils and a sophisticated visual system. The siphon, located ventrally, is used for jet propulsion and maneuvering. See Mantle (anatomy) and Siphon (anatomy). - Appendages: Octopuses have eight arms; squids and cuttlefishes possess eight arms plus two longer feeding tentacles in many species; nautiluses have numerous short tentacles. See Octopus, Squid and Cuttlefish. - Nervous system and behavior: Cephalopods host one of the most sophisticated nervous systems among invertebrates, enabling problem solving, short-term learning, and flexible foraging strategies. Their behavior is the subject of extensive study in neuroscience and animal cognition. See Nervous system and Cephalopod intelligence. - Sensory biology: They generally have well-developed eyes and acute sensing capabilities, enabling detection of prey, predators, and mates in low light. See Eye and Vision. - Defense and camouflage: Chromatophores (pigment-containing cells) and related tissues permit rapid color and pattern changes, while ink sacs provide a defensive spray in many species. See Chromatophore and Ink (cephalopod). - Shells and buoyancy: Nautiluses retain external shells with gas-filled chambers for buoyancy, whereas most coleoid cephalopods have lost shell buoyancy or carry a vestigial internal structure. See Nautilus and Cuttlefish.

Diversity, ecology, and life history - Diversity: The class Cephalopoda includes a variety of forms, with the most species-rich groups being the octopuses, squids, and cuttlefishes (collectively the Coleoidea). See Octopus, Squid, Cuttlefish. - Ecology: Cephalopods occupy a wide range of habitats, from shallow reefs and kelp beds to the deep pelagic zone and abyssal environments. They are both predators and prey, forming crucial links in marine food webs. See Marine ecosystem. - Reproduction and life cycles: Reproductive strategies vary; many octopuses are semelparous (have a single reproductive season and die after spawning), while nautiluses and some other cephalopods have different life histories. Eggs and larval development differ among groups, with some species releasing eggs that hatch into free-swimming paralarvae. See Life cycle and Paralarva. - Growth and longevity: Growth rates and lifespan are highly species-specific; some cephalopods mature quickly and live only a year or two, while others may persist longer under favorable conditions. See Growth (biology).

Ecology in human contexts - Fisheries and food security: Cephalopods such as squid and octopus are important commodities in global fisheries and regional cuisines. Their harvest can be sustainable when managed carefully, but overfishing and bycatch can threaten local populations. See Fisheries and Conservation status. - Research and ethics: Cephalopods have become increasingly prominent in scientific research due to their cognitive and behavioral complexity, raising discussions about animal welfare in laboratories and in aquaculture settings. See Animal welfare and Science ethics. - Conservation: Some cephalopod populations are stable, while others face pressures from habitat changes, climate variability, and fishing pressures. Conservation assessments range from least concern to threatened for individual species. See Conservation biology.

See also - Octopus - Squid - Cuttlefish - Nautilus - Mollusca - Marine biology - Evolutionary biology - Camouflage