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Cephalopods

Mike Vecchione, who is a MAR-ECO scientist and a cephalopod specialist presents a brief, popular description on cephalods.

Description

The Class Cephalopoda includes (1) decapodiforms, such as squids and cuttlefishes, (2) octopodiforms, including the vampire squid as well as a variety of octopods, and (3) their more distant relatives, the chambered nautiluses.

These molluscs are characterised by a well developed head that contains a circumoral (surrounding the mouth) crown of arms that bear suckers and/or hooks (except in Nautilus). The mouth has chitinous beak-like jaws and a chitinous tongue-like radula (band of teeth).

The shell is reduced, modified, or absent and is enclosed by the mantle. An external shell occurs only in Nautilus (restricted to Indo-Pacific), although a shell-like egg case is found in female argonauts. The size of adult cephalopods ranges from about 2 cm to over 20 m in total length; largest specimens may weigh over 1 ton.

Cephalopods are soft-bodied animals; their primary skeletal features include a cartilaginous cranium and, in most forms, a rigid structure in the mantle composed of chitin (gladius or "pen"), calcium carbonate (cuttlebone), or cartilage (fin supports in finned octopods). They have one pair of gills, except for the Nautilus, which has two pairs. The central nervous system is highly developed especially the well-organised eyes.  

View a photogallery showing some physical features in cephalopods collected by Dr. Vecchione during a research cruise to Bear Sea Mount, on the western side of the mid-Atlantic Ridge.

Coloration

Coloration is variable depending on group and habitat; most species are provided with numerous chromatophores (pigment sacs controlled by neuromuscular action) and iridocytes (shiny, reflective platelets) in the skin.

Rapid changes in colour and colour patterns are an integral part of their behaviour.
While shallow-living cephalopods are able to conceal themselves by chromatophore-produced colour patterns and chameleon-like colour changes, many deep-sea forms camouflage themselves by producing bioluminescent light from photophores (light-producing organs) which eliminates their silhouettes against the downwelling light in the dimly-lit mid-depths.

Respiration and locomotion

Water is taken into the mantle (body) cavity for respiration. A funnel or siphon (a small ventral tube) expels the water from the mantle cavity for propulsion and elimination of waste products, in addition to completion of the respiratory cycle. Locomotion is achieved by drawing water into the mantle cavity followed by its jet-like expulsion through the funnel, and by fins on the mantle, as well as by crawling along the bottom on the arms (mostly octopods). Fins also provide balance and steering.

Life history

The sexes are separate. Many, though not all, cephalopod species exhibit external sexual dimorphism, either in structural or size differences. Males of many forms possess a modified arm (hectocotylus) for mating. The hectocotylus may consist of modified suckers, papillae, membranes, ridges and grooves, flaps, etc., but in any case it functions to transfer the spermatophores (sperm packets) from the male to an implanta-tion site on the female. The spermatophores may be implanted inside the mantle cavity, around the mantle opening on the neck, in a pocket under the eye, around the mouth, etc. The mode of reproduction and egg-laying is unknown for many forms, especially oceanic and deep-sea species.

The life expec-tancy appears to be about one to two years in most forms, but larger species of squids and octopus, for example, the giant squid (Architeuthis spp.) and the giant octopus (Enteroctopus spp.), must live at least somewhat longer. Conversely, small oceanic species such as pyroteuthids may complete their life cycles in less than six months. Some species die after spawning, but this phenomenon is not universal.

The female lays eggs that are heavily yolked. Cephlapod development is direct, without true metamorphic stages. Cephalopod eggs may vary in size from about 1.7 cm long in some Octopus species to 0.8 mm long in Argonauta, both octopods. Time of embryonic development also varies widely, from a few days to many months, depending on the species and temperature conditions. Hatching may occur rapidly from a single clutch or be extended over a period of 2 to 3 weeks. Eggs have one or more layers of protective coatings and generally are laid as egg masses. Egg masses may be benthic or pelagic, varying among major taxonomic groups. Hatchlings from benthic eggs may be either benthic, and morphologically similar to the older stages, or planktonic. Pelagic hatchlings are planktonic and, in some species, very different from more developed conspecifics (members of the same species). The term "paralarva" has been adopted for early stages of cephalopods that differ morphologically and ecologically from older stages.

Systematics

The total number of living species of cephalopods currently recognized is fewer than 1 000. The status of the systematics of cephalopods is rapidly changing, as research has increased significantly in the past 25 years. Cephalopods occur in all marine habitats of the world, though none are found at salinities less than about 17.5 PSU. The range of depths extends from 0 to over 5,000 m. Many species of oceanic cephalopods undergo diel (daily) vertical migrations, wherein they occur at depths of about 400 to 1000 m during the day, then ascend into the uppermost 200 m or so during the night. Abundance of cephalopods varies (depending on group, habitat, and season) from isolated territorial individuals (primarily benthic octopods) through small schools with a few dozen individuals to huge schools of oceanic species with millions of squids.

View a photogallery showing a diverse selection of squids collected by Dr. Vecchione during a research cruise to Bear Sea Mount, on the western side of the mid-Atlantic Ridge.

Cephalopods of the northern Atlantic Ocean

Two groups of cephalopods, decapods and octopods, are common in the deep waters of the Atlantic Ocean. The major groups are easily distinguished by external characteristics. The squids have an elongate body with lateral fins, and eight circumoral arms with stalked suckers in two or more rows and bearing chitinous rings (sometimes modified into hooks), plus two longer tentacles with an organized cluster (tentacular club) of two to many rows of suckers (or hooks) at the distal end. The octopods have a short, sac-like body generally with no lateral fins (some deep-sea octopods have a pair of paddle-like fins), and eight circumoral arms only (no tentacles) with unstalked suckers without chitinous rings along the length of the arms. Sepiolid decapods (bobtail squids) also have a short, sac like body, but have fins and an arm crown similar to that of the squids.

Cephalopods in the food chain

Cephalopods are active predators that feed upon shrimps, crabs, fishes, other cephalopods, and, in the case of octopods, on bivalved molluscs. In turn, cephalopods are major food items in the diets of toothed whales, seals, pelagic birds (penguins, petrels, albatrosses, etc.), and both benthic and pelagic fishes (e.g., sea basses, lancetfishes, tunas, billfishes).

Cephalopods are extremely important as food for human consumption, and well over 1 million metric tons are caught each year. The fisheries are especially intense in Japan, the Orient and in the Medi-terranean/Eastern Atlantic waters. Fishing techniques include small traps (octopods), wiers, lures and jigs (some cuttlefishes and squids), lampara nets (nearshore squids), and midwater and otter trawls (squids and octopods). Certain species of squids are attracted to light, then jigged or seined. Occasionally cuttlefishes and octopods are caught in hand-nets or are speared.

Cephalopods in science

Cephalopods are important experimental animals in biomedical research with direct application to man. Because of the highly developed brain and sensory organs, cephalopods have a great capacity to learn and remember, rendering them valuable in behavioural and comparative neuroanatomical studies. In addition, some cephalopods possess extremely large single nerve axons and these are used extensively in all aspects of neurophysiological research.

Links

Informative web sites developed by Mike and some colleages: Histioteuthis reversa, Lampadiotheuthis and Cephalopoda Cuvier 
CephBase - another informative site by James B. Wood and Catriona Day


 

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