Cephalopod

A cephalopod is any member that belongs to the Class Cephalopoda(octopus Phylum) included under the phylum Mollusca. Cephalopods are an exclusively marine, highly organised and advanced group of animals. Have you wondered what is the phylum of octopus? Cephalopods examples include - octopus, squid, cuttlefish, etc. Cephalopods were greatly diverse in the Mesozoic and the late Paleozoic times. There is presently a lesser number of cephalopods in comparison to their extinct forms. Extinct cephalopod molluscs include - ammonites, belemnites, etc. 

In addition to learning what is a cephalopod, in this article, we will attempt to learn cephalopod meaning, Cephalopoda characteristics, cephalopod ecology, behaviour and more. 

Cephalopod Meaning

The term ‘cephalopod’ has been derived from the Greek word ‘kephalópodes’ which means head-feet. The cephalopod also implies an organism with tentacles i.e long parts resembling arms around the head, as seen in octopus phylum Mollusca. 

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Cephalopoda Characteristics

The basic structure of the cephalopod molluscs shows much resemblance to the rests of the members of the phylum Mollusca. 

• The cephalopod molluscs characteristically possess about 8 to 10 arms or tentacles in most species. However, the number of these tentacles goes up to 90 in Nautilus. 

• The characteristic molluscan shell is also greatly reduced and modified in all the members of cephalopods except for Nautilus. 

• The size of cephalopods also differs greatly from the largest Architeuthis species, which are giant squids of about 60 feet or 20 metres to the smallest squid Idiosepius which rarely ensures up to an inch. The arms of an octopus are usually about 12 inches or 30 centimetres. Instances of arms being more than a metre or 39 inches are usually rare. However, in Octopus dofleini, arms can reach as far as 30 feet or 9 metres. 

• There are presently more than 800 species of living cephalopods and are majorly found in the Earth’s oceans. 

• Cephalopods are not known to inhabit freshwater for their inability to tolerate it except for Lolliguncula Brevis, present in the Chesapeake Bay, as it is able to tolerate brackish water.

• They are known to occupy the depths of the oceans mostly, with maximum diversity near the equatorial regions which gradually decreases towards the poles. 

Cephalopod Structure

The structure of cephalopods varies from streamlined, elongated oceanic organisms to slow-moving, saccular drifting and bottom-dwelling forms. In most of the cephalopods, their body plan indicates their mode of life and their dwelling habitats. 

In a generalized cephalopod, an elongated or dome-shaped sheath of muscle known as the mantle encapsulates the viscera. The mantle is anteriorly connected to the head. The mantle is free towards the posterior and encloses the mantle cavity within. The gills of the cephalopods project into this mantle cavity and the reproductive and excretory systems open into it. 

The head-foot is present anterior to the mantle and bears the funnel which is the ventral muscular tube. In the order Octopoda, eight prehensile, long arms with suckers surround the mouth. In cuttlefishes and squids two tentacles and eight arms provided with horny ringed suckers that teeth or hooks are seen. On the other hand, about 90 small appendages without suckers are seen in Nautilus.

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Cephalopods Behaviour

Intelligence and Learning: The brains of cephalopods are considerably complex resulting from the unison of the major ganglionic centres. The degree of cerebralization and cephalization in cephalopods makes them unique among invertebrates. Several studies have demonstrated that the Octopus is considerably intelligent and capable of learning.  The variation in the mode of life between octopuses and squids also causes their behaviour to differ. 

Colour Change: Based on behavioural conditions, cephalopods are also known to exhibit colour change. Chromatophores or colour pigment cells along with iridocytes or reflecting cells are present in the skin of most cephalopods. The colours such as black, red, yellow, brown or orange-red are exposed upon the expansion of the chromatophores by the brain-controlled nerves. Camouflage, rest, attack on prey, and alarm or defence are the specific behavioural conditions that cause the colours and patterns of colour to exhibit accordingly. The most readily recognisable condition is the alarm pattern. It comprises a strong contrast between dark and light areas, peripheral dark outlines and bars of vividly displaying spots such as huge eyes. 

Ink Release: Cephalopods are known to utilise their ink for escape as well as defence. The ink of the Octopus is seen to render its predator moray eel in a state of paralysis. The ink is seen paralyzing the sense of smell and sight of the latter. The spindle-shaped mass of the squid ink ejected roughly resembles the squid’s size itself. The ink coagulates with water leaving behind a ‘dummy’ allowing the squid to become almost transparent by means of contracting its chromatophores and thus, escaping the predator. 

Camouflage: Cephalopods are often seen attempting to camouflaging or concealing themselves by imitating objects found in the sea bottom such as coral, sand or seaweed. This behavioural modification involves changing the texture of the skin which includes curling or the arms and erecting spikelike or branched papillae.

Bioluminescence: A number of cephalopods with the exception of Octopus and Nautilus are known to possess photophores or special light organs. These organs emit bioluminescence or chemical light. Distributed over the body of the cephalopods, these photophores are employed at night or to attract prey, in defence, as camouflage, even as an aid in schooling, recognising the sexes or mid-way during mating plays. The enzymatic reaction between luciferase and light produces light. In sepiolids or bottle-tailed squids, however, light is produced indirectly by luminescent bacterial cultures. In the squid Histioteuthis, the photophores are highly complex comprising a light source, directive muscles, reflector, lens, window, diaphragm, and colour screens.

In addition to these cephalopods behaviour, squids, octopuses and cuttlefishes are quite intelligent and skilled when it comes to luring prey, patiently stalking and hunting. The use of the tips of the arms to resemble worms in order to lure small fishes has been observed in cuttlefishes and octopuses. 

Locomotion in Cephalopods

Swimming, crawling or jet propulsion are the major locomotion methods employed by cephalopods, of which the latter is the most favourable. The mantle in cephalopods is associated with locomotion by becoming highly muscular and losing its rigid shell almost entirely. The contraction and expansion of the mantle draw in water into the mantle cavity and expels it through the funnel, thus producing a locomotory water current. Thus, the cephalopod animals can quickly execute forward and backward movements owing to the jet of water ejecting rapidly. 

The relaxing of the circular muscles draws water into the mantle cavity which causes the mantle to expand. The aperture or the neck region of the mantle is where the water enters, or through the funnel as in the case of certain deep-sea octopuses. When the mantle contracts, the locking mechanisms combined with the anterior ring muscle contraction closes the aperture forcing the water out through the funnel. The construction of this flexible tube which can be rotated in any direction is akin to a jet nozzle. This makes the animal highly flexible in terms of steering motion.

For hovering or slow movement in squids, lateral or terminal fins are present. Rapid undulations in the fins’ outer edges facilitate locomotion. Swimming keels or lateral expansions on the outer surface of the squids’ third pair of arms help them move through the water. Certain squids such as Thysanoteuthis and Onychoteuthis are capable of flying up to several feet when pursued by predatory dolphins and birds. The powerful thrusts provided by the jets drive these squids into the air which are then able to glide on their arm keels and expanded fins. Owing to the presence of the large cuttlebone, cuttlefishes by nature, are less active. Most of their lives are spent hovering just above the bottom or lying. They make use of both fins and jets for their locomotion with the use of fins being more frequent. 

Most bottom dwellers and active swimmers apparently lack any hydrostatic organ. The cuttlefish are known to adjust their buoyancy by regulating the amount of gases present in the porous cuttlebone. Nautilus, usually seen swimming in mid-water or above the bottom, slowly adjusts its buoyancy by regulating the amount of gases in its chambered shell. 

Cephalopod Reproduction 

Sexual dimorphism or the presence of separate sexes is usually seen in cephalopods. The difference is usually in the sizes and proportion of the different parts. The reproductive system in female cephalopods is relatively simple comprising paired oviducts and posterior ovary. In the species that lay eggs enclosed in gelatinous heavy capsules, nidamental glands are present. The reproductive system in males comprises a series of sacs or chambers running along the course of vas deferens. The vas deferens produce spermatophores or long tubes that contain spermatozoa. The Needham’s organ, which is the final sac, is used for storing spermatophores. The spermatophores are quite complicated as they contain a cement body, cap, sperm reservoir, along with a delicate triggering mechanism that releases the tube and cements it onto the body of the female. When the eggs reach maturity and get ready to be laid, the sperm is released into the body of the female. 

In the course of courtship, spermatophores are deposited by the male in the female either on the below-mouth pad or within the mantle cavity. The deposition of spermatophores by the male takes place by means of a hectocotylus or specially modified arm. In octopuses, the hectocotylized arm is deeply grooved on one side and ends in a spoon-like terminal organ. A larger portion of the arm is modified in the case of squids with degenerate suckers and rows of slender papillae present on the arm’s distal half. Special flaps and pouches may also be found on the modified arms of the squids. In Nautilus, the modified arm is known as the spadix. 

The mating habits of a majority of cephalopods is relatively unknown. The female and the male in the common octopus are stationed some distance apart. The male, using its hectocotylized arm’s tip, caresses the female before inserting the tip into the mantle cavity of the female and remains there for an hour or more. During this time, the spermatophores glide down the arm’s spermatophoral groove. In the cuttlefish, the male and female are seen swimming side by side with a certain type of courtship behaviour being shown by the make with its arms. Mating eventually occurs when the pair intertwine their arms and remain together while the spermatophores are being placed on the inner side of the mouth membrane of the female.

The eggs in cephalopods may be laid within a short period of time after mating or after a prolonged maturation period during which the sperm remains viable. Most cephalopod eggs are enclosed within a gelatinous capsule that may be transparent as in the case of Loligo squids or leather and opaque as in cuttlefishes and octopuses. Eggs may be either laid singly, in clusters or gelatinous masses. The incubation time of the various cephalopod species varies, about 40 days in Loligo squids and 50 days in octopus. 

Information on the life span of cephalopods is relatively less. As per studies, it is believed that smaller inshore species of cephalopods usually have a lifespan of about a year, rarely extending to 2 or 3. The life span of oceanic squids is not confirmed but Architeuthis (which are giant squids) are believed to attain bulk after 4 to 5 years. Observational data in many smaller squids and octopuses indicate that many males die after they mate while many females die after the first major spawning.

Cephalopod Ecology and Distribution

In all exclusivity, the Cephalopoda are marine animals with several littoral species. A few, however, are seen dwelling in brackish water with Lolliguncula Brevis, occurring along the coast in bays of Florida where the salinity plummets to 8.5 parts per thousand (one-fourth as compared to an open ocean).

Less is known about the geographic and vertical distribution of cephalopods. Certain open-ocean bathypelagic and pelagic forms are widely distributed in temperate and warm waters such as the Cranchia scabra and Onychoteuthis banksi. Some others may be limited by their genera to continental waters or particular oceans. In fact, some species dwelling in a bathypelagic habitat is limited to a single habitat. 

The local distribution of certain cephalopods is remarkably affected by the breeding season. In the spring and summer seasons, the common cuttlefish migrates to shallow water for breeding Certain squids such as the Alloteuthis, Illex, Loligo are also known to display similar migratory patterns. 

The movement of Nautilus that lives near the bottom is vertically through the water and can be seen at a depth of 1800 ft or 550 metres. The Sepiidae, among the cuttlefishes, are littoral as opposed to the Sepiolidae dwelling on or near the bottom at considerable depths. The Myopsida squids are the coastal forms whereas the Oegopsida squids are oceanic that are found from the surface to the deepest depths of about 16,400 feet or 5000 metres or more. Similarly, the various Octopoda also range in their habitats from the open ocean’s surface (Tremoctopus) to the floor of the ocean (Bentheledone, Cirroteuthis, Pareledone) at about 16,400 feet or more. 

Diet of Cephalopods

The cephalopods of the subclass Coleoidea are carnivorous in nature. They primarily feed on other cephalopods and small fishes. Squids are known to display cannibalistic behaviour. In the squid Illex, the young mackerel are fed upon by the adults while the adult mackerel prey upon young squid. Octopuses are known to feed on decapod crustaceans and bivalve molluscs. Smaller oceanic squids are also believed to primarily feed on copepods, heteropods, small fish, and caridean shrimp. The Cirrata are more likely to feed upon small plankton or bottom dwellers, given their reduced radula and musculature which are indicative of their reduced masticatory power and activity. 

Predators of Cephalopods

Several marine mammals, sea birds and large fish are among the predators of cephalopods. Squids are being fed upon by several toothed whales and sperm whales.

Importance of Cephalopods to Humans

The cephalopods are known to occur in large numbers and comprise one of the oceans’ most important potential food resources. They are consumed across the world with being accepted as a part of the general diet in northern Europe and North America. Cephalopoda also bears indirect importance to humans as they comprise a large part of the diet of seals, fishes, smaller whales, sperm whales and seabirds.