Cephalopods are a group of mollusks characterized by a bilaterally symmetric body and a prominent head with appendages. They belong to the Cephalopoda class, which includes squid, octopus, cuttlefish, and nautilus.
Although they exclusively inhabit marine environments, some, like the Atlantic brief squid, can tolerate brackish water. Fishermen sometimes call cephalopods ‘inkfish’ because they can spurt ink at their enemies.
They derive their name from the Greek word kephalópodes, meaning ‘head-feet.’ Currently, there are over 800 extant species divided into two subclasses: Coleoidea and Nautiloidea.
These mollusks are as tiny as the Thai pygmy squid, measuring 10 mm (0.3 inches), or as large as the largest colossal squid, which is 14 m (45.1 ft) in length.
The only living cephalopods that possess a true external shell are the nautiluses. These shells are usually ectodermic in origin.
The outer wall of the cephalopod shell comprises three layers: an outer spherulitic layer, a middle laminar (nacreous) layer, and an inner prismatic layer. In modern cephalopods, the layers are composed of aragonite, a form of calcium carbonate.
The feeding organ or radula comprises multiple symmetrical rows of teeth (nine in extant and thirteen in extinct varieties). The teeth may be homodont (similar in form), heterodont (not similar), or ctenodont (comb-like), with the number of cusps and dimensions varying with species. Although the teeth pattern is repeated, each row is not necessarily identical to the last one (In octopus, the sequence repeats every five rows).
Certain octopus species and members of Spirula lack this feeding organ altogether.
Many muscular appendages extend from the head and surround the mouth of cephalopods. In coleoids, they are usually eight to ten in number and serve multiple purposes, like feeding, movement, and reproduction.
There are two distinctly long arms called tentacles that lengthen in as little as 15 milliseconds. In the giant squid, they may be as long as 8 meters, culminating in a broad, club-like structure with suckers. The tentacles comprise a central nerve cord surrounded by circular and radial muscles. When they lengthen, the contracting circular muscles decrease their radius.
There are four pairs of short, accessory arms, also equipped with suckers, that help capture prey. Some shelled cephalopods belonging to Nautilus and Allonautilus possess around ninety partly retractable, finger-like appendages that usually lack suckers.
All cephalopods (except the family Nautilidae and octopus belonging to the suborder Cirrina) possess a characteristic ink sac that releases a smokescreen-like cloud of melanin to ward off predators. The melanin is often mixed with mucous to increase the thickness of the cloud.
They are the only mollusks with a closed circulatory system characterized by gill hearts (or branchial hearts) that move blood through the gill capillaries. After oxygenation, the blood circulates throughout the body with the help of a single systemic heart.
The oxygen is transported through hemocyanin, a copper-containing pigment in the hemolymph that adds blue color to the oxygenated blood in cephalopods.
Cephalopods respire using gills attached to the ventral surface of the mantle cavity. As soon as water enters the mantle cavity through the gills, it closes, followed by contractions of the muscular mantle. Water between the mantle cavity and the funnel is then forced through the gills. The radial and circular mantle cavity muscles control this flow of water.
Since most cephalopods move slowly and do not usually need to achieve high velocity, they have gills with large surface areas to ensure constant water lashing (even if the animal is stationary). They are supported by a hard, fibrous skeleton and can also excrete ammonium into surrounding water.
Nitrogenous wastes in the form of ammonia are produced in the gill hearts and expelled through usually a single pair of large nephridia. Each gill heart is connected to the nephridium by a narrow canal that delivers the excreta to a renal sac, besides resorbing excess water from it. The sac constantly inflates and deflates by the action of the outgrowths of the lateral vena cava that project into the organ. This constant movement causes the waste to be easily released into the mantle cavity through an excretory pore.
As an exception, Nautilus has four nephridia, none connected to the gill hearts.
They are the most intelligent invertebrates, with an extremely complex nervous system, well-developed senses, and large brains protected in cartilaginous cranium. A series of thick nerve fibers emanate from the mantle of these mollusks.
Though cephalopods primarily rely on vision to perceive their surroundings, they have a variety of chemical sense organs to help them sense the environment.
Eyes
Their eyes lack a cornea, have an everted retina, and are sensitive to plane-polarized light. They are as sensitive as the eyes of sharks and can distinguish between the brightness, size, shape, and orientation of objects. However, unlike most cephalopods, nautiluses have a simple pinhole eye that lacks a lens and has less accurate vision.
Most cephalopods have only a single type of photoreceptor and cannot determine color by comparing photon intensity (color blindness). However, color vision is observed in the sparkling enope squid (Watasenia scintillans) with three different types of photoreceptors. This squid uses retinal molecules like A1, A3, and A4 to detect light across different visible spectrum wavelengths. For example, the A1 photoreceptor is most sensitive to green-blue light (484 nm), while the A2 and A4 photoreceptors are sensitive to blue-green (500 nm) and blue (470 nm) lights, respectively.
Skin
Cephalopods possess a variety of skin cells that interact with light, such as iridophores, leucophores, chromatophores, and photophores.
Coleoids possess pigment-containing cells called chromatophores that facilitate rapid change of body color. Three pigments, red, yellow, and brown, are contained in elastic sacs called the cytoelastic sacculus found in each cell. They also possess iridophores (thin, layered protein cells) to cover the other colors of the visible spectrum. These pigment cells help cephalopods actively camouflage themselves against coral reefs or sandy sea floors to distract their enemies (disruptive camouflage).
Apart from almost 800 living species in the Coleoidea and Nautiloidea subclasses, most extinct species, like ammonites and belemnites, belong to the subclasses Ammonoidea and Belemnoidea.
The following classification is based on the Current Classification of Recent Cephalopoda (May 2001).
While the genera Nautilus and Vampyroteuthis are regarded as outgroups, Spirula is considered a sister group to Sepia and Metasepia.
Cephalopods are found in all oceans worldwide and cannot tolerate freshwater environments. They occupy all depths, ranging from abyssal plains to the topmost layer of the ocean. However, some cephalopods, like the Atlantic brief squid (Lolliguncula brevis) of Chesapeake Bay, United States, can tolerate brackish water.
They have the highest diversity near the equator, gradually dropping as we move to the poles.
Cephalopods usually draw their prey using their tentacles and nibble on it. They then secrete digestive juices (formed using symbiotic algae) from their salivary glands to separate the flesh from the shell or bones and digest the tissues. A small tooth in the salivary gland is poked into the prey’s flesh to inject the secretion deep into its body.
The ingested food passes through the crop, stomach, and caecum before moving into the intestine for further breakdown. A digestive gland or liver (linked to the junction of the stomach and the caecum) absorbs the nutrients in the prey. The cells of this gland release excretory pigments into the gut lumen, which are then coated and mucous and passed through the anus as long dark strings.
Most cephalopods propel themselves underwater through jet propulsion, an efficient mechanism that makes them the fastest marine invertebrates. In this propagation method, oxygenated water is taken into the mantle cavity from the gills, followed by the muscular contraction of the mantle cavity. Upon contraction, the water is expelled forcefully, like a jet, through the single siphon called the hyponome. This jet is supported by the fins that flap each time the water is released, improving the thrust. Water reenters the body through both the funnel and the orifices. Though the resultant motion of jet propulsion is backward, the direction can be easily altered by pointing the siphon at different angles.
Nautiluses produce jets by drawing their bodies into shells or undulating their funnels. Although their velocity is less than that of coleoids, less energy is expended in the same.
Some cephalopods can crawl on the ocean bed (some octopuses), while others, like squids and cuttlefish, move short distances by rippling through the water using the muscles around the mantle.
Octopus vs. Squid
The movement of octopuses and squids underwater varies considerably. While an octopus barely swims long distances and is usually found hovering near the sea bed, squids travel vast distances (some have been found to cover around 2000 km in 2.5 months). Similarly, during jet propulsion, an octopus must actively flex its longitudinal muscles to maintain the same mantle length; however, a squid does not require such muscle flexing during propulsion.
Although most cephalopods float, different species employ unique methods to attain buoyancy. While nautiluses allow gas to enter the gap between the mantle and the shell, others force out denser salt water from their kidneys.
Squids undergo negative buoyancy, constantly regulating their vertical position by jetting out water or undulating their bodies, thus expending considerable energy. Some species, like Ocythoe tuberculata and Haliphron atlanticus, possess true swim bladders for floating.
These mollusks employ chromatophores to perform two types of camouflage: mimicry and color matching. For example, Sepioteuthis sepioide mimics the color of the non-threatening herbivorous parrotfish to catch prey unalerted, whereas the octopus Thaumoctopus mimicus mimics multiple venomous animals to fend off their predators. On the other hand, in color matching, they develop specific color patterns to blend seamlessly with their backgrounds and dodge enemies’ eyes.
Cephalopods also change their body color to ensure conspecific, intraspecific, and interspecific communication. For example, the octopus Callistoctopus macropus puts on a striking bright red-brown color dotted with contrasting white to startle the attacker.
Cephalopods primarily reproduce sexually, with most species being semelparous (reproduce only once before dying). However, the vampire squid, the lesser Pacific striped octopus, and the nautilus are iteroparous (multiple reproductive cycles throughout their life).
The attainment of sexual maturity in cephalopods is marked by the enlargement of gonads and accessory sexual organs in both males and females. Most males develop a hectocotylus, an arm tip modified for transferring their spermatozoa into the female mantle cavity; however, some develop a secondary sexual organ, the spadix. Some deep-water species have a penis longer than their own body length, which they use to deposit a spermatophore on the female’s body.
Adult females are generally larger than males, with some species, like blanket octopus females, growing 10,000 to 40,000 times larger than the males.
In cephalopods like cuttlefish, females choose their potential mates using olfactory cues. They sometimes accept multiple males (polyandrous) and store their spermatophores either in the buccal cavity or internal sperm-storage receptacles. Storing spermatophores increases the chances of male-male competition, thereby engaging these animals in aggressive pre-mating behavior.
When a female is nearby, the males charge each other violently and flail their arms. If none of the males recede, they expose their mouths and bite the arm tips of the other with full force (biting). Another way of warding off competitors is flushing, in which water is forced into the buccal cavity of the females to flush out spermatophores placed by some other male.
Sometimes, the smaller males mimic the females, distracting the larger males, and quickly insert drop-like sperm in the female’s seminal receptacle.
The male grabs the female using his tentacles and wraps her tightly in his arms. He then contracts the mantle cavity and releases the spermatozoa in water. Since cephalopods mate multiple times, the males may increase these contractions and mate for longer durations to increase the chances of fertilization. The females release their eggs in the water column in clutches, covering each egg with a protective coat. Sometimes, these eggs release a sperm-attracting peptide to direct the spermatozoa to itself. In all cephalopods, except octopuses, the gametes are fertilized externally.
Different cephalopod species resort to different mating strategies. For instance, giant Pacific octopus females usually lay large eggs in a den and die shortly after, whereas the Japanese flying squid releases egg masses that float at the interface between water layers with different densities.
Cephalopods lay eggs (1 to 30 mm in diameter) with egg yolk on one pole and a disc of germinal cells on the other. The disc gradually spreads out and covers the entire yolk, giving rise to the embryo. As it grows, the tentacles and arms first appear at the posterior end of the body, later shirting to the anterior head. In no time, the funnel forms on the top of the head, while the mouth develops on its opposite. A shell develops from the ectoderm and is later mineralized.
Unlike most mollusks, cephalopods lack a distinct larval stage and develop directly into juveniles called paralarvae. They grow allometrically (proportional growth of all body parts) and become independent with time.
Cephalopods are hunted by sharks, bony fishes, sea birds, aquatic mammals, and other cephalopods.
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