Cephalopod Class: Species, Habitats, and Diets

Cephalopods are mollusks (Cephalopoda), a class which includes octopuses, squid, cuttlefish, and nautilus. These are ancient species that are found in all of the world's oceans, and are thought to have originated about 500 million years ago. They include some of the most intelligent creatures on the planet.

Description

Cephalopods are highly intelligent, highly mobile ocean-dwelling creatures that are remarkably diverse in size and lifestyle. All of them possess at least eight arms and a parrot-like beak. They have three hearts that circulate blue blood—cephalopod blood is copper-based, rather than iron-based like red-blooded humans. Some cephalopod species have tentacles with suckers for grabbing, camera-like eyes, color-changing skin, and complex learning behaviors. Most cephalopod eyes are quite like humans, with an iris, pupil, lens, and (in some) a cornea. The shape of the pupil is specific to species.

Cephalopods are intelligent, with relatively large brains. The largest is the giant squid (30 feet long and weighing 440 pounds); the smallest are the pygmy squid and California lilliput octopus (under 1/2 inch and 2/10 of an ounce). Most live only one to two years, with a maximum of five years, except for nautiluses which can live as long as 15 years.

Species

There are over 800 living species of cephalopods, loosely divided into two groups called clades: Nautiloidea (of which the only surviving species is the nautilus) and Coleoidea (squids, cuttlefish, octopuses, and the paper nautilus). The taxonomic structures are under debate.

• Nautiluses have a coiled shell, are slow-moving, and are only found in deep water; they have more than 90 arms.
• Squids are by and large torpedo-shaped, fast-moving, and have a thin, flexible internal shell called a pen. The pupils of their eyes are circular.
• Cuttlefish look and behave like squid but they have stouter bodies and a broad internal shell called a "cuttlebone." They navigate by undulating their body fins and live in the water column or on the sea floor. Cuttlefish pupils are shaped like the letter W.
• Octopuses live mostly in deep water, have no shell, and can swim or walk on two of their eight arms. Their pupils are rectangular.

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Types of Cephalopods

By Jennifer Kennedy

Habitat and Range

Cephalopods are found in all of the major water bodies in the world, primarily but not exclusively salt water. Most species live at depths between seven and 800 feet, but a few can survive at depths near 3,300 feet.

Some cephalopods migrate following their food sources, a characteristic that may well have allowed them to survive for millions of years. Some migrate vertically every day, spending most of the day in the dark depths hiding from predators and rising to the surface at night to hunt. 

Diet

Cephalopods are all carnivorous. Their diet varies depending on the species but can include everything from crustaceans to fish, bivalves, jellyfish, and even other cephalopods. They are hunters and scavengers and have several tools to assist them. They grasp and hold their prey with their arms and then break it into bite-sized pieces using their beaks; and they further process the food with a radula, a tongue-like form edged with teeth that scrapes the meat and pulls it into the cephalopod digestive tract.

Behavior

Many cephalopods, especially octopuses, are intelligent problem solvers and escape artists. To hide from their predators—or their prey—they can eject a cloud of ink, bury themselves in the sand, change color, or even make their skin bioluminesce, emit light like a firefly. Skin color changes are engineered by expanding or contracting pigment-filled bags in the skin called chromatophores.

Cephalopods move through the water in two ways. Traveling tail-first, they move by flapping their fins and arms. Traveling head first, they move by jet propulsion: muscles fill their mantle with water and then expel it in a burst that propels them forward. Squids are the fastest of any marine creature. Some species can move in bursts up to 26 feet per second, and in sustained migrations for up 1 foot per second.

Reproduction

Cephalopods have both male and female sexes, and mating usually includes a courtship often involving skin color changes, varying with the species. Some species of cephalopods gather together in great masses to mate. The male transfers a sperm packet to the female through her mantle opening via either a penis or a modified arm; the females are polyandrous, meaning they can be fertilized by multiple males. The females lay large yolky eggs in clusters on the ocean floor, creating 5 to 30 egg capsules with four to six embryos each.

In many species, males and females both die shortly after spawning. Octopus females, however, stop eating but live on to watch over their eggs, keeping them clean and protecting them from predators. Gestation periods can last for months, depending on species and conditions: one deep-sea octopus, Graneledone boreopacifica, has a gestation period of four and a half years.

Identifying the young of different cephalopod species is difficult. Some juvenile cephalopods swim freely and feed on "marine snow" (bits of food fragments in the water column) until they mature, while others are adept predators at birth. 

Conservation Status

There are 686 species listed in the class Cephalopoda in the International Union for Conservation of Nature (IUCN) Red List. One species is listed as Critically Endangered (Opisthoteuthis chathamensis), two are Endangered (O. mero and Cirroctopus hochbergi), two are Vulnerable (O. calypso and O. massyae) and one is Near Threatened (Giant Australian Cuttlefish, Sepia apama). Of the rest, 304 are Least Concern and 376 are Data Deficient. The Opisthoeuthis genus of octopus live in the most shallow waters of the oceans, and they are the species which is most threatened by commercial deep-water trawling. 

Cephalopods reproduce rapidly and over-fishing is not typically a problem. Nacre from the nautilus is prized in the United States and elsewhere, and although nautiluses are not listed in the IUCN Red List, they have been protected under the Convention of International Trade in Endangered Species (CITES) since 2016. 

Sources

• Bartol, Ian K., et al. "Swimming Dynamics and Propulsive Efficiency of Squids Throughout Ontogeny." Integrative and Comparative Biology 48.6 (2008): 720–33. Print.
• "Cephalapoda - Class." IUCN Red List.
• "Cephalopoda Cuvier 1797." Encyclopedia of Life, 2010.
• Hall, Danielle. "Cephalopods." Ocean. Smithsonian Institution, 2018.
• Vendetti, Jann. "The Cephalopoda: Squids, octopuses, nautilus, and ammonites." Lophotrochozoa: Mollusca, University of California at Berkeley, 2006.
• Young, Richard E., Michael Vecchione, and Katharina M. Mangold. "Cephalopoda Cuvier 1797 Octopods, squids, nautiluses, etc." Tree of Life, 2019.
• Wood, James B. The Cephalopod Page, University of Hawaii, 2019.