Cephalopods – Squids, Octopuses, Nautilus, and Ammonites


Cephalopods are the most intelligent, most mobile, and the largest of all molluscs. Squid, octopuses, cuttlefish, the chambered nautilus, and their relatives display remarkable diversity in size and lifestyle with adaptations for predation, locomotion, disguise, and communication.


These “brainy” invertebrates have evolved suckered tentacles, camera-like eyes, color-changing skin, and complex learning behavior. Their lengthy evolutionary history spans an impressive 500 million years and the abundant fossils they’ve left behind (mostly shelled nautiloids and ammonoids) record repeated speciation and extinction events. From myths about their enigmatic fossilized remains to fantastic accounts of tentacled sea monsters, cephalopods also figure prominently in the literature and folklore of human societies around the world.

Today, biologists and paleontologists continue to captivate the human mind and imagination with details of these molluscs’ behavior, natural history, and evolution.

Some of the most innovatively adapted invertebrates belong to class Cephalopoda, Greek for “head-footed.” Over 700 species of cephalopods have been identified, and they are divided into the subclasses Coleoidea (cuttlefish, squid, and octopus) and Nautiloidea (nautiluses).

Cephalopods inhabit all of the world’s oceans and occur at a wide range of depths, from ocean bottoms (benthic species) to open waters (pelagic species). They are also diverse in size, ranging from the centimeters-long Californian octopus (Octopus micropyrsus) to giant squid (Architeuthis), some of which measure over 18 meters long.

Although cephalopod species have many differences among them, they all share several common features:

CHANGING COLOR: Cephalopods have an amazing ability to change color very rapidly. They accomplish this feat using numerous pigment-filled bags, called chromatophores. Chromatophores are found in the skin, and expand and contract to reveal or conceal small dots of color (left). They can be so densely concentrated that 200 may be found in a patch of skin the size of a pencil eraser! Additionally, an iridescent dermal tissue can also be manipulated by some cephalopods to aid in camouflage, courtship rituals, or accompany color changes.

Some pelagic squids possess an additional color-changing structure; the light organ. A pair of these light organs is located within the mantle cavity on the underside of the squid. Each contains a crypt and a lens. A crypt is a small sac that houses luminous bacteria and a lens is a complex stack of tiny reflecting plates that controls the luminescence of this bacteria. Light from the bacteria projects downward and the squid can manipulate its intensity to match any light coming from above. This masks the squid’s own silhouette, protecting it from potential predators.

THE BRAIN: Finally, one of the most intriguing aspects of cephalopods is their intelligence. With a centralized brain, the largest of all invertebrates, and highly developed eyes and other sense organs, they are able to remember and learn by example or through trial and error. Cephalopods have the largest brains of any invertebrate, and species of octopus, squid, and cuttlefish are capable of learning and retaining information.

HABITIAT: All cephalopods inhabit marine environments. There are no freshwater cephalopod species.

DIET: Cephalopods are strictly carnivorous. They all possess a hard beak used for defense and tearing prey. Most species hunt for prey, while some are scavengers.

LIFE SPAN: Generally, cephalopods grow quickly and have short life spans. Most live from one to two years, with the exception of the nautilus, which may live more than 15 years.

BODY PLAN: The basic cephalopod body plan consists of a body, head, and foot. A muscular bag called the mantle contains the cephalopod’s organs. The mantle has assumed many of the protective functions served by a shell in other mollusks. The cephalopod head contains the brain and sense organs. The foot, consisting of the grasping appendages, is fixed to the head, (hence “head-footed.”)

APPENDAGES: Cephalopods are famous for the gangly limbs encircling their mouths. Octopuses, squid, and cuttlefish possess cone-shaped limbs studded with rows of suckers and referred to as arms. In addition, squid and cuttlefish brandish a pair of tentacles—long, elastic structures that dart out beyond the arms and are used exclusively for prey capture.

THREE HEARTS AND BLUE BLOOD: Cephalopods usually have three hearts. Two hearts pump blood to the gills, and one central heart pumps oxygenated blood to the body. Cephalopod blood is blue because it binds oxygen using a blue, copper-containing protein called hemocyanin. Human blood is red because the oxygen-binding protein hemoglobin contains iron.

INK RELEASE: Most cephalopods are endowed with a small ink-producing gland within an ink sac, embedded in their digestive systems. When threatened by predators, the sac expels highly concentrated melanin to obscure the animal from predators.

SKIN: Cephalopod skin can take on wild patterns, colors, and shapes. Tens of thousands of organs called chromatophores form color patterns on the skin similar to the way pixels form images on a computer screen. The chromatophores are controlled by nerves, allowing color change to occur instantaneously.

JET PROPULSION: When they feel the need for speed, cephalopods propel themselves through the water using jet propulsion. Other methods of swimming involve fins or propulsion with the arms and web.

References and resources: UC BERKELY EDU, Smithsonian National Zoo

Related Articles & Information: Octopus did you know, Blue Ring Octopus

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September 7, 2016 |

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