Category: Molluscs: Cephalopods

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The Cephalopoda are a group of molluscs characterised by large heads, muscular tentacles (modifications of the molluscan 'foot') and in most cases internalisation or loss of the typical molluscan shell. They move about in the oceans with the aid of fins, tentacles and jet-propulsion (water is taken into the gills and forced out through a fold in the soft mantle, termed the hyponome). Cephalopods include the octopuses, squid and cuttlefish, well known for their incredible colour-changing chromatophores and ability to squirt ink (which is convergent with gastropods such as the sea hare Aplysia ). Other important cephalopod groups include the pearly Nautilus, and the extinct Ammonoidea (including ammonites and goniatites), both of which have external, coiled shells.

Cephalopods are a rich source of insights into evolutionary convergence, perhaps the most celebrated example being the camera-eye, which includes also the pupil, correction for spherical aberration, retinal sensitivity, polarized vision, lateral asymmetry of eye use, and crystallin proteins. Other convergences in the sensory systems include the lateral line system and the oculomotor balancing reflex. The cephalopod brain shows interesting convergences, both in terms of anatomy (e.g. blood-brain barrier and cerebellum-like structures) as well as intelligence manifested in aspects of learning, memory, psychology, personality, social play and even a sort of tool use.

Complex behaviour among cephalopods includes lekking and use of a nuptial dance during courtship. Some cephalopods, notably cuttlefish and octopus, show highly accomplished mimicry and camouflage, based largely on changes in the colour and pattern of their chromatophores. Aspects of convergence in terms of anatomy range from the vertebrate-like structure of the cephalopod aorta to the possession of 'statolith' balancing organs and in one octopus (Ocythoe) even a fish-like swim-bladder. Regarding limbs and locomotion, the movement of octopus tentacles rather remarkably converges on the operation of a jointed limb even though there is effectively no anatomical similarity, and a number of small octopuses and the cuttlefish Metasepia pfefferi are known to engage in a form of bipedal locomotion.

Examples of convergence at the molecular level include the blood pigment haemocyanin, which is found in most molluscs and also arthropods, and elastic proteins in the aorta, which are reminiscent of those in the vertebrate heart. Powerful bioluminescence is employed by certain deep sea squid, and other squid have been found to possess bioluminescent 'reverse eyes', uniting them with many other marine and terrestrial animals that employ bioluminescence for signalling or vision.

The extinct ammonoids (ammonites and their predecessors such as goniatites) are well known for their coiled shell, and within the ammonoids we see interesting examples of convergence of shell shape. In addition to echoing patterns of change in shell shape in gastropods, a particularly important example of convergence among ammonoids concerns the pattern of morphospace re-occupation after a mass extinction.

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Topic Title Teaser text Availablity
Light producing chemicals: how to make bioluminescence

The most remarkable luciferin in terms of its distribution is known as coelenterazine. This nitrogen-ring based molecule is found in nine separate groups, ranging from radiolarians to fish.


Flying through the air on a summer's evening or sparkling in the ocean you may see magical flashes of light that signal some of nature's most enchanting creatures, those that are bioluminescent.

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Lateral line system in fish and other animals

Some cavefish are completely blind, so how do they manage to navigate through their environment with astonishing ease?

Dicyemids and chromidinids: enigmatic endoparasites

Dicyemids and chromidinids are tiny, worm-like or 'vermiform' creatures that typically live inside the kidneys ('renal organs') of cephalopod molluscs such as octopus, squid and cuttlefish.

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Ink production in cephalopods and gastropods

A series of striking convergences can be found in the sea-hares (Aplysia), a group of gastropods and only remotely related to the cephalopods.  Not only do they emit ink clouds (the colour is derived from ingested red algae), but they also employ chemical cues that assist in defense.

Gliding reptiles

In the reptiles, different forms of skin membrane (called ‘patagia’) and in some extinct species, primitive feathers, have evolved convergently as adaptations for gliding.

Hearts in cephalopods and vertebrates

There is a striking convergence between the aorta of the cephalopod and vertebrate heart, notably in its structure and the employment of elastic proteins.

Pinhole eyes in Nautilus and giant clam

The pinhole eye has evolved not only in the Pearly Nautilus, but also in another group of molluscs, the bivalves and specifically the giant clams (Tridacna).

Sleep in animals

Suffering from insomnia? Fruit flies do as well...

Haemocyanin in arthropods and molluscs

The degree of similarity between the active sites in arthropod and molluscan haemocyanin has been called “remarkable” and “startling”, but actually suggests that wherever in the universe life employs copper for aerobic respiration it will call upon haemocyanin.

Transparent tissues: eyes, bodies and reflective surfaces

Read on if you want to know about the numerous animal equivalents to the invisible man...

Play in birds, mammals and octopus

Social play is the hall-mark of the most intelligent of this planet’s species, and there is a particularly striking convergence between birds and mammals.

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“Colour vision” in Firefly squid

The Japanese firefly squid (Watasenia scintillans), which inhabits the deep ocean, has three visual pigments located in different parts of the retina that are likely to allow colour discrimination as they each have distinct spectral sensitivities.

Bipedal locomotion in vertebrates and cephalopods

In effect, two of octopus' arms are applied sucker-side down to the sea-floor and unfurl along their length to provide a rolling locomotion that kinematically can be classified as walking.

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Statoliths and balance in animals

An almost universal, but convergent, method to detect changes in orientation is for small grains (statoliths) to be attached to fine hairs, whose movement triggers nervous impulses.

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Retinal sensitivity changes in vertebrates and cuttlefish

In vertebrates the sensitivity of the retina changes during the growth of the animal. In invertebrates this only occurs in the cephalopods, or at least cuttlefish, where this sensitivity has been acquired convergently.

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Polarized light detection in arthropods, fish and cephalopods

In bees detection of polarized light from different quadrants of the sky is an important component in their navigation.

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Simple tool use in owls and cephalopods

Burrowing owls place pieces of collected dung. These attract insects such as beetles that are then eaten by the owls.

Personality in vertebrates and cephalopods

Personality in the vertebrates might, therefore, be deeply embedded in their phylogeny, although this does not rule out the convergent appearance of more complex personality traits in more advanced vertebrates, notably birds and mammals.

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Lekking in birds, fish, mammals and cephalopods

Complex interactions between males and females prior to mating have evolved independently many times. Amongst the most familiar examples are leks.

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Swim bladders of fish and the octopus Ocythoe

Swim bladders have evolved independently in fish and in Ocythoe octopus females.

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Octopus arm function

If you want to see a truly remarkable example of convergence, then present an octopus with a piece of food and have a high-speed camera ready…

Mimicry in insects and other arthropods

Defensive mimicry is usually Batesian, where an innocuous species adopts the colouration of a toxic species, but Mullerian mimicry is also known whereby one species, already toxic, converges on the colouration of a more common toxic species.

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Blood-brain barrier of vertebrates, cephalopods and other invertebrates

A very important component of complex brains is an effective barrier between the blood vascular system and the brain, both to regulate electrolyte balance (e.g. potassium) and exclude potentially harmful substances.

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Asymmetric eye use in octopus, dolphins and birds

In a number of cases one eye is used in preference to another. This convergent phenomenon is found in octopus (cephalopods), dolphins, birds, and other animals.

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Camouflage and mimicry in cephalopods

The most familiar examples of colour change in cephalopods are related to camouflage, but there are also striking examples of sexual and defensive (Batesian) mimicry.

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Sexual mimicry in mammals and cephalopods

Sexual mimicry is widespread, and the most famous example is probably the male-like genitalia in the females of the hyaena.

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Bioluminescent reverse eyes in squid

Normally one thinks of an eye as a structure that allows light to pour into the body, but in at least some squid (cephalopods) the opposite has been achieved.

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Learning and memory in vertebrates and cephalopods

Convergence in learning (and by implication memory) is important not only because it will give us clues as to the nature of consciousness, but it will also have a bearing on the reality (or otherwise) of pain and suffering in “primitive” animals.

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Cephalopod brains: convergence with vertebrates

Cephalopod brains show some striking convergences with those of vertebrates, including optic lobes and a cerebellum-like region.

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Ammonoids: fossil insights into convergence

Ammonoids, perhaps most familiar from the Mesozoic ammonites, are abundant as fossils and typically occur as planispiral forms. They show extensive homeomorphy, that is the same shapes repeatedly evolve.

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Camera eyes of cephalopods

The remarkable similarity between the camera eyes of cephalopods and vertebrates is one of the best-known examples of evolutionary convergence.

Reflective tissues

Other cephalopods achieve reflectivity by employing collagen fibrils, of which the deep-sea Vampyroteuthis is perhaps the most striking example.

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Ultraviolet (UV) absorption in vertebrates and cephalopods

In some vertebrates (fish, mammals) and cephalopods we find an interesting convergence whereby some of the incoming ultraviolet is screened out.

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Octopus and other cephalopods: convergence with vertebrates

What could be more different from us than the alien-like octopus? Hold on. Look it in the eye and think again.

Camera eyes in vertebrates, cephalopods and other animals

Camera eyes are superb optical devices, so it is not surprising that they have evolved several times. But why, of all animals, in the brainless jellyfish? Or for that matter in a slow-moving snail?