Topic: 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.
H.G. Wells was fascinated with the alien appearance of the octopus, and it is no coincidence that the aliens controlling the sinister tripods in ‘The War of the Worlds’ are based on these marine creatures. And why not? After all, what could be more different from us (as mammals and vertebrates) than the octopus? Not only marine, but with eight flexible arms and moving by jet propulsion… Yet despite these obvious differences, there are many striking similarities. Evolutionary convergence reveals that differences are often only skin-deep. Some of the most important of these convergences, relating to the octopus itself, or other close relatives such as the cuttlefish and squid (all of which belong to the cephalopod molluscs) are summarised below.
Camera eyes and vision
Undoubtedly the most famous cephalopod-vertebrate convergence is the camera eye. The eyes of most cephalopods are built in a manner very similar to ours, consisting of two chambers separated by a lens that projects an image onto the light-sensitive retina. In both cases, the eyeball is moved by extra-ocular muscles. The lens must by definition be transparent, which is achieved by crystallin proteins, one of the classic examples of molecular convergence. The lens acts, of course, to focus the light, but in a spherical lens there is the risk of spherical aberration, with the result that the image on the retina is no longer in focus. Both cephalopods and vertebrates have solved the problem in exactly the same way, changing the refractive index of the lens from the centre to the edge. Furthermore, the eye has a pupil that is capable of remarkably quick contraction. So too at least cuttlefish show a change in retinal sensitivity as they grow, as do vertebrates (but few other invertebrates). Despite these extraordinary similarities, there are some differences between vertebrate and cephalopod camera eyes. Whilst the former are outgrowths of the brain, the latter develop as invaginations of the skin, thus having an everted retina. Focussing occurs through a change in lens shape in most vertebrates, whereas the lens is moved relative to the eyeball in cephalopods. Camera eyes are not unique to cephalopods and vertebrates but also found in polychaete annelids, gastropod (snail) molluscs and, amazingly, in the cubozoan jellyfish.
Although cephalopods are highly adept at changing colour and blending in with their environment, most species do not possess colour vision (the only hitherto documented exception is the firefly squid Watasenia scintillans, which has three instead of only one visual pigment). However, some cephalopods are sensitive to polarised light, mediated by the orthogonal orientation of neighbouring photoreceptors. As common cuttlefish (Sepia officinalis) show a prominent polarisation pattern on different body parts that changes with an individual’s behaviour, it has been hypothesised that polarisation vision might be involved in intraspecific recognition and communication.
Octopuses have, of course, two eyes, but evidence now exists for lateral asymmetry of use – one eye is preferred to the other. This convergent phenomenon is also found in, for example, dolphins and birds and in some cases associated with sleep (which itself has evolved many times).
Lateral line system and balance
The camera eye is not the only cephalopod sensory system that shows convergence. Independently, cuttlefish, squid and octopuses have evolved a pressure-sensitive system highly reminiscent of the fish lateral line system. Head and arms bear similar lines of ciliated epidermal cells, which act as mechanoreceptors. Not much is known about functional similarities with the fish, but so close is the cephalopod lateral line analogue to that of fish, it almost certainly serves to detect predators and prey, localise objects and maybe even assist in schooling behaviour when vision is limited.
Also strikingly convergent are the balancing reflexes of cephalopods and vertebrates. Both are mediated by statoliths, small particles that bend the hairs of mechanosensory cells when the body changes position, and there are strong similarities in terms of nervous activity and related physiologies. Interestingly, statocysts, the receptacles that house the statoliths, seem to be involved in low-frequency hearing in cephalopods (which lack ears). The statocysts of squid and cuttlefish were furthermore shown to possess analogues of the semicircular canals of vertebrates that monitor rotational movements in these fast-moving predators. These structures are grooves rather than complete tubes, but cartilaginous projections limit the flow of endolymph in certain directions.
Cephalophods in general and the octopus in particular are famous for their intelligence, but that too is convergent and arises from a large brain that has evolved completely separately from that of mammals, yet again shows interesting similarities. Features of octopus intelligence include short-term memory (and also long-term potentiation, an enhancement in neuronal transmission that is regarded one of the major cellular mechanisms underlying memory), as well as learning (including associative, spatial and possibly observational capacities). Not surprisingly, therefore, we can study the psychology of octopus, with evidence for personality, including play. So far, the octopus is the only invertebrate shown to use simple tools. In addition to individuals deliberately placing stones, shells and other objects to form a wall constricting the aperture to their den, it has recently been documented that veined octopuses (Amphioctopus marginatus) use discarded coconut shells as a shelter, after having retrieved, manipulated, transported and reassembled them. It has been suggested that the predatory lifestyle of cephalopods might have been a driving force in the evolution of their intelligence, and their foraging techniques can be quite extraordinary indeed (e.g. octopuses have been observed to visit lobster traps and even climb aboard fishing boats).
Limbs and movement
The overall body shape of an octopus is most unlike that of a vertebrate, but there are actually many anatomical convergences. Despite their flexibility, when the arms grasp food, they show remarkable similarity to a human arm, being reconfigured into a stiffened structure consisting of three units that articulate via a series of “pseudo-joints”. Cephalopods normally swim by jet propulsion, a very fast, but energy-consuming mode of locomotion (that evolved independently several times). In squid, the most capable of mollusc swimmers, the muscular mantle consists of three distinct zones that differ in structure and metabolic activity. This differentiation is strongly reminiscent of the red and white muscle types of fish. Remarkably, octopuses can also stroll across the sea floor by bipedal locomotion (which is familiar not only from us, but also from birds and their presumed ancestors, the theropod dinosaurs, and even from some skates). Two of the eight arms are applied sucker-side down and unfurl along their length to provide a rolling locomotion that kinematically can be classified as walking.
In male octopuses, one of the arms (known as the hectocotylus) is modified for copulation and used to transfer a spermatophore to the oviduct of the female. In at least one species, the California two-spot octopus (Octopus bimaculoides), the arm tip is erectile, which is highly unusual among invertebrates, and structurally strongly convergent on mammalian penises. It has been suggested that the erectile tissue could reflect an evolutionary compromise – a large arm tip could probably transfer more sperm (thus potentially increasing reproductive success) but would be conspicuous and thus likely to attract predators. Incidentally, some squid show a nuptial dance, and have a number of other reproductive similarities with such fish as the anchovy.
What about the internal anatomy of cephalopods? Their circulatory system is closed and like in the vertebrates has a dual arrangement with separate supply to the respiratory organs versus the rest of the body. Blood leaving the heart travels along the aorta and this vessel shows striking similarities in its construction to that of mammals, including the employment of elastic proteins. The respiratory pigment is haemocyanin, which is copper-based and not bound to cells. This is a nice example of molecular convergence, having evolved independently of arthropod haemocyanin.
So next time you look at an octopus, remember that its camera eye looking back at you is only one of many examples of convergence.
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Map of Life - "Octopus and other cephalopods: convergence with vertebrates"
September 19, 2021