If brains are the crown of creation then the crown of the Mollusc Phylum goes to the Cephalopods. They can’t match human intelligence, but for invertebrates, an octopus is about as clever as it gets. There are about 700 known species of these fascinating animals, all marine. Cephalopods have never conquered freshwater or the land.
The cephalopods show enough molluscan characteristics to be included in this phylum but have modified many to suit a lifestyle quite different from other molluscs, especially the shelled varieties. The Nautilus shows us where the cephalopods started, complete with a complicated and beautiful shell. But the Nautilus already leads a free swimming existance and is a predator and its tentacles come out of its head, like the rest of the class. It is not hard to see how the squid and octopus have gone the next steps to internalise or shed the shell and use speed, stealth, camouflage and intelligence to survive without it.
In cephalopods, the head and foot become merged during development and cannot be separated. That is why they get the name ‘head-foot’. There is a ring called the circumoral ring surrounding the head which is probably derived from an ancient posterior appendage and from it the tentacles develop. This is the equivalent I think, of having our legs and arms coming out of our ears. Nautilus can have up to 90 tentacles, sepioids (squid) have ten and the octopoids of course have eight.
This ‘head-foot’ is what makes cephalopods so different from other molluscs, as Meglitsch (p. 344) explains: “Cephalopod evolution has resulted in the complete shifting of the functional axes of the body. With the elongation of the visceral hump, and with the development of the hyponome from the posterior end of the foot, the primitive ventral surface was changed to the functional anterior end… In discussing the cephalopods, functional rather than morphological axes will be used: head and tentacles will be considered anterior and the funnel ventral. In other words, what looks like the ‘head’ to us is in a real sense the tail of these strange animals.
The Nautilus is different enough from the other cephalopods to occupy its own subclass, the Tetrabranchiata, so named because of having two pairs of gills instead of the only one, as seen in the other subclass, the Dibranchiata, which are the ockies and squid. On shell type, paleontologists divide the Class into three groups: Nautiloidea, Ammonoidea and Coleoidea. There are many more extinct cephalopods than living species. The Ammonoids once dominated the ancient seas but now they are all gone. Only the Nautilus continues to sail the seas, the last of the shelled cephalopods.
Nautilus has some other unique characteristics. It has a muscular dorsal hood which can be used to close off the animal in its shell in times of danger, much like the operculum of other shelled molluscs. It has large eyes which look out from its living chamber. Behind the current living chamber are all the smaller and smaller rooms, separated by perforated partitions, in which the younger animal spent its time. The empty chambers can be filled with gas to help with buoyancy. In order to swim, the Nautilus forces water out of the mantle cavity by retracting its body and contracting its funnel muscles.
The first nautiloids had a straight conical shell, much like the first gastropods, but later species have first slightly curved and then more tightlly coiled shells. Meanwhile the octopi were getting rid of their shells altogether while the squid were reducing it to a kind of ‘backbone’ for support, in the form of the familiar cuttlebones that we find washed up on the shore after storms.
All cephalopods are carnivores, catching food with their tentacles and then passing it to the hard beak in the mouth for crushing. Some ambush or trap their prey but most actively hunt. Shell reduction in squid and octopi has certainly increased their mobility. They swim by shooting out jets of water through the ventral funnel. The funnel can point both forward and back in squids so they can swim in both directions. Squids use fins on the sides of the mantel as well for hovering or for rapid swimming. These fins also act as stabilisers, so cuttlefish can be very graceful and acrobatic swimmers. Octopi can swim but usually crawl around on the sea floor, saving jet propulsion for dangerous situations.
Squid and octopi use chromatophores in their mantle skin to change colours, for camouflage and sexual and social signalling. They are the masters of the art form, producing rapid color changes by expanding and contracting various chromatorphores, each of which contains a single pigment of yellow, red, brown or blue. Some blink rapidly giving the effect of a rapidly changing neon sign. They can also be used to perfectly match the background for camouflage. I have snorkelled a lot over the barrier reef and only once spotted an octopus because he moved at just the right moment. He was pink and white in a perfect match for the surrounding plate corals and I had to really look to keep seeing him as I moved around.
There is of course much more that can be said about these amazing animals. Their ability to learn to negotiate mazes, their interesting sex lives and their cleverness in zoo and laboratory situations. I hope that other people will write about them also and fill the gaps that I have left. More information can also be found on the websites below.
Reference: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press. http://www.thecephalopodpage.org/ http://www.cephbase.utmb.edu/ http://animaldiversity.ummz.umich.edu/site/accounts/information/Cephalopoda.html