A common or European cuttlefish, Sepia officinalis, trapped in monofilament bottom set nets, Brixham Harbour. Each year nets are set around seagrass beds (Zostera marina) where cuttlefish come in to breed. Image No. MBI000305.

Intelligent life. Few divers who have ever encountered an octopus or cuttlefish underwater cannot have been struck by a sense of an alien intelligence staring hard at them, assessing whether you are friend or foe and what you are likely to do next. That cephalopods (octopi, cuttlefish, squid and their kin) are bright has now become common knowledge; yet this is still something that sits uneasily with their molluscan nature. Great apes, whales and dolphins are easier to accept; they are mammals and so not that distantly related to us. But cephalopods? They are not even vertebrates; first cousins to slugs, snails and slipper limpets. Their blood is greeny-blue not red as the oxygen carrying molecule is copper-based rather than the iron-based haemoglobin in all mammals. They really are shape-shifting aliens from inner space. And yet, when a cuttlefish rises out of the sand or seagrass to hover in front of you, you get a real sense of cogs turning and a logical decision-making process taking place. That may, of course simply be down their binocular vision; we tend to almost intuitively associate this with intelligence (it is designed, after all, to assess distance, form mental 3D images of the World and judge when and how to strike at prey). That there is real intelligence behind those eyes is most elegantly demonstrated by the octopus, which has joined the elite group of animals that have demonstrated the use of tools to manipulate their environment in the wild. The veined octopus has recently been filmed collecting and stacking discarded coconut shells halves to use as a shelter. So far it is the only invertebrate to do so.

Cuttlefish breeding. The seagrass beds of South Devon have long been a favourite dive habitat of mine. They provide gentle, sunlit dives where one can float along in the hope of encountering a pipefish, mating sea hares and, at the right time of year, cuttlefish arriving to lay eggs. Watching a female cuttlefish lay eggs is a fascinating experience. Each pointed black egg is attached to a tuft of seagrass or weed one at a time. The female will hover before it, then after a few minutes contemplation will move forward to firmly grasp the stem with her tentacles and pull it towards her. After a minute she draws back again, to reveal a new shiny, pointed black eggs bound to the stem by a band extending out of the egg case. This process with continue for over an hour, until the stem is wrapped in what resembles a bunch of black, pointed grapes.

A female common or European cuttlefish, Sepia officinalis, trapped in monofilament bottom set nets, Brixham Harbour. A male hovers nearby and will eventually also be entangled. Other males hover in the background. Image No. MBI000270.

While the eggs are laid a gang of males hover in attendance. When cuttlefish mate the male transfers the spermatophores (sperm packages) to within the females buccal cavity, using his modified fourth arm (the hectcotylus). The hectocotylus is then used to break open the spermatophores, releasing the sperm which is then temporarily stored within the female’s buccal cavity. If she subsequently mates with another male it will direct jets of water into her buccal cavity to attempt to flush out earlier sperm deposits. Consequently the most recent mate will hover above her, warding off other males that will hang around, to ensure that it is his genetic material that is passed on rather than that of his upstart rivals.

Cuttlefish in nets. Cuttlefish are, of course, a valuable catch for fishermen. They are caught in trawls out at sea and also in set nets deployed around their inshore breeding grounds. Cuttlefish grow quickly and most die shortly after breeding, so those caught after they have laid their eggs will not have any real effect on future populations. Unfortunately however, nets are often laid around the edges of breeding grounds such as seagrass nets, thus cuttlefish are caught not only when leaving the breeding ground but also when arriving, before they breed. The nets do not kill them, but they are often quite badly damaged as they twist and the fine line cuts into their flesh. Indeed I often wonder how they can be sold after their flesh is so ripped up. If a female is caught then inevitably a number of males arriving will hang around her until they too are caught. A trapped female may remain their for up to twelve hours before the nets are hauled, so the potential for catching large numbers of breeding cuttlefish is quite high. So far, this practice does not seem to have noticeably affected local cuttlefish populations. It is nonetheless rather disconcerting to watch such lovely animals twisting and turning for hours on end.

Update. In the past couple of years I, and other local divers, have seen notably fewer cuttefish hanging around in the shallow bays of South Devon during the breeding season.  This may simply be due to factors like the lousy weather we’ve had during the past couple of summers; it may also be due to the almost impenetrable ring of nets set on the edge of these bays.

Images. All the images in this blog are available to license.  You can search all my online stock images at my www.colinmunroimages.com  Cuttlefish images, Sepia officinalis images, fishing images, stock images.