The Dive Shack Tides: Creature Feature: Cuttlefish

Ngerchong Inside dive site (Palau)
Photo courtesy of Liz Tuttle

Often called the "chameleon of the sea" because of its ability to change the color of its skin at will, the cuttlefish is actually not a fish at all, but really a mollusk. Belonging to the class Cephalopoda (which means head-footed), they are the cousin of the squid, octopus, and nautilus.

The cuttlefish can range in size from 1 inch to 3 feet (and even bigger in the case of the giant Australian cuttlefish which may grow to the size of a small man). It possesses eight sucker-lined arms and two prehensile tentacles (which can be withdrawn into pouches under the eyes) and a sharp, beak-like mouth which allows it to cut open flesh like a pair of scissors, so it can use its tentacles to tear out meat. It hunts during daylight, feeding primarily on small fish, crustaceans and other mollusks.

It moves about in the water by way of a complex propulsion and bouyancy system (much like a submarine). "The cuttlefish propels itself using a series of spurts, drawing water into a compression chamber which it squeezes to jet the water out a funnel under the head. Direction changes can be made by swivelling the nozzle of this funnel, and narrowing the funnel controls speed. Like a submarine, the cuttlefish fills tiny compartments in its cuttlebone with gas to help maintain neutral buoyancy. This helps the cephalopod hover above the ocean floor, because although it has a sophisticated propulsion system its large cuttlebone does not allow it to be overly active, or quick in the water. " [1]

Staying Neutral

Redrawn after Clarkson, E.,
Invertebrate Palaeontology and Evolution,
George Allen and Unwin, London,
1979 (Seventh impression 1984), p.167.

The cuttlefish is a bottom-dweller which often lies in ambush for smaller animals. For this way of life, it needs to keep itself at neutral buoyancy, so that it neither sinks nor rises...in order to be able to operate at varying depths and water densities, cuttlefish need to be able to adjust their overall density so as to always remain ‘neutral’ in the water. The cuttlefish does this by an ingenious mechanism. The bony shell actually has many narrow chambers. If these were all filled with gas, they would give a lift of up to 4% of the animal’s weight. However, they are only part-filled with gas—the darker areas shown are where it is part-filled with liquid. The cuttlefish is able to pump liquid in and out of that section as needed to keep the buoyancy ‘just right’. [1]

The cuttlefish actually has 3 hearts - one for each set of gills and one for the remaining body. Unlike our own blood which gets its red color from the pigment hemoglobin, the cuttlefish's heart pumps blue-green blood which uses hemocyanin pigment to carry oxygen. Its skin is comprised of 3 layers of color pigment cells called chromatophores. These cells are controlled by the nervous system which signals them to expand or contract at a remarkable speed allowing for the hypnotic transformation of the cuttlefish's coloration and pattern.

"A strange fact about the cuttlefish is that it can adopt the female coloration along one side of the body as well as retain the male coloration on the other side. Male cuttlefish are extremely alert when it comes to the females, especially if she is laying the eggs. The male will guard aggressively over the female and this is when another approaching male will use his ability to have a dual look to access the female and trick the male." [2]

Close-up of a cuttlefish eye
Photo courtesy of Wikipedia

Oddly enough, even though it can put on a spectacular color display, the cuttlefish does not see color. Its w-shaped pupil only perceives polarized light which enhances its perception of contrast. Their retina has two areas of concentrated sensor cells called foveae, one to look forward and one to look more backward. There is no blind spot like we have because the optic nerve is positioned behind the retina. The eye changes focus by reshaping the entire eye, instead of reshaping the lens as in mammals.

Cuttlefish are considered to be highly intelligent, like their cousin the octopus. In fact, they have one of the largest brain-to-body size ratios of all invertebrates. Cuttlefish have ink, like squid and octopuses, which they use as a last resort to help evade predators when their camoflague fails. Their predators include dolphins, sharks, fish, seals, seabirds and other cuttlefish. Their life expectancy is about one to two years.

They are mostly shallow-water animals which inhabit tropical to temperate waters, although they have been know to descend to depths of 2,000 feet. Cuttlefish are generally found along the coasts of east and south Asia, western Europe, the Mediterranean, as well as all coasts of Africa and Australia.