Cuttlefish are from the same family as Octopodes and Squid, belonging to the class Cephalopoda. They are cephalods (meaning “head-footed”), as have only two distinct body parts: the soft body and then 5 pairs of tentacles/arms. Despite the name, Cuttlefish are in fact molluscs rather than fish.
Globally, the Molluscs are found in both tropical and temperate climates, almost everywhere in the world except for the coasts of the Americas (it is thought they were perhaps unable to cross the cold Atlantic to colonize there). They are able to tolerate depths up to 600m, but generally prefer to reside close to the seabed in lagoons and warm shallow waters. We find Cuttlefish on all of our reefs,
Cuttlefish are thought to be highly intelligent, with one of the largest brain to body ratios of all invertebrates. Their internal shell (referred to as the cuttlebone) is comprised of aragonite and has a number of chambers which are used to control buoyancy. They regulate the ratio of liquid and gas within the chambers in order to maintain a density close to that of the sea water, a method not dissimilar to that submarines use. The Cuttlebone is commonly used as parakeet food (due to its high calcium content) and by jewelers to cast small objects.
Cuttlefish can also change their colour and texture, using tiny elastic sacs on the skin. Each sac contains pigments that can expand or retract in response to the environment. Adults tend to use this to creep up on prey, whereas juveniles can hide from predators.
Their eyesight is also highly developed and used in both hunting and protection from predation. The eyes will be easily noticeable to a diver – large with a strangely shaped pupil providing 360degree vision. Interestingly though, although colour plays an important role in both camouflage and courtship, they are unable to detect colour.
One of the most fascinating anatomical features of the Cuttlefish is their triple circulatory system. Unlike humans, which use the iron-based molecule haemoglobin to transport oxygen within the blood, Cuttlefish use hemocyanin, a copper-based carrier. Hemocyanin is less efficient than haemoglobin and causes the fish to have blue blood. In order to compensate for this, they have three hearts, one for each gill and another to pump blood to round the rest of the body.
Like Octopi, Cuttlefish have ink, which is used as their primary defence. mechanism. Ink is squirted if the fish is approached by a predator, creating a noxious black cloud. While this distracts and disorientates a predator, the Cuttlefish uses a jet propulsion to flee the area, by ejecting water through a siphon within the body cavity. Historically the ink has been used as dye called sepia, but this practice widely has been replaced by the use of artificial dyes.
Cuttlefish reach sexual maturity at 13-18 months, signalling the end of the solitary first phase of their lives. They begin to collect in groups where the males use rhythmic displays of courtship behaviour to impress females. Once the female is satisfied, the males use a specialised tentacle for mating, which deposits sperm sac in an opening close to the mouth. Even at this point however, intrasexual competition prevails and the fertilising male must guard the female, as competing males attempt to use jet propulsion to flush out the sperm sac. It is possible for a female to mate with twice and therefore have two sacs; when this occurs she is able to choose which to use to fertilise her eggs.
Each female will spawn, on average, between 150-4000 eggs, each in its own 1-2cm egg case. The eggs are covered in ink for camouflage and take residence on grass/weeds/shells/rocks. One week before hatching, the egg sac will clear and thin, providing a window to the external environment. The babies will begin learning at this stage, for example by observing small shrimps that become prey immediately after they hatch.
Cuttlefish hunt in a number of stages. Firstly the camouflage is used to sneak up on prey within suitable proximity to strike. Then they shoot their tentacles to grab the prey, using the suckers for extra grip. In spite of a lack of hard tissue, Cuttlefish have a surprisingly sharp beak which will bite the prey. Finally, whatever is caught is torn up using a Radula (a rasp-like structure) and swallowed.