Cuttlefish

Cuttlefish are cephalopods that are perhaps best known for their subtle movement and ability to change their color to blend into almost any surrounding. The cuttlefish skin possesses up to 200 chromatophores (pigment cells) per square millimeter. In addition, the cuttlefish has one of the largest brain-to-body size ratios of any invertebrate making it a most formidable predator.

Anatomy
Cuttlefish are soft-bodied Mollusks with short, flattened bodies and a large head. Cuttlefish skin is soft, and slides easily back and forth over a mantle of muscle that is attached to the internal cuttlefish bone. It is covered with chromatophores or pigment sacks which allow the cuttlefish to change color for camouflage purposes, mating rituals, and to show emotions. Cuttlefish have eight sucker-lined tentacles that are attached to the head in a ring around the mouth. They also have skin flaps along each side which shield two retractable feeding tentacles that are longer than the rest and are flattened at the end. On the stomach there is an opening cavity that contains the gills and openings for the gut, kidneys, and sexual organs. There is a siphon just under the head which helps the cuttlefish to steer and swim. Cuttlefish have large eyes with a W-shaped lens and a transparent cover and lid. The body is edged by a thin frill-like fin which circles horizontally around the body. Cuttlefish usually grow to between 5 and 30 centimeters long, but the giant cuttlefish grow much bigger. Giant cuttlefish are only found in the waters off southern Australia and they produce cuttlefish bones up to 1 meter long. Cuttlefish are usually seen in large numbers near the shore in winter when they gather on the shallow reefs to mate and spawn.

Reproduction
Fertilization is internal. Mating takes place when the male places his spermatophore in a pouch under the female's mouth. The capsules burst, releasing sperm into the female's mantle, thus fertilizing the eggs. After mating a female will lay about 200 golfball-size eggs among crevices in the reef, which hatch into miniature adults several months later. The eggs develop in 2-3 centimeter egg masses which are often attended by the female until the young emerges. Depending on temperature, the hatchlings are about 50 mm long excluding tentacles. As they mature, the egg case becomes thinner and more translucent. Hatchlings use the sharp end of their cuttlebone to back out of the egg case where they immediately change from white to a yellow-brown.

Ecology
Its highly specialized array of color-changing structures allow it to blend itself perfectly into the background. This lets it sneak up on prey and shoot out its two long tentacles at lightning speed, which have suckers on the widened pads at the tips. Once captured, its prey is paralyzed by poisonous saliva or crushed by the strong beak. When feeding, the cuttlefish often blows water over the sand in order to expose small crabs or shrimp. It uses these suckers to grab a hold of its prey and bring it back towards its beak. Cuttlefish eat crustaceans and fish. Cuttlefishes primary predators are large fish like sharks, monkfish, and swordfish. 

Movement
Cuttlefish have fins that can move very differently than that of other fish's fins. They move by moving their back fins in a wave-like motion and the cuttlefish is able to move slowly backwards or forwards. They glide smoothly through the water and can vary their depth easily. The internalised shell, the cuttlebone, controls their buoyancy. By filling this porous " backbone" with gas or releasing gas, the cuttlefish can regulate its buoyancy.