Cuttlefish belong to the class which also includes squid and octopses. Despite their name, cuttlefish are not fish but molluscs. Recent studies indicate that cuttlefish are among the mostintellegent invertebrates. Cuttlefish also have one of the largest brain-to-body size ratios of all invertebrates. They have an internal shell (the cuttlebone), large W-shaped pupils, and eight arms and two tentacles furnished with denticulated suckers, with which they secure their prey. They generally range in size from 15 cm (5.9 in) to 25 cm (9.8 in), with largest species reaching 50 cm (20 in) in length and over 10.5 kg (23 lb) in weight. Cuttlefish eat small crabs, shrimp, fish and other cuttlefish. Their predators include dolphins, sharks, fish, seals and other cuttlefish. Their life expectancy is about one to two years.