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The most distictive feature of decapods is the modification of the fourth pair of arms into tentacles (see title photographs). In some species these are long slender structures that are several times the length of the body and probably function much like a fishing line. In some others, they are short, muscular structures that can be quickly extended (literally shot forward) to grasp prey then completely retracted into pockets in the head.
The Decapodiformes includes species with a wide range of body forms and habits. Muscular sepioids bury in the sand during the day; gelatinous squids swim via slow jet propulsion in the deep-sea while in the same habitat some squids with weak mantle muscles use large muscular fins for swimming; surface-dwelling squid with powerful jet propulsion may undertake migrations of a thousand miles and, when disturbed, dart from the water and glide over the ocean surface like a flying fish. In size they range from Idiosepius with a mantle length of 8 mm at maturity to the giant squid Architeuthis which reaches nearly 18 m in total length. Cephalopods are primarily visual animals even though they possess lateral line analogs, very low frequency hearing and olfaction. The dominance of the visual system is particularly true of decapods. For example, the eye diameter of the small Iridoteuthis is over 40% of the mantle length and large squids have eyes that surpass the eye-size of all other animals big or small. Among decapods, the oegopsid squids characteristically produce small pelagic eggs and the paralarvae develop in the near-surface plankton whereas myopsids and sepioids have benthic eggs that often produce large demersal hatchlings.
The suckers of all decapods have horny rings. These rings often carry sharp claw-like teeth and in some species these have been modified into hooks. The advantages of hooks are uncertain. They could be designed for sinking into soft-bodied prey, such as other squid, or they could act as grappling hooks for hard-body spiny prey where suckers would be ineffective (Young, et al., 1999).
Figure. Oral-lateral view of a portion of the tentacular club of a preserved Ancistrocheirus lesueurii showing hooks with sheaths. Photograph by R. Young.