containing ammonium ions and occupying about two-thirds the volume of the animal, allowing it to float at the required depth. About half of the 28 families of squid use this mechanism to solve their buoyancy issues. Largest and smallest The majority of squid are no more than long, although the giant squid may reach . The smallest species are probably the benthic pygmy squids Idiosepius, which grow to a mantle length of , and have short bodies and stubby arms. In 1978, sharp, curved claws on the suction cups of squid tentacles cut up the rubber coating on the hull of the USS Stein. The size suggested the largest squid known at the time. In 2003, a large specimen of an abundant but poorly understood species, Mesonychoteuthis hamiltoni (the colossal squid), was discovered. This species may grow to in length, making it the largest invertebrate. In February 2007, a New Zealand fishing vessel caught the largest squid ever documented, weighing and measuring around off the coast of Antarctica. Dissection showed that the eyes, used to detect prey in the deep Southern Ocean, exceeded the size of footballs; these may be among the largest eyes ever to exist in the animal kingdom. Development The eggs of squid are large for a mollusc, containing a large amount of yolk to nourish the embryo as it develops directly, without an intervening veliger larval stage. The embryo grows as a disc of cells on top of the yolk. During the gastrulation stage, the margins of the disc grow to surround the yolk, forming a yolk sac, which eventually forms part of the animal's gut. The dorsal side of the disc grows upwards and forms the embryo, with a shell gland on its dorsal surface, gills, mantle and eyes. The arms and funnel develop as part of the foot on the ventral side of the disc. The arms later migrate upwards, coming to form a ring around the funnel and mouth. The yolk is gradually absorbed as the embryo grows. Some juvenile squid live higher in the water column than do adults. Squids tend to be short-lived; Loligo for example lives from one to three years according to species, typically dying soon after spawning. In a well-studied bioluminescent species, the Hawaiian bobtail squid, a special light organ in the squid's mantle is rapidly colonized with Aliivibrio fischeri bacteria within hours of hatching. This light-organ colonization