The family
Cranchiidae comprises the approximately 60 species of
glass squid, also known as
cockatoo squid,
cranchiid,
cranch squid, or
bathyscaphoid squid. Cranchiid squid occur in surface and midwater depths of open oceans around the world. They range in
mantle length from to over , in the case of the
colossal squid. The common name, glass squid, derives from the
transparent nature of most species. Cranchiid squid spend much of their lives in partially sunlit shallow waters, where their transparency provides
camouflage. They are characterised by a swollen body and short arms, which bear two rows of
suckers or hooks. The third
arm pair is often enlarged. Many species are
bioluminescent organisms and possess light organs on the undersides of their
eyes, used to cancel their shadows. Eye morphology varies widely, ranging from large and circular to telescopic and stalked. A large, fluid-filled chamber containing
ammonia solution is used to aid
buoyancy. This buoyancy system is unique to the family and is the source of their common name "bathyscaphoid squid", after their resemblance to a
bathyscaphe. Often the only
organ that is visible through the transparent tissues is a cigar-shaped
digestive gland, which is the
cephalopod equivalent of a
mammalian liver. This is usually held in a vertical position to reduce its silhouette and a light organ is sometimes present on the lower tip to further minimise its appearance in the water.