Sunday, July 25, 2010
July 25, 2010 : Glass Squid
The family Cranchiidae comprises the approximately 60 species of glass squid, also known as cranchiid or cranch squid. Cranchiid squid occur in surface and midwater depths of open oceans around the world. They range in mantle length from 10 centimetres (3.9 in) to over 3 metres (9.8 ft), 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 shadow. 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. 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.
Like most squid, the juveniles of cranchiid squid live in surface waters, descending to deeper waters as they mature. Some species live over two kilometres below sea level. The body shape of many species changes drastically between growth stages and many young examples could be confused for different species altogether.