continuing our marine biology lesson: 2. The Blue Shark - Prionace glauca - is a slender, moderately-sized shark (up to 4m) of the open oceans. Named for its beautiful colouration, it is a very attractive and efficient killer. Its large eyes help it to see at depths of up to 350m as it hunts its prey of fish and squid. Widespread, it is found in all tropical, sub-tropical and temperate oceans. It can be dangerous, especially when excited or in a 'feeding frenzy' but it is not generally considered a threat to humans. The female bears live young, in litters of typically 25-50, sometimes many more.