78 PELAGIC FAUNA 



which under other conditions are colourless may be a vivid red both in 

 the cold ocean abyss and in surface water near the poles. 



Many animals live mainly in the upper portion of the bathypelagic 

 region, where there is still faint light. Probably the temperature, rather 

 than the light of the surface, keeps them at these lower depths. In the 

 tropical and subtropical belts there is a rather sharp dividing line between 

 the warm surface water and the cold deep water. The boundary is marked 

 by the isotherm of 10°, which at the Equator is at a depth of about 250 

 metres, and at the North and South Tropics about 500 metres. Above 

 this level there is a rapid rise in temperature and below it a rapid drop, 

 resulting in a "discontinuity layer". Many pelagic animals collect round 

 this level, making it a good hunting-ground for swift-moving predacious 

 animals such as fish, squids, and prawns. Many of the small creatures rise 

 nearer the surface at night and are followed by their hunters. 



Even at the lowest depths it is never totally dark. Luminescent animals 

 live at all levels, and especially at deep levels where there is no other 

 light. Most deep-sea creatures are either dead or dying when brought 

 up from the cold deep through the warm surface water, especially in 

 the Tropics, and so we do not know how many of them are luminescent. 

 There will certainly be many more than we know which can radiate light 

 from some part of their body. Numerous bathypelagic fish, cephalopods, 

 and crustaceans have special light organs, often of a highly complex 

 structure. There are many variations, but in the most highly developed a 

 luminous substance is secreted in glandular cells; the light is reflected by 

 a cup-shaped reflector with a hood of dark pigment, and is emitted through 

 a lens. In some it can be dimmed. Light organs may be sited in the oddest 

 of places and their purpose is controversial. The stalked lantern borne by 

 angler-fishes on their nose doubtless acts as a lure to prey; the distinctive 

 patterns of luminiscent dots and spots of many fishes and cephalopods may 

 serve to identify the sexes or help to keep the shoal together; some animals 

 may illuminate their surroundings in order to find food. But in many cases 

 the value of luminescence is a mystery, and particularly so in the case of 

 the many luminescent creatures of the surface water, nearly all of which 

 belong to the lowest orders and never in any way seek their prey but take 

 what they chance to encounter. Many riddles remain to be solved in 

 connection with the evolution of light organs and eyes in deep-sea animals. 

 The Galathea Expedition, which fished at much greater depths than 

 any of its predecessors, should provide many valuable clues when the mate- 

 rial has been fully studied. 



Nearly every group of pelagic fauna found in the surface water is also 



