BEHAVIOUR 



when plankton is there all right, and the disturbance is there, wc hardly 

 get any sparklers. Sometimes at sea the light will flash in rhythm with the 

 ship's propeller — understandable as vibrations are carried so well in water, 

 but more inexplicably it is reported to respond to the ship's radar, dis- 

 appearing when the radar is switched off as if something in the transmission 

 stimulates the organisms to light up. At other times a flash of light can be 

 seen to move across the water at a speed of lOO knots, reminiscent of the 

 streamers of the aurora, and possibly stimulated by electrical or magnetic 

 disturbances in the earth. 



What are photophores, and how does the hght work? There are various 

 differences in detail, but mostly each consists of a ball with a silvery re- 

 flector behind and a transparent lens in front so that the maximum efficiency 

 is obtained — and it is efficient too, only about lO per cent of the light energy 

 is lost in heat. Some photophores work by a chemical reaction which 

 results in the production of light and this seems to be at the control of the 

 owner. Others are cells containing luminous bacteria and some of these 

 can be controlled by a shutter like an eyelid. Some squids can change the 

 colour of their lights by covering them with a red skin. 



Most photophores point downwards (Plate XXXI) and the reason is 

 not really understood. Photophores, as distinct from a general all-over 

 phosphorescence, seem most common amongst those organisms that live in 

 the middle depths and migrate towards the surface after dark (see below). 

 One plausible reason is that one creature below another would see it as a black 

 silhouette against the slightly illuminated blue background above. Bluish 

 photophores under the body would tend to neutralize this effect, but if 

 photophores were on the back, then the aninial would show up against the 

 black background of the depths. This is, in effect, the same type of adaptation 

 that makes a fish silvery underneath but dark on top. 



The second form of swimming behaviour to be mentioned in this chapter 

 is that of 'diurnal migration' where so many species move towards the 

 surface at night and go down again to deeper layers in the daytime. There 

 seems to be no doubt at all that this is related to the light intensity, even 

 though such a large proportion of the species that do so are sightless in the 

 accepted sense. Nevertheless, they are very sensitive to light and cannot 

 tolerate an excess. In order to keep at their own preferred light intensity, 

 they remain submerged during the day, but as sunset approaches they start 

 to climb upwards reaching the surface layers in the twilight. In total dark- 

 ness there is no directional stimulus so they become scattered, only to re- 

 assemble near the surface as dawn approaches. As the light increases down 

 they go again (Fig. 37). The copepod Calmnis will daily climb and sink 

 again through some 300 feet. Although only about \ inch long, it can climb 



L 155 



