324 THE BIOLOGY OF MARINE ANIMALS 



arthropods, there is a marine nudibranch, Elysia viridis, which shows this 

 response clearly. In a horizontal beam of light it responds by oriented 

 movements and crawls in a straight line. When the position of the light is 

 moved, the sea-slug responds by moving in a new direction, and by repeat- 

 ing this procedure it can be shown that the direction of locomotion pur- 

 sued after each shift of the light source bears a constant relation to the 

 direction of the light. This relationship is expressed in the angle between 

 the longitudinal axis of the body and a line extending from the animal's 

 eye to the light source. Each animal maintains its orientation angle con- 

 stant, within certain limits which lie between 45° and 135°. The latter 

 limitation has been explained on the basis of the structure of the eyes, which 

 are pigmented cup-shaped ocelli provided with a lens, and so situated 

 that only light falling within an angle of about 35 c to 130° can reach the 

 retina. Light-compass reactions are also reported in other marine ani- 

 mals — periwinkles, crabs and polychaetes. This type of response enables an 

 animal to use a light source as a sensory guide for orienting itself while 

 foraging over a wide radius, and releases it from stereotyped progression 

 confined to the path of the light rays (49, 113a). 



Vertical Migration. Many planktonic animals are known to make regular 

 vertical migrations through the water column each day. The phenomenon 

 has been observed in many groups: tintinnids, siphonophores, polychaetes, 

 pteropods, crustaceans, chaetognaths, appendicularians, fish, etc. Migra- 

 tory planktonic species usually occur at some considerable depth below 

 the surface during the day, and in the evening they migrate towards the 

 surface from the day-depth (Fig. 8.15). Many oceanic species, however, do 

 not necessarily reach the surface but merely rise to a higher level. Sub- 

 sequent temporal phasing involves: a departure from the surface at or 

 before midnight; a return to the surface just before dawn; and a sharp 

 descent to the day-depth when the sunlight starts to penetrate the water. 

 The subject is conveniently reviewed by Russell (126) and Cushing (34). 



The depth range is sometimes very great, for example, up to 400 m 

 or more for species of pelagic decapod crustaceans in the North Atlantic. 

 Different species inhabit different depths during the day, but even in one 

 species-population the day-depth is variable, depending on age, season, 

 weather conditions, etc. 



The major determining factor in diurnal migration is light intensity. The 

 depth to which light penetrates changes continuously during the day and 

 from day to day, and it has proved possible to correlate changes in depth- 

 distribution of a species with these fluctuations of light intensity. The daily 

 ascent from the day-depth takes place during falling light intensity. At 

 great depths, where the amount of daylight is always low, the ascent may 

 begin early, in some cases even at midday. Descent in the morning occurs 

 during increasing light intensity. The midnight sinking noticed in many 

 species is seemingly due to a passive condition induced by total darkness. 

 The short rise at dawn represents a return by the animals to the mean 

 optimal light intensity for the population. 



