11. ECOLOGY O F DISTRIBUTION
335
tropical waters while Euphausia americana replaces E. krohnii in tropical
regions. A further three species occur in restricted geographical areas ;
Nyctiphanes simplex has only been recorded from the Caribbean and
N . capensis from South Africa while Euphausia hanseni seems to be
2 5
FIG.
2 5
20
15
?5
c "
2 0
15
25
20
15
10
15
10
15
10
15
T,o-12
124. The relationship between the abundance of a species, surface temperature,
illumination. Relative abundance shown by
and temperature at the depth of
density of shading. (After Moore, 1952.)
limited to the sea areas close to the coasts of West Africa. Little can
as yet be said about the separate distributions of Thysanopoda aequalis
and T . subaequalis and of Nematoscelis microps and N . atlantica because
of the difficulties involved in separating these pairs of species and the
probability that the records in the literature are confused.