240
THE BIOLOGY OB EUPHAUSIIDS
is a t 4 9 0 m p Astaxanthin, however, is only bleached by very long
exposure to light and so would appear to be of little use to the animal
in a visual function. On the other hand, astaxanthin does respond to
light and dark by moving to and from the bases of the rhabdoms and
its absorption maxima are not very far from those of any ambient light
in the environment.
Chun (1896) studied the eyes of several species and, of course,
observed that many deep sea species have bilobed eyes. He considered
that the upper lobes, with their small numbers of large ommatidia, were
capable of perceiving movement of other animals in front of the
euphausiid but without any very definite image being formed. The
lower lobe, on the other hand, with its large number of small ommatidia,
would probably produce a much more detailed image. Further, the
field of vision of the lower lobe would be illuminated by the photophore
in the eye stalk. Consequently, Chun put forward the theory that deep
sea euphausiids with bilobed eyes lead a predacious existence and,
moreover, he pointed out that no deep sea Crustacea which live on the
ocean floor show any evidence of bilobed eyes, an adaptation of
significance only to pelagic organisms. If, however, reference is made
to Table I11 in which the mesopelagic and bathypelagic species of
euphausiids are listed, evidence of a contradiction to Chun’s hypothesis
is found. The eight Thysanopoda species occurring in this list do not
have divided eyes, yet they are pelagic and live a t deep levels in the
oceans. Most of the other species do have distinctly divided eyes but
it is probable that the detailed visual characteristics of a severely
divided eye, like that of Stylocheiron suhmii, differ from those of an
eye like that of Nematoscelis dificilis which has two lobes of more or
less equal size. Nobody has examined the possibility of bifocal vision
in the upper eyes of species such as Stylocheiron suhmii. The ommatidia
in the upper eyes are extremely few in number (Fig. 80), are almost
parallel with each other and with those of the other upper eye in the
animal and so could probably be used t o determine the range
(distance from the eyes) of a prey organism.
The ommatidia of the upper and lower lobes of species of Thysanoessa that have bilobed eyes are of equal size, and males have larger eyes,
relative to body size, than females ; the upper lobe is smaller than the
lower lobe in these species and in species of Stylocheiron and Tessarabrachion, is larger than the lower lobe in species of Nematobrachion, and
varies in relative size in species of Nematoscelis (Nemoto, 1966).
As pointed out previously, bilobed eyes are only possessed by species
which have elongated thoracic limbs which are probably used to grasp
prey. It is therefore tempting, and probably correct, to assume that
THE BIOLOGY OB EUPHAUSIIDS
is a t 4 9 0 m p Astaxanthin, however, is only bleached by very long
exposure to light and so would appear to be of little use to the animal
in a visual function. On the other hand, astaxanthin does respond to
light and dark by moving to and from the bases of the rhabdoms and
its absorption maxima are not very far from those of any ambient light
in the environment.
Chun (1896) studied the eyes of several species and, of course,
observed that many deep sea species have bilobed eyes. He considered
that the upper lobes, with their small numbers of large ommatidia, were
capable of perceiving movement of other animals in front of the
euphausiid but without any very definite image being formed. The
lower lobe, on the other hand, with its large number of small ommatidia,
would probably produce a much more detailed image. Further, the
field of vision of the lower lobe would be illuminated by the photophore
in the eye stalk. Consequently, Chun put forward the theory that deep
sea euphausiids with bilobed eyes lead a predacious existence and,
moreover, he pointed out that no deep sea Crustacea which live on the
ocean floor show any evidence of bilobed eyes, an adaptation of
significance only to pelagic organisms. If, however, reference is made
to Table I11 in which the mesopelagic and bathypelagic species of
euphausiids are listed, evidence of a contradiction to Chun’s hypothesis
is found. The eight Thysanopoda species occurring in this list do not
have divided eyes, yet they are pelagic and live a t deep levels in the
oceans. Most of the other species do have distinctly divided eyes but
it is probable that the detailed visual characteristics of a severely
divided eye, like that of Stylocheiron suhmii, differ from those of an
eye like that of Nematoscelis dificilis which has two lobes of more or
less equal size. Nobody has examined the possibility of bifocal vision
in the upper eyes of species such as Stylocheiron suhmii. The ommatidia
in the upper eyes are extremely few in number (Fig. 80), are almost
parallel with each other and with those of the other upper eye in the
animal and so could probably be used t o determine the range
(distance from the eyes) of a prey organism.
The ommatidia of the upper and lower lobes of species of Thysanoessa that have bilobed eyes are of equal size, and males have larger eyes,
relative to body size, than females ; the upper lobe is smaller than the
lower lobe in these species and in species of Stylocheiron and Tessarabrachion, is larger than the lower lobe in species of Nematobrachion, and
varies in relative size in species of Nematoscelis (Nemoto, 1966).
As pointed out previously, bilobed eyes are only possessed by species
which have elongated thoracic limbs which are probably used to grasp
prey. It is therefore tempting, and probably correct, to assume that