The Design of Compound Eyes and the Illumination of Natural Habitats
207
animals can spot prey floating above, or against which they themselves can be seen
from below. Not surprisingly, this peculiar light habitat has led to very specialized
eye designs, especially among the compound eyes.
In the dim blue depths of the sea one can find compound eyes of tremendous
sensitivity, a fact that applies not only to superposition eyes, but to apposition eyes
as well. A very sensitive superposition eye is that of the deep sea shrimp Oplophorus
(Land 1976, 1981), an animal which lives at a depth of about 500 m. With an Fnumber of0.38, a reflective tapetum, and enormous rhabdoms (32 x 100 J..lm), these
eyes attain a very high sensitivity of3300 J..lm 2 sr, one of the largest values known in
the animal kingdom. Unfortunately, they probably cannot distinguish point light
sources closer than 15° apart (Land 197 6), but presumably they do not need to either.
Even greater sensitivity can be found, amazingly, in an apposition eye. The deep sea
isopod Ciro/ana has apposition eyes possessing 60 ommatidia of giant proportions
(Nilsson and Nilsson 1981 ). Huge corneal lenses (150 J..lm wide) of short focal length
(1 00 J..lm), and gigantic rhabdoms (90 x 90 J..lm) sitting in cups of reflective pigment,
endow Ciro/ana with an incredible sensitivity of 5091 J..lm 2 sr. As in Oplophorus this
high sensitivity comes only at the cost of resolution: in Cirolana, the ommatidia
have visual fields of about 45°, and are separated from one another by an angle of
between 10° and 20°. But for the scavenging life style that Cirolana leads, sensitivity
is probably of greater importance. Indeed, its close relative A ega has 300 ommatidia
and is an active hunter, a life style that probably demands better visual acuity (Nilsson
andNilsson 1981).
The darkness of the deep sea has not been the only driving force in the evolution
of aquatic vision. The fact that almost all available daylight comes directly from
above has also been a major force. To catch as much of the downwelling light as
possible, and to enhance the discrimination of any small object that might be
silhouetted against it, many aquatic eyes have evolved narrow, dorsally directed
visual fields. This is particularly obvious in compound eyes.
The large apposition eyes of hyperiid amphipods are an excellent example, as
shown in the lovely study of Land ( 1989b ). These highly mobile crustaceans can be
found at all depths down to 1 000 m, with different species being adapted for life at
different depths (and thus different light intensities). Their eyes reflect this adaptation
too. Near the surface the position of the sun determines the direction from which
most light is incident, but due to underwater scattering, animals just beneath the
surface see light from other directions as well. This all-round visual world is clearly
reflected in the eyes of surface living species like Thamneus. This amphipod has
eyes of quite uniform construction (Fig. 1 OA), with rather even (but somewhat low)
anatomical resolution in all directions of the visual field (Fig. lOB). A species
inhabiting a depth of around 200 m experiences a light environment quite different
to that of Thamneus. Compared to the surface, the visual world is no longer allaround: daylight is 1000 times dimmer and incident almost entirely from above (ie.
from within ca. ± 50° of vertical: Jerlov 1976). The eyes of amphipods living at
these depths, like Parapronoe (Fig. 1 OC), differ accordingly. In Parapronoe the dorsal
part of eye has become greatly enlarged, with enormous corneal facets adapted to
the dim downwelling light (122 J..lm wide compared to 35 J..lm in Thamneus). In
207
animals can spot prey floating above, or against which they themselves can be seen
from below. Not surprisingly, this peculiar light habitat has led to very specialized
eye designs, especially among the compound eyes.
In the dim blue depths of the sea one can find compound eyes of tremendous
sensitivity, a fact that applies not only to superposition eyes, but to apposition eyes
as well. A very sensitive superposition eye is that of the deep sea shrimp Oplophorus
(Land 1976, 1981), an animal which lives at a depth of about 500 m. With an Fnumber of0.38, a reflective tapetum, and enormous rhabdoms (32 x 100 J..lm), these
eyes attain a very high sensitivity of3300 J..lm 2 sr, one of the largest values known in
the animal kingdom. Unfortunately, they probably cannot distinguish point light
sources closer than 15° apart (Land 197 6), but presumably they do not need to either.
Even greater sensitivity can be found, amazingly, in an apposition eye. The deep sea
isopod Ciro/ana has apposition eyes possessing 60 ommatidia of giant proportions
(Nilsson and Nilsson 1981 ). Huge corneal lenses (150 J..lm wide) of short focal length
(1 00 J..lm), and gigantic rhabdoms (90 x 90 J..lm) sitting in cups of reflective pigment,
endow Ciro/ana with an incredible sensitivity of 5091 J..lm 2 sr. As in Oplophorus this
high sensitivity comes only at the cost of resolution: in Cirolana, the ommatidia
have visual fields of about 45°, and are separated from one another by an angle of
between 10° and 20°. But for the scavenging life style that Cirolana leads, sensitivity
is probably of greater importance. Indeed, its close relative A ega has 300 ommatidia
and is an active hunter, a life style that probably demands better visual acuity (Nilsson
andNilsson 1981).
The darkness of the deep sea has not been the only driving force in the evolution
of aquatic vision. The fact that almost all available daylight comes directly from
above has also been a major force. To catch as much of the downwelling light as
possible, and to enhance the discrimination of any small object that might be
silhouetted against it, many aquatic eyes have evolved narrow, dorsally directed
visual fields. This is particularly obvious in compound eyes.
The large apposition eyes of hyperiid amphipods are an excellent example, as
shown in the lovely study of Land ( 1989b ). These highly mobile crustaceans can be
found at all depths down to 1 000 m, with different species being adapted for life at
different depths (and thus different light intensities). Their eyes reflect this adaptation
too. Near the surface the position of the sun determines the direction from which
most light is incident, but due to underwater scattering, animals just beneath the
surface see light from other directions as well. This all-round visual world is clearly
reflected in the eyes of surface living species like Thamneus. This amphipod has
eyes of quite uniform construction (Fig. 1 OA), with rather even (but somewhat low)
anatomical resolution in all directions of the visual field (Fig. lOB). A species
inhabiting a depth of around 200 m experiences a light environment quite different
to that of Thamneus. Compared to the surface, the visual world is no longer allaround: daylight is 1000 times dimmer and incident almost entirely from above (ie.
from within ca. ± 50° of vertical: Jerlov 1976). The eyes of amphipods living at
these depths, like Parapronoe (Fig. 1 OC), differ accordingly. In Parapronoe the dorsal
part of eye has become greatly enlarged, with enormous corneal facets adapted to
the dim downwelling light (122 J..lm wide compared to 35 J..lm in Thamneus). In