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J.C. Montgomery and J.A. Macdonald
Both the gross anatomy of the eye and the fine structure and
biochemistry of the retina may be altered to enhance visual sensitivity.
Mesopelagic fishes have relatively large eyes, although relative eye size
decreases for fish inhabiting depths below the level to which daylight
penetrates (> 1000 m). By comparison, Antarctic fishes have a relative
eyesize (eye diameter as a proportion of head length) roughly equivalent to
that found in shallow water teleosts (Fig. 3). A number of deep-sea fishes
also have a tubular eye, which essentially allows a large eye in a small
head. Also common in deep-sea fishes is a distinct gap between the lens
and the iris, the so-called aphakic space. The aphakic space allows light
from obliquely placed point sources to be picked up by the whole of the
lens thereby increasing the intensity of the retinal image (without an
aphakic space the iris shields part of the lens). An aphakic space improves
the detection of weak punctate bioluminescent sources, but can impair
vision in the presence of a diffuse background illumination. A comparable
aphakic space has not been reported in Antarctic fishes. The relatively
small space seen on the anterior margin of the lens provides for lens
movement during accommodation (Fig. 2B).
Pure rod retinas are not uncommon in deep-sea fish, but are unrecorded
for Antarctic fishes. In his comprehensive study of the retinae of
notothenioids, Eastman [13] showed that only Dissostichus mawsoni had a
rod-dominated retina. In D. mawsoni, the ratio of rods to cones is 57:1,
whereas other notothenioids had much lower ratios (e.g. T bernacchii 2: 1,
T hansoni 7: 1). Rod outer segments in Pleuragramma antarcticum are the
longest recorded [13] at about 44 !-lm, which is an order of magnitude less
than the exceptionally long rods (400-600 /-lm) described in the retina of
Diretmus [14). Multiple rod banks, not seen in the notothenioids, are
another obvious low light adaptation in deep-sea fishes, with for example
up to 28 layers of rods in the fovea of Bajacalifornia drakei [15]. The
convergence ratio of receptor cells to ganglion cells also correlates with
light sensitivity. Of the Antarctic fish, D. mawsoni again shows the highest
convergence ratio (58:1), much less than the convergence ratios of about
600:1 reported for a number of deep-sea species (orange roughy 615:1,
black oreo dory 597:1, hoki 700:1, hake 623:1) [16). Finally at the level of
the fine structure of the retina, Antarctic fishes also lack the tapetum
lucidum typical of deep-sea fishes [13].
In the deep-sea, the absorption spectrum of the visual pigments matches
the spectrum of available light. The only information available in Antarctic
fishes is an electroretinogram study [11] which shows the fish to have an
action spectrum with "max of 482-528 nm which brackets the wavelength of
maximum light penetration (515 nm) during the Antarctic spring.
J.C. Montgomery and J.A. Macdonald
Both the gross anatomy of the eye and the fine structure and
biochemistry of the retina may be altered to enhance visual sensitivity.
Mesopelagic fishes have relatively large eyes, although relative eye size
decreases for fish inhabiting depths below the level to which daylight
penetrates (> 1000 m). By comparison, Antarctic fishes have a relative
eyesize (eye diameter as a proportion of head length) roughly equivalent to
that found in shallow water teleosts (Fig. 3). A number of deep-sea fishes
also have a tubular eye, which essentially allows a large eye in a small
head. Also common in deep-sea fishes is a distinct gap between the lens
and the iris, the so-called aphakic space. The aphakic space allows light
from obliquely placed point sources to be picked up by the whole of the
lens thereby increasing the intensity of the retinal image (without an
aphakic space the iris shields part of the lens). An aphakic space improves
the detection of weak punctate bioluminescent sources, but can impair
vision in the presence of a diffuse background illumination. A comparable
aphakic space has not been reported in Antarctic fishes. The relatively
small space seen on the anterior margin of the lens provides for lens
movement during accommodation (Fig. 2B).
Pure rod retinas are not uncommon in deep-sea fish, but are unrecorded
for Antarctic fishes. In his comprehensive study of the retinae of
notothenioids, Eastman [13] showed that only Dissostichus mawsoni had a
rod-dominated retina. In D. mawsoni, the ratio of rods to cones is 57:1,
whereas other notothenioids had much lower ratios (e.g. T bernacchii 2: 1,
T hansoni 7: 1). Rod outer segments in Pleuragramma antarcticum are the
longest recorded [13] at about 44 !-lm, which is an order of magnitude less
than the exceptionally long rods (400-600 /-lm) described in the retina of
Diretmus [14). Multiple rod banks, not seen in the notothenioids, are
another obvious low light adaptation in deep-sea fishes, with for example
up to 28 layers of rods in the fovea of Bajacalifornia drakei [15]. The
convergence ratio of receptor cells to ganglion cells also correlates with
light sensitivity. Of the Antarctic fish, D. mawsoni again shows the highest
convergence ratio (58:1), much less than the convergence ratios of about
600:1 reported for a number of deep-sea species (orange roughy 615:1,
black oreo dory 597:1, hoki 700:1, hake 623:1) [16). Finally at the level of
the fine structure of the retina, Antarctic fishes also lack the tapetum
lucidum typical of deep-sea fishes [13].
In the deep-sea, the absorption spectrum of the visual pigments matches
the spectrum of available light. The only information available in Antarctic
fishes is an electroretinogram study [11] which shows the fish to have an
action spectrum with "max of 482-528 nm which brackets the wavelength of
maximum light penetration (515 nm) during the Antarctic spring.