[Paleopsych] SW: On the Optical Structure of Animal Eyes
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Evolution: On the Optical Structure of Animal Eyes
http://scienceweek.com/2005/sw050805-2.htm
The following points are made by Michael F. Land (Current Biology 2005
15:R319):
1) The ability to respond to light is common to many forms of life,
but eyes themselves -- structures that break up environmental light
according to its direction of origin -- are only found in animals. At
its simplest, an eye might consist of a small number of
light-responsive receptors in a pigmented pit, which shadows some
receptors from light in one direction, and others from a different
direction. This definition distinguishes an eye from an organ with a
single photoreceptor cell, which may indeed be directional because of
screening pigment, but which does not allow for spatial vision -- the
simultaneous comparison of light intensities in different directions
[1]. An alternative starting point for an eye would be for each
receptor to have its own pigmented tube, the assemblage forming a
convex cushion. In these two proto-eye structures we have the
beginnings of the two mutually exclusive ways of building an eye: the
single-chambered range of eyes, often misleadingly called "simple",
and the compound eyes.
2) Although no eyes survive in fossils from the Precambrian (more than
550 million years ago) it seems certain that eyes like these were
present from early in the evolution of the Bilateria [2], long before
the Cambrian explosion. Simple pit eyes are still present in
flatworms, annelid worms, and molluscs, and in many larval eyes.
Proto-compound eyes occur in ark clams and some tube-dwelling
polychaetes, where they act as detectors of moving predators. Genetic,
developmental, and morphological evidence indicates that from the
earliest times eyes had access to two different photoreceptor types:
ciliary receptors, in which the photosensitive pigment is displayed on
outgrowths of cilia, and rhabdomeric receptors, in which the expanded
pigment-containing membrane consists of microvilli. The two receptor
types use different transducer cascades, and their opsins -- the
protein component of the photopigments -- are also different. Ciliary
receptors are typical of deuterostomes (echinoderms and chordates) and
rhabdomeric receptors of the protostomes (annelids, molluscs and
arthropods), but both types can be found in both lineages. The
development of cerebral eyes in both of these lineages has been
associated with the Pax-6 control gene, evidently from early in
bilaterian evolution.
3) In the Cambrian period, carnivory became important as a way of life
and both predators and prey needed better vision. During the hundred
million years from about 550 millions years ago, compound and then
single-chambered eyes increased greatly in size, in their ability to
resolve, and in optical sophistication. One way to improve the
performance of a single-chambered proto-eye is to make the eye bigger
and the aperture smaller, so that it becomes a genuine pinhole eye.
This is a far from ideal solution, because the small aperture lets in
little light, and so makes for a very insensitive eye, and increasing
the aperture diameter drastically reduces the ability of the eye to
resolve. For reasons that remain obscure, this design has been
retained in the quite large (1 cm) eyes of the cephalopod Nautilus,
even though its relatives (octopus and squid) have eyes with excellent
lenses. Giant clams also have small pinhole eyes around their mantles,
which do allow them to detect the presence of browsing fish.
4) A much better solution is to provide the eye with a lens, usually
spherical in marine animals as a sphere provides the shortest focal
length for a structure of a given diameter, and hence the most compact
design. Such a structure might be made of protein, or some other
substance with a refractive index higher than that of water.
Refraction at each surface would bend rays and produce an image behind
the lens. There is, however, a serious problem with a lens of this
kind. Rays striking the outer regions of the lens are bent too much,
so that they are focussed much closer to the lens than rays nearer to
the lens center. This defect is known as spherical aberration, and in
a spherical lens this is so severe that the image would be effectively
unusable. The solution (attributed to James Clerk Maxwell) is for the
lens to have a gradient of refractive index, highest in the center and
falling to close to that of water in the periphery [3]. Peripheral
rays are then bent much less, and overall the focal length of the eye
becomes much shorter -- about 2.5 lens radii as opposed to 4 radii for
a homogeneous lens. This makes for a lens that resolves well, and has
a very high light-gathering power --an F-number of 1.25.[4,5]
References (abridged):
1. Land, M.F. and Nilsson, D.-E. (2002). Animal Eyes. Oxford
University Press
2. Arendt, D. and Wittbrodt, J. (2001). Reconstructing the eyes of
Urbilateria. Philos. Trans. R. Soc. Lond. B Biol. Sci. 356, 1545-1563
3. Jagger, W.S. (1992). The optics of the spherical fish lens. Vision
Res. 32, 1271-1284
4. Land, M.F. (1984). Crustacea. In: Ali, M.A. (Ed.), Photoreception
and Vision in Invertebrates.. (1984). Plenum, New York
5. Kröger, R.H.H., Campbell, M.C.W., Fernald, R.D., and Wagner, H.-J.
(1999). Multifocal lenses compensate for chromatic defocus in
vertebrate eyes. J. Comp. Physiol. [A] 184, 361-369
Current Biology http://www.current-biology.com
--------------------------------
Related Material:
ON THE EVOLUTION OF THE EYE
The following points are made by E.J.W. Barrington (citation below):
1) Two main types of highly differentiated photoreceptor system have
appeared in the invertebrates: the compound eyes of arthropods and the
camera-type eyes of cephalopods. Enough is known of the mode of
functioning of these, and of their probable past history, to show that
they represent the evolution, along two very different lines, of
organs that have some striking points of similarity with the
vertebrate eye, not only in their pigments but also in certain details
of their structural organization. Indeed, this is an aspect of animal
organization which is of considerable significance -- a convergence
resulting from the widespread distribution of a common biochemical
ground plan. In this instance the common feature is, of course, the
nature of the photosensitive pigments.
2) Simple types of eyes are seen in the free-living Platyhelminthes
and in the Annelida, where they are often composed of sensory cells
associated with screening pigment cells. In their simplest form they
may be no more than pigment spots, forming part of the general
epithelium, but more usually they sink inwards to form cups. In the
Turbellaria the pigment cells are often arranged to form the wall of
an open bowl, the bipolar receptor cells projecting into this through
its aperture. In such an eye there can be no possibility of forming an
image, for there is no refractive structure. These organs are
doubtless restricted to the differentiation of light and darkness, and
in this way they make it possible for the animal to orientate itself
with respect both to the intensity and to the source of the
illumination. The distal ends of the receptor cells are differentiated
to form a rod border, in which longitudinal striations can be seen
with the light microscope...
3) Cup-like arrangements of pigment cells are common in the eyes of
polychaetes, but a higher level of differentiation is reached in this
group. Not only do the receptor cells themselves have a rod like tip,
but the epithelium of the cup may produce secretions that fuse to form
one or more lenses. Moreover, groups of sensory cells may be closely
collected together to form ommatidia, recalling the unit structures of
the compound eye of arthropods. Indeed, in sabellids (Branchiomma, for
example) the ommatidia themselves may be grouped together to form a
rudimentary type of compound eye. No doubt a similar tendency played
an important part in the ancestors of arthropods, contributing to the
establishment of their characteristic compound eyes. Convergence was
probably involved in the process of arthropodization, so much so that
it is necessary to envisage the possibility of an independent
evolution of compound eyes in more than one line. The situation in
annelids goes some way to make the possibility of the independent
evolution of compound eyes acceptable, although it does not reveal the
actual ancestry of these organs.
Adapted from: E.J.W. Barrington: Invertebrate Structure and Function.
Nelson 1967, p.282.
--------------------------------
Related Material:
ON THE MAMMALIAN CLOCK-EYE
T. Roenneberg and M. Merrow (University of Munich, DE) discuss
mammalian clocks, the authors making the following points:
1) Even without time cues from the environment, physiological events,
from gene expression to behavior, recur with a high regularity but not
necessarily in a precise 24 hour rhythm --hence the term "circadian"
("about one day"). Circadian rhythms are controlled by endogenous
"clocks" which are synchronized, or "entrained", to the 24-hour day
predominantly by light [1]. The central circadian pacemaker in mammals
resides above the optic chiasm in the suprachiasmatic nuclei (SCN). It
has long been known that light entrainment in mammals requires the
eyes, but it was unclear through which photoreceptor the signal was
processed. It came as a surprise that the circadian clock remains
perfectly entrainable by light in mutant mice devoid of rods and cones
[2].
2) Researchers are racing to identify the novel receptor in the
mammalian retina. Its spectral characteristics have been defined in
mice and, more recently, in humans [3,4]. In addition to its role in
entrainment, the novel photoreceptor is responsible for several other
non-visual light responses, such as melatonin suppression, pupillary
constriction and direct effects of light on motor-activity
("masking"), or for many other "vegetative" light effects, for example
on cortisol levels or heart rate. Hankins and Lucas (5) have taken our
understanding of this novel light input pathway a step further,
showing that its influence is already apparent in the primary steps of
intra-retinal signal processing.
3) The authors discuss vision vs. irradiation detection. Vision
capitalizes on photons, using rods or cones as "pixels" to create a
retinal image that is processed in the thalamus and the cortex. While
a memory of these "pictures" may be stored in the brain, the retinal
picture itself has to be renewable within milliseconds for instant
detection of any changes. Visual processing thus requires both fast
kinetics and high spatial resolution. In contrast, a detector for the
assessment of day and night should not care about a flash of lightning
or the shadow of a flying object. Its task is to integrate photons
over a long time. This integration mechanism is partially responsible
for the difficulties that shift workers have in adjusting their
biological clocks to socially enforced schedules -- the competition
between indoor and outdoor light cannot be won. A worker who is
exposed to 500 lux over an eight-hour night shift collects a similar
quantity of photons waiting 15 minutes for the bus, even on a cloudy
day. As a result, the circadian system remains entrained to the "real"
day -- it cannot adjust to the implemented night shift, so workers try
to be active and alert when their physiology is tuned to sleep. In
fact, workers on night shifts, with most of the rest of the day free
to spend outdoors, may collect more day light than their non-shifting
colleagues. The invention of artificial light has ironically created a
biological shadow world because we spend more time indoors.
4) In summary: Light is the most reliable environmental signal for
adjusting biological clocks to the 24-hour day. Mammals receive this
signal exclusively through the eyes, but not just via rods and cones.
New evidence has been uncovered for a novel photoreceptor that may be
responsible for more than just adjusting the clock.
References (abridged):
1. Roenneberg T. and Foster R.G. (1997) Twilight Times-light and the
circadian system. Photochem. Photobiol., 66:549-561
2. Freedman M.S., Lucas R.J., Soni B., von Schantz M., Munoz M.,
David-Gray Z.K. and Foster R. (1999) Non-rod, non-cone ocular
photoreceptors regulate the mammalian circadian behavior. Science,
284:502-504
3. Brainard G.C., Hanifin J.P., Greeson J.M., Byrne B., Glickman G.,
Gerner E. and Rolag M.D. (2001) Action spectrum for melatonin
regulation in humans: evidence for a novel circadian photoreceptor. J.
Neurosci., 21:6405-6412
4. Thapan K., Arendt J. and Skene D.J. (2001) An action spectrum for
melatonin suppression: evidence for a novel non-rod, non-cone
photoreceptor system in humans. J. Physiol., 535:261-267
5. Hankins, M.W. and Lucas, R.J. (2002). A novel photopigment in the
human retina regulates the activity of primary visual pathways
according to long-term light exposure. (in press)
Current Biology 2002 12:R163
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