This document discusses the evolution of photoreception and vision. It describes the different types of photoreceptor cells and eyes that evolved in lower invertebrates including pigmented epithelia in corals, eye cups in flatworms, and lensed eyes in cubomedusae jellyfish. It then focuses on the advanced camera eyes that evolved independently in molluscs like cephalopods and the compound eyes of arthropods. The evolution of eyes is considered one of the prime examples of convergent evolution, with estimates that eyes evolved over 40 times across different phyla utilizing similar genetic regulatory mechanisms.

1. 1
I. Overview of photoreception and vision
II. Photoreception in Lower Invertebrates
III. Photoreception in Protostomes
A. Mollusc camera eyes
B. Arthropod compound eyes
IV. Evolution of Eyes
Photoreception and Vision
The process involves…..
Light energy
Reception
Transformation
Transduction
Integration
(biochemical energy)
(nerve impulse)
(photosensitive
molecule)
Rhodopsin metarhodopsin
+
biochemical energy
Light energy
Types of photoreceptor cells
(contain rhodopsin)
Ciliary type: photosensitive membrane
derived from or associated with cilia
Rhabdomeric type: photosensitive membrane
is of microvilli, not associated with cilia
There
has been some
debate
over the
phylogenetic
significance
of receptor
cell types
II. Photoreceptors in “Lower Invertebrates”
A. Pigmented epithelia: detect light intensity
ex. Corals, most medusae; very small # cells
B. Eye cup or ocellus: pit of pigmented cells
II. Photoreceptors in “Lower Invertebrates”

2. 2
B. Eye cup or ocellus: pit of pigmented cells
discern direction of light
light
II. Photoreceptors in “Lower Invertebrates”
B. Eye cup or ocellus: pit of pigmented cells
flatworms
Also cnidaria, nemerteans etc.
II. Photoreceptors in “Lower Invertebrates”
C. Lensed Eyes: Cubomedusae
II. Photoreceptors in “Lower Invertebrates”
• Tiny spherical lenses 100 um wide form sharp images free
of spherical aberrations (better peripheral imaging)
• However the focal plane lies behind the retina thus the
image perceived is blurry.
C. Lensed Eyes: Cubomedusae
II. Photoreceptors in “Lower Invertebrates”
Why should evolution have produced such sophisticated
optics that have only poor resolution? Isn’t higher
resolution always better?
In each rhopalium Box jellies have two types of lensed
eyes one looking upward and the other horizontally as
well as two other simple eyes
“Reverse neurobiology” : optics are known but
functions are not
Medial dorsal eye of copepod
III. Photoreceptors in Protostomes
Many species in the molluscs and arthropods
have only simple eyes or no eyes.
III. Photoreceptors in Protostomes
But Mollusca and Arthropoda have independently evolved
eyes that rank among the best:
compound eyes camera eyes

3. 3
III. Photoreceptors in Protostomes
A. Mollusca:
- most with simple eye cups2
- some snails with lensed, image forming eyes
- bivalves without ocular organs except for the
scallop Pecten (more on this)
- most cephalopods have image forming
camera eyes and acute vision
III. Photoreceptors in Protostomes
A. Mollusca:
1. scallops have as many as 60 lensed eyes lining
the edge of the mantle; in some species these eyes are
elaborate and functionally unusual
III. Photoreceptors in Protostomes
A. Mollusca:
2. nautiloids have a large pair of pin hole eyes
III. Photoreceptors in Protostomes
A.2. nautiloid eyes work by the same
principle as a pinhole camera.
III. Photoreceptors in Protostomes
cornea
Some differences between the cephalopod eye
and the human (chordate) eye:
Direct v. indirect eye arrangement
Rhabdomeric v. ciliary chromophores
Photoreceptors from nervous system
in chordates but from epidermis in
cephalopods
Focusing ability of cornea ( I.e. the
refractive index of water and air)
-- A more dense lens is needed to
focus light originating in a fluid
medium; complications in focusing
Aquatic lens
eye
Cornea lens
eye

4. 4
III. Photoreceptors in Protostomes
B. Arthropod Compound eyes
Amphipod
crustacean
with simple
eye and
compound
eye
Made up of hundreds to thousands
independent optical units: ommatidia
facets
Single
ommatidium
Lens and cone
help direct the light
Retinula cells produce
the photosensitive
rhabdomes
Pigment cells can keep
light from moving into
adjacent ommatidia
• The higher the # of ommatidea the sharper the image
• But no system to adjust focus; image is grainy
• Whole eyes are sensitive to motion
• In some species, lateral inhibition improves sensitivity
The
performance
of compound
eyes
• Erect, compound images
• Wide field of vision
Each facet must
Represent 10,000
ommatidea for this
compound eye to
reach the visual
acuity of the
human eye

5. 5
Among Compound Eyes, the House fly eyes achieve best focus
Sharp image
Trinocular vision
Use polarized light
Can be pivoted on
eye stalk, increasing
the visual field to
nearly 360 degrees
Among Crustaceans, mantis shrimp
have the most elaborate eyes
Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very
imperfect and simple, each grade being useful to its possessor, can be shown to exist; if
further, the eye does vary ever so slightly, and the variations be inherited, which is certainly
the case; and if any variation or modification in the organ be ever useful to an animal under
changing conditions of life, then the difficulty of believing that a perfect and complex eye could
be formed by natural selection, though insuperable by our imagination, can hardly be
considered real."
Darwin's eye and the enigma
of biological perfection
"To suppose that the eye, with all its inimitable
contrivances for adjusting the focus to different
distances, for admitting different amounts of
light, and for the correction of spherical and
chromatic aberration, could have been formed
by natural selection, seems, I freely confess,
absurd in the highest possible degree...
IV. How many times did eyes evolve?
1993 - Developmental geneticists discover “eye opening”
gene called eyeless in fruit flies
1994 - similar gene found in mice and human eyes
more recently even in squid eyes (pax-6 etc.)
(90% similarity)
1995 - experiments in which eyeless was activated in
other parts of fly body by human and mouse genes
implanted in fly embryos
1977- Ernst Mayr and colleagues estimate at least 40 times,
although 6 phyla have developed image-forming eyes
: convergent evolution to vision
When eyeless is turned on
in parts of the body where
it is inactive it could initiate
development of topical eyes
in unusual places... and in
other species!
Work of Walter Gehring and colleagues