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sensory information processing varies with body type, brain design and spectrum of actions

1. Eyes: diversity and similarities
2. Spatial resolution and sensitivity of the eyes
3. Filtering


Dusenbery (1992)

see notes


vision w/ and w/o spatial resolution (Land, 2014, 2018)

· Cnidaria – corals, anemones, jellyfish (radial symmetry)
· Porifera – sponges
· Bilateria – animals with bilateral symmetry as embryo
· Protostomia/Deuterostomia – differ in blastula development in the embryonic stage
· Chordata – vertebrates
· Annelida – worms
· Platyzoa – Planarian flatworms
· Mollusca – snails, mussels, squid, octopus
· Arthorpoda – insects, crabs, crayfish, mantis shrimps, scorpions, spiders, mites, ticks
Spatial resolution = image forming eyes

see notes


High-resolution vision has evolved to process more spatial information for increasing task complexity (Nilsson, 2013)

Some directional info

see notes


High-resolution vision has evolved to process more spatial information for increasing task complexity (Nilsson, 2013) research

Lamb (2013)

Pradeep et al. (2020)


Lamb (2013)

Evidence is reviewed from a wide range of studies relevant to the evolution of vertebrate photoreceptors and phototransduction, in order to permit the synthesis of a scenario for the major steps that occurred during the evolution of cones, rods and the vertebrate retina. The ancestral opsin originated more than 700 Mya (million years ago) and duplicated to form three branches before cnidarians diverged from our own lineage. During chordate evolution, ciliary opsins (C-opsins) underwent multiple stages of improvement, giving rise to the ‘bleaching’ opsins that characterise cones and rods. Prior to the ‘2R’ rounds of whole genome duplication near the base of the vertebrate lineage, ‘cone’ photoreceptors already existed; they possessed a transduction cascade essentially the same as in modern cones, along with two classes of opsin: SWS and LWS (short- and long-wave-sensitive). These cones appear to have made synaptic contact directly onto ganglion cells, in a two-layered retina that resembled the pineal organ of extant non-mammalian vertebrates. Interestingly, those ganglion cells appear to be descendants of microvillar photoreceptor cells. No lens was associated with this two-layered retina, and it is likely to have mediated circadian timing rather than spatial vision. Subsequently, retinal bipolar cells evolved, as variants of ciliary photoreceptors, and greatly increased the computational power of the retina. With the advent of a lens and extraocular muscles, spatial imaging information became available for central processing, and gave rise to vision in vertebrates more than 500 Mya. The ‘2R’ genome duplications permitted the refinement of cascade components suitable for both rods and cones, and also led to the emergence of five visual opsins. The exact timing of the emergence of ‘true rods’ is not yet clear, but it may not have occurred until after the divergence of jawed and jawless vertebrates.


Pradeep et al. (2020)

○ A histological understanding of the layers of the eye is essential for appreciating disease pathophysiology and also understanding certain therapeutic approaches. Broadly, from an anatomical perspective, the eye can be viewed as a series of overlapping layers of tissue.
○ External structures of the eye include the eyelashes, lids, muscles, accessory glands, and conjunctiva.
○ The internal structures of the eye consist of three layers of tissue arranged concentrically:
w The sclera and cornea make up the exterior layers.
w The uvea is the vascular layer in the middle, subdivided into the iris, ciliary body, and choroid.
w The retina constitutes the innermost layer and is made up of nervous tissue.
All of these layers can further subdivide and undergo histological classification.[1]


Forming an image: shared principles in the 2 basic designs of eyes with spatial resolution

see notes


Forming an image: shared principles in the 2 basic designs of eyes with spatial resolution research

Schoenemann and Clarkson (2020)


Schoenemann and Clarkson (2020)

In all arthropods the plesiomorphic (ancestral character state) kind of visual system commonly is considered to be the compound eye. Here we are able to show the excellently preserved internal structures of the compound eye of a 429 Mya old Silurian trilobite, Aulacopleura koninckii (Barrande, 1846). It shows the characteristic elements of a modern apposition eye, consisting of 8 (visible) receptor cells, a rhabdom, a thick lens, screening pigment (cells), and in contrast to a modern type, putatively just a very thin crystalline cone. Functionally the latter underlines the idea of a primarily calcitic character of the lens because of its high refractive properties. Perhaps the trilobite was translucent. We show that this Palaeozoic trilobite in principle was equipped with a fully modern type of visual system, a compound eye comparable to that of living bees, dragonflies and many diurnal crustaceans. It is an example of excellent preservation, and we hope that this manuscript will be a starting point for more research work on fossil evidence, and to develop a deeper understanding of the evolution of vision.


Evolution of vertebrate eyes (Lamb, 2013)

· Protostomes: mollsucs, annelids and arthropods

Around 420MYA the jawed vertebrates (gnathostomes) evolved, ancestors to all modern vertebrates

· Hag fish (slime eels) and lamprey are jawless vertebrates, living species in evolutionary distinct lines that shares a common ancestor with the gnathostomes

see notes


Evolution of vertebrate eyes (Lamb, 2013) research

Kim et al. (2016)

Bringmann et al. (2018)


Kim et al. (2016)

Vertebrate ancestors had only cone-like photoreceptors. The duplex retina evolved in jawless vertebrates with the advent of highly photosensitive rod-like photoreceptors. Despite cones being the arbiters of high-resolution color vision, rods emerged as the dominant photoreceptor in mammals during a nocturnal phase early in their evolution. We investigated the evolutionary and developmental origins of rods in two divergent vertebrate retinas. In mice, we discovered genetic and epigenetic vestiges of short-wavelength cones in developing rods, and cell-lineage tracing validated the genesis of rods from S cones. Curiously, rods did not derive from S cones in zebrafish. Our study illuminates several questions regarding the evolution of duplex retina and supports the hypothesis that, in mammals, the S-cone lineage was recruited via the Maf-family transcription factor NRL to augment rod photoreceptors. We propose that this developmental mechanism allowed the adaptive exploitation of scotopic niches during the nocturnal bottleneck early in mammalian evolution.


Bringmann et al. (2018)

A fovea is a pitted invagination in the inner retinal tissue (fovea interna) that overlies an area of photoreceptors specialized for high acuity vision (fovea externa). Although the shape of the vertebrate fovea varies considerably among the species, there are two basic types. The retina of many predatory fish, reptilians, and birds possess one (or two) convexiclivate fovea(s), while the retina of higher primates contains a concaviclivate fovea. By refraction of the incoming light, the convexiclivate fovea may function as image enlarger, focus indicator, and movement detector. By centrifugal displacement of the inner retinal layers, which increases the transparency of the central foveal tissue (the foveola), the primate fovea interna improves the quality of the image received by the central photoreceptors. In this review, we summarize ‒ with the focus on Müller cells of the human and macaque fovea ‒ data regarding the structure of the primate fovea, discuss various aspects of the optical function of the fovea, and propose a model of foveal development. The “Müller cell cone” of the foveola comprises specialized Müller cells which do not support neuronal activity but may serve optical and structural functions. In addition to the “Müller cell cone”, structural stabilization of the foveal morphology may be provided by the 'z-shaped' Müller cells of the fovea walls, via exerting tractional forces onto Henle fibers. The spatial distribution of glial fibrillary acidic protein may suggest that the foveola and the Henle fiber layer are subjects to mechanical stress. During development, the foveal pit is proposed to be formed by a vertical contraction of the centralmost Müller cells. After widening of the foveal pit likely mediated by retracting astrocytes, Henle fibers are formed by horizontal contraction of Müller cell processes in the outer plexiform layer and the centripetal displacement of photoreceptors. A better understanding of the molecular, cellular, and mechanical factors involved in the developmental morphogenesis and the structural stabilization of the fovea may help to explain the (patho-) genesis of foveal hypoplasia and macular holes.


Eye spot of the New Zealand hagfish (Lamb, 2013)

· 76 species
· Eyes without lens
- They have eye sports rather then eyes


Eye spot of the New Zealand hagfish (Lamb, 2013) research

Dong and Allison (2018)

Locket and Jorgensen (1998)


Dong and Allison (2018)

Hagfish eyes are markedly basic compared to the eyes of other vertebrates, lacking a pigmented epithelium, a lens, and a retinal architecture built of three cell layers – the photoreceptors, interneurons & ganglion cells. Concomitant with hagfish belonging to the earliest-branching vertebrate group (the jawless Agnathans), this lack of derived characters has prompted competing interpretations that hagfish eyes represent either a transitional form in the early evolution of vertebrate vision, or a regression from a previously elaborate organ. Here we show the hagfish retina is not extensively degenerating during its ontogeny, but instead grows throughout life via a recognizable Pax6+ ciliary marginal zone. The retina has a distinct layer of photoreceptor cells that appear to homogeneously express a single opsin of the rh1 rod opsin class. The epithelium that encompasses these photoreceptors is striking because it lacks the melanin pigment that is universally associated with animal vision; notwithstanding, we suggest this epithelium is a homolog of gnathosome Retinal Pigment Epithelium (RPE) based on its robust expression of RPE65 and its engulfment of photoreceptor outer segments. We infer that the hagfish retina is not entirely rudimentary in its wiring, despite lacking a morphologically distinct layer of interneurons: multiple populations of cells exist in the hagfish inner retina that differentially express markers of vertebrate retinal interneurons. Overall, these data clarify Agnathan retinal homologies, reveal characters that now appear to be ubiquitous across the eyes of vertebrates, and refine interpretations of early vertebrate visual system evolution.


Locket and Jorgensen (1998)

Though probably functional light receptors, hagfish eyes are small, that of Myxine glutinosa only 500 mu m diameter, and degenerate. Demonstrated extraocular photoreception may be more important for hagfish behaviour. Eptatretus species eyes are beneath an unpigmented skin patch, but Myxine glutinosa eyes are buried beneath muscle. AIL hagfishes have only an undifferentiated corneo-scleral layer, and extraocular muscles are absent. We found no lens in any hagfish examined. Eptatretus species have a vitreous cavity, with scattered collagen fibrils, some forming dense aggregates. Choroidal capillaries, but not pigment, occur in all species examined. Eptatretus retain a hollow optic cup, but at the margin epithelium and neuroretina are continuous, without extension to ciliary body or iris, both of which are absent. Developmental anomalies are common in peripheral retina in all. The Myxine optic cup has no lumen, the margins meeting at a fibrous plug. Eptatretus species retinas contain photoreceptors, with clear outer segments in the periphery, but few or none in the fundus. Myxine has few, degenerate outer segments, indenting the opposing epithelium. Receptor synapses are sessile. Synaptic bodies, like vertebrate ribbons, occur in Eptatretus, but only simple synapses in Myxine. Myxine optic nerve contains a few hundred thin axons only.


Lateral eyes of the lamprey (Lamb, 2013)

· Also known as nine-eyes eel
· 38 species, some of which are fish parasites
· Lamprey eyes are very similar to those of other vertebrates which supports the hypothesis derived from fossil founds that the vertebrate lens eye evolved early in the evolution of vertebrates

A. Ammocoete – its rudimentary ‘eyes’ cannot be seen as they are embedded beneath the skin
B. Downstream migrant
Upstream migrant

see notes


Lateral eyes of the lamprey (Lamb, 2013) research

Saitoh et al. (2007)

Gustafsson et al. (2010)


Saitoh et al. (2007)

The intrinsic function of the brain stem-spinal cord networks eliciting the locomotor synergy is well described in the lamprey-a vertebrate model system. This study addresses the role of tectum in integrating eye, body orientation, and locomotor movements as in steering and goal-directed behavior. Electrical stimuli were applied to different areas within the optic tectum in head-restrained semi-intact lampreys (n = 40). Motions of the eyes and body were recorded simultaneously (videotaped). Brief pulse trains (< 0.5 s) elicited only eye movements, but with longer stimuli (< 0.5 s) lateral bending movements of the body (orientation movements) were added, and with even longer stimuli locomotor movements were initiated. Depending on the tectal area stimulated, four characteristic response patterns were observed. In a lateral area conjugate horizontal eye movements combined with lateral bending movements of the body and locomotor movements were elicited, depending on stimulus duration. The amplitude of the eye movement and bending movements was site specific within this region. In a rostromedial area, bilateral downward vertical eye movements occurred. In a caudomedial tectal area, large-amplitude undulatory body movements akin to struggling behavior were elicited, combined with large-amplitude eye movements that were antiphasic to the body movements. The alternating eye movements were not dependent on vestibuloocular reflexes. Finally, in a caudolateral area locomotor movements without eye or bending movements could be elicited. These results show that tectum can provide integrated motor responses of eye, body orientation, and locomotion of the type that would be required in goal-directed locomotion.


Gustafsson et al. (2010)

The sharpness and thus information content of the retinal image in the eye depends on the optical quality of the lens and its accurate positioning in the eye. Multifocal lenses create well-focused color images and are present in the eyes of all vertebrate groups studied to date (mammals, reptiles including birds, amphibians, and ray-finned fishes) and occur even in lampreys, i.e., the most basal vertebrates with well-developed eyes. Results from photoretinoscopy obtained in this study indicate that the Dipnoi (lungfishes), i.e., the closest piscine relatives to tetrapods, also possess multifocal lenses. Suspension of the lens is complex and sophisticated in teleosts (bony fishes) and tetrapods. We studied lens suspension using light and electron microscopy in one species of lamprey (Lampetra fluviatilis) and two species of African lungfish (Protopterus aethiopicus aethiopicus and Protopterus annectens annectens). A fibrous and highly transparent membrane suspends the lens in both of these phylogenetically widely separated vertebrate groups. The membrane attaches to the lens approximately along the lens equator, from where it extends to the ora retinalis. The material forming the membrane is similar in ultrastructure to microfibrils in the zonule fibers of tetrapods. The membrane, possibly in conjunction with the cornea, iris, and vitreous body, seems suitable for keeping the lens in the correct position for well-focused imaging. Suspension of the lens by a multitude of zonule fibers in tetrapods may have evolved from a suspensory membrane similar to that in extant African lungfishes, a structure that seems to have appeared first in the lamprey-like ancestors of all extant vertebrates.


Why do animals have pairs of eyes? Already onee ye is enough to guide directional responses and see objects and scenes by itself

o Due to bilateral symmetry and developmental processes which automatically result in paired structures
o It increases the size of the visual field whilst reducing the need to move the head or body
The brain can make additional comparisons from different areas relative to their body halves


A pessimistic estimate of the time required for an eye to evolve (Nilsson and Pelger, 1994)

“Only a few hundred thousand years” to evolve an eye with high spatial resolution

see notes


A pessimistic estimate of the time required for an eye to evolve (Nilsson and Pelger, 1994) research

Maclver et al. (2017)

Kumar (2019)


Maclver et al. (2017)

The evolution of terrestrial vertebrates, starting around 385 million years ago, is an iconic moment in evolution that brings to mind images of fish transforming into four-legged animals. Here, we show that this radical change in body shape was preceded by an equally dramatic change in sensory abilities akin to transitioning from seeing over short distances in a dense fog to seeing over long distances on a clear day. Measurements of eye sockets and simulations of their evolution show that eyes nearly tripled in size just before vertebrates began living on land. Computational simulations of these animal’s visual ecology show that for viewing objects through water, the increase in eye size provided a negligible increase in performance. However, when viewing objects through air, the increase in eye size provided a large increase in performance. The jump in eye size was, therefore, unlikely to have arisen for seeing through water and instead points to an unexpected hybrid of seeing through air while still primarily inhabiting water. Our results and several anatomical innovations arising at the same time suggest lifestyle similarity to crocodiles. The consequent combination of the increase in eye size and vision through air would have conferred a 1 million-fold increase in the amount of space within which objects could be seen. The “buena vista” hypothesis that our data suggest is that seeing opportunities from afar played a role in the subsequent evolution of fully terrestrial limbs as well as the emergence of elaborated action sequences through planning circuits in the nervous system.


Kumar (2019)

Eye evolution is far from resolved and despite the considerable interest and decades of study, many questions remain on the evolutionary scenarios of eyes and photoreceptor cells. While examining their ultrastructure has been an important way for comparative studies, the high degree of variation of eye structures/complexities within species (and closely related species) calls for more detailed studies at the molecular level. As with many lophotrochozoan taxa, annelids also display simple to elaborate eye structures. Although general homology of the cerebral rhabdomeric eyes is assumed, this has not been firmly established leaving the scene in the annelid ancestor unanswered. To gain an understanding of the situation in a long pelagic annelid larva, we studied Malacoceros fuliginosus. The larvae possess multiple eyespots and therefore suitable for studies on how different eyespots develop and integrate into the nervous system. We used ultrastructure and gene expression studies to understand the eyespot structure and development. Our phylogenetic analysis of annelid r-opsins revealed the existence of two r-opsin paralogs - r-opsin1 and r-opsin3 within the two main annelid groups - sedentaria and errantia, whereas in basal branching annelids only a single r-opsin type is present. In comparison with the well-studied annelid Platynereis dumerilii, we find that the rhabdomeric eyes have several similarities in terms of spatial and temporal development, r-opsin expression dynamics and axonal connectivity. This suggests homology of the two rhabdomeric eyes and the more complex dorsal eyes in P. dumerilii is likely a case of augmentation of a simple eyespot. Apart from visual r-opsins, the eye PRCs in M. fuliginosus also expresses the newly classified opsin type, xenopsin. Inspection of the eye structure also revealed the existence of a prominent cilium in both rhabdomeric eyes. Additionally, we also identified a c-opsin in an extraocular cell type thereby making it the only species so far having both c-opsin and xenopsin. Taken together, our data provide insights into the eye organization of the annelid ancestor and adds information on how eye evolution is shaped by opsin gain and loss.
The second topic of interest is the nervous system development in the M. fuliginosus larva. The evolution of the bilaterian nervous system is a topic of long-standing debate inciting the need for studies at multiple levels along with broader species sampling. A major question is whether the centralized nervous system seen across taxa is derived from a common ancestor or independently originated multiple times. Characterization of the nervous system has been mainly done at the level of gene expression patterns along the major body axes, anterior-posterior and dorsal-ventral. One aspect that has been overlooked particularly in lophotrochozoans is the development of pioneer neurons that give rise to the early neuronal scaffold. In M. fuliginosus, we identify at least three pioneer neurons that are responsible to form the complete early neuronal scaffold. While a posterior neuron pioneers the path for the ventral nerve cord, pair of neurons form the prototroch ring nerve and a ganglion cell near the apical organ with descending axons prefigures the central ganglia. Here we focused on the development of the posterior pioneer neuron and distinguish it from the rest of the neurons. It is one of the earliest cells to differentiate along with other ciliated cells of apical tuft and prototroch cells which are known to have mosaic development. The posterior neuron does not express the well-characterized proneural genes such as Ascl1, Olig, NeuroD, and Ngn and moreover, they even lack Prox1 and Elav which are represented by most other neurons. From a molecular perspective, the posterior pioneer neuron is indeed distinct from the rest of the neurons and may develop in a cell-autonomous manner.


main determinants of visual performance in image-forming eyes

· Spatial resolution
o Viewing distances, size and density of relevant features or objects in the visual scene
o Density and number of photoreceptors
o Eye size and curvature of retina
· Light sensitivity
o Intensity range in which receptors operate (dim or bright light)
o Eye size
o Size of the lens(es)
o Decreases with higher spatial resolution
· Temporal resolution
o Speed of movements
Fast or slow photoreceptors


Larger eyes have higher spatial resolution (Lythgoe, 1979)

1 = humans
· 2 = peregrine falcon
· 13 = honeybee
· 18 = drosophila
· 8 = Myotis (bat)
17 = Metaphidippus (jumping spider, better vision than bat)

see notes


Larger eyes have higher spatial resolution (Lythgoe, 1979) research

Bagheri et al. (2020)

Pusch et al. (2013)