A retina with at least ten spectral types of photoreceptors in a mantis shrimp

Evolution of anatomical and physiological specialization in the compound eyes of stomatopod crustaceans

Stomatopod crustaceans have complex and diverse visual systems. Among their many unique features are a specialized ommatidial region (the midband) that enables the eye to have multiple overlapping visual fields, as well as sets of spectral filters that are intercalated at two levels between tiers of photoreceptors involved in polychromatic color vision. Although the physiology and visual function of stomatopod eyes have been studied for many years, how these unique visual features originated and diversified is still an open question. In order to investigate how stomatopods have attained the current complexity in visual function, we have combined physiological and morphological information (e.g. number of midband rows, number of filters in the retina, and the spectral properties of filters) with new phylogenetic analyses of relationships among species based on nucleotide sequence data from two nuclear (18S and 28S rDNA) and two mitochondrial [16S and cytochrome oxidase I (COI)] genes. Based on our recovered phylogenetic relationships among species, we propose two new superfamilies within the Stomatopoda: Hemisquilloidea and Pseudosquillodea. Maximum likelihood ancestral state reconstructions indicate that ancestral stomatopod eyes contained six midband rows and four intrarhabdomal filters, illustrating that the visual physiological complexity originated early in stomatopod evolutionary history. While the two distal filters contain conservative sets of filter pigments, the proximal filters show more spectral diversity in filter types, particularly in midband row 2, and are involved in tuning the color vision system to the photic environment. In particular, a set of related gonodactyloid families (Gonodactylidae, Protosquillidae, Takuidae) inhabiting shallow, brightly lit coral reef waters contain the largest diversity of filter pigments, which are spectrally placed relative to the underlying photoreceptors to take advantage of the broad spectrum of light available in the environment.

Molecular diversity of visual pigments in Stomatopoda (Crustacea)

Stomatopod crustaceans possess apposition compound eyes that contain more photoreceptor types than any other animal described. While the anatomy and physiology of this complexity have been studied for more than two decades, few studies have investigated the molecular aspects underlying the stomatopod visual complexity. Based on previous studies of the structure and function of the different types of photoreceptors, stomatopod retinas are hypothesized to contain up to 16 different visual pigments, with 6 of these having sensitivity to middle or long wavelengths of light. We investigated stomatopod middle- and long-wavelength-sensitive opsin genes from five species with the hypothesis that each species investigated would express up to six different opsin genes. In order to understand the evolution of this class of stomatopod opsins, we examined the complement of expressed transcripts in the retinas of species representing a broad taxonomic range (four families and three superfamilies). A total of 54 unique retinal opsins were isolated, resulting in 6–15 different expressed transcripts in each species. Phylogenetically, these transcripts form six distinct clades, grouping with other crustacean opsins and sister to insect long-wavelength visual pigments. Within these stomatopod opsin groups, intra- and interspecific clusters of highly similar transcripts suggest that there has been rampant recent gene duplication. Some of the observed molecular diversity is also due to ancient gene duplication events within the stem crustacean lineage. Using evolutionary trace analysis, 10 amino acid sites were identified as functionally divergent among the six stomatopod opsin clades. These sites form tight clusters in two regions of the opsin protein known to be functionally important: six in the chromophore-binding pocket and four at the cytoplasmic surface in loops II and III. These two clusters of sites indicate that stomatopod opsins have diverged with respect to both spectral tuning and signal transduction.

Circular polarization vision in a stomatopod crustacean

We describe the addition of a fourth visual modality in the animal kingdom, the perception of circular polarized light. Animals are sensitive to various characteristics of light, such as intensity, color, and linear polarization [1, 2]. This latter capability can be used for object identification, contrast enhancement, navigation, and communication through polarizing reflections [2-4]. Circularly polarized reflections from a few animal species have also been known for some time [5, 6]. Although optically interesting [7, 8], their signal function or use (if any) was obscure because no visual system was known to detect circularly polarized light. Here, in stomatopod crustaceans, we describe for the first time a visual system capable of detecting and analyzing circularly polarized light. Four lines of evidence-behavior, electrophysiology, optical anatomy, and details of signal design-are presented to describe this new visual function. We suggest that this remarkable ability mediates sexual signaling and mate choice, although other potential functions of circular polarization vision, such as enhanced contrast in turbid environments, are also possible [7, 8]. The ability to differentiate the handedness of circularly polarized light, a visual feat never expected in the animal kingdom, is demonstrated behaviorally here for the first time.

Stomatopod eye structure and function: a review

Stomatopods (mantis shrimps) possess apposition compound eyes that contain more photoreceptor types than any other animal described. This has been achieved by sub-dividing the eye into three morphologically discrete regions, a mid-band and two laterally placed hemispheres, and within the mid-band, making simple modifications to a commonly encountered crustacean photoreceptor pattern of eight photoreceptors (rhabdomeres) per ommatidium. Optically the eyes are also unusual with the directions of view of the ommatidia of all three eye regions skewed such that over 70% of the eye views a narrow strip in space. In order to scan the world with this strip, the stalked eyes of stomatopods are in almost continual motion. Functionally, the end result is a trinocular eye with monocular range finding capability, a 12-channel colour vision system, a 2-channel linear polarisation vision system and a line scan sampling arrangement that more resembles video cameras and satellite sensors than animal eyes. Not surprisingly, we are still struggling to understand the biological significance of stomatopod vision and attempt few new explanations here. Instead we use this special edition as an opportunity to review and summarise the structural aspects of the stomatopod retina that allow it to be so functionally complex.

Scanning behavior by larvae of the predacious diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae) enlarges visual field prior to prey capture

Larvae of the predaceous diving beetle Thermonectus marmoratus bear six stemmata on each side of their head, two of which form relatively long tubes with linear retinas at their proximal ends. The physical organization of these eyes results in extremely narrow visual fields that extend only laterally in the horizontal body plane. There are other examples of animals possessing eyes with predominantly linear retinas, or with linear arrangements of specific receptor types. In these animals, the eyes, or parts of the eyes, are movable and perform scanning movements to increase the visual field. Based on anatomical data and observations of relatively transparent, immobilized young larvae, we report here that T. marmoratus larvae are incapable of moving their eyes or any part of their eyes within the head capsule. However, they do perform a series of bodily dorso-ventral pivots prior to prey capture, behaviorally extending the vertical visual field from 2 degrees to up to 50 degrees. Frame-by-frame analysis shows that such behavior is performed within a characteristic distance to the prey. These data provide first insights into the function of the very peculiar anatomical eye organization of T. marmoratus larvae.

Electrophysiological evidence for linear polarization sensitivity in the compound eyes of the stomatopod crustacean Gonodactylus chiragra

Gonodactyloid stomatopod crustaceans possess polarization vision, which enables them to discriminate light of different e-vector angle. Their unusual apposition compound eyes are divided by an equatorial band of six rows of enlarged, structurally modified ommatidia, the mid-band (MB). The rhabdoms of the two most ventral MB rows 5 and 6 are structurally designed for polarization vision. Here we show, with electrophysiological recordings, that the photoreceptors R1-R7 within these two MB rows in Gonodactylus chiragra are highly sensitive to linear polarized light of two orthogonal directions (PS=6.1). They possess a narrow spectral sensitivity peaking at 565 nm. Unexpectedly, photoreceptors within the distal rhabdomal tier of MB row 2 also possess highly sensitive linear polarization receptors, which are in their spectral and polarization characteristics similar to the receptors of MB rows 5 and 6. Photoreceptors R1-R7 within the remainder of the MB exhibit low polarization sensitivity (PS=2.3). Outside the MB, in the two hemispheres, R1-R7 possess medium linear polarization sensitivity (PS=3.8) and a broad spectral sensitivity peaking at around 500 nm, typical for most crustaceans. Throughout the retina the most distally situated UV-sensitive R8 cells are not sensitive to linear polarized light.

Ecology and evolution of primate colour vision

More than one hundred years ago, Grant Allen suggested that colour vision in primates, birds and insects evolved as an adaptation for foraging on colourful advertisements of plants--fruits and flowers. Recent studies have shown that well developed colour vision appeared long before fruits and flowers evolved. Thus, colour vision is generally beneficial for many animals, not only for those eating colourful food. Primates are the only placental mammals that have trichromatic colour vision. This may indicate either that trichromacy is particularly useful for primates or that primates are unique among placental mammals in their ability to utilise the signals of three spectrally distinct types of cones or both. Because fruits are an important component of the primate diet, primate trichromacy could have evolved as a specific adaptation for foraging on fruits. Alternatively, primate trichromacy could have evolved as an adaptation for many visual tasks. Comparative studies of mammalian eyes indicate that primates are the only placental mammals that have in their retina a pre-existing neural machinery capable of utilising the signals of an additional spectral type of cone. Thus, the failure of non-primate placental mammals to evolve trichromacy can be explained by constraints imposed on the wiring of retinal neurones.

Compensation for longitudinal chromatic aberration in the eye of the firefly squid, Watasenia scintillans

The camera eyes of fishes and cephalopods have come forth by convergent evolution. In a variety of vertebrates capable of color vision, longitudinal chromatic aberration (LCA) of the optical system is corrected for by the exactly tuned longitudinal spherical aberration (LSA) of the crystalline lens. The LSA leads to multiple focal lengths, such that several wavelengths can be focused on the retina. We investigated whether that is also the case in the firefly squid (Watasenia scintillans), a cephalopod species that is likely to have color vision. It was found that the lens of W. scintillans is virtually free of LSA and uncorrected for LCA. However, the eye does not suffer from LCA because of a banked retina. Photoreceptors sensitive to short and long wavelengths are located at appropriate distances from the lens, such that they receive well-focused images. Such a design is an excellent solution for the firefly squid because a large area of the retina is monochromatically organized and it allows for double use of the surface area in the dichromatically organized part of the retina. However, it is not a universal solution since compensation for LCA by a banked retina requires that eye size and/or spectral separation between photopigments is small.

Adaptive color vision in Pullosquilla litoralis (Stomatopoda, Lysiosquilloidea) associated with spectral and intensity changes in light environment

Some stomatopod crustacean species that inhabit a range of habitat depths have color vision systems that adapt to changes in ambient light conditions. To date, this change in retinal function has been demonstrated in species within the superfamily Gonodactyloidea in response to varying the spectral range of light. Intrarhabdomal filters in certain ommatidia within the specialized midband of the eye change spectrally, modifying the sensitivity of underlying photoreceptors to match the spectrum of available light. In the present study, we utilized Pullosquilla litoralis, a member of the superfamily Lysiosquilloidea that also has a wide depth range. Individuals were placed within one of three light treatments: (1) full-spectrum, high-intensity 'white' light, (2) narrow-spectrum 'blue' light and (3) full-spectrum, reduced-intensity 'gray' light. After 3 months, the intrarhabdomal filters in Row 3 ommatidia of the midband in blue- and gray-light-treated animals were short-wavelength shifted by 10-20 nm compared with homologous filters in animals in white-light treatments. These spectral changes increase the relative sensitivity of associated photoreceptors in animals that inhabit environments where light spectral range or intensity is reduced. The adaptable color vision system of stomatopods may allow animals to make the best use of the ambient light occurring at their habitat regardless of depth. The major controlling element of the plasticity in lysiosquilloid stomatopod color vision appears to be light intensity rather than spectral distribution.

Sensory adaptation. Tunable colour vision in a mantis shrimp

Systems of colour vision are normally identical in all members of a species, but a single design may not be adequate for species living in a diverse range of light environments. Here we show that in the mantis shrimp Haptosquilla trispinosa, which occupies a range of depths in the ocean, long-wavelength colour receptors are individually tuned to the local light environment. The spectral sensitivity of specific classes of photoreceptor is adjusted by filters that vary between individuals.

Spectral tuning and the visual ecology of mantis shrimps

The compound eyes of mantis shrimps (stomatopod crustaceans) include an unparalleled diversity of visual pigments and spectral receptor classes in retinas of each species. We compared the visual pigment and spectral receptor classes of 12 species of gonodactyloid stomatopods from a variety of photic environments, from intertidal to deep water (> 50 m), to learn how spectral tuning in the different photoreceptor types is modified within different photic environments. Results show that receptors of the peripheral photoreceptors, those outside the midband which are responsible for standard visual tasks such as spatial vision and motion detection, reveal the well-known pattern of decreasing lambdamax with increasing depth. Receptors of midband rows 5 and 6, which are specialized for polarization vision, are similar in all species, having visual lambdamax-values near 500nm, independent of depth. Finally, the spectral receptors of midband rows 1 to 4 are tuned for maximum coverage of the spectrum of irradiance available in the habitat of each species. The quality of the visual worlds experienced by each species we studied must vary considerably, but all appear to exploit the full capabilities offered by their complex visual systems.

Evolution of color vision

Color vision is achieved by comparing the inputs from retinal photoreceptor neurons that differ in their wavelength sensitivity. Recent studies have elucidated the distribution and phylogeny of opsins, the family of light-sensitive molecules involved in this process. Interesting new findings suggest that animals have evolved a strategy to achieve specific sensitivity through the mutually exclusive expression of different opsin genes in photoreceptors.

Visual discrimination: Seeing the third quality of light

Objects can differ in brightness and colour. At least that is what our own visual system tells us. It now seems that stomatopod shrimps, and possibly also cephalopod molluscs, can see the direction of the electric vector of light, in much the same way we see colour.

Two visual pigment opsins, one expressed in the dorsal region and another in the dorsal and ventral regions, of the compound eye of a dragonfly, Sympetrum frequens

This paper describes the primary structure of two visual pigment opsins (DfRh1 and DfRh2) in the regionalized compound eye of a dragonfly, Sympetrum frequens. The amino acid sequences were deduced from the nucleotide sequences of cDNAs isolated from a cDNA library of the dragonfly retina. The two opsins both consist of 379 amino acids with 81.3% identity. Analysis of hydropathy indicated that the sequences have seven transmembrane domains like those of previously described opsins. Expression analysis using RT-PCR revealed that DfRh1 was present only in the dorsal region whereas DfRh2 was detected in both the dorsal and the ventral regions of the eye.

The compound eyes of mantis shrimps (Crustacea, Hoplocarida, Stomatopoda). I. Compound eye structure: the detection of polarized light

Stomatopod crustaceans possess compound eyes divided into three distinct regions: two peripheral retinae - the dorsal and ventral hemispheres — and the mid-band. Throughout the eye, in particular in the midband, there are many structural adaptations that potentially enable different portions of the eye to perform different visual tasks. A high degree of optical overlap between these eye regions allows the parallel sampling of various parameters of light from one direction in space. In consecutive papers, we present structural evidence that stomatopods have the receptors necessary for colour and polarization vision. The first paper describes the retinal structures that suggest the existence of polarization sensitivity in stomatopods. mid-band rows five and six, together with the hemispheres, are probably involved in this visual process. By using two strategies, rhabdomal modification and varying the orientation of similar ommatidial units in the three eye regions, stomatopods have the capacity to analyse polarized light in a very detailed manner. All the species included in this study live in shallow, tropical waters where polarized light signals are abundant. It therefore seems likely that their eyes have evolved to take advantage of such environmental cues. Structural evidence also suggests that all retinular cells in rows one to four of the mid-band, and the distal most retinular cells (R8) over most of the retina, are not sensitive to polarized light. These mid-band rows are instead adapted for colour detection. This function of the stomatopod retina and structural features concerned with colour sensitivity are described in paper II ( Phil. Trans. R. Soc. Lond. B 334, 57—84 (1991)).

Specialization of retinal function in the compound eyes of mantis shrimps

Visual function and its specialization at the level of the retina were studied in 13 species of stomatopod crustaceans, representing three superfamilies: Gonodactyloidea, Lysiosquilloidea, and Squilloidea. We measured attenuation and irradiance spectra in the environment of each species, at the actual depths and times of activity where we observed individuals. We also characterized the intrahabdomal filters of all study species and determined the absolute spectral sensitivity functions and approximate photon capture rates of all photoreceptor classes below the level of the 8th retinular cell in seven of these species. Shallow-water gonodactyloid species have four distinct classes of intrarhabdomal filters, producing photoreceptors that are relatively insensitive but which have the broadest spectral coverage of all. Deep-water gonodactyloids and all lysiosquilloids have filters that are spectrally less diverse. These species often discard the proximal filter classes of one or more receptor types. As a result, their retinas are more sensitive but have reduced spectral range or diversity. The single squilloid species has the most sensitive photoreceptors of any we observed, due to the lack both of intrarhabdomal filters and tiered photoreceptors. Photon absorption rates, at the times of animal activity, were similar in most photoreceptor classes of all species, whether the receptors were tiered or untiered, or filtered or unfiltered. Thus, the retinas of stomatopods are specialized to operate at similar levels of stimulation at the times and depths of actual use, while evidently maintaining the greatest possible potential for spectral coverage and discrimination.

Ultraviolet photoreception in mantis shrimp

An UV-sensitive class of photoreceptors exists in all regions of the retinas of mantis shrimps. UV photosensitivity apparently resides in rhabdomeres of the eighth retinular cell (R8) that lies atop each rhabdom; and in ommatidia where the R8 rhabdomere consists of microvilli parallel in a single direction, sensitivity is maximal when the e-vector of plane-polarized light is parallel to the microvilli. Spectral sensitivity of the UV photoreceptor peaks at 345 nm and is best explained by the presence of a photopigment with lambda max near 325 nm overlain by material that absorbs UV light at wavelengths below approximately 350 nm. Rhabdomeres of R8 cells in several different retinal regions of a variety of species examined contain a photopigment absorbing maximally below 340 nm. Under appropriate conditions, a metapigment with lambda max near 460 nm can be formed. UV vision may be useful for enhancing the visual contrast of midwater predators or prey.

The intrarhabdomal filters in the retinas of mantis shrimps

The intrarhabdomal filters in the photoreceptors of compounds eyes of 32 species of mantis shrimps (Crustacea: Stomatopoda), representing seven families within the superfamilies Lysiosquilloidea and Gonodactyloidea, were surveyed by microspectrophotometry of filters in fresh cryosections of the retina. A total of up to four classes of filters exist in stomatopods: two each in Rows 2 and 3 of the midband. All lysiosquilloid species lacked the proximal filter in Row 3; a few also lacked the proximal Row 2 class. While most gonodactyloid species had all four possible classes, in some species the proximal filters of Row 3 and (in one case) Row 2 were missing. In all, at least 11 distinct spectral classes of pigments were found. Absorption spectra suggested that the filter pigments were probably carotenoids or carotenoproteins, although the distal filter of Row 3 was often exceptional, appearing to contain a mixture of pigments. While the types of pigments found in the filters of the various species generally followed taxonomic lines, numerous exceptions were found that were apparently related to the ecological requirements of the various species.

The retinoids of seven species of mantis shrimp

Eyes of stomatopod crustaceans, or mantis shrimps, contain the greatest diversity of visual pigments yet described in any species, with as many as ten or more spectral classes present in a single retina. In this study, the eyes of seven species of mantis shrimp from three superfamilies of stomatopods were examined for their content of retinoids. Only retinal and retinol were found; neither hydroxyretinoids nor dehydroretinoids were detected. The principal isomers were 11-cis and all-trans. The eyes of most of these species contain stores of 11-cis retinol, principally as retinyl esters, and in amounts in excess of retinal. Squilla empusa is particularly noteworthy, with over 5000 pmoles of retinol per eye.

The distribution and nature of colour vision among the mammals

1. An oft-cited view, derived principally from the writings of Gordon L. Walls, is that relatively few mammalian species have a capacity for colour vision. This review has evaluated that proposition in the light of recent research on colour vision and its mechanisms in mammals. 2. To yield colour vision a retina must contain two or more spectrally discrete types of photopigment. While this is a necessary condition, it is not a sufficient one. This means, in particular, that inferences about the presence of colour vision drawn from studies of photopigments, the precursors of photopigments, or from nervous system signals must be accepted with due caution. 3. Conjoint signals from rods and cones may be exploited by mammalian nervous systems to yield behavioural discriminations consistent with the formal definition of colour vision. Many mammalian retinas are relatively cone-poor, and thus there are abundant opportunities for such rod/cone interactions. Several instances were cited in which animals having (apparently) only one type of cone photopigment succeed at colour discriminations using such a mechanism. it is suggested that the exploitation of such a mechanism may not be uncommon among mammals. 4. Based on ideas drawn from natural history, Walls (1942) proposed that the receptors and photopigments necessary to support colour vision were lost during the nocturnal phase of mammalian history and then re-acquired during the subsequent mammalian radiations. Contemporary examination of photopigment genes along with the utilization of better techniques for identifying rods and cones suggest a different view, that the earliest mammals had retinas containing some cones and two types of cone photopigment. Thus the baseline mammalian colour vision is argued to be dichromacy. 5. A consideration of the broad range of mammalian niches and activity cycles suggests that many mammals are active during photic periods that would make a colour vision capacity potentially useful. 6. A systematic survey was presented that summarized the evidence for colour vision in mammals. Indications of the presence and nature of colour vision were drawn both from direct studies of colour vision and from studies of those retinal mechanisms that are most closely associated with the possession of colour vision. Information about colour vision can be adduced for species drawn from nine mammalian orders.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial, temporal and spectral pre-processing for colour vision

Fourier transforms of the spectral radiance of natural objects were investigated. The average spectral power spectrum Sc(fc) is well described by Sc(fc) = exp (-beta fc), with fc the spectral frequency (cycles micron-1), and beta = 0.419 +/- 0.097 microns. Average spectral contrast (Cc = [epsilon fc not equal to 0 Sc(fc)/Sc(0)]1/2) was 0.224 +/- 0.127. Optimal filters for colour pre-processing were derived using a recently developed theory of early vision (van Hateren (J. comp. Physiol. A 171, 157 (1992))). The theory assumes that the surrounding world is first sampled spatially, temporally and spectrally by an array of pre-filters, and subsequently filtered by an array of neural filters that maximize the information delivered to an array of noisy information channels. These optimal filters show lateral inhibition and spectral opponency for high signal-to-noise ratios (SNRs) and low temporal frequencies (ft). Decreasing SNR or increasing ft eventually produce filters that are spatially and spectrally low-pass, resulting in a visual system lacking lateral inhibition and spectral opponency. The optimal filters for high SNR lead to responses in the spectral channels approximately independent of the spectrum of the illumination, which is a first step towards colour constancy. Finally, the optimal spectral pre-filter has a half-width of about 100 nm; this is close to that of the common rhodopsins.