Physiological properties of the visual system in the Green Weaver ant, Oecophylla smaragdina.

The Green Weaver ants, Oecophylla smaragdina are iconic animals known for their extreme cooperative behaviour where they bridge gaps by linking to each other to build living chains. They are visually oriented animals, build chains towards closer targets, use celestial compass cues for navigation and are visual predators. Here, we describe their visual sensory capacity. The major workers of O. smaragdina have more ommatidia (804) in each eye compared to minor workers (508), but the facet diameters are comparable between both castes. We measured the impulse responses of the compound eye and found their response duration (42 ms) was similar to that seen in other slow-moving ants. We determined the flicker fusion frequency of the compound eye at the brightest light intensity to be 132 Hz, which is relatively fast for a walking insect suggesting the visual system is well suited for a diurnal lifestyle. Using pattern-electroretinography we identified the compound eye has a spatial resolving power of 0.5 cycles deg-1 and reached peak contrast sensitivity of 2.9 (35% Michelson contrast threshold) at 0.05 cycles deg-1. We discuss the relationship of spatial resolution and contrast sensitivity, with number of ommatidia and size of the lens.

Vision in sharks and rays: Opsin diversity and colour vision.

The visual sense of elasmobranch fishes is poorly studied compared to their bony cousins, the teleosts. Nevertheless, the elasmobranch eye features numerous specialisations that have no doubt facilitated the diversification and evolutionary success of this fascinating taxon. In this review, I highlight recent discoveries on the nature and phylogenetic distribution of visual pigments in sharks and rays. Whereas most rays appear to be cone dichromats, all sharks studied to date are cone monochromats and, as a group, have likely abandoned colour vision on multiple occasions. This situation in sharks mirrors that seen in other large marine predators, the pinnipeds and cetaceans, which leads us to reassess the costs and benefits of multiple cone pigments and wavelength discrimination in the marine environment.

Enhanced short-wavelength sensitivity in the blue-tongued skink Tiliqua rugosa.

Despite lizards using a wide range of colour signals, the limited variation in photoreceptor spectral sensitivities across lizards suggests only weak selection for species-specific, spectral tuning of photoreceptors. Some species, however, have enhanced short-wavelength sensitivity, which probably helps with the detection of signals rich in ultraviolet and short wavelengths. In this study, we examined the visual system of Tiliqua rugosa, which has an ultraviolet/blue tongue, to gain insight into this species' visual ecology. We used electroretinograms, opsin sequencing and immunohistochemical labelling to characterize whole-eye spectral sensitivity and the elements that shape it. Our findings reveal that T. rugosa expresses all five opsins typically found in lizards (SWS1, SWS2, RH1, RH2 and LWS) but possesses greatly enhanced short-wavelength sensitivity compared with other diurnal lizards. This enhanced short-wavelength sensitivity is characterized by a broadening of the spectral sensitivity curve of the eye towards shorter wavelengths while the peak sensitivity of the eye at longer wavelengths (560 nm) remains similar to that of other diurnal lizards. While an increased abundance of SWS1 photoreceptors is thought to mediate elevated ultraviolet sensitivity in a couple of other lizard species, SWS1 photoreceptor abundance remains low in this species. Instead, our findings suggest that short-wavelength sensitivity is driven by multiple factors which include a potentially red-shifted SWS1 photoreceptor and the absence of short-wavelength-absorbing oil droplets. Examining the coincidence of enhanced short-wavelength sensitivity with blue tongues among lizards of this genus will provide further insight into the co-evolution of conspecific signals and whole-eye spectral sensitivity.

Visual Opsin Diversity in Sharks and Rays.

The diversity of color vision systems found in extant vertebrates suggests that different evolutionary selection pressures have driven specializations in photoreceptor complement and visual pigment spectral tuning appropriate for an animal's behavior, habitat, and life history. Aquatic vertebrates in particular show high variability in chromatic vision and have become important models for understanding the role of color vision in prey detection, predator avoidance, and social interactions. In this study, we examined the capacity for chromatic vision in elasmobranch fishes, a group that have received relatively little attention to date. We used microspectrophotometry to measure the spectral absorbance of the visual pigments in the outer segments of individual photoreceptors from several ray and shark species, and we sequenced the opsin mRNAs obtained from the retinas of the same species, as well as from additional elasmobranch species. We reveal the phylogenetically widespread occurrence of dichromatic color vision in rays based on two cone opsins, RH2 and LWS. We also confirm that all shark species studied to date appear to be cone monochromats but report that in different species the single cone opsin may be of either the LWS or the RH2 class. From this, we infer that cone monochromacy in sharks has evolved independently on multiple occasions. Together with earlier discoveries in secondarily aquatic marine mammals, this suggests that cone-based color vision may be of little use for large marine predators, such as sharks, pinnipeds, and cetaceans.

Ocellar spatial vision in Myrmecia ants.

In addition to compound eyes, insects possess simple eyes known as ocelli. Input from the ocelli modulates optomotor responses, flight-time initiation, and phototactic responses - behaviours that are mediated predominantly by the compound eyes. In this study, using pattern electroretinography (pERG), we investigated the contribution of the compound eyes to ocellar spatial vision in the diurnal Australian bull ant Myrmecia tarsata by measuring the contrast sensitivity and spatial resolving power of the ocellar second-order neurons under various occlusion conditions. Furthermore, in four species of Myrmecia ants active at different times of the day, and in European honeybee Apis mellifera, we characterized the ocellar visual properties when both visual systems were available. Among the ants, we found that the time of activity had no significant effect on ocellar spatial vision. Comparing day-active ants and the honeybee, we did not find any significant effect of locomotion on ocellar spatial vision. In M. tarsata, when the compound eyes were occluded, the amplitude of the pERG signal from the ocelli was reduced 3 times compared with conditions when the compound eyes were available. The signal from the compound eyes maintained the maximum contrast sensitivity of the ocelli as 13 (7.7%), and the spatial resolving power as 0.29 cycles deg-1. We conclude that ocellar spatial vison improves significantly with input from the compound eyes, with a noticeably larger improvement in contrast sensitivity than in spatial resolving power.

Can the Dynamic Colouration and Patterning of Bluelined Goatfish (Mullidae; Upeneichthys lineatus) Be Perceived by Conspecifics?

Bluelined goatfish (Upeneichthys lineatus) exhibit dynamic body colour changes and transform rapidly from a pale, buff/white, horizontally banded pattern to a conspicuous, vertically striped, red pattern when foraging. This red pattern is potentially an important foraging signal for communication with conspecifics, provided that U. lineatus can detect and discriminate the pattern. Using both physiological and behavioural experiments, we first examined whether U. lineatus possess visual pigments with sensitivity to long ("red") wavelengths of light, and whether they can discriminate the colour red. Microspectrophotometric measurements of retinal photoreceptors showed that while U. lineatuslack visual pigments dedicated to the red part of the spectrum, their pigments likely confer some sensitivity in this spectral band. Behavioural colour discrimination experiments suggested that U. lineatuscan distinguish a red reward stimulus from a grey distractor stimulus of variable brightness. Furthermore, when presented with red stimuli of varying brightness they could mostly discriminate the darker and lighter reds from the grey distractor. We also obtained anatomical estimates of visual acuity, which suggest that U. lineatus can resolve the contrasting bands of conspecifics approximately 7 m away in clear waters. Finally, we measured the spectral reflectance of the red and white colouration on the goatfish body. Visual models suggest that U. lineatus can discriminate both chromatic and achromatic differences in body colouration where longer wavelength light is available. This study demonstrates that U. lineatus have the capacity for colour vision and can likely discriminate colours in the long-wavelength region of the spectrum where the red body pattern reflects light strongly. The ability to see red may therefore provide an advantage in recognising visual signals from conspecifics. This research furthers our understanding of how visual signals have co-evolved with visual abilities, and the role of visual communication in the marine environment.

Underwater hearing in sea snakes (Hydrophiinae): first evidence of auditory evoked potential thresholds.

The viviparous sea snakes (Hydrophiinae) are a secondarily aquatic radiation of more than 60 species that possess many phenotypic adaptations to marine life. However, virtually nothing is known of the role and sensitivity of hearing in sea snakes. This study investigated the hearing sensitivity of the fully marine sea snake Hydrophis stokesii by measuring auditory evoked potential (AEP) audiograms for two individuals. AEPs were recorded from 40 Hz (the lowest frequency tested) up to 600 Hz, with a peak in sensitivity identified at 60 Hz (163.5 dB re. 1 µPa or 123 dB re. 1 µm s-2). Our data suggest that sea snakes are sensitive to low-frequency sounds but have relatively low sensitivity compared with bony fishes and marine turtles. Additional studies are required to understand the role of sound in sea snake life history and further assess these species' vulnerability to anthropogenic noise.

Miniaturisation reduces contrast sensitivity and spatial resolving power in ants.

Vision is crucial for animals to find prey, locate conspecifics and navigate within cluttered landscapes. Animals need to discriminate objects against a visually noisy background. However, the ability to detect spatial information is limited by eye size. In insects, as individuals become smaller, the space available for the eyes reduces, which affects the number of ommatidia, the size of the lens and the downstream information-processing capabilities. The evolution of small body size in a lineage, known as miniaturisation, is common in insects. Here, using pattern electroretinography with vertical sinusoidal gratings as stimuli, we studied how miniaturisation affects spatial resolving power and contrast sensitivity in four diurnal ants that live in a similar environment but vary in their body and eye size. We found that ants with fewer and smaller ommatidial facets had lower spatial resolving power and contrast sensitivity. The spatial resolving power was maximum in the largest ant Myrmecia tarsata at 0.60 cycles deg-1 compared with that of the ant with smallest eyes Rhytidoponera inornata at 0.48 cycles deg-1 Maximum contrast sensitivity (minimum contrast threshold) in M. tarsata (2627 facets) was 15.51 (6.4% contrast detection threshold) at 0.1 cycles deg-1, while the smallest ant R. inornata (227 facets) had a maximum contrast sensitivity of 1.34 (74.1% contrast detection threshold) at 0.05 cycles deg-1 Miniaturisation thus dramatically decreases maximum contrast sensitivity and also reduces spatial resolution, which could have implications for visually guided behaviours. This is the first study to physiologically investigate contrast sensitivity in the context of insect allometry.

Retinal Morphology and Visual Specializations in Three Species of Chimaeras, the Deep-Sea R. pacifica and C. lignaria, and the Vertical Migrator C. milii (Holocephali).

The majority of holocephalans live in the mesopelagic zone of the deep ocean, where there is little or no sunlight, but some species migrate to brightly lit shallow waters to reproduce. This study compares the retinal morphology of two species of deep-sea chimaeras, the Pacific spookfish (Rhinochimaera pacifica) and the Carpenter's chimaera (Chimaera lignaria), with the elephant shark (Callorhinchus milii), a vertical migrator that lives in the mesopelagic zone but migrates to shallow water to reproduce. The two deep-sea chimaera species possess pure rod retinae with long photoreceptor outer segments that might serve to increase visual sensitivity. In contrast, the retina of the elephant shark possesses rods, with an outer-segment length significantly shorter (a mean of 34 µm) than in the deep-sea species, and cones, and therefore the potential for color vision. The retinal ganglion cell distribution closely follows that of the photoreceptor populations in all three species, but there is a lower peak density of these cells in both deep-sea species (215-275 cells/mm2 vs. 769 cells/mm2 in the elephant shark), which represents a significant increase in the convergence of visual information (summation ratio) from photoreceptors to ganglion cells. It is evident that the eyes of deep-sea chimaeras have increased sensitivity to detect objects under low levels of light, but at the expense of both resolution and the capacity for color vision. In contrast, the elephant shark has a lower sensitivity, but the potential for color discrimination and a higher visual acuity.

Ancestral duplications and highly dynamic opsin gene evolution in percomorph fishes.

Single-gene and whole-genome duplications are important evolutionary mechanisms that contribute to biological diversification by launching new genetic raw material. For example, the evolution of animal vision is tightly linked to the expansion of the opsin gene family encoding light-absorbing visual pigments. In teleost fishes, the most species-rich vertebrate group, opsins are particularly diverse and key to the successful colonization of habitats ranging from the bioluminescence-biased but basically dark deep sea to clear mountain streams. In this study, we report a previously unnoticed duplication of the violet-blue short wavelength-sensitive 2 (SWS2) opsin, which coincides with the radiation of highly diverse percomorph fishes, permitting us to reinterpret the evolution of this gene family. The inspection of close to 100 fish genomes revealed that, triggered by frequent gene conversion between duplicates, the evolutionary history of SWS2 is rather complex and difficult to predict. Coincidentally, we also report potential cases of gene resurrection in vertebrate opsins, whereby pseudogenized genes were found to convert with their functional paralogs. We then identify multiple novel amino acid substitutions that are likely to have contributed to the adaptive differentiation between SWS2 copies. Finally, using the dusky dottyback Pseudochromis fuscus, we show that the newly discovered SWS2A duplicates can contribute to visual adaptation in two ways: by gaining sensitivities to different wavelengths of light and by being differentially expressed between ontogenetic stages. Thus, our study highlights the importance of comparative approaches in gaining a comprehensive view of the dynamics underlying gene family evolution and ultimately, animal diversification.

Evolution of Vertebrate Phototransduction: Cascade Activation.

We applied high-throughput sequencing to eye tissue from several species of basal vertebrates (a hagfish, two species of lamprey, and five species of gnathostome fish), and we analyzed the mRNA sequences for the proteins underlying activation of the phototransduction cascade. The molecular phylogenies that we constructed from these sequences are consistent with the 2R WGD model of two rounds of whole genome duplication. Our analysis suggests that agnathans retain an additional representative (that has been lost in gnathostomes) in each of the gene families we studied; the evidence is strong for the G-protein α subunit (GNAT) and the cGMP phosphodiesterase (PDE6), and indicative for the cyclic nucleotide-gated channels (CNGA and CNGB). Two of the species (the hagfish Eptatretus cirrhatus and the lamprey Mordacia mordax) possess only a single class of photoreceptor, simplifying deductions about the composition of cascade protein isoforms utilized in their photoreceptors. For the other lamprey, Geotria australis, analysis of the ratios of transcript levels in downstream and upstream migrant animals permits tentative conclusions to be drawn about the isoforms used in four of the five spectral classes of photoreceptor. Overall, our results suggest that agnathan rod-like photoreceptors utilize the same GNAT1 as gnathostomes, together with a homodimeric PDE6 that may be agnathan-specific, whereas agnathan cone-like photoreceptors utilize a GNAT that may be agnathan-specific, together with the same PDE6C as gnathostomes. These findings help elucidate the evolution of the vertebrate phototransduction cascade from an ancestral chordate phototransduction cascade that existed prior to the vertebrate radiation.

Visual Pigments, Ocular Filters and the Evolution of Snake Vision.

Much of what is known about the molecular evolution of vertebrate vision comes from studies of mammals, birds and fish. Reptiles (especially snakes) have barely been sampled in previous studies despite their exceptional diversity of retinal photoreceptor complements. Here, we analyze opsin gene sequences and ocular media transmission for up to 69 species to investigate snake visual evolution. Most snakes express three visual opsin genes (rh1, sws1, and lws). These opsin genes (especially rh1 and sws1) have undergone much evolutionary change, including modifications of amino acid residues at sites of known importance for spectral tuning, with several tuning site combinations unknown elsewhere among vertebrates. These changes are particularly common among dipsadine and colubrine "higher" snakes. All three opsin genes are inferred to be under purifying selection, though dN/dS varies with respect to some lineages, ecologies, and retinal anatomy. Positive selection was inferred at multiple sites in all three opsins, these being concentrated in transmembrane domains and thus likely to have a substantial effect on spectral tuning and other aspects of opsin function. Snake lenses vary substantially in their spectral transmission. Snakes active at night and some of those active by day have very transmissive lenses, whereas some primarily diurnal species cut out shorter wavelengths (including UVA). In terms of retinal anatomy, lens transmission, visual pigment spectral tuning and opsin gene evolution the visual system of snakes is exceptionally diverse compared with all other extant tetrapod orders.

Electrophysiological measures of temporal resolution, contrast sensitivity and spatial resolving power in sharks.

In most animals, vision plays an important role in detecting prey, predators and conspecifics. The effectiveness of vision in assessing cues such as motion and shape is influenced by the ability of the visual system to detect changes in contrast in both space and time. Understanding the role vision plays in shark behaviour has been limited by a lack of knowledge about their temporal resolution, contrast sensitivity and spatial resolution. In this study, an electrophysiological approach was used to compare these measures across five species of sharks: Chiloscyllium punctatum, Heterodontus portusjacksoni, Hemiscyllium ocellatum, Mustelus mustelus and Haploblepharus edwardsii. All shark species were highly sensitive to brightness contrast and were able to detect contrast differences as low as 1.6%. Temporal resolution of flickering stimuli ranged from 28 to 44 Hz. Species that inhabit brighter environments were found to have higher temporal resolution. Spatial resolving power was estimated in C. punctatum, H. portusjacksoni and H. ocellatum and ranged from 0.10 to 0.35 cycles per degree, which is relatively low compared to other vertebrates. These results suggest that sharks have retinal adaptations that enhance contrast sensitivity at the expense of temporal and spatial resolution, which is beneficial for vision in dimly lit and/or low contrast aquatic environments.

Retinal temporal resolution and contrast sensitivity in the parasitic lamprey Mordacia mordax and its non-parasitic derivative Mordacia praecox.

Lampreys and hagfishes are the sole extant representatives of the early agnathan (jawless) vertebrates. We compared retinal function of fully metamorphosed, immature Mordacia mordax (which are about to commence parasitic feeding) with those of sexually mature individuals of its non-parasitic derivative Mpraecox We focused on elucidating the retinal adaptations to dim-light environments in these nocturnally active lampreys, using electroretinography to determine the temporal resolution (flicker fusion frequency, FFF) and temporal contrast sensitivity of enucleated eyecups at different temperatures and light intensities. FFF was significantly affected by temperature and light intensity. Critical flicker fusion frequency (cFFF, the highest FFF recorded) of M. praecox and M. mordax increased from 15.1 and 21.8 Hz at 9°C to 31.1 and 36.9 Hz at 24°C, respectively. Contrast sensitivity of both species increased by an order of magnitude between 9 and 24°C, but remained comparatively constant across all light intensities. Although FFF values for Mordacia spp. are relatively low, retinal responses showed a particularly high contrast sensitivity of 625 in M. praecox and 710 in M. mordax at 24°C. This suggests selective pressures favour low temporal resolution and high contrast sensitivity in both species, thereby enhancing the capture of photons and increasing sensitivity in their light-limited environments. FFF indicated all retinal photoreceptors exhibit the same temporal response. Although the slow response kinetics (i.e. low FFF) and saturation of the response at bright light intensities characterise the photoreceptors of both species as rod-like, it is unusual for such a photoreceptor to be functional under scotopic and photopic conditions.

Visual pigments in a palaeognath bird, the emu Dromaius novaehollandiae: implications for spectral sensitivity and the origin of ultraviolet vision.

A comprehensive description of the spectral characteristics of retinal photoreceptors in palaeognaths is lacking. Moreover, controversy exists with respect to the spectral sensitivity of the short-wavelength-sensitive-1 (SWS1) opsin-based visual pigment expressed in one type of single cone: previous microspectrophotometric (MSP) measurements in the ostrich (Struthio camelus) suggested a violet-sensitive (VS) SWS1 pigment, but all palaeognath SWS1 opsin sequences obtained to date (including the ostrich) imply that the visual pigment is ultraviolet-sensitive (UVS). In this study, MSP was used to measure the spectral properties of visual pigments and oil droplets in the retinal photoreceptors of the emu (Dromaius novaehollandiae). Results show that the emu resembles most other bird species in possessing four spectrally distinct single cones, as well as double cones and rods. Four cone and a single rod opsin are expressed, each an orthologue of a previously identified pigment. The SWS1 pigment is clearly UVS (wavelength of maximum absorbance [λmax] = 376 nm), with key tuning sites (Phe86 and Cys90) consistent with other vertebrate UVS SWS1 pigments. Palaeognaths would appear, therefore, to have UVS SWS1 pigments. As they are considered to be basal in avian evolution, this suggests that UVS is the most likely ancestral state for birds. The functional significance of a dedicated UVS cone type in the emu is discussed.

Multiple rod-cone and cone-rod photoreceptor transmutations in snakes: evidence from visual opsin gene expression.

In 1934, Gordon Walls forwarded his radical theory of retinal photoreceptor 'transmutation'. This proposed that rods and cones used for scotopic and photopic vision, respectively, were not fixed but could evolve into each other via a series of morphologically distinguishable intermediates. Walls' prime evidence came from series of diurnal and nocturnal geckos and snakes that appeared to have pure-cone or pure-rod retinas (in forms that Walls believed evolved from ancestors with the reverse complement) or which possessed intermediate photoreceptor cells. Walls was limited in testing his theory because the precise identity of visual pigments present in photoreceptors was then unknown. Subsequent molecular research has hitherto neglected this topic but presents new opportunities. We identify three visual opsin genes, rh1, sws1 and lws, in retinal mRNA of an ecologically and taxonomically diverse sample of snakes central to Walls' theory. We conclude that photoreceptors with superficially rod- or cone-like morphology are not limited to containing scotopic or photopic opsins, respectively. Walls' theory is essentially correct, and more research is needed to identify the patterns, processes and functional implications of transmutation. Future research will help to clarify the fundamental properties and physiology of photoreceptors adapted to function in different light levels.

Comparative assessment of phototherapy protocols for reduction of oxidative stress in partially transected spinal cord slices undergoing secondary degeneration.

BACKGROUND: Red/near-infrared light therapy (R/NIR-LT) has been developed as a treatment for a range of conditions, including injury to the central nervous system (CNS). However, clinical trials have reported variable or sub-optimal outcomes, possibly because there are few optimized treatment protocols for the different target tissues. Moreover, the low absolute, and wavelength dependent, transmission of light by tissues overlying the target site make accurate dosing problematic. RESULTS: In order to optimize light therapy treatment parameters, we adapted a mouse spinal cord organotypic culture model to the rat, and characterized myelination and oxidative stress following a partial transection injury. The ex vivo model allows a more accurate assessment of the relative effect of different illumination wavelengths (adjusted for equal quantal intensity) on the target tissue. Using this model, we assessed oxidative stress following treatment with four different wavelengths of light: 450 nm (blue); 510 nm (green); 660 nm (red) or 860 nm (infrared) at three different intensities: 1.93 × 10(16) (low); 3.85 × 10(16) (intermediate) and 7.70 × 10(16) (high) photons/cm(2)/s. We demonstrate that the most effective of the tested wavelengths to reduce immunoreactivity of the oxidative stress indicator 3-nitrotyrosine (3NT) was 660 nm. 860 nm also provided beneficial effects at all tested intensities, significantly reducing oxidative stress levels relative to control (p ≤ 0.05). CONCLUSIONS: Our results indicate that R/NIR-LT is an effective antioxidant therapy, and indicate that effective wavelengths and ranges of intensities of treatment can be adapted for a variety of CNS injuries and conditions, depending upon the transmission properties of the tissue to be treated.

Fluorescence characterisation and visual ecology of pseudocheilinid wrasses.

BACKGROUND: Wrasses represent the second largest family of marine fishes and display a high diversity of complex colours linked to ecological functions. Recently, red autofluorescent body colouration has been reported in some of these fishes. However, little is known about the distribution of such fluorescent body patterns in wrasses or the animals' ability to perceive such colours. RESULTS: Against this background, we (1) investigated long-wavelength emission autofluorescence in thirteen species of pseudocheilinid wrasses and (2) characterised the spectral absorbance of visual pigments in one of the examined species, the fairy wrasse Cirrhilabrus solorensis. Spectrophotometric analysis revealed that fluorescent body colouration is widespread and diverse within this clade, with considerable variation in both fluorescent pattern and maximum emission wavelength between species. Characterisation of visual pigments in retinal photoreceptors showed a single class of rod and three spectrally distinct cone photoreceptors, suggesting possible trichromacy. CONCLUSION: Combining the emission characteristics of fluorescence body colouration and the spectral sensitivity data of retinal cells suggests that the visual system of C. solorensis is sensitive to pseudocheilinid fluorescence.

Visual resolution and contrast sensitivity in two benthic sharks.

Sharks have long been described as having 'poor' vision. They are cone monochromats and anatomical estimates suggest they have low spatial resolution. However, there are no direct behavioural measurements of spatial resolution or contrast sensitivity. This study estimates contrast sensitivity and spatial resolution of two species of benthic sharks, the Port Jackson shark, Heterodontus portusjacksoni, and the brown-banded bamboo shark, Chiloscyllium punctatum, by recording eye movements in response to optokinetic stimuli. Both species tracked moving low spatial frequency gratings with weak but consistent eye movements. Eye movements ceased at 0.38 cycles per degree, even for high contrasts, suggesting low spatial resolution. However, at lower spatial frequencies, eye movements were elicited by low contrast gratings, 1.3% and 2.9% contrast in H portusjacksoni and C. punctatum, respectively. Contrast sensitivity was higher than in other vertebrates with a similar spatial resolving power, which may reflect an adaptation to the relatively low contrast encountered in aquatic environments. Optokinetic gain was consistently low and neither species stabilised the gratings on their retina. To check whether restraining the animals affected their optokinetic responses, we also analysed eye movements in free-swimming C. punctatum We found no eye movements that could compensate for body rotations, suggesting that vision may pass through phases of stabilisation and blur during swimming. As C. punctatum is a sedentary benthic species, gaze stabilisation during swimming may not be essential. Our results suggest that vision in sharks is not 'poor' as previously suggested, but optimised for contrast detection rather than spatial resolution.

Spatial resolving power and spectral sensitivity of the saltwater crocodile, Crocodylus porosus, and the freshwater crocodile, Crocodylus johnstoni.

Crocodilians are apex amphibious predators that occupy a range of tropical habitats. In this study, we examined whether their semi-aquatic lifestyle and ambush hunting mode are reflected in specific adaptations in the peripheral visual system. Design-based stereology and microspectrophotometry were used to assess spatial resolving power and spectral sensitivity of saltwater (Crocodylus porosus) and freshwater crocodiles (Crocodylus johnstoni). Both species possess a foveal streak that spans the naso-temporal axis and mediates high spatial acuity across the central visual field. The saltwater crocodile and freshwater crocodile have a peak spatial resolving power of 8.8 and 8.0 cycles deg(-1), respectively. Measurement of the outer segment dimensions and spectral absorbance revealed five distinct photoreceptor types consisting of three single cones, one twin cone and a rod. The three single cones (saltwater/freshwater crocodile) are violet (424/426 nm λmax), green (502/510 nm λmax) and red (546/554 nm λmax) sensitive, indicating the potential for trichromatic colour vision. The visual pigments of both members of the twin cones have the same λmax as the red-sensitive single cone and the rod has a λmax at 503/510 nm (saltwater/freshwater). The λmax values of all types of visual pigment occur at longer wavelengths in the freshwater crocodile compared with the saltwater crocodile. Given that there is a greater abundance of long wavelength light in freshwater compared with a saltwater environment, the photoreceptors would be more effective at detecting light in their respective habitats. This suggests that the visual systems of both species are adapted to the photic conditions of their respective ecological niche.