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

Abstract

Crocodilians are apex amphibious predators that occupy a range of tropical habitats. In this study, we examined whether their semiaquatic 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, 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 λ max ), green (502/510 nm λ max ) and red (546/ 554 nm λ max ) sensitive, indicating the potential for trichromatic colour vision. The visual pigments of both members of the twin cones have the same λ max as the red-sensitive single cone and the rod has a λ max at 503/510 nm (saltwater/freshwater). The λ max 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.

Synchrotron microtomography of a Nothosaurus marchicus skull informs on nothosaurian physiology and neurosensory adaptations in early Sauropterygia

“…Although morphologically unpronounced, they do occupy one of the broadest cephalic domains, which would be consistent with a certain degree of visual acuity. Nevertheless, although extant crocodylians feature optic systems adapted for their respective niches [ 84 ], their optic lobes are poorly delimited due to the locally particularly thick dural envelope [ 56 , 85 , 86 ]. In crocodylians, bulges observed in this region typically reflect a portion of the venous blood system [ 87 ].…”

“…Aquatic ambush predation guided by visual cues characterizes crocodylians (e.g. [ 84 ] and references therein), whereas vision also represents a crucial sense in short-distance ambush predation by various aquatic snakes and sea kraits [ 201 , 202 ].…”

Crocodiles Alter Skin Color in Response to Environmental Color Conditions

“…However, light of shorter wavelengths (350–450 nm) produced a much stronger response. Saltwater ( C. porosus ) and Australian freshwater ( C. johnsonii ) crocodiles have cone photoreceptors which absorb light at 424 and 426 nm, respectively 37 . Since the wavelength of light for these photoreceptors closely match the energy for light that stimulates this response, it is reasonable to expect that they may facilitate this physiological response.…”

The Diversity and Adaptive Evolution of Visual Photopigments in Reptiles

“…The crocodilian visual system has been studied to some degree, but not nearly to the level of detail of the squamates, an outcome that may be related to the difference in the number of extant species that are easily attainable for study (10,221 squamates, 24 crocodilians, Uetz et al, 2019). Thus far, information regarding the visual photopigments of five species of crocodilians has been collected (Dartnall and Lythgoe, 1965;Govardovskii et al, 1988;Sillman et al, 1991;Nagloo et al, 2016).…”

“…Most recently Nagloo et al (2016) described and compared the visual systems of two Australian crocodiles, Crocodylus porosus (a saltwater crocodile) and C. johnstoni (a freshwater crocodile). The retinae of both species possessed three different types of single cones (deemed to be distinct photoreceptors based on their size and spectral sensitivity), as well as a double cone and a rod.…”