Evidence for Control of Cutaneous Oxygen Uptake in the Yellow-Lipped Sea Krait Laticauda colubrina (Schneider, 1799)

Global analysis of the influence of environmental variables to explain ecological niches and realized thermal niche boundaries of sea snakes

Understanding the factors affecting species distributions is a central topic in ecology and biogeography. However, most research on this topic has focused on species inhabiting terrestrial environments. At broad scales, abiotic variables consistently serve as primary determinants of species' distributions. In this study, we investigated the explanatory power of different abiotic variables in determining the distribution patterns of sea snakes on a global scale. Additionally, as the boundaries of realized thermal niches have significant implications for the ecology of species and their geographic distributions, we evaluated the asymmetry of realized thermal limits (i.e., differences in variances between the upper and lower limits of the realized thermal niche). We obtained 10 marine environmental variables from global databases along with >5000 occurrence records for 51 sea snake species in 4 genera across the group's entire known geographic range. Using these data, we employed correlative ecological niche modeling to analyze the influence of the individual variables in explaining species' distributions. To estimate the realized thermal limits of each species, we extracted the mean, minimum, and maximum temperature values at four depths (superficial, mean benthic, minimum benthic, and maximum benthic) for each occurrence record of the species. We then evaluated the asymmetry of the realized thermal niche by measuring and comparing the variances in the upper and lower limits. Both analyses (the importance of variables and realized thermal limit asymmetry) were performed at three taxonomic levels (sea snakes as a lineage of marine-adapted elapids [true sea snakes + sea kraits], subfamily, and genus) and two spatial resolutions. Overall, we found that temperature, silicate, nitrate, salinity, and phosphate concentrations were the most influential factors in explaining the spatial distribution patterns of sea snakes, regardless of taxonomic level or spatial resolution. Similarly, we observed that the realized thermal limits were asymmetric, with a higher variance in the lower limits, and that asymmetry decreased as the taxonomic level and spatial resolution increased.

Visual adaptation of opsin genes to the aquatic environment in sea snakes

BACKGROUND

Evolutionary transitions from terrestrial to aquatic life history cause drastic changes in sensory systems. Indeed, the drastic changes in vision have been reported in many aquatic amniotes, convergently. Recently, the opsin genes of the full-aquatic sea snakes have been reported. However, those of the amphibious sea snakes have not been examined in detail.

RESULTS

Here, we investigated opsin genes and visual pigments of sea snakes. We determined the sequences of SWS1, LWS, and RH1 genes from one terrestrial, three amphibious and four fully-aquatic elapids. Amino acid replacements at four and one spectra-tuning positions were found in LWS and RH1, respectively. We measured or predicted absorption of LWS and RH1 pigments with A1-derived retinal. During their evolution, blue shifts of LWS pigments have occurred stepwise in amphibious sea snakes and convergently in both amphibious and fully-aquatic species.

CONCLUSIONS

Blue shifted LWS pigments may have adapted to deep water or open water environments dominated by blue light. The evolution of opsins differs between marine mammals (cetaceans and pinnipeds) and sea snakes in two fundamental ways: (1) pseudogenization of opsins in marine mammals; and (2) large blue shifts of LWS pigments in sea snakes. It may be possible to explain these two differences at the level of photoreceptor cell composition given that cone and rod cells both exist in mammals whereas only cone cells exist in fully-aquatic sea snakes. We hypothesize that the differences in photoreceptor cell compositions may have differentially affected the evolution of opsins in divergent amniote lineages.

Coming up for air: thermal-dependence of dive behaviours and metabolism in sea snakes

Cutaneous gas exchange allows some air-breathing diving ectotherms to supplement their pulmonary oxygen uptake, which may allow prolongation of dives and an increased capacity to withstand anthropogenic and natural threatening processes that increase submergence times. However, little is known of the interplay between metabolism, bimodal oxygen uptake and activity levels across thermal environments in diving ectotherms. Here, we show in two species of sea snake (spine-bellied sea snake; Hydrophis curtus and elegant sea snake; H. elegans) that increasing temperature elevates surfacing rates, increases total oxygen consumption, and decreases dive durations. The majority of dives observed in both species remained within estimated maximal aerobic limits. While cutaneous gas exchange accounted for a substantial proportion of total oxygen consumption (up to 23%), unexpectedly it was independent of water temperature and activity levels, suggesting a diffusion-limited mechanism. Our findings demonstrate that rising water temperature and a limited capability to up-regulate cutaneous oxygen uptake may compromise the proficiency with which sea snakes perform prolonged dives. This may hinder their capacity to withstand ongoing anthropogenic activities like trawl fishing, and increase their susceptibility to surface predation as their natural environments continue to warm.