Hydrophis platurus xanthos, Bessesen & Galbreath, 2017

All-yellow snakes, Hydrophis platurus xanthos View in CoL

Smith (1926) recognized different color morphs of H. platurus , which conceivably might be related to variations in physical conditions or predation pressures. Kropach (1971b), Solórzano (2011), and Bessesen (2012) reported a population in the northern basin of Golfo Dulce that is exclusively composed of xanthic (all-yellow or primarily yellow) sea snakes that are rarely seen outside the Gulf along the southwestern coast of Costa Rica. Later studies revealed a geographically restricted marine habitat in which 100% of the sampled sea snakes exhibited a diagnostic character ( Bessesen and Galbreath 2017). The xanthic population is spatially separated from the oceanic bicolored snakes by a gap of about 22 km and has been given subspecies status ( Bessesen 2012; Bessesen and Galbreath 2017), although the taxonomy remains controversial ( Solórzano and Sasa 2024). The topography of Golfo Dulce and associated currents generally prevent free exchange between the deeper waters of the inner basin and the adjacent coastal water ( Svendsen et al. 2006; Bessesen and Galbreath 2017), but various observations suggest that some snakes likely wash out from the Gulf to the Pacific from time to time ( Bessesen and Galbreath 2017; Solórzano and Sasa 2024). Moreover, the presence of some bicolored specimens inside Golfo Dulce (even mating with yellow specimens) suggests that the movement of snakes occurs in both directions ( Solórzano and Sasa 2024).

Clearly, the xanthic H. p. xanthos snakes are not cryptic, as they are highly visible when floating on the surface of the ocean (Fig. 9). Noteworthy differences in behavior between the xanthic and bicolor populations of H. platurus have been reported. Unlike the bicolor snakes that spend time resting or foraging while floating on the surface during daylight hours, the xanthic snakes tend to be markedly nocturnal with respect to this behavior ( Bessesen 2012; Lillywhite et al. 2015; Solórzano and Sasa 2024). They tend not to be attracted to slicks like the bicolor snakes, and they surface in rougher water. Why the xanthic subspecies lost its dark coloration is not known, but the lighter yellow skin likely risks damage from solar radiation and presumably exposes the snakes to avian predators ( Bessesen and González-Suárez 2022) and harassment by dolphins (Bessessen et al. 2021). Although multiple factors (including surface temperatures) might have influenced the evolution of nocturnal surfacing behavior of the xanthic snakes, the reversal of activity that is seen in the less conspicuous bicolor snakes is clearly compatible with the hypothesis for crypsis in the latter population.

Conservation of Hydrophis platurus

Sea snakes are important components of the vertebrate marine fauna, and the very broad range of H. p. platurus is especially significant. The bicolored subspecies can therefore serve as a possible harbinger for climate change, both in contexts of temperature and drought as well as changes in trophic ecology and community structure. Sea snakes have been generally acknowledged as important bioindicators that are relevant to monitoring marine habitats and assessing the effects of climate change, changes or loss of habitat, declines in biodiversity, and other anthropogenic impacts ( Lillywhite et al. 2018; Udyawer et al. 2018; Rasmussen et al. 2020). Increasing our knowledge and insights related to the biological properties, behavior, and survival of sea snakes is especially important, particularly in the pelagic species.

Crypsis is seemingly important for this species because of its periodic association with the ocean’s surface. Intermittent air-breathing is essential and foraging while floating on slicks is an evolved behavior that is necessary for successful foraging. The pelagic H. platurus consumes at least 34 species of fishes from 27 different families, and thus its foraging ecology is a centrally important aspect of its pelagic life history ( Brischoux and Lillywhite 2013). Crypsis might well be important to the successful foraging of H. p. platurus on slicks.

In addition, access to fresh water for osmoregulation is important for sea snakes, although it is limited in marine environments, and drinking from lenses of rainwater is critical for hydration in H. platurus (reviewed by Rash and Lillywhite 2019). Sea snakes were once thought to drink seawater and eliminate excess salts by means of sublingual salt glands. However, recent investigations have demonstrated that sea snakes do not drink seawater. Dehydrated sea snakes avoid drinking seawater but rather drink fresh water to maintain their water balance ( Lillywhite et al. 2012). Hydrophis platurus dehydrate at sea during periods of drought, and in offshore waters near Guanacaste, Costa Rica, they potentially withstand as long as six months of drought. When the dry season ends, these snakes drink from the freshwater lenses that form on the surface of the ocean during periods of heavy rainfall ( Lillywhite et al. 2014, 2019). Following drought, these snakes are in a low state of body condition and have significantly lower total body water than conspecifics captured during the wet season ( Lillywhite et al. 2014). Logically, free-ranging snakes would not be in a dehydrated condition if they drank and processed seawater. Regional precipitation over the ocean is reduced during the dry season, and climatically induced stochastic periods of drought could pose serious threats to the well-being of sea snake populations. Cryptic features of H. platurus could be important in protecting snakes from aerial predation during critical behaviors such as drinking fresh water from oceanic surface lenses.

In summary, cryptic coloration is important to the wide-ranging H. p. platurus because of the diel behaviors that position these snakes on the ocean’s surface where they are subject to possible predation from above as well as attacks from fishes or mammals in pelagic waters. In contrast, the conspicuously colored xanthic subspecies H. p. xanthos has evolved more nocturnal behavior that involves resting or floating at the surface, and they do not appear to aggregate specifically along slicks. Both subspecies and color forms are subject to predation, and the brilliant yellow coloration is likely to function in aposematic contexts, reflecting the significant pressures of predation upon this species. Actual observations or other evidence of predation on the xanthic snakes in Golfo Dulce are less numerous than expected, which offers some credence for the effectiveness of the aposematic signal ( Solórzano and Sasa 2024). Conservation is important in the context of the broader ranging services of this subspecies as a bioindicator of marine health and climatic impacts, and for the xanthic subspecies in the context of its limited geographic isolation where it is inherently vulnerable to climatic and anthropogenic impacts ( Bessesen et al. 2023). Indeed, with the lack of much genetic exchange with sea snakes from the broader Indo-Pacific, H. p. xanthos has become a highly discrete organism and ‘evolutionarily significant unit’ with unknown risks of extinction, and it may ultimately be considered for status as a full species ( Bessesen and Galbreath 2017). Monitoring populations of both subspecies of H. platurus may provide insights into the status of other marine fauna and the health of marine habitats.

Acknowledgements. —I am grateful to numerous persons who assisted me in the field during numerous visits to Costa Rica, over which time I developed ideas concerning the camouflage of Yellow-bellied Sea Snakes. Adán Barrera provided excellent boat transportation and assistance in locating sea snakes. I am grateful to A. Solórzano and M. Sasa for managing the permits (018-2009-ACAT, DNOP-002-2010, DGT-013-04-2010, ACG-PI-012-2010, 129-2011-SINAC, 069-2012-ACAT, PI-ACAT-053-2012, DGVS-171- 2013, ACT-OR-DR-055-17). I also thank C. Sheehy III, J. Pfaller, J. Wixson, H. Heatwole, M.-C. Tu, M. Edwards, M. Sandfoss, J. Lillywhite, S.R. Lillywhite, S.M. Lillywhite, and J. Crowe-Riddell for assistance with the observations of snakes. S. Boucher provided accommodation during our studies and was helpful in many ways. Harold Heatwole and Coleman Sheehy III provided comments for improving an earlier draft version of the manuscript. This research was conducted within guidelines and approval of the University of Florida IACUC (200902798; IC00000701; IC00001629; IC00003534) and was supported by the National Science Foundation (IOS-0926802 to HBL).