Safe from sunburn: The divergent diel pattern of a Hydrophis sea snake

Abstract

Diel activity patterns are an important aspect of wildlife ecology and evolution and provide valuable information for conservation and monitoring, yet for many species, activity patterns remain unstudied and may be presumed to mirror related taxa. Here, we describe the distinct diel patterns of an endemic population of venomous sea snakes Hydrophis platurus xanthos inhabiting a narrow range (circa 320 km2) in Golfo Dulce, Costa Rica. To investigate, we conducted a systematic visual survey over five 24‐h cycles and evaluated 339 h of previously obtained sighting data from different studies spanning a decade. While sporadic diurnal surfacing does occur, mostly for respiration, our observations revealed marked crepuscular peaks with regular surfacing through the night. We also report on observed surface behaviors that were also found to vary in frequency at different phases of the photoperiodic cycle. In particular, we show feeding as more common at night. Hydrophis platurus xanthos has developed a circadian rhythm that differs noticeably from its taxonomic parent (H. p. platurus is reported as diurnal across its Indo‐Pacific range), and no congeners have been categorized as crepuscular. Our work thus contributes to the ecological knowledge of this evolutionarily distinct marine elapid and offers insights into the potential role of environmental conditions in shaping animal activity.

Population Abundance and Density Estimates for Costa Rica’s Endemic Sea Snake, Hydrophis platurus xanthos

“…Inside Golfo Dulce, that team published an encounter rate of ~21 xanthic sea snakes per hour during two nights of collection (17/h the first night and 25.7/h the second night). This is comparable to the highest encounter rate for xanthic sea snakes ever recorded by the lead author (29/ hr during an around-the-clock diel study; Bessesen & González-Suárez, 2022); however, across all studies, variance between nightly counts was apparent, likely associated to environmental conditions that are more or less suitable for surfacing. Hence, Lillywhite's team may have benefitted from a small sample collected under ideal conditions.…”

“…Transects were surveyed from 5-13 February 2020 following protocols in Buckland et al, 1993;Buckland et al, 2001;Buckland et al, 2015). As H. p. xanthos is nocturnal (Bessesen and González-Suárez, 2022), surveys were completed at night with observation hours typically beginning just after sunset and ending between 2200-0200 h. An expert local boat captain navigated our 7-m research vessel at 8-13 km per hour (generally 12 km/h) using a handheld Garmin GPSMAP 64 to maintain course following the transects. Three LED lamps were attached to the bow of the boat to illuminate forward waters from 90 degrees port (left) to 90 degrees starboard (right) and extending out to approximately 20 m distance.…”

“…To address potential biases resulting from this, we developed an activity bias correction method. We estimated surface activity rates using sighting data from a diel study with constant hourly survey effort across multiple 24-hour periods (Bessesen and González-Suárez, 2022). This was done by fitting a kernel density in the R package 'activity' (Rowcliffe, 2021), with a 1.5 kernel bandwidth multiplier as recommended by Rowcliffe et al (2014), to calculate surface activity rates for each clock hour.…”

“…These corrections required making some assumptions, especially using dive data from a related species when diving behaviors may differ. For example, pelagic sea snakes dive to avoid turbulent waters (Rubinoff et al, 1986) while xanthic sea snakes regularly surface in higher sea states (BWF 3-5;Bessesen and González-Suárez, 2022). If having more flexibility in surfacing conditions and/or a steady diel pattern results in H. p. xanthos spending more time at the surface, our corrections could have overestimated the population size.…”

“…obs. ), weather, lunar phase, productively, and other factors that have not yet been identified (Lillywhite et al, 2015;Bessesen & González-Suárez, 2022). Precipitation, for example, could draw more individuals to the surface and/ or extend the surfacing duration at a particular moment since sea snakes are known to drink from fresh-water lenses formed by rain (Lillywhite et al, 2014.…”