Pelagic sea snakes dehydrate at sea

Abstract

Secondarily marine vertebrates are thought to live independently of fresh water. Here, we demonstrate a paradigm shift for the widely distributed pelagic sea snake, Hydrophis (Pelamis) platurus, which dehydrates at sea and spends a significant part of its life in a dehydrated state corresponding to seasonal drought. Snakes that are captured following prolonged periods without rainfall have lower body water content, lower body condition and increased tendencies to drink fresh water than do snakes that are captured following seasonal periods of high rainfall. These animals do not drink seawater and must rehydrate by drinking from a freshwater lens that forms on the ocean surface during heavy precipitation. The new data based on field studies indicate unequivocally that this marine vertebrate dehydrates at sea where individuals may live in a dehydrated state for possibly six to seven months at a time. This information provides new insights for understanding water requirements of sea snakes, reasons for recent declines and extinctions of sea snakes and more accurate prediction for how changing patterns of precipitation might affect these and other secondarily marine vertebrates living in tropical oceans.

Keywords: Hydrophis (Pelamis) platurus; dehydration; drought; pelagic marine vertebrate; precipitation.

Figure 1.

Spatial and temporal patterns of precipitation during the years 2010–2012. (a) Daily average rainfall for the tropical Indo-Pacific during the years 2010–2012. (b) Daily average rainfall during the dry season, December–May, 2010–2012. (c) Daily average rainfall during the wet season, June–November, 2010–2012. Data are from NASA metadata project TRMM v. 7, multisatellite precipitation. Red star represents Golfo de Papagayo, Costa Rica.

Figure 2.

Patterns of monthly rainfall and the percentages of sea snakes (H. platurus) drinking fresh water (FW) during three drought cycles at Golfo de Papagayo, Costa Rica. Plots for rainfall are monthly totals, and the snakes drinking are percentages of the snakes sampled (n = numbers next to data points) that drank FW immediately following capture from the open sea. Note that FW drinking increases following periods of several months without significant rainfall. FW drinking decreases following periods having large amounts of precipitation (see text for further explanation of the patterns). Data for rainfall are from NASA, TRMM 3B43 v. 7, and reflect monthly totals for a 25 × 25 km quadrat of ocean centred at the area from which snakes were collected. The data point representing a single snake (right-hand ‘dry season’) is included for completeness, although unusual conditions of a red tide combined with cold water, turbidity and high winds prevented a larger sampling of snakes at that time.

Figure 3.

BCI related to consumption of fresh water by sea snakes (H. platurus) collected over three wet–dry cycles at Golfo de Papagayo, Costa Rica. Snakes that drink fresh water immediately following capture have significantly lower BCI than do those not drinking (a) (ANOVA, log-transformed data, p ≤ 0.0001), and the amount of water ingested (% original body mass) varies inversely with the BCI (b) (r² = 0.067, p = 0.0156).