Temperature Physiology of the Sea Snake Pelamis Platurus
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Proc. Nat. Acad. Sci. USA Vol. 68, No. 6, pp. 1360-1363, June 1971 Temperature Physiology of the Sea Snake Pelamis platurus: An Index of Its Colonization Potential in the Atlantic Ocean (sea-level canal/surface isotherms/distribution/thermal death) J. B. GRAHAM*, I. RUBINOFF*, AND M. K. HECHT * Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Canal Zone; and Department of Biology, Queens College, City University of New York, Flushing, N.Y. 11367 Communicated by Carl L. Hubbs, April 21, 1971 ABSTRACT The yellow-bellied sea snake Pelamis therms that delimit its distribution. It is apparent (Fig. 1) platurus occurs throughout the tropical Indian and that the 18'C isotherm closely agrees with the distribution Pacific Oceans from east Africa to Central America. Its latitudinal distribution limits coincide with the 18'C limits of P. platurus. The capacity of this species for survival surface isotherm. P. platurus has upper and lower thermal at a particular latitude depends on seasonal temperatures. tolerances of 36.0 and 11.70C. With rapid cooling, P. Presumably, the northern- and southernmost dispersal rec- platurus stops feeding at 16-18'C; however, it has a high ords were for the summer months of each hemisphere, but this resistance to cold temperature and can withstand 50C for 1 hr. After 10 days' exposure, P. platurus does not cannot be confirmed from the literature. If we use the 18'C acclimate to 17'C and, thus, would not be able to survive isotherm as an index to the potential distribution of P. plat- for long periods in water this cold. urus, it appears that large areas of the Atlantic Ocean are In the event of its transit through the proposed Central thermally suitable for this species (Fig. 1). Our principal American Sea-Level Canal, P. platurus would colonize the objectives have been to determine the potential extremes of Atlantic Ocean and, during the summer months, would be able to extend its north Atlantic distribution to as far its distribution, given its thermal tolerances, and to verify the as Cape Cod and the English Channel. reliability of the 18'C isotherm as a basis for explaining both its present distribution in the Indian and Pacific Oceans and The relatively weak swimming habits of the yellow-bellied sea predicting its potential dispersal in the Atlantic Ocean. In snake Pelamis platurus (Linnaeus), together with its tendency addition to determining tolerances, we have examined the to stay in drift lines for feeding (1), facilitate the transport of capacity of P. platurus for thermoregulation and low-tempera- this species by oceanic currents. P. platurus normally occurs ture acclimation and have observed its response to rapid throughout the tropical Pacific and Indian Oceans from East changes in temperature. Africa to Central America, and occasionally penetrates into subtropical and temperate latitudes (Fig. 1) (2-8). The dis- MATERIALS AND METHODS persal of P. platurus into the high latitudes of the western All experiments were run, within 30 days of collection, on 67 Pacific is facilitated by the warm Kuroshio Current that has snakes collected in Panama Bay near the Pearl Islands. The occasionally carried the snake as far north as the coastal snakes were held in large tanks at ambient temperatures (27- waters of southern Siberia (2). To the south, the East Aus- 28°C) and salinities (26-300/oo). Total lengths were 370-740 tralian Current has extended P. platurus to as far south as mm (median = 620) and weights were 19-150 g (median = Tasmania (3). The Agulhas Current occasionally carries a 91). few P. platurus from the coast of East Africa to Table Bay, beyond the Cape of Good Hope. The cold, upwelled coastal Lethal temperature determination waters of the Benguela Current ensure that no individuals Upper and lower tolerance temperatures of P. platurus were penetrate into the Atlantic Ocean (6, 7, 9). determined by the Up-Down procedure (14), which, in gen- In the eastern Pacific, P. platurus is able to extend its eral, allows estimation of means and their variances in ex- summer range north through most of the Gulf of California periments that in some way alter the constitution of an and to the outer coast of Baja California. This is facilitated by organism so that it cannot be further tested (as would the northerly movement of equatorial Pacific water that dis- exposure to near lethal temperature). The Up-Down places the California Current seaward (10). The southern procedure concentrates testing near the mean value and, range limit of P. platurus in the eastern Pacific is held fairly because it requires separate testing of each snake, it is constant at about 50C during all seasons because of the influ- accurate with smaller sample sizes; however, it does not ence of the cold Peru Current (9). allow the determination of the lethal temperature range. In The potential colonization of P. platurus in the Atlantic determining the thermal tolerances of P. platurus, each snake Ocean, if a sea-level canal were to be constructed across Cen- was tested only once, by being placed directly into the test tral America, has been discussed by a number of authors (11- temperature and left for 24 hr. It was then removed to ambient 13). We have examined the relationship between the present sea water (26-27°C) and monitored for 12 hr. The particular distribution of P. platurus in the tropical Pacific and Indian test temperature to which the snake was exposed was con- Oceans and the hemispheric winter and summer surface iso- sidered lethal if the snake died within the 36-hr period. 1360 Downloaded by guest on September 24, 2021 Proc. Nat. Acad. Sci. USA 68 (1971) Temperature and Pelamis platurus 1361 FiG. 1. Relationship between the 18'C February (dashed lines) and August (solid lines) surface isotherms and the distribution of P. platurus (dots) in the Indian and Pacific Oceans. The 180C isotherms in the Atlantic Ocean constitute the approximate potential distribu- tion limits of P. platurus in that ocean. Isotherm data are from Neumann and Pierson (8). Distribution records are from various published sources (2-7), and from collections in the Museum of Comparative Zoology and the National Museum of Natural History. Response to temperature change snakes in the holding-tank water column was observed. Snake To determine the response of P. platurus to gradually changing positions were scored arbitrarily as either at the surface (upper temperatures, specimens were heated or cooled at constant 3 cm of water) or not. rates from ambient sea water. For cooling, temperatures at RESULTS AND DISCUSSION which eight snakes (a) stopped swimming, (b) could not hold Thermal tolerance limits their heads up, and (c) became torpid were recorded. Another The upper and lower lethal temperatures of P. platurus are group of six snakes was heated, and the temperature at which 36.0 + 0.30C and 11.7 i 0.20C (mean 4i SE). After prelimi- they entered heat shock and died was recorded. nary testing, eight snakes were used to determine the upper- Thermal acclimation lethal limit. The test-temperature sequence was 34-35-36- 37-3635-36350C. Ten snakes were used to determine the To test the capacity of P. platurus for low-temperature accli- lower-lethal limit, the test sequence was: 12-11-12-11-12-11- mation, five snakes were acclimated to 170C for 10 days and 12-13 12-11'C. Lethal temperatures are underlined. were then tested for their response to changing temperatures The lethal temperatures found by this method agree with as above. the data of Ehlert (15), who found upper- and lower-lethal Feeding response with changing temperature ranges of 33-370C and 10-120C for a Mexican population of P. platurus. This species never experiences temperatures as Six P. platurus were fed daily on small (20-50 mm) fish. Pre- extreme as 36 and 110C and, as with most organisms, the liminary tests showed that the snakes could eat between 10 and range of thermal tolerances exceeds the range of environ- 15 fish at each feeding. Snakes were cooled and the persistence mental temperatures (16). This points out the ecological of the feeding response with changing temperature was ob- importance of sublethal temperatures that influence distribu- served. One or two fish were offered at each test temperature tions by affecting such processes as reproduction and feeding. to prevent satiation of the snake. Response to changes in temperature Thermal regulation Specimens of P. platurus that were heated from ambient tem- P. platurus may be able to regulate its body temperature perature at a constant rate (50C/hr) became hyperactive at by basking under the sun at the surface to absorb heat 360C. At 390C, the snakes became comatose and soon died. and by diving into deeper, cooler water to lose heat. To Attempts to reverse the comatose condition by returning the test this idea, we measure the body temperatures of five animals to water at 260C were unsuccessful (Table 1). This snakes exposed to sunlight in a shallow (40-cm) pool at snake can be cooled rapidly to temperatures below its lethal different water temperatures. Cloacal temperatures were tolerance limit. At a cooling rate of 170C/hr the snakes lost measured with a Schultheis thermometer within 30 sec after their capacity to swim at 8.50C. Below 7.5CC, they were the snake had been caught and held in a net. Care was taken unable to raise their heads to the surface and respire. At to keep the snakes in water and the experimenters wore gloves temperatures below 60C, they were in deep torpor and did not to insure minimal conduction of body heat to the snakes.