We investigated non-climatic thermal adaptation in the tropical, eulittoral-fringe snail, Echinolittorina malacanna
), by determining the role of solar radiation in controlling maximum body temperature (Tb(max)
) and the relationship between Tb(max)
and the thermal limits of physiological performance. We studied a population of E. malacanna
snails found between 2 and 5 m above chart datum (mean high water level approx. 2.5 m CD) on artificial seawalls at Pantai Tungku in Brunei Darussalam (4°32′ N; 114°43′ E). When immersed in air, this snail species withdraws its foot, cements its shell to the rock surface and closes its operculum, a behaviour that prevents heat conduction (by excluding tissue contact with the rock substratum) and limits evaporative cooling (snails lose water at around 0.01% body mass h−1
over 32 days at 30°C, D. J. Marshall 2009, unpublished data; McMahon (1990)
). Such behaviour can be maintained for weeks during continuous aerial emersion when tidal conditions are calm.
Field operative body temperature (Tb), rock surface temperature (Tr), habitat air temperature (Tah; 7 mm above the rock surface), and solar irradiance were recorded at 1 min intervals for 21 days (August/September 2009). Tb was measured within silicone-filled shells (7 mm length) glued to horizontal rocks, with maximum exposure to direct sunlight. Tb was determined for two rock types with different thermal properties: a light sandstone rock (‘light’) and a darker, ferruginous sandstone rock (‘dark’). Only the light rock, which resembled the habitat substratum, was used for all other temperature measurements. Live snail body temperature (Tb(live)) closely correlated with the model's body temperature on the light rock (r = 0.9086, p < 0.001, n = 60; for Tb(light) between 35°C and 43.5°C determined in direct sunlight). Simultaneous recordings of local ambient air temperature (Tam; 600 mm above the rock surface), Tb(light) and solar irradiance were also taken every 10 min over 7 days, after the 21 day recording period. Temperature was measured using fine, K-type thermocouples (shielded from the wind and sun), and logged using Fluke 54 Series II recording thermometers. Global solar irradiance was recorded using a CM11 Kipp and Zonen pyranometer and logged using an ADInstruments PowerLab 4/SP and Chart v. 5. Data analyses were based on recordings for 12.30 to 14.30 each day, or for local ambient air temperature, between 10.00 and 14.30.
Climatically relevant, regional ambient air temperature (Tam-reg, independent of the re-radiative heating on the rocky shoreline) was determined from within a Stevenson screen at the Brunei International Airport (6200 m from the study site, 5200 m inland and approx. 10 m in altitude; courtesy of the Meteorological Service, Ministry of Communications). Monthly maxima for Tam-reg showed no seasonal variation (mean ± s.d: 34.29 ± 1.08°C, n = 33 for 2006, 2008 and 2009 until September).
Heart function was used to assess physiological adaptation to temperature (see Somero 2002
). Active snails (shell length, 7–9 mm) were returned to the laboratory, where they were rinsed in filtered sea water, dried with blown air and fitted with infrared sensors (Vishay Semiconductors, CNY70). Signals, sampled at 40 Hz, were amplified and filtered using a custom-built preamplifier, and digitally logged (PowerLab/4SP, Chart v. 5). Heart rate (HR), determined from triangular Bartlett smoothed traces, was logged every 1 min for snails (n
= 7) held in bags in a Grant GP200 programmable water bath and experiencing constant heating (0.25°C min−1
), between 30°C and 60°C. The Arrhenius breakpoint temperature (ABT) for heart function was determined from piecewise regressions (Statistica
v. 6.1, StatSoft) from the plot of the natural log of HR against the inverse Kelvin temperature, between 45°C and 57°C. Lethal temperature was based on the endpoint of heart function (EPT), when the heart stopped beating; this closely matches the LT50
of this species (D. J. Marshall 2009, unpublished data). Although heart function often recovered when the temperature was lowered after EPT was reached, no snails re-emerged from their shells within 24-h at 30°C, a timeframe typically used to test lethal effects (Somero 2002