Vertical zonation and heat tolerance of three littorinid gastropods on a rocky shore at Tanjung Chek Jawa, Singapore

Three common rocky shore littorinid taxa, i.e., Littoraria spp. (a collective term for L. strigata and L. articulata), Echinolittorina malaccana and E. vidua occupied different tidal heights based on fiobservations carried out at Tanjung (=Cape) Chek Jawa, Pulau (=Island) Ubin, Singapore in 2002. Littoraria spp. were consistently observed at a lower level on the shore than E. malaccana and E. vidua. Manually translocated littorinids returned to their preferred zones in the fi eld, i.e., E. malaccana returned to the region above the MHWS level of 2.7 m while Littoraria spp. remained below the region occupied by E. malaccana. Further, E. malaccana individuals with their shell nodules removed by fi ling did not occupy a lower zone than intact conspecifi cs in the fi eld. Although signifi cant temperature differences (TD) were observed between rock and shell surfaces in the laboratory (TDLittoraria ≈ TDfi led E. malaccana < TDE. malaccana), preference for a particular height on the shore is probably governed more by physiological factors (e.g., heat tolerance) than by morphological adaptation (e.g., shell morphology). As the upper shore is exposed to the sun for longer periods of time, it is expected that the heat tolerance of inhabitants of the higher shore would be greater than those occupying the lower regions. Results of a 1-hour lethal temperature experiment support this hypothesis: the temperature at which 50% mortality (1h LT50) was observed was highest for E. malaccana (50.4°C) compared to E. vidua, (48.1 °C) and Littoraria spp. (47.5°C), possibly refl ecting their relative tidal positions on the shore. Enzyme stability may also account for high heat tolerance of E. malaccana. Incubation of the enzyme, glutamate oxaloacetate transaminase (GOT) at 55°C showed that E. malaccana sustained high GOT activity compared to those of Littoraria spp. and E. vidua which decreased sharply at the same temperature, again suggesting inherent physiological adaptation in E. malaccana.

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