Behavioural ecology of the sand-bubbler crab Scopimera intermedia Balss, 1934 in Hong Kong

Abstract

Environmental heterogeneities at different scales impose strong selection pressures on the evolution of flexible behaviours among species. Intertidal species, in particular, experience strong fluctuations in environmental conditions, with temperature, desiccation, food supply and other factors varying over different temporal (tidal and seasonal cycles) and spatial (within and between shores) scales. To survive and reproduce in such dynamic environments, intertidal species demonstrate diverse and functional behavioural repertoires for processes such as energy acquisition, thermoregulation and reproduction.

The deposit-feeding sand-bubbler crab Scopimera intermedia Balss, 1934, which inhabits the mid to high shore of soft sediment shores in Hong Kong, demonstrates such flexibility in behaviour. Temperature plays an overarching role in governing the crabs’ activity level: the crabs emerge to be active on the sediment surface only when air temperature exceeds ~ 20 °C, below which thermal performance is low (reaching a critical thermal minimum at 13 °C). When the crabs are active, they spend ~ 80 % of their time feeding, which contributes to half the variation in the behavioural time budgets of the crabs between seasons. During the cool season, activity time reduces and foraging responses diminish (the crabs travel at lower speeds over smaller foraging areas with shorter tracks), but subsequently increase during the hot season (with longer activity times, higher speeds, larger foraging areas and longer tracks).

To optimize energy gain when foraging on sediments, which are spatially and temporally variable in food value, the crabs employ a context-dependent feeding threshold to determine their movements, based on sampling estimates of the average food value from a finite length of past foraging experience. Such a strategy allows the crabs to increase their foraging time in food-rich areas and, as a result, maximize their energy gain within the limited time set by both tidal and circadian cycles. Foraging is sustained by the crab’s multifunctional sponging behaviour (the uptake of water from sediments by pressing their bodies onto the surface), which supplies water necessary for flotation feeding, as well as evaporative cooling. Sponging reduces body temperatures of the crabs by ~ 1.3 °C (up to ~ 4 °C) on hot summer days. Such behavioural thermoregulation is critical for the crabs’ survival on the high shore, where air temperature above the sediment surface can exceed their critical thermal maximum (39 °C) for 80 % of their activity time. Waving (flexing of the chelipeds) followed sponging more than would be expected by chance, and may enhance such an evaporative cooling effect, in addition to functioning as a courtship signal.

By flexibly adjusting their time budgets on feeding, sponging, waving and other behaviours, the crabs can effectively respond to small-scale environmental variations in food distribution and temperature over their foraging areas during their activity periods. Such flexibility and the evolution of multifunctional behaviours enhance the survival of species in environments such as intertidal shores, and represent a cost-effective strategy for individuals living in these challenging habitats with strong temperature fluctuations and variable food supply according to the tidal and seasonal cycles.