Life history strategies of the limpet Cellana toreuma and its physiological response to environmental stress / y Karen Agnes Villarta, M.Sc. Biology, University of the Philippines

Abstract

Species are generally distributed within certain environmental and biotic conditions, which are usually variable and change spatially and temporally. As a result, individuals of species vary their allocation of energy and resources into growth, reproduction, defense or self-maintenance depending on variability in environmental conditions, and this partitioning is a central concept of life history theory. Hong Kong, with its strong seasonal climate, presents a combination of physical and biological challenges to many intertidal animals. Using the limpet, Cellana toreuma (Reeve), as a model species, this thesis investigates the different strategies which enable this limpet to survive in a seasonally variable and stressful environment.

Cellana toreuma shows a highly dynamic annual population pattern, with a steep decline in abundance (from ~100 ind m^(-2) in winter to ~1.2 ind m^(-2) in summer). Mortality is linked to the physically stressful conditions in summer, when many individuals suffer from thermal and desiccation stress, as well as reduced food availability. Although some individuals survive the summer, the majority of the population (40-90%) recruits in early winter when environmental conditions are more favourable and juveniles settle to form the dominant cohort. Growth is also maximal at this time (0.21 mm 〖day〗^(-1)), as is investment in gonads which is high (gonadosomatic index, GSI values > 50%), and most individuals spawn at the end of winter preceding the summer die-off.

Physiological experiments showed that overall thermal tolerance is low and metabolic rate is high in Cellana toreuma, but differs between size (=age) classes. Under heat stress, heart rates, water loss and haemolymph osmotic concentrations were greater in small (=young) than larger (=old) individuals, while Arrhenius break temperatures (ABT) showed a lower thermal threshold for small (38.1˚C) as compared to larger limpets (41.0˚C). Similarly, variation in food supply also contributed towards the response of C. toreuma to heat stress, where ABTs decreased with both acute and chronically reduced food levels, although this was not statistically significant.

Reproductive investment (GSI) showed that C. toreuma invests heavily in reproduction. GSI, together with shell size and shell robustness, however, varied between limpets collected from different sites with varying levels of physical and biological factors. Such differences suggest variation in resource partitioning between individuals from the different sites (within 200 m to 3.5 km) into reproduction and self-maintenance.

In general, Cellana toreuma adopts a different life history strategy as compared to Hong Kong’s other limpet species, exhibiting an almost annual pattern. Being thermally sensitive, C. toreuma is vulnerable to mass mortalities in summer, but is still able to persist through adopting heavy investment in growth and reproduction and subsequent strong annual recruitment which appears to be an effective strategy in Hong Kong’s highly stressful dynamic environment. Further investigations, such as a Dynamic Energy Budget model approach, would clarify how individuals assimilate energy, and subsequently utilize this energy for physiological processes and allow predictions of how successful the life history strategy of C. toreuma may be in a changing climate.