Seasonal changes in feeding rates and respiratory metabolism in two cohorts ofHelix aspersa (Müller) across an extreme heat wave: laboratory and field studies

Abstract: Seasonal evolutions of organic growth rates, metabolic rates and organic assimilationrates of two actively growing age classes of brown garden snail (Helix aspersa) from a wildpopulation have been studied during 2003-2004. Organic growth rate was calculated for each ageclass as differences between samplings divided by time elapsed. Maximal rates were observedthroughout the spring season once snails had aroused from hibernation (~50 to 100J d-1). Anextreme heat wave induced an aestivation period associated to weight loss (in September minimalgrowth rates are recorded: ~ -15.7 to -24.7J d-1) degrowth increasing in smaller animals. Positivenet growth was recovered in autumn before the hibernation period, activity being resumed in thenext spring. Continuous increase of metabolic rate observed during the spring, due mainly toindividual weight gain (effects of temperature increase being exclusive to the month of June)allows higher activity levels and subsequent higher ingestion and assimilation rates. Maximalassimilation rates occur in July: 225J d-1 for young immature snails and 360J d-1 for adultreproductive specimens. Minimal and maximal routine metabolic rates of active snails recordedin this study oscillate between 25 to 60J d-1 in April (for non-reproducing and mature snailsrespectively) to 175 to 275J d-1 in August. Although oxygen consumption of hibernating snailswas not recorded, values registered at the peak of negative organic growth rate in mature snailscould be taken as an approaching value: 25.8J d-1 animal-1. Since 114.7J d-1 animal-1 can be takenas winter active metabolic rate acute metabolic depression would represent 22.5% of that ofactive snails at the onset of hibernation. A 3 month period of hibernation seems to occurrepeatedly (2005, 2006), whereas the recorded aestivation period was unexpected, beingattributed to the singular weather conditions during the summer of 2003, indicating that Helixaspersa dwelling in this locality would behave as facultative aestivators.