Environmentally induced responses of Donax obesulus and Mesodesma donacium (Bivalvia) inhabiting the Humboldt Current System

Abstract

The Humboldt Current System (HCS) is one of the most productive ecosystems on earth which is deeply modified by the El Nino Southern Oscillation (ENSO). El Nino (EN, the warm phase of ENSO) and La Nina (LN, the cold phase of ENSO) cause changes in vitally important environmental parameters including water temperature, salinity and oxygen concentration. The changes elicit distinct responses in coastal populations and communities. Thus are documented for the surf clams Donax obesulus Reeve, 1854 and Mesodesma donacium Lamarck, 1818 dominating the benthic communities of sandy beaches of the HCS. Existing knowledge gleaned from field data indicates that EN and LN events affect populations of each species differently, but understanding of the mechanisms underlying the effects are lacking. The life cycle of bivalve species is characterized by a highly mobile meroplanktonic larval stage, lasting from a few weeks to several months. In the wake of EN events, meroplanktonic larval stages may define the medium- and long-term stability of shallow water-species. Unfortunately, the effects of environmental changes and temperature thresholds on both species and the larval stages of D. obesulus have not yet been addressed. The effect of salinity (on M. donacium and larvae of D. obesulus) and temperature on growth and mortality of these clams under controlled experimental conditions were tested. For this, both species were exposed to reduced salinity under normal (NTC), El Nino (ENTC) and La Nina (LNTC) temperature conditions. Growth, mortality and development of D. obesulus larvae were recorded under the different temperature regimes. During a second 48 hour in vitro-experiment at NTC and ENTC, activity and mortality of larvae under reduced salinities (25, 15 and 5 /-1) was documented. Results of the in vitro-experiments with adult and larval D. obesulus indicated that compared with NTC, adults showed lower tolerance to LNTC and higher tolerance to ENTC. Similarly, D. obesulus larvae reared under ENTC grew and developed faster compared to those reared under NTC. Larvae were highly tolerant of salinity changes (15-35). In contrast, M. donacium showed lower tolerance to ENTC, although juveniles are more tolerant than adults when exposed to ENTC and lower salinity. The distinct responses observed to environmental factors may be related to the different evolutionary origins of the two families, which are tropical for the Donacidae and temperate for the Mesodesmatidae, supporting previous hypotheses suggesting that thermal tolerance is a factor in the observed distributional changes of both species during warm and cold episodes of ENSO. The enhanced osmotic tolerance of larvae and juveniles compared to adult specimens may represent an adaptation in early life stages permitting colonization of vacant areas close to outlets of estuaries. Results of this thesis and data concerning the former and current distributions of each species clearly indicate that D. obesulus is better adapted to ENTC than to LNTC. M. donacium on the other hand, appears better adapted to normal temperate conditions of the HCS than to ENTC. Beside the in vitro-experiment of Donacidae and Mesodesmatidae the taxonomy of D. obesulus and D. marincovichi Coan, 1983 was revised. The use of shell morphological characteristics to separate both species frequently revealed incongruences in taxonomically important morphometric parameters. Thus the taxonomy of the two sympatric surf clams was revised by genetics and sperm morphology. Genetic analyses showed no significant mitochondrial differentiation within or among populations, a conclusion supported by observations of sperm ultrastructure, which proved indistinguishable between morphotypes. Thus it is concluded that the two morphotypes should be included together under D. obesulus Reeve, 1854.