Thriving and declining: Population dynamics of the macha (Mesodesma donacium, Bivalvia, Mesodesmatidae) along a latitudinal gradient of the Humboldt Current Upwelling System

The mass mortality of the highly dominant and abundant bivalve Mesodesma donacium along Peruvian sandy beaches represents one of the most conspicuous cases of impacts of strong El Nimo events (EN) in nearshore ecosystems, causing considerable shifts in the whole community. More than 25 years after EN-related mass mortalities began, the northern geographic distribution of this commercially important clam is still confined to northern Chile, illustrating its low ability to regain the former distribution. The present work uses a combination of ecophysiological experiments, field work and a literature data compilation. These data were used to assess (i) the significance of the biotic interactions with the spionid polychaete Polydora bioccipitalis in terms of the population persistence of M. donacium; (ii) the response of M. donacium to main abiotic changes expected during EN; (iii) the demographic processes of M. donacium at the current northern range boundary and (iv) the geographical trends in population dynamics of this species and the environmental factors driving them. Features of the association between P. bioccipitalis and M. donacium confirm that the polychaete is a parasite causing deleterious effects on the clam's performance. Evidence of the infestation in fossil shells of M. donacium from fossiliferous deposits of central and northern Chile strongly suggests a long-term association, at least since the Middle Pleistocene, between these species. Even during weak environmental stress events, this parasite can cause the mortality of adult clams, which represents a loss of overall fecundity amounting to over 90% that may delay or prevent northward recolonization. M. donacium was highly sensitive to high temperature, suggesting that the northward distribution is limited by high sea surface temperature. In contrast, it was remarkably tolerant to low salinity, particularly juveniles, which may be related to the selection of river mouths as suitable areas for recruitment. Strong freshwater-input seasonality best explained the patterns in the reproductive cycle of this clam. Owing to the limited upper thermal tolerance, the long-term expansion and dominance of M. donacium from the Pliocene/Pleistocene transition until present seems closely linked to the establishment and development of the cold Humbold Current Upwelling System (HCUS). Consistent patterns of increasing abundance, growth performance, mortality and productivity of M. donacium with decreasing latitude seem mainly related to large-scale differences in primary productivity associated to upwelling centres and river input in the HCUS. Tropical and subtropical sandy beaches in the HCUS are highly productive but unpredictable environments for M. donacium. Hence, populations developed there are ephemeral and act as sink populations, whereas those in central and southern Chile act as sources and should, therefore, be given priority in management and conservation strategies.