Environmentally-driven variations in reproductive traits in a sandy beach bivalve throughout its geographic range

Abstract

The yellow clam Amarilladesma mactroides (Reeve, 1854) inhabits the intertidal zone of sandy beaches along thetemperate Atlantic coast from Brazil to Argentina (24◦-41◦S). Over the past decades, there has been a noticeabledecline in its abundance, underscoring the importance of understanding their reproductive strategies andphenotypic plasticity for conservation efforts. This study explored large-scale variations in the size at first sexualmaturity (SL50), the age at maturity (A50), the vitellogenic oocyte area (VOA), and a reproductive age index (RAI)that combines A50 and the lifespan of each clam population, in relation to local habitat characteristics (e.g.,beach morphodynamics) and large-scale environmental variables, such as salinity, sea surface temperature (SST)and chlorophyll-a. Four sandy beach clam populations from Uruguay and Argentina (latitudinal range: 33◦45′S -38◦56′S) were sampled monthly between June and August during the reproductive season. SL50 increased linearlywith the mean and maximum SST (SSTmean, SSTmax), while VOA and RAI decreased with tidal range andwidth of intertidal zone, respectively. In beaches that had higher SSTmean and SSTmax, yellow clams reachedmaturity earlier and spent more than 70% of their lifespan in reproduction. The yellow clam population at thelowest latitude showed the largest oocyte sizes and a shorter lifespan of 3.5 years, while the population at thehighest latitude had a longer lifespan (7 years) and the smallest VOA values. Populations at intermediate latitudesshowed delayed sexual maturity, a 50–60% investment of their lifespan in reproduction, the longestlifespan (up to 9 years) and intermediate VOA values. The influence of the Rio de la Plata and Bahia Blancaestuaries modified the expected latitudinal gradient in reproductive traits, but local habitat conditions prevailedover large-scale environmental variables as explanatory factors of the reproductive strategy of the yellow clam.Therefore, the species displays phenotypic plasticity in its reproductive aspects to ensure population success.