Description: One of the central issues in ecology is the identification of processes affecting the population structure and dynamics of species with complex life cycles. In such species, variation in both the number of larvae that enter a population and their phenotype are important drivers of survival and growth after metamorphosis. Larval experience can have strong effects on key post-metamorphic traits, but the temporal scale of such ‘trait-mediated effects’ may be short, and their magnitude may depend on the environment experienced after metamorphosis. We used an intertidal barnacle to study the long-term consequences of trait-mediated effects under different post-metamorphic conditions by manipulating larval food concentration and monitoring patterns of survival and growth in juveniles at 2 intertidal levels over a 5 mo period. In 2 replicated experiments, higher food levels resulted in increased body size, mass and reserves (measured from elemental composition) in the settling larval stage and increased body size of newly metamorphosed juveniles. In Expt 1, high food concentration reduced juvenile mortality at low intertidal levels, while on the upper intertidal, mortality was high for all larval food concentrations. By contrast, in Expt 2, low larval food concentration decreased juvenile survival at both shore levels. When present, effects were established early (Weeks 1 or 2) and persisted for over 10 wk in Expt 1 and 22 wk in Expt 2. Interactive effects of the larval and juvenile environments can have important implications for population size: trait-mediated effects may persist for long periods, helping to explain patterns of adult abundance.

Description: In species with complex life cycles, laboratory studies have shown that variations in the traits of settling larvae can affect post-settlement survival and influence recruitment and benthic− pelagic coupling. However, we still know little about the magnitude and spatial scale of natural trait variation. We studied spatial variation in body size and nutritional reserves (carbon, nitrogen and lipids) of settled cyprids of the barnacle Semibalanus balanoides along the coast of West Scotland. We quantified variation among regions (north vs. south: range ~700 km), locations (~50 km), shores (~10 km) and within shores (~10 m). We also evaluated trait responses to gradients in chlorophyll and shore openness and compared swimming vs. settled cyprids in order to infer the likely influence of costs of substratum search on trait variation. Variability between regions was large, with higher trait values (e.g. carbon cyprid−1: 35 to 50% higher) in the north. Most traits correlated negatively with pelagic chlorophyll a (a proxy for larval/juvenile food availability); this counter-gradient pattern suggests an adaptive role of increased reserves, buffering benthic juveniles from low food availability during the critical early post-settlement period. Body size and nitrogen content correlated positively with shore openness; lower than expected carbon content suggest increased costs of substratum search on open shorelines. Higher nitrogen content but lower percent carbon was found in settled vs. swimming larvae, suggesting costs of sub - stratum search at the time of settlement. Overall, we uncovered the spatial scales at which trait variation, shaped by pelagic processes, can affect post-metamorphic survival, recruitment and benthic−pelagic coupling.