The 'ghost of settlement past' determines mortality and fecundity in the barnacle, Semibalanus balanoides

Intraspecific competition is common in many organisms. For many sessile marine invertebrates high settlement densities can lead to competition with conspecifics that can affect the probability of mortality, morphology, and reproductive output. This work aims to determine the affect of recruit density on shell morphology and fecundity and the temporal nature of intraspecific competition for the acorn barnacle, Semibalanus balanoides . Two tile arrays were used; experiment 1 was deployed at the start of the settlement season (43,264 recruits, 4.1% survivorship to adult), experiment 2 was deployed in the middle of the season (5,454 recruits, 42.8% survivorship). Experiment 2 displayed density dependent mortality between recruits and juveniles and adults, whereas no relationship was found for exp. 1. Experiment 2 had higher egg production per individual, but also higher egg production per area of tile compared to exp. 1. A negative relationship was found between recruit density and egg production, however during the summer period, this relationship switched to a positive relationship between juvenile density and egg production, due to high mortality on tiles with high recruit density. Cumulative population density (CPD) was calculated as the cumulative mean density of barnacles per cm^-2 of tile and was used as an index of intraspecific competition. There was a linear negative relationship between mass of eggs per individual and CPD. However, CPD over the summer period more fully explained egg production per individual. A time window was identified when cumulative intraspecific competitive effects were shown to have a strong negative effect on egg production and an increase in mortality, causing a flip from density independent fecundity to density dependent fecundity. These results suggest that recruit density is an important driver of subsequent population processes.