Examination of the larval shell (prodissoconch) of molluscs with planktotrophic development can provide valuable information on their planktonic and early benthic life. We examined temporal variability of abundance and size among 11,994 veligers of the blue mussel (Mytilus edulis) in a coastal lagoon during settling periods between 1995 and 2009. Size and date at metamorphosis during the recruitment season were determined for 1,925 postlarvae (shell length, 255–900 µm) with prodissoconch II (PII) measurements. Emphasizing the recurrence of metamorphosis delay in the field, our study reveals a net increase in mean size at metamorphosis through time, with means for PII size ranging from 255–288 µm early in summer (after peak spawning events) to 400–422 µm (PII) during late July to early September. By estimating the “true” settlement date using the amount of dissoconch secreted after metamorphosis, such time-series analyses appropriately recapitulated the temporal pattern of mean pediveliger (competent larvae) size in the plankton. Our results demonstrate that greater settlement success rates were related to small size at metamorphosis—in particular, less than 320 µm. Seasonal increase in mean PII size occurring during the latter part of the settling period may be explained by competent veligers remaining adrift and delayed metamorphosis as a result of the lack of favorable encounters with a suitable substrate or the absence of specific trophic signals, or cues, required for stimulating settlement, thus forcing larvae to continue planktonic growth. The difference between the smallest and largest means for PII size corresponds to 122 µm of larval shell growth, or 47.8%, potentially representing a 322% difference in larval body mass at settlement.