Planktonic larval durations (PLDs) affect the ecology and evolution of benthic marine populations through their effects on dispersal distance, the spread of sibling larvae, the level of larval mortality, and population connectivity. Despite the importance of PLDs, field estimates of PLDs in marine invertebrates are rare and restricted to specific taxa. The size of the first post-metamorphic feeding structure (the ancestrula lophophore) in Bugula neritina has been shown previously to decline with increases in larval duration. We hypothesised that if the relationship between larval duration and the size of the ancestrula lophophore can be estimated in the laboratory, then it should be possible to infer the PLD of B. neritina settlers in the field. We manipulated the length of the larval period in the laboratory and quantified the relationship between larval duration and the volume of the ancestrula lophophore. We then measured the size of lophophores in field-settled individuals at 2 sites on multiple days over a 2 yr period, and predicted their planktonic durations based on our laboratory-derived classifications. The average percentage of individuals in the field with short (0 to 1.5 h), intermediate (1.5 to 6.5 h), and long (6.5 to 32 h) PLDs was 54, 27, and 19 , respectively. Despite being competent to settle upon release from the colony, a significant proportion of settlers in the field experienced metamorphic delays that would incur significant post-settlement costs. Furthermore, there was a positive relationship between settlement density and the proportion of settlers in any one cohort that had experienced short PLDs. The extended planktonic periods that we predicted are likely to affect the dynamics of adult populations directly via reductions in settlement density and indirectly via reductions in the post-settlement performance of individuals that experienced a metamorphic delay before settling.