Seasonality of the reproductive cycle for Necora puber was determined using three biological indicators: extruding sperm plugs (mating period), extrusion of eggs (spawning period) and presence of empty egg capsules (hatching period). Sperm plugs were found only in recently moulted females, indicating that mating coincides with moulting periods. Necora puber mating indicators were observed during the summer months and into autumn; spawning was observed year-round but peaked in the summer; hatching was observed throughout the summer into late autumn. Size at maturity was assessed using a variety of reproductive and morphometric criteria; estimates from reproductive criteria are generally larger than those from morphometric criteria and these differences are often indicative of ‘functional’ vs ‘behavioural’ maturity. Size at maturity estimates for N. puber in Shetland ranged from 40.0–49.9 mm CW in males and 51.8–59.5 mm CW in females; sizes at functional maturity were larger than sizes derived from morphometric indicators.