There is some disagreement in the literature concerning the reproductive type of Crangon crangon. Boddeke (1989) proposed that Crangon crangon was a protandrous hermaphrodite with mature males 30-55 mm long and females >44 mm long. Males mate once and then change into to females, taking 2 months to do so. Other authors, e.g. Lloyd & Yonge (1947), stated that Crangon crangon was gonochoric but males were smaller and had a shorter lifespan than females. It was reported from the Solway Firth that the abundance of males varied between 6 and 82% of the adult population over the course of a year (Abbott & Perkins, 1977). This could be due to differential mortality of males and females or due to males changing sex.

Similar to lobsters and crabs, female Crangon crangon carry their eggs glued to the abdominal appendages (the pleopods) for a period of 4-13 weeks, depending on temperature (Boddeke, 1989). Egg-bearing (berried) females can be found for 46 weeks of the year but there are two peaks in numbers of berried females in the southern North Sea (Boddeke, 1989) and one in the Irish Sea (Oh et al., 1999).

Peak reproductive periods occur between April and September, when females carry up to 4,500 small 'summer' eggs approximately 370 µm across. The number of berried females decreases sharply in September but then increases again in October/November as females produce up to 2,800 larger 'winter' eggs approximately 430 µm across (Boddeke, 1982; 1989).

Onset of maturity may be temperature dependent. Maturity was reported to occur in the second year of life in the Solway Firth (Abbott & Perkins, 1977). Maturity probably occurs in the first year of life in southerly areas (Gelin et al., 2000; ICES, 2001) considering that mature males are >30 mm in length and mature females >44 mm in length (Boddeke, 1989) and that growth can be from ripe egg to 54 mm body length in the first 4 months (Boddeke et al., 1986), and up to 25 mm in the first month (Beukema, 1992).

Male Crangon crangon do not have copulatory organs. Instead, packets of sperm (spermatophores) are deposited adjacent to the genital openings of the female (Lloyd & Yonge, 1947). Copulation and spawning occur within 48 hours of mating (Abbott & Perkins, 1977), and egg extrusion takes between 4 and 8 minutes. The eggs are attached to the pleopods after copulation with secretions from a cement gland, which takes a further 30 minutes (Lloyd & Yonge, 1947).

The larvae that hatch from summer eggs are 2.14 mm long, while those from winter eggs are larger at 2.44 mm in length (Boddeke, 1982), presumably to improve survivorship at a time of year when planktonic productivity is low.