Size at sexual maturity is generally taken as the size (carapace length) at which 50% of animals will be sexually mature, also known as CL50. In females, sexual maturity is assigned based on reproductive state; namely the presence of a spermatophore and/or egg mass. For males, assigning sexual maturity is more difficult as there is no obvious external indicator. However, research has indicated that sexual maturity can be assigned based on a change in the relationship between the length of the male merus of the second pereiopod (the segment closest to the body of the second walking leg, Image 1) and carapace length (Melville-Smith & de Lestang 2006).
Mean Size at Sexual Maturity
Females 65.0 mm – 87.5 mm (Melville-Smith & de Lestang 2006).
Males 72.2 mm – 95.3 mm (Melville-Smith & de Lestang 2006).
Females reach sexual maturity at smaller sizes than males, generally by around 5 – 10 mm (Melville-Smith & de Lestang 2006). Because juvenile males and females appear to have similar growth rates, this difference indicates that females reach sexual maturity younger than males. The wide range in the size of maturity found in western rock lobsters is due to the broad temperature regime found off Western Australia.
Water temperature
Size at sexual maturity generally increases with latitude, i.e. larger in the south and smaller in the north. Females reach sexual maturity (CL50) at 87.5 mm in Fremantle, decreasing to 74.9 mm in Dongara, and 65.0 mm at the offshore Abrolhos Islands. Males exhibit the same pattern, with an CL50 of 95.3 mm at Fremantle, 84.6 mm in Dongara, and 72.2 mm at the Abrolhos. (Figures 1 and 2).
This variation has been attributed to the latitudinal gradient in water temperature throughout the lobster’s distribution. There is a strong correlation between size at sexual maturity and water temperature, with smaller size at sexual maturity obtained in warmer temperatures (Melville-Smith & de Lestang 2006). Because lobsters grow faster in warmer waters, not slower, variation in growth rates is unable to explain this pattern. Instead, lobsters must be smaller because they experience precocious maturation in warmer waters, maturing at a younger age and therefore a smaller carapace length (Johnston et al. 2008, Melville-Smith et al. 2010, de Lestang 2018).
