The reproductive cycle of Arbacia dufresnii was studied in two contrasting populations on the eastern coast of Patagonia (Nuevo Gulf and San Jorge Gulf) from January 2008 to August 2010. Sea urchins from San Jorge Gulf had larger test diameter and heavier gonads than did sea urchins from Nuevo Gulf. A. dufresnii showed an annual cycle in both populations, with gametogenesis occurring mainly in the autumn and the winter. An extended spawning period took place during the spring and the summer. Sea urchins from Nuevo Gulf had a strong seasonal pattern of reproduction, with presence of gonadal stages showing re-absortion and accumulation of reserves in nutritive cells at different seasons, whereas individuals from San Jorge Gulf had mature gametes during most of the year. Interannual variation of seawater temperature affected the gamete production and spawning duration in both populations. Higher temperatures during gametogenesis enhanced gonadal growth and gametes were released early. The major population differences found in gonadal cell composition highlight the plasticity in reproductive traits of A. dufresnii, and may be responsible for the wide distribution of A. dufresnii in different environments.

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The heart urchin Abatus cavernosus shows sexual dimorphism characterized by the development of external brood pouches and the enlargement of gonopores in brooding females. Relationships between body size, gonopore size, and gonadal maturation in each sex were examined for inflection points using piecewise regression models (PRM). Opening of the gonopore occurred at 15.5 mm test length. Inflection points in the gonadal growth and gonopore diameter trajectories were clustered at smaller sizes in males (23 and 24.2 mm, respectively) than in females (25.1 and 25.9 mm), indicating sex‐specific differences in sexual maturation. Gonopore growth showed positive allometry at pre‐adult stages of development in both sexes, but isometry and negative allometry in adult females and males, respectively. Gonadal growth was initiated at smaller sizes and proceeded at a higher rate with increasing body size in males than in females. Identification of inflection points in gonopore and gonadal growth trajectories, using objective PRM, allows the determination of life stages and sexual maturation for individuals, thus providing a complementary tool for population studies.