The differences between male and female lobsters can only be seen when they are turned on their backs. In the male lobster the two swimmerets (forked appendages used for swimming) nearest the carapace (the solid shell) are hard, sharp, and bony; in the female the same swimmerets are soft and feathery. The female also has a receptacle that appears as a shield wedged between the third pair of walking legs. During mating the male deposits sperm into this receptacle, where it remains for as long as several months until the female uses it to fertilize her eggs as they are laid.

The gender of an alligator is determined by the temperature at which the eggs are incubated. High temperatures of 90 to 93°F (32–34°C) result in males; low temperatures of 82 to 86°F (28–30°C) yield females. This determination takes place during the second and third week of the two-month incubation. Further temperature fluctuations before or after this time do not alter the gender of the young. The heat from the decaying matter on top of the nest incubates the eggs.