A distinguishing feature in the sexuality of crustaceans from that of insects is the prevalence of hermaphroditism in different taxa. Fertilization occurs in hermaphroditic crustaceans both by selfing (self-fertilization) and by outcrossing (mating with separate individuals). When selfing extends across successive generations, genetic variation (heterozygosity) within each genetic lineage quickly declines. Nevertheless, many crustacean species resort to self-fertilization, enforced by environmental constraints. In others, individuals may outcross exclusively, as in the simultaneously hermaphroditic barnacles, in which sperm are deposited in the mantle cavity of the female-acting hermaphrodite (pseudocopulation), often by more than one adjacent hermaphrodite (Charnov, 1979b). The spermatozoa of barnacles such as Balanus balanoides are usually immobile in the seminal fluid but become extremely active after being discharged through the penis into the mantle cavity of another individual. Oviposition normally takes place in these barnacles as a result of the stimulus of copulation (Crisp and Davies, 1955). When these balanoids were separated by a distance of 5 cm or more from its neighbors, no fertilization of eggs could happen in these isolated barnacles. The distance of 5 cm represents the maximum extension of the penis in these cirripedes. Although such obligatory cross-fertilization might be the rule for all cirripedes, isolated individuals of species like Chthamalus stellatus, Verruca stroemia, and Balanus perforates have been found to have self-fertilization (Barnes and Crisp, 1956). Nevertheless, self-fertilization is less efficient, as the eggs are less viable most of the time.

In the androdioecious species Eulimnadia texana, the hermaphrodites either undergo self-fertilization or mate with free-living males. The hermaphrodites cannot pair with another hermaphrodite for reciprocal fertilization because they do not possess the claspers. Only the males possess them to facilitate mate guarding of the hermaphrodite until mating (Sigvardt and Olesen, 2014).

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