Bisexual breeding, also known as hermaphroditism, describes a biological condition where an organism possesses both male and female reproductive organs or capabilities. A single individual can produce both male gametes (sperm or pollen) and female gametes (eggs or ovules). This reproductive strategy is observed across various species.
Types of Organisms with Bisexual Reproduction
Organisms exhibiting bisexual reproduction fall into two main forms: simultaneous hermaphroditism and sequential hermaphroditism.
Simultaneous hermaphrodites possess functional male and female reproductive organs throughout their adult lives, producing both sperm and eggs concurrently. Examples include earthworms, many snail species, slugs, barnacles, and certain fish like the chalk bass.
Sequential hermaphroditism involves a change in sex during an organism’s lifetime. Individuals are born as one sex and later transition to the other. There are two types: Protandry describes organisms that are male first and then change to female, as seen in clownfish. Protogyny refers to organisms that are female first and later change to male, a common occurrence in many wrasses and moray eels.
How Bisexual Organisms Reproduce
Bisexual organisms employ various mechanisms for reproduction, including self-fertilization and cross-fertilization.
Self-fertilization, or “selfing,” occurs when an individual organism fertilizes its own eggs with its own sperm. This strategy is observed in some plants, such as the mangrove killifish, and certain parasitic animals like tapeworms.
Cross-fertilization, or “outcrossing,” involves two bisexual organisms exchanging gametes. In many simultaneous hermaphrodites, like earthworms and some snails, individuals act as both male and female during mating, reciprocally exchanging sperm. This exchange promotes genetic diversity in the offspring. Many hermaphroditic species capable of self-fertilization also retain the ability for cross-fertilization.
Why Bisexual Reproduction Evolves
Bisexual reproduction offers several evolutionary advantages, particularly in certain ecological contexts. One benefit is mate assurance, allowing reproduction even when mates are scarce or difficult to find, which is especially beneficial for sessile organisms or those in low-density populations.
Bisexual reproduction can also lead to increased reproductive output. In some species, an individual can produce more offspring by functioning as both sexes than if it were a single sex. Sequential hermaphrodites can optimize resource allocation by changing sex based on factors like size or social hierarchy within a group.
While offering advantages, bisexual reproduction also involves trade-offs. Maintaining two sets of reproductive organs can be energetically demanding, requiring a greater investment of resources compared to specializing in one sex. Self-fertilization, while ensuring reproduction, can lead to reduced genetic diversity and an accumulation of harmful recessive traits, a phenomenon known as inbreeding depression. However, species that regularly self-fertilize may exhibit less inbreeding depression due to the purging of deleterious genes over time.