Sixty-nine under the sea: fascinating sexuality

The underwater sexuality is a domain as rich as it is astonishing, where the behaviors of marine animals defy our conventions. Among the most intriguing behaviors, there are practices that might make one smile, but reveal unique reproductive strategies. 

The underwater sexuality is a domain as rich as it is astonishing, where the behaviors of marine animals defy our conventions. Among the most intriguing behaviors, there are practices that might make one smile, but reveal unique reproductive strategies. 

Two colorful nudibranchs moving on an algae, illustrating underwater biodiversity.
These Mediterranean slugs Felimare fontandraui have just finished their mating and are starting to lay the fertilized eggs. 

An Unconventional Sexuality

In the marine world, the traditional classification of sexes does not always apply. Many animals, such as synchronous hermaphrodites, possess both male and female reproductive organs. These creatures, like the serrans, are capable of fertilizing their own eggs, illustrating a fascinating reproductive flexibility.

Marine Hermaphrodites

Opisthobranch mollusks, often called nudibranchs or sea slugs, are among the most remarkable examples of this sexual diversity. These animals can reproduce by aligning head-to-tail, an act reminiscent of the famous « sixty-nine« . 

  • Reproductive Mechanism: Each mollusk extends a sort of trunk from its right side, which acts as both a « penis » and a « vagina« . One inserts its trunk into the other’s orifice to exchange sperm, and vice versa. This process ensures better gene distribution, without any embarrassment.
Two delicately colored nudibranchs meet in a vibrant underwater environment.
 Detail of the copulation organs of a Nembrotha couple. Visayas, Philippines. 

Underwater Orgies

The reproductive behaviors of nudibranchs sometimes lead to spectacular orgies. In these scenes, one individual can fertilize another, which in turn fertilizes a third, forming long chains of mollusks in full copulation. These true « subaquatic orgies » testify to the diversity and richness of marine biology.

Conclusion

Marine biology, far from being boring, reveals fascinating and often surprising behaviors. The sexual practices of marine animals, whether it be hermaphroditic reproduction or orgies, illustrate the complexity of underwater life. Next time you dive, think of these astonishing behaviors unfolding before your eyes.

This article, initially written by Steven Weinberg for our print edition, has been adapted for the web
Photos: Steven Weinberg

Colorful nudibranchs evolving on a rich and vibrant underwater background.
 Two Nembrotha chamberlaini caught in full “69” at Alona Beach, Panglao, Philippines.

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