In 1862 Charles Darwin said: “We do not in the least know the final cause of sexuality; why new beings should be produced by the union of the two sexual elements”. This statement still holds true today. Having watched the lesson, you now know that camouflage groupers have sex in enormous spawning aggregations packed densely at around two fish per square meter with around 700 hungry sharks hunting after them. Animals that cannot move, such as corals and algae broadcast their gametes into the ocean. Many fish, from wrasses over moray eels to clown fishes, are capable of having both sexes at once or in sequence. Cuttlefish show us that sex doesn’t always involve putting up a fight. And their deep sea cousins, the jumbo squid, enjoy passing their sperm packages to other males as much as to females. However, these scientific findings are just an inkling of the enormous phenomenon that is sex.

To give you a taste, here are some of the big questions relating to sex that scientists are currently working on: Why did sex evolve in the first place and how is it beneficial? Why did some organisms lose the ability to have sex? How did sexual selection evolve? How common is sexual selection in animals like corals that can't move? How did same- and different-sex sexual behaviour evolve? How did different genitals and symbioses evolve to facilitate gamete transfer? Why did separate sexes evolve? Why is there internal fertilization? Why do some organisms have clearly distinguishable sexes while others don’t? Why are there different sex determination systems like U/V, XX/XY and ZW/ZZ chromosomes? How is sex coopted for different purposes? How and where do certain species have sex? etc. etc.

Questions about sex lead on to questions about life cycles. Just as sex is often erroneously assumed to always involve male individuals internally fertilizing female individuals, an alternation of diploid multicellular (individuals) and haploid unicellular (gametes) stages is considered the norm. However, many organisms, from towering giant kelp over ancient jellyfish to phytoplankton roaming the ocean, have completely different life cycles. For instance, giant kelp doesn’t produce sperm and eggs. Instead, it releases spores that settle to become a seemingly different lifeform. These tiny gametophytes then produce the gametes that fuse into the plant we know. Jellyfish don’t have gametophytes, but their eggs and sperm also don’t make baby jellies. Instead, creatures emerge that settle on the seafloor and look like anemones. Sometimes years elapse before these polyps, which live as seemingly unrelated organisms, suddenly release several little upside-down umbrellas: the familiar jellyfish. These processes are called alternation of generations.

Clearly, studying hidden life beneath the waves is important to understand anything from the origin of life to its persistence through reproduction. There is still a long way to go until we have worked out these questions but as always, appreciating diversity will get us there. Here are some links to resources that will hopefully help you learn more:

https://www.ted.com/talks/marah_j_hardt_are_we_interrupting_the_kinky_sex_lives_of_fish
https://us.macmillan.com/books/9781250118592
https://ncseagrant.ncsu.edu/coastwatch/previous-issues/2017-2/summer-2017/everything-you-always-wanted-to-know-about-sex-in-the-sea/
https://www.onezoom.org/life.html/@=304358?img=best_any&anim=flight#x1340,y667,w0.3896
https://www.cell.com/trends/ecology-evolution/fulltext/S0169-5347(19)30257-5
https://www.khanacademy.org/science/high-school-biology/hs-evolution/hs-phylogeny/v/discovering-the-tree-of-life
https://www.penguin.co.uk/books/26016/the-diversity-of-life
https://natureecoevocommunity.nature.com/posts/55757-queering-evolution-from-practice-to-theory
https://www.algaebase.org
http://www.marinespecies.org
https://www.fishbase.se/search.php

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