How Do Fish Have Sex?

How Do Fish Have Sex?

The ocean is full of fascinating tales of reproduction! By adding a humorous twist about sex, we can seamlessly integrate important conservation messages. After all, successful reproduction lies at the core of sustainability. It fuels the abundance of life in the ocean, from providing food to nurturing reefs safeguarding our coastlines!

Let's Begin…

In the crystal waters of Koh Tao's reef, thousands of fish engage in what appears to be an underwater orgy—Yet, While an orgy might sound flashy for breeding, sea creatures sure know how to get creative with their reproduction!


Fascinating journey of sexual transformation

Grouper at Koh Tao Chumphon pinnacle Dive site

Beginning as females, groupers produce eggs before undergoing "sex reversal" to become males. Some groupers, however, bypass the female stage entirely, maturing directly as males.
Switching sexes benefits groupers in many ways. The biggest and oldest males are great at defending territory. If a male dies, a female can become a male without searching for a new partner. However, some groupers are at risk of overfishing due to their strict sex-changing habits, which slows down their population rebound compared to species with distinct sexes.
Groupers' unusual reproduction has intrigued scientists, shedding light on fish sexual development. And guess what? This research has fueled innovations in aquaculture, benefiting the industry.

Groupers are frequently spotted at various dive sites around Koh Tao, such as Chumphon Pinnacle.

Let's debunk the myth of Finding Nemo.

When we scrutinize the storyline from a scientific perspective, the portrayal of Clownfish as a single father raising a son doesn't quite match reality!
In their natural habitat, Clownfish typically have a dominant female, not a male. 'Coral', the mother, would have been the largest and dominant family member, with multiple male mates rather than just 'Marlin', the father. Additionally, Clownfish begin their lives as males and later transition into females. If the dominant female were to die, the largest male would indeed undergo a sex change to become the new female. In the movie's context, this would mean that 'Marlin', as the remaining male, would eventually transform into 'Coral' 2.0 after her demise, and 'Nemo' would become his mate, not his son!

Clownfish are often seen at the Twins dive site.

Harems and affairs

In Koh Tao's coral reefs, numerous fish species exhibit sequential hermaphroditism, possessing both male and female reproductive organs but utilizing them one at a time. Examples include wrasse and parrotfish, known for forming harems with a dominant big male and several smaller females. During spawning, the male releases a large cloud of sperm as the females release their eggs, simultaneously fertilizing all eggs. When the big male dies, the next largest female assumes the role and gradually transitions into the breeding male.
In Cleaner wrasse, this transformation is completed within a week. In contrast, many goby species are protogynous fish, meaning they can't choose which sex to be! Put them with a larger female, and they will become male. Plop down a bigger, meaner male, and this goby turns back into a female!

Wrasse, Parrotfish, and gobies are commonly spotted at various dive sites around Koh Tao, including Leam Thian.

House husband Champions

Seahorses are the true house-husbands of the ocean! They believe in equality and strong bonds, often sticking to one partner. Each morning starts with an elaborate dance between the male and female before they go about their day. When it's time to mate, the female deposits her eggs into a pouch on the male's belly, where he fertilizes them. (Sharing is caring).

Seahorse, Mango bay Koh Tao dive site

The male then nurtures the developing eggs in his pouch, providing them with nutrients and oxygen, much like a mammalian placenta. After a few weeks of tender care, he goes through a marathon birthing session, pushing out as many as 1,500 tiny seahorse babies! Although there are other instances of paternal care among fish, seahorses are champions in the underwater world.

Seahorses are often seen at Mango Bay Dive site.


Underwater Kama Sutra

Cuttlefish mating is like something out of the Kama Sutra! The male wraps his arms around the female's head and delivers his sperm directly into her mouth. The female then stores it either in her mouth or a receptacle below her beak. It's a bit unconventional, but hey, it works for them! Once fertilized, she lays her eggs in crevices, ensuring they're safe since she will perish at the breeding season's end. To increase their chances of reproducing, cuttlefish mate with multiple partners. Yet, it's not all about quantity for these cephalopods - they can be pretty picky when it comes to mating. Females show a "Sorry, I'm taken" white stripe across the body after getting lucky!
Males, on the other hand, prefer unfamiliar females or are drawn to those who have yet to mate, hoping for a chance to impress. This suggests they use specific cues, possibly chemicals, to judge mating success. Scientists are on the case, trying to decode these underwater dating secrets and see who's making the most giant swing in the romance department.

Seahorses are often seen at White Rock Dive site.

You'll never see Nudibranchs the same way again!

The term 'nudi' in nudibranch comes from 'nude' (nudus in Latin), reflecting their naked! These sea slugs boast stunningly vibrant colors and lack the protective shells that their ancestors had millions of years ago. Unlike humans, who are either male or female, nudibranchs and other sea slugs are hermaphrodites.  Their reproductive system is intriguingly complex: both male and female reproductive organs reside closely within their bodies. Though the details may differ among species, typically, the female system comprises a vagina, semen receptor, and various egg-producing glands. Conversely, the male system includes the prostate, sperm duct, and ejaculatory duct.

When nudibranchs mate, they're all about equality—they fertilize each other and both get to lay eggs! It's a team effort: they line up their genital pores (those openings on their right sides of their body) and then get busy with the copulation.  Talk about multitasking—Then can mutually and simultaneously penetrate each other! Certain species possess disposable penises; an unusual evolutionary trait. Yep, shed their penis after use, regrowing a new one within 24 hours, equipped with coils for future use! Some other species of nudibranchs like to inject their partner with prostate fluid in the head! Scientists are still scratching their heads over this peculiar behavior. 

Nudibranchs - Hypselodoris tryoni - Shark island Koh Tao dive site

Storing sperm next to the brain might seem like a strange choice, but hey, who are we to judge? Maybe it's their way of spicing up the relationship or ensuring their legacy lives on in their mate's offspring! 

Nudibranchs are frequently spotted at various dive sites around Koh Tao, such as Shark island dive site.

Love me tender

The love lives of lobsters are unconventional. Before getting down to business, they engage in some quirky foreplay, like showering each other with mutual urine—a sort of love potion, if you will! Once that's out of the way, the female performs a charming ritual: she lifts her big claw and taps the male on each side as if knighting him, signaling her readiness for action!
Then comes the vulnerable part: she molts, leaving her soft-bodied and defenseless. But fear not; the male is a tender lover, there to offer gentle support. He cradles her in his legs, rolls her onto her back, and, in the missionary position, they proceed to get busy.

Lobsters are frequently spotted at various dive sites around Koh Tao, such as Southwest Pinnacle dive site.


The ocean truly holds some of the most bizarre sex stories! Who wouldn't be tempted to sneak a peek into the love lives of fish? Diving in Koh Tao offers breathtaking sights and the opportunity to observe the intimate behaviors of underwater creatures. And you know what's even more fascinating? When you start asking questions about sex, it inevitably leads to questions about life cycles!

Stay tuned for the next blog of "Sex and the Sea at Koh Tao," coming soon!

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