Image credit: Nick Hobgood, CC BY-SA 3.0, Image Cropped

If you’ve ever seen the movie Finding Nemo, you might’ve also heard the fun tidbit that Nemo’s dad, Marlin, should have become a female when Nemo’s mother Coral died. As strange as it may sound, this is true for many species of fish on earth. Every clownfish community has one female in charge (the only female in the community), and that female only mates with one male, the largest male in the community. The rest of the community is made up of smaller, immature, non-breeding males. When the female dies, the breeding male will become the new female, and the largest of the immature males will take the role of the breeding male. Simple enough, right? 

The reproductive strategies of fish are far more complex than you’d probably expect, and are much more varied than other animals. The ocean has many different ecosystems made up of many unique species. With so many species sharing the same spaces, some of them need to find different strategies to survive and reproduce, like keeping their offspring somewhere different to try to outsmart other species for whom those offspring are an easy meal. Fish have been around for much longer than many other living things, so they’ve had hundreds of millions of years to find the best ways to live and reproduce. 

Every individual’s goal is to have the highest possible fitness, or individual reproductive success. This is quantified by the number of offspring from that individual that are born, or how many times the individual gets to pass on its genes to the next generation, compared to other individuals. This can be hard, because if an animal isn’t careful, its offspring could get eaten before they’ve even had a chance at life. That’s why having unique reproductive strategies can be extremely beneficial. 

Forms of Hermaphroditism in Fish

There are many examples of hermaphroditism, where an individual can have both male and female reproductive organs (called gonads) at some point throughout its life, in nature. In fact, studies have shown that approximately 2% of fish species exhibit some sort of hermaphroditism. 

Simultaneous hermaphroditism is when an individual has both sets of gonads at once, meaning any individual of a species can mate with any other individual of the same species. For a small number of species that utilize simultaneous hermaphroditism, it’s even possible to have self-fertilization, eliminating the need for a mate. Other species will start off with one set of gonads and switch to another set at some point, which is called sequential hermaphroditism. 

Other species can go back and forth in no particular order, which is called bidirectional hermaphroditism. In fish, large body size is generally a good indicator of fitness for females (a larger body means that the female is able to have more eggs), but body size doesn’t have as much of an impact on fitness for males, so being able to switch between sexes allows individuals to have a higher fitness throughout a lifetime depending on the individual’s size at the time. 

Clownfish are considered protandrous sequential hermaphrodites, meaning every single clownfish is born male, with active male reproductive organs and undeveloped female organs, and some individuals will become females at some point in their life, but they cannot redevelop their male reproductive organs after that’s happened. There are also protogynous sequential hermaphrodites: fish that are all born female and can become male at some point. These changes can happen as a result of social or environmental cues.  For clownfish species in particular, this change is triggered by a social cue: the disappearance of the female. At that point, the breeding male’s reproductive organs are absorbed by the body as the female reproductive organs become mature. Once the breeding male has become a female, its behavior will change to become more dominant and aggressive. 

Parental Care

Parental care is another aspect of fish reproduction that people have misconceptions about, but believe it or not, fish can actually be fairly good parents. Parental care is considered to be any behavior that increases the survival of an individual’s offspring. This can range from maintaining and protecting a nest, to having offspring spend more time developing in the mother before being born, to eggs being hidden in rock crevices.  Unlike most other animals, the dads are more often the ones caring for offspring, not the mothers.

Parental care looks different based on which parent is doing the care. For species that have internal fertilization, parental care often means the mother has a longer pregnancy, where she can provide offspring with nutrients and keep them safe. For instance, in some shark species, the offspring will hatch inside the mother, and then something shocking takes place. The largest of the offspring will eat the others until there are only two left: one in each uterus. The two individuals that survive will be larger, stronger, and better prepared to survive after being born. 

For species that have external fertilization of eggs, parental care is more convenient for males, as the males can fertilize and protect multiple clutches of eggs at once, while defending their regular territory. One example of how hard some males work to protect their young is the cardinalfish. After a female cardinalfish release eggs, the male quickly fertilizes them and then gently picks them up with his mouth! The eggs will stay safe in his mouth for one month, and during that time, the male is unable to eat anything. Once the eggs hatch and leave their father’s mouth, they already have an advantage against other species whose eggs are left in the environment, vulnerable to predators. 

Seahorses are yet another example of male parental care in fish. However, seahorse dads take parental care to the next level by actually being the parent that goes through a pregnancy, rather than the female! The female seahorse deposits her eggs into a special pouch on the male’s body, called a brood pouch. While the eggs spend time incubating in the pouch, the male provides them with the nutrients and oxygen that they need to survive and grow. After keeping his offspring safe for what would have been one of the most dangerous parts of their development, a male seahorse can give birth to hundreds of babies at the end of his pregnancy. 

The binary way of seeing gender as being either male or female and having the same reproductive role from birth to death falls apart when it comes to fish. Over millions of years, fish have had to evolve unique ways to stay alive and raise offspring in a challenging environment where chances of survival for offspring can be incredibly low, which has led them to becoming incredibly diverse animals. 

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Holly Albrecht is currently an educator/zookeeper, and holds a Bachelors degree in Conservation Biology from the University of Arizona. She is passionate about ichthyology, herpetology, and using education and outreach to get people interested in conservation and the natural world. 

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Read More

Hermaphroditism in fishes: an annotated list of species, phylogeny, and mating system – Kuwamura et al.

Fish are the sex-switching masters of the animal kingdom – BBC Earth

Parental Care in Fishes – Blashine & Sloan