The underwater world is mesmerizing. Last month, we discovered some of the most fascinating courtship methods. What is even more incredible is their sexual organs. Most of the underwater world is hermaphrodite (both female and male reproductive organs in the same individual). This can occur in many ways, but most commonly, a fish starts off as one sex, and changes to the other at a certain stage in life (called sequential hermaphroditism).
At least 27 species of fish can change their sex during their life cycle in 3 different patterns:
- a change from female to male is called protogyny (from Greek “proto” = first and “gyno” = female),
- a change from male to female, called is called protandy (“proto” = first and “andros” = male) and
- serial bi-directional sex change
All three types of sex change occur across their life, which suggest that it has evolved multiple times – but why?
There are different theories:
- One sex has a much higher reproductive success at a certain size. For example, males that are in strong competition with other males will have higher reproductive success if they are larger and can out-compete smaller males. This is the case with Parrotfish where the males will be more successful at mating when they become larger, but to keep from missing out on mating while the individual is small, it makes sense to take advantage of this time by continuing to mate as a female instead, where size does not matter as much.
- The size will also matter when the mating system is based on the way in which a male is capable of defending the spawning site where females lay their eggs. So only dominant males that are big enough are more likely to mate. The best option here again is to be a female when you are small and once you grow large enough, change into a male. This is the case with blue headed wrasses
- Fish produce exponentially more eggs the larger they get. In some cases, for protandrous (change from male to female) fish like the anemone fish, it makes more sense to use the time when the individual is small to mate as a male and then switch to mating as a female once the individual is large enough to make a lot more eggs.
- Fish that change back and forth between sexes is much rarer but this is the strategy adopted by some species of coral goby. It is not explained by the size advantage model. Instead, it is thought that coral gobies experience limited mating opportunities because they live in specific crevices and rarely come out to play due to predation risk. The ability to change sex in either direction makes it easier to form a pair without moving and also reduces the time between breeding events.
So, why are not all animals hermaphrodites? Although hermaphrodites are able to maximise their reproductive success, they put a lot of energy into doing so. It costs a lot of energy to produce both male and female gametes at the same time (and it may result in producing fewer gametes overall).
How does sex change occur?
In many sequential hermaphrodite fish, tissues of both sexes are present in the gonad prior to sex change whereas in others species reproductive tissues are completely replaced by the secondary sex.
Example of simultaneous hermaphrodites
Perhaps one of the most amazing and remarkable sex lives in fish is found in Caribbean Hamlets. Every individual is both male and female at the same time. But to avoid self-fertilisation that will decrease the genetic variability of the offspring, they trade roles in a strategy known as “egg trading”. During the spawning event, the first time that both sexes rises in the water column, the fish acting as a male embraces the one acting as a female while releasing sperm, and the fish behaving as a female releases eggs. A few minutes later, they rise again but now the roles are inverted!
Example of protogynous hermaphrodites (Female to male)
Protogynous hermaphrodites are most often haremic fish. These fish form monoandric harems consisting of 1 male overseeing numerous females for life. The two primary responsibilities of the male are to defend its territory against other conspecific males, and to court and fertilise females of its territory. If the male dies, the dominant female of the harem will undergo a sex change from female to male. This sex change may take as little as 5 days. Sex change happens in stages. First, just a few hours after the male goes missing, the dominant female starts behaving as a male. This behavioural change triggers hormonal shifts that cause her gonads to become male gonads and her colours to change into male colours. The entire process is completed in just a few weeks.
Examples: wrasses, dwarf angelfish, anthias
Example of protandrious hermaphrodites (male to female)
Protandrious hermaphrodites do not keep harems, rather, a single pair of breeding adults (with a female usually much larger in size). If the dominant female dies, the larger male changes into a female and the next immature clownfish in the social order becomes a male to form a new breeding pair.
Examples: clownfish, damselfish, blue ribbon eels, moray eels. These are common in coral reef systems, many coral reef fishes are hermaphrodites.
Example of bi-directional sex reversal
An individual will recognise its place in a dominance hierarchy and then exhibit a sexual phenotype accordingly. Examples of this are cleaner wrasses and gobies. Male cleaner wrasses (which are born females and change into males) can change back to females when there are only males in a group.
In case of gobies, if a larger, more dominant male appears, the newly transformed male will? change back to a female.
It’s kind of poetic that the ocean is home to such elegant gender fluidity!