by Matthew Norton
The ultimate goal of every living organism is to pass on their genes to the next generation through reproduction, the individual itself is only a temporary and short lived holder of these genes. Sexual reproduction within a species (usually) requires the combination of genes from a male and female, which produce sperm and egg cells respectively. However, in some species each individual can be both male and female, a phenomenon called hermaphroditism, of which there are two types. In simultaneous hermaphroditism the individual is both genders at the same time, whereas in sequential hermaphroditism the individual shifts from one gender to the other at a key point in their life.
The latter occurs in clownfish, a subfamily of fish (Amphiprioninae) containing around 30 fish species which live among anemones, protecting by the stinging tentacles for which they have developed a tolerance. Within each occupied anemone there is one breeding pair, the female being the larger, and a series of gradually smaller juvenile (sub-adult) males. When the female is lost the breeding male is gradually reformed into the new female.
Within the first 24 hours the male, due to changes in brain activity, changes its behaviour to better suit its new role, becoming increasingly aggressive to juvenile males and potential intruders. Later on, further changes in brain activity triggers the change in their reproductive organs from testes to ovaries, which produce sperm and egg cells respectively. Even in fully male clownfish there is a small portion of immature ovarian tissue within their testes, containing the bare essentials to make the sex change possible. During the transition this ovarian tissue grows and develops as the testicular tissue is reabsorbed. Meanwhile each juvenile male moves up in the pecking order with the largest juvenile male becoming the new breeding male.
It is remarkable that clownfish can make this transition so quickly and smoothly, but you may be wondering why they undergo such an extreme change instead of sticking to one gender for their entire lives. Some have suggested the energy requirements for sperm and egg cell production may be involved, egg cells are far larger, and more expensive to produce, than sperm cells. As a result only egg cell production is limited by body size, which directly controls the energy reserves they can invest in reproduction.
To maximise the number of offspring they produce it makes sense that they are male when they are young and small, when they don’t have a lot of energy reserves, and then female when they are older, larger and thus have the energy to invest in egg cells. There is also the lifestyle of clownfish to consider. Due to their reliance on anemones for protection they need to avoid straying too far, or they’ll be easy prey. Therefore among the surviving male clownfish the breeding female can be easily replaced without having to move to another anemone.
From a human perspective
Clownfish have become a famous group of fish thanks to the popular animated film “Finding Nemo”. Unfortunately, despite the positive conservation message, there was a surge in the aquarium trade of clownfish for years after the film’s release with many wanting their own Nemo. As a result millions are taken from the wild which is causing major population declines and, in some areas of Southeast Asia, local extinctions. Worse still, some of the fishing methods are very destructive, causing considerable damage to the surrounding coral reefs. For example the use of cyanide as a fish anaesthetic, while not as destructive as blast fishing, can cause corals to expel their symbiotic algae, a phenomenon called coral bleaching, on which they rely for food.
So what can be done to stop this destructive trend from continuing? The most commonly discussed solution is for most of the clownfish demand to be satisfied from aquaculture, fish bred in captivity, relieving the fishing pressure on wild populations. This approach can also benefit those involved in the aquarium trade, ensuring a consistent supply of clownfish and the opportunity to create new ‘designer’ varieties.
There are however, some issues that need to be addressed. A common issue with aquaculture is how to feed the fish, catching other marine organisms, especially other fish species, would defeat the purpose of breeding fish in captivity in the first place fortunately, the clownfish diet mainly consists of invertebrate larvae, which makes using cultured insects as a food source is a viable alternative.
Also, while their methods are destructive, the trade in wild caught clownfish does provide a source of income for fishermen in developing countries, which aquaculture could take away. One possible solution is to introduce captive bred clownfish into the wild, safeguarding both wild populations and fishermen’s incomes. However, for this to be effective fishermen need to be persuaded to use less destructive methods and captive bred clownfish need to be in contact with their particular anemone species, otherwise they won’t develop their tolerance to the stinging tentacles.
The most important aspect is that we as the potential consumers buy clownfish from a sustainable source. On a more general point we need to be careful that popular media (e.g. films, books, TV) about charismatic marine creatures does not cause us take actions that, while with good intentions, ultimately cause more harm than good. The impact that we have had on clownfish after the release of Finding Nemo is a sobering example, but I am hopeful that we can learn from our mistakes so that history does not repeat itself in the years following the sequel “Finding Dory”.
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