Nemo Goes the Other Way

The Strange Sex Lives of Clownfish

Female to male sex change (protogyny) is by far the most common form of sequential hermaphroditism. But some species, including the charismatic clownfish, do the reverse. That is, they first reproduce as males, but once they reach a certain relative size, they transform into egg laying females. Nemo belongs to one of these male to female sex changing species.

Sometimes the common name, clownfish, is reserved for two anemonefishes (Amphiprion ocellaris and Amphiprion percula). There are around of 30 species of anemonefishes all of which share the protandrous life cycle. Anemonefish are so called because they dwell in large sea anemones. That they can do so is remarkable, as the anemone’s tentacles release tiny toxic barbs of a potency sufficient to kill most fishes of that size.

Not only do clownfishes manage to survive in this seemingly hostile environment, they cannot survive without the sea anemones. The stinging tentacles are a strong deterrent for clownfish predators. This protection is essential because clownfish are lousy swimmers—except in short spurts—and likely fall prey to countless predatory fishes should they stray far from an anemone. There is some anemone switching under certain circumstances that are beyond the purview of this post. Their fidelity to their anemone, and reluctance to disperse is the key to their mating system: long-term monogamy. By long-term, I mean years, potentially, though each can stray if the opportunity arises.

All anemonefish species have preferences when it comes to the of kind of anemone they will inhabit. The two clownfish species are particularly particular in that regard. Of all reef anemones they choose to inhabit only two species. The suitable anemones can be few and far between, discouraging forays across open water for those seeking to trade up regarding either houses or mates. Their externally enforced monogamy—think of a permanent covid lockdown during a first date—in turn, explains why they are protandrous, more on which later.

But how do a male and female clownfish find themselves together on the same anemone in the first place? To answer that question, we need to examine the clownfish lifecycle. All clownfish begin life as eggs attached to rocks at the base of an anemone, primarily under the care of the male. Both he and the female zealously chase off any fish who happens to wander by. When I call this behavior zealous, I’m not exaggerating. Their zealotry can in part be attributed to their ancestry, the family of fish to which clownfish belong, called damselfishes (Family Pomacentridae). Damselfish is a misnomer; there is nothing damsel-ish about most damselfish. Though generally quite small, such that you would expect them to be shy and retiring, they went completely the other way when it comes to aggression. Damselfishes are the bad-asses of the sea, especially in the breeding season. Undaunted by larger fishes and non-fishes alike, they spend most of their time driving intruders out of their territories, including divers. Damselfish aggression is especially intense when guarding their eggs. (In this video shot by Justin Rhodes at the Cabo Pulmo Marine Reserve, Baja California, a male giant damselfish attacks his son, Avi, who got too close to the brood. Note that the giant damsefish(Microspathodon dorsalis) is named for its large size relative to other damselfishes. This one is at least 15X the size of a clownfish.)

I have been entertained over the years by damselfishes of numerous species attempting to drive me off while caring for a brood. Clownfishes are about average for a damselfish when it comes to aggressive defense. So clownfish eggs are well protected.

Once Nemo leaves the comfort of the egg, however, the world becomes a much more menacing place. Upon hatching he will float up into the water column, then transported by the vagaries of ocean currents for 10-15 days. He may find himself hundreds of miles from his birthplace. Chances are slight that he will survive this pelagic stage. Should he be one of the few, his odds of survival odds improve somewhat once he heads for the reef. He must quickly find a suitable anemone before any one of innumerable predators finds him.

If Nemo does find a suitable anemone, chances are it is already occupied. At a minimum there would be a mated pair; more often than not there are also immature individuals of varying sizes in the host anemone. All residents conform to a strict linear dominance hierarchy based on size. The largest and hence alpha individual is the female. Next in line is her male mate, followed by the largest juvenile, and so on down to the smallest resident of the anemone. This dominance hierarchy goes unchallenged, no rank skipping. When Nemo joins the group, he is the omega and must wait his turn to become a reproductive male.

(This BBC clip, narrated by David Attenborough, illustrates the dominance hierarchy and male egg tending. Note, however, that there is little evidence that a female will evict a male mate when his babysitting doesn’t meet her demanding standards.) (https://www.wonderstrucktv.com/video/clownfish-life-in-the-anemone)

Opportunities for moving up in rank most frequently arise when one of the larger juveniles is evicted or dies. If the reproductive male dies, all the juveniles advance one level in the hierarchy. But for the largest of the juveniles that includes maturation into an adult male, who becomes the new mate for the alpha female. If the female goes missing, things get really interesting. Her erstwhile male mate takes on her role, and not just with respect to social status. He becomes a she.

Nemo, like all the non-reproductive members of the hierarchy probably functions as backup, should one of the adults die. If it was the female who met that fate, the resident male will transform into a female, but it may take months. In the meantime, Nemo, formally sexually repressed, will transform from juvenile to male, and we have a new mated pair

As is apparent from the foregoing clownfish females are always larger than their male mates. This was not apparent in the film Finding Nemo. Nemo’s dad Marlin was bigger than his mom, Carol, which doesn’t happen in nature. Moreover, in nature, Carol would physically dominate Marlin. Perhaps it was assumed that kids would not be psychologically prepared to have their world rocked in this way. Probably most parents as well. But it makes perfect evolutionary sense, as we will see down the line.

There is a more practical way in which Finding Nemo did a disservice to clownfish. Just as the movie Jawswas bad for Great White sharks and their kin, Finding Nemo resulted in the decimation of clownfish populations, albeit from “love” rather than hate. Everyone wanted a Nemo. Most were ill equipped to care for them properly. Those that were, got greedy. Collecting wild clownfish became quite lucrative in parts of the developing world, relative to other avenues of employment. I saw an example of this in the reefs off Bunaken Island, near Sulawesi, Indonesia. I had traveled there with Justin Rhodes—an expert on clownfishes—and his graduate student, Ross DeAngelis, for a pilot study. Justin had come equipped with a report of previous research on clownfish in that area, in which the location of many clownfish families was conveniently recorded.

It quickly became apparent, though, that the population had been wiped out by Nemo collectors. The right anemones were there but they were uninhabited. We did find one anemone with a male, female and juvenile but they were unusually small. They had obviously settled in quite recently. As always, the largest of the three was a female, but she would have been a male under normal circumstances, perhaps a juvenile. This group of tiny clownfish was testimony to the adaptability by way of sexual flexibility of this species. In the next post I will consider the evolutionary forces that produced such flexibility in this and many other fish species. I will consider as well why sex change, given its obvious advantages, isn’t more common in other vertebrates, including mammals.