male clownfish becomes female -- 7/19/23

19 Jul 2023

male clownfish becomes female -- 7/19/23

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Today's selection -- from Life on the Edge by Johnjoe McFadden and Jim Al-Khalili. When the testes of a male clownfish cease to function, it can become a queen female:

“Tucked within the arms of a deadly sea anemone that is fastened to a coral reef off the coast of Isla Verde in the Philippines is a pair of small orange-and-white striped fish known as common clownfish or, more properly, anemonefish, or, even more properly, Amphiprion ocellaris. One of the pair, a female, has led a more interesting life than most vertebrates, because she has not always been female. Like all anemonefish, she started life as a smaller male who had been subordinate to the one female in the group of fish inhabiting this particular anemone. Anemonefish have a rigid social structure, and as a male this one had competed with the other males until eventually becoming dominant and enjoying the honor of mating with the sole female. But when its mate was eaten by a passing eel, the ovaries that had lain dormant in its body for several years matured, its testes ceased to function and the male clownfish became the queen female ready to mate with the next male in the pecking order.

“Anemonefish are common inhabitants of coral reefs from the Indian Ocean to the western Pacific, feeding on plants, algae, plankton and animals such as mollusks and small crustaceans. Their small size, bright colors and absence of spines, sharp fins, barbs or spikes make them easy prey for the eels, sharks and other predators that rove the reef. When threatened, their principal means of defense is to dash between the tentacles of their host anemone, from whose poisonous sting they are protected by a thick layer of mucus covering their scales. In turn, the anemone benefits from its colorful tenants who chase off unwelcome intruders, such as grazing butterflyfish.

“It was in this setting that the anemonefish came to be most familiar to us in the animated film Finding Nemo. The challenge facing Nemo’s dad, Marlin, was to find his son, who had been abducted from his home in the Great Barrier Reef and carried all the way to Sydney. But the challenge that besets real anemonefish is to find their way back home.

Ocellaris clownfish (Amphiprion ocellaris)

“Each anemone may be host to an entire colony of anemonefish that contains a dominant male and female together with several juvenile males vying with one another for the role of queen's consort. The unusual capability of the dominant male to change sex on the death of the queen fish, a capability known as protandrous hermaphroditism, may be an adaptation to life in the dangerous reef, as it allows the colony to survive the demise of the single reproductive female without ever having to leave the protection of the host anemone. But although an entire colony of fish may remain resident on a single anemone for many years, the progeny of those fish must leave the safety of their home. And, eventually, they will need to find their way back.

“A full moon is the cue for spawning of most coral fish. As the moon begins to wane over the ocean, the female of the pair busies herself laying a dutch of eggs to be fertilized by the dominant male. Thereafter, her work is done; guarding the eggs and chasing away carnivorous reef fish is the job of the male anemonefish. After about a week of his custodianship, the eggs hatch and hundreds of larvae are launched into the currents.

“Larval anemonefish are only a few millimeters in length and almost completely transparent. For about a week they drift in the pelagic currents, feeding on zooplankton. As anyone who has dived off coral reefs will know, drifting in an ocean current will soon take you far from your starting point; so anemonefish larvae can be carried many kilometers from their natal reef. Most are eaten, but some survive; after about a week, these lucky few swim to the sea floor and, within a day, metamorphose (like our frog in chapter 3) into their juvenile form, a smaller version of the adult fish. Lacking the protection of the poisonous anemone, the brightly colored juvenile is very vulnerable to predators that cruise the benthic waters. If it is to survive, it must quickly find a coral reef where it can gain sanctuary.”