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Coral Bleaching Hits Clownfish Habitat Selection
With the pace of coral bleaching picking up in recent years, prospects for fish that depend on them as habitat aren’t looking too good. Take clownfish, a species that is fiercely reliant on anemones over the course of their lives.
Found exclusively in the Indo-Pacific, clownfish are symbiotic animals that only live in sea anemones, a close relative of corals that don’t have a hard outer shell. The anemone provides a home and protection for the clownfish, while the clownfish provides food for the anemone.
As global concern grows for Australia’s Great Barrier Reef, which has experienced one of the worst bleaching events in history recently thanks to high ocean temperatures and El Niño, scientists at the University of Delaware have completed a study looking at just how vulnerable clownfish are to the increased frequency of bleaching events.
To get at that question, the investigators worked with others from Southern Cross University in Australia to determine if clownfish can identify, through smell alone, if their potential home is bleached or healthy. From there, they studied whether or not the fish could shift from one species of anemone to another to obtain the best habitat for themselves.
In the study, clownfish were individually placed in a small vessel called a choice flume containing two streams of water flowing parallel at identical rates but containing different chemical cues. Scientists recorded the fish’s location every five seconds over a two-minute period and then switched the water source and repeated the test with chemical cues on the opposite side of the flume to validate the data.
These efforts revealed that all clownfish were able to distinguish between bleached and unbleached anemones. And given the option of a healthy or bleached anemone of their host species, all clownfish chose the healthy option. But given the choice between a bleached anemone of their preferred host and another anemone species that was healthy, the clownfish chose the bleached host.
This can prove problematic because, once established, clownfish cannot relocate easily due to the species’ innate hierarchy. They are sequential hermaphrodites, which means that all clownfish are born male and the largest on a particular anemone becomes the female. The next largest male is the reproductive male, while the others queue up behind waiting for their chance to reproduce.
So if an anemone bleaches, adult clownfish who have been waiting on their chance to reproduce can’t just move to the next one. Clownfish in that anemone will be highly aggressive and protective of their place in line to newcomers.
Results of the work highlight the potentially harmful indirect effects that declining habitat quality can have on species like clownfish who are very specific about where they live. Full findings are published in the journal Proceedings of the Royal Society B.
Top image: A University of Delaware study reveals clownfish aren’t very flexible in selecting a habitat, making them vulnerable to increased bleaching due to rising ocean temperatures. (Credit: University of Delaware)