The importance of parrotfish poo
Why parrotfish change sex
Humans, usually, tend to have one gender and stick with it. Fish are much more interesting gender-wise. Some species grow up as male, others as female, and change sex later in life. Some fish are born entirely without a gender and develop one when needed. Did you know that the adorable little anemonefish family you see living in an anemone is not a family at all? The biggest fish is the female, the smaller one is the male, and the “kids” are not their offspring — they’re genderless. When something happens to mum, dad turns into mum, and one of the “kids” turns into dad.
Other fish species are slightly less bizarre. They simply start life as female and turn male when necessary, like the parrotfish. But it’s not only their change in sex that makes parrotfish interesting.
How parrotfish turn reef into sand
Parrotfish have a useful diet. They scrape algae from rubble, and sometimes from the reef itself, preventing algae overgrowth. Because of this scraping behaviour, they ingest a lot of extra material they don’t actually eat — and that extra material is what matters most to us beach lovers. Parrotfish grind coral rock and reef into fine sand, which they then excrete. Stay close to a parrotfish for a while and you’ll notice they poo constantly; you’ll regularly see clouds of sand trailing behind them as they graze.
That sand eventually ends up on shorelines, and the crystal-clear white sand on many tropical beaches is, quite literally, evidence of parrotfish at work on the reef nearby. A 2015 study published in the journal Geology by Professor Chris Perry and colleagues at the University of Exeter found that parrotfish generate more than 85% of the new sand-grade sediment produced on the reef around Vakkaru Island in the Maldives — meaning the island itself is, to a striking degree, made of parrotfish poo. You can read the University of Exeter’s summary of the research here.
The bumphead parrotfish: biggest producer, most destructive
The species that produces the most poo — and by a wide margin — is the bumphead parrotfish (Bolbometopon muricatum). Researchers estimate that a single large bumphead can erode reef and produce more than five tonnes of sand a year (Bellwood et al., 2003) — more than the weight of an adult African elephant, from one fish.
That scale comes at a cost. Bumphead parrotfish are heavily overfished, in part because they’re large and feed in schools, making them an easy and prized catch. Their powerful jaws let them eat significant amounts of live hard coral alongside algae, and in large numbers they can do real damage to a reef. Blue Planet II from the BBC shows this feeding behaviour clearly.
What MCP is finding on our reefs
MCP is interested in which fish species occur where across our different dive sites, as part of our baseline study. Once we know this, we can set up a long-term monitoring programme and watch for any disturbances, natural or human-caused. Right now we’re working to catalogue which parrotfish species we have, alongside several other families.
Anybody who’s ever paid attention to parrotfish will tell you that identifying different species isn’t easy — our volunteers will readily agree. At first glance they all look more or less the same, but if you look closely (especially in photos, where you have more time to study them) you start to notice the differences. Even now, after more than a dozen dives looking for them, we discover new species every week. We haven’t recorded a bumphead parrotfish at our sites, but with 21 other species logged, the variety here is already substantial. [Suggested internal link: MCP’s current fish-survey / baseline-monitoring page]
We welcome all volunteers, but if you happen to be an ichthyologist (that’s a fish expert, if your Greek needs a little brushing up) specialising in parrotfish, now would be a really, really good time to come.