Parrotfish poop

Giovanna Fasanelli | VOLUME 31, ISSUE 3

The fine sand on a tropical beach might be more than you bargained for …

You’re enjoying a romantic stroll along the fine, white sandy beach of your dream holiday resort, the glow of the setting sun bathing your smiling faces. Encountering a young child happily humming to itself while putting the finishing touches to a sandcastle, you remark upon the fine nature of the creation. The child responds nonchalantly: “Yes … fish poo makes a great building material!”

You’ve discovered the unromantic truth: most of the sand caressing your toes has been pooped out by parrotfish.

Parrotfish inhabit all shallow tropical and subtropical oceans of the world. To fishermen, they’re known for their tasty flesh and, to underwater divers and snorkelers, for their astounding colours.

Like cattle are to grassland, so are parrotfish to a reef. Any form of algae, or coral covered in algae – or anything that photosynthesises, for that matter – is in serious danger of becoming parrotfish food. As these voracious grazers move throughout the reef, they use their hardened, beak-like teeth to scrape algae off the substrate and, in doing so, collect layers and chunks of the reef itself. Once their pharyngeal mill (a special grinding apparatus in the throat) has suitably ground the hard calcium carbonate into cement-like ooze, it passes through the extensive digestive tract, which extracts the nutrient-rich, living material. Then the parrotfish releases the ‘cleaned’ reef powder into the water column – some of it makes its way, through incessant wave and wind action over time, onto your dream beach.

The bumphead, or humphead, parrotfish (Bolbometopon muricatum) is the ‘Big’ Bill Cavubati of the reef fish world. At 46kg and 1.3m length, it is the largest of the 80 or so species in the Scaridae family. Competing males can be seen smashing their scarred, bulbous heads together in showdowns of brute strength. Unfortunately, these huge fish are targeted by fishermen throughout their range in the Pacific and Indian Oceans, which makes finding them rather challenging. Protection within marine reserves has enabled impressive schools to form – a sight to behold and well worth seeking out as an underwater observer.

Usually, their presence is heard before it is seen, as the collective crunching sound of dozens of these heavyweight fish is very audible. Like a band of careless marauders, they move over the reef, smashing their heads into the substrate while biting, scraping and snapping off thumb-sized pieces of coral. When they excrete the remains, the clouds of crushed reef particles fall through the water column like spilt milk. It is estimated that an individual bumphead parrotfish can consume and excrete over five tons of coral each year. As such, they’re one of the reef’s most important agents of bioerosion.

Another unromantic truth about parrotfish is that they are classic models for sex change. A female can undergo a radical transformation, during which her gonads switch from featuring ovaries, to testes. She grows bigger, more muscular and, in many cases, ends up looking completely different … so much so that you may think she-turned-he was a different species altogether.

In many parrotfish societies, a large, flashy-looking male dominates a harem of females, but if something were to happen to him a female, or subordinate male, will undergo the necessary morphological changes to take his place.

So, with all this in mind … will those romantic, tropical beach strolls ever be the same again?


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