Parrotfish: Rainbows in the Reef

Beaches have long been popular tourist destinations, romanticised for picture-perfect sunsets and walking barefoot in the sand. Did you know the same sand between your toes came from the decomposition of rocks, byproducts of living organisms and, more interestingly, fish poop?

Parrotfish are brightly coloured fish found in tropical and subtropical coral reefs of the world and closely resemble wrasses (another group of fast-swimming, brightly coloured fish). Parrotfish belong to the family Scaridae. They have elongated bodies, blunt to rounded heads, and are swimming rainbows in shades of blue, green, pink, orange, and purple. Around 90 species have been recorded globally, most from the Indo-Pacific belt.

In appearance, parrotfish are the marine counterparts of parrots and are named for their bright colours and beak-like mouths. While it is possible to tell a parrotfish apart from other fish, identifying different species can be extremely difficult due to the highly variable colouration of parrotfish individuals. Individuals of one single species might show as many as three to four different morphs! Males differ from females, juveniles differ from adults, and sometimes, they have a completely different transitionary intermediate phase of colouration.

Parrotfish are relatively large fish and can live up to 10 years (some average 20 years). The smallest parrotfish is the bluelip parrotfish (Cryptotomus roseus) with an average body length of 12 cm, while the largest parrotfish, the green humphead parrotfish or bumphead parrotfish (Bolbometopon muricatum) can be 150 cm long.

Sand makers and grazers

Parrotfish are diurnal (active during the day) and spend most of their time feeding on a variety of plants and invertebrates. They are voracious feeders, clocking in up to 20 bites of coral per minute. They consume coral polyps as well as epilithic algae (algae growing on rocks or coral) which comprise a large portion of their diet, while invertebrates like sponges and plankton supplement it in smaller quantities. They are equally prolific poopers, and estimates suggest a large individual can excrete up to 450 kg of sand annually! Possibly, most of the coral sand found at reefs can be attributed to parrotfish.

How do they bite off and digest hard, solid calcium carbonate? Parrotfish have strong beak-like mouths formed by the fusion of around 15 rows of nearly a thousand teeth. Another set of strong teeth in the throat crunches up the coral. This complex, called a pharyngeal mill, is very effective at grinding the coral to sand, which is then excreted. A parrotfish’s teeth are stronger than gold and copper!

Hermaphrodites and cocoons

Parrotfish find space under or in between corals at night, but some also exhibit another unique sleeping habit. Special glands in the gills secrete a cocoon of mucous around their bodies, which serves multiple purposes. The mucous primarily protects them from blood-sucking parasites that might attach to them while they sleep. However, some suggest it also masks their scent so nocturnal predators like moray eels and sharks cannot detect them easily.

Like clownfish, hawkfish and sea basses, parrotfish also exhibit sequential hermaphroditism, starting life as females and later transitioning into males as adults (protogynous hermaphrodites). Socially, parrotfish usually organise themselves in harems. The females occupy small territories or a collective home range, and one large, brightly coloured male defends the smaller, duller females from other males. Upon the male’s death, the largest, dominant female changes sex and takes over as the dominant male. Interestingly, when the population is thriving, some individuals may be born as males (primary males) that resemble females and remain males for their entire lives.

Not all parrotfish exhibit hermaphroditism. Marbled parrotfish (Leptoscarus vaigiensis) males and females look alike and never change their sex.

Some parrotfish, like the bumphead parrotfish, exhibit an elaborate mating behaviour called lek-mating or lekking — previously observed in some species of birds and insects. Around the time of the full moon, they form spawning aggregations where males engage in competitive, elaborate displays to attract females. The males occupy small patches, bumping heads to defend these territories as females swim by in large numbers.

Declining numbers and reef health

Coral reef health is globally threatened by factors ranging from rising ocean temperatures, ocean acidification, pollution, and the introduction of invasive species to overfishing. In their crucial role as grazers, parrotfish keep algae in check, preventing an overgrowth from covering and suffocating corals completely. The cascading events arising from the absence of parrotfish in the reef ecosystem can be catastrophic. This has occurred in the Caribbean, where live corals have declined by 50 per cent since the 1970s, an alarming trend coinciding with the decline of parrotfish and other grazers like sea urchins. In the absence of these key grazers, algae take over reefs, making them less resilient.

Parrotfish carry out the crucial function of keeping a reef clean. They scrape off algae that cover corals, allowing for healthier and better coral growth. One can often see bite marks on corals where parrotfish have been feeding, and for a long time, this feeding behaviour was considered harmful to corals. However, the patches cleared by parrotfish, especially on bleached coral overgrown by algae, allow baby corals to settle, attach and grow — assisting in reef recovery. While parrotfish do bite off healthy coral, causing some damage, the overall effect is net positive.

Parrotfish populations have declined by 30-70 per cent across several regions of the world, with severe impacts on the coral reefs. Species like the bumphead parrotfish are threatened due to overfishing, pollution, and habitat decline, facing threats even in Indian waters. These long-lived species have slower reproduction rates, making it more difficult for the populations to recover. Classified as “Vulnerable” on the IUCN Red List, these fish face a high risk of local extinction.

Some parrotfish species migrate in large numbers daily between shallower feeding sites and deeper, night-refuge sites — a chance sighting that has stayed with me for years. Hopefully, conservation efforts will drive significant change to protect these marine habitats and lifeforms, ultimately ensuring that migrating schools of parrotfish stay as large and magnificent as ever.