Parrotfish – Engineers of the Reef
Parrotfish are a clade of 95 species. They are a group first classified by Constantine Samuel Rafinesque as Scaridae, but now technically named scarine labrids, derived from the family Labridae (wrasses) and the subfamily Scarinae. They are small (1-4 feet) pelagic fish that sport a variety of groovy colours, thanks to their polychromatic life history. The largest identified species is the Green humphead parrotfish (Bolbometopon muricatum) which measure up to 4.3 feet long. The smallest are the Marbled parrotfish (Eptoscarus viagiensis) which are often smaller than 1 foot. Depth ranges of the marbled parrotfish are short (1-15m), this is typical of many parrotfish species.
Many species are sequential hermaphrodites, meaning they change their sex throughout their life to increase sexual reproductive potential. Another curiosity of the parrotfish is the ability for the Queen parrotfish (Scarus vetula) to envelop themselves in a mucus based cocoon before sleeping in order to defend themselves from being eaten by tropical bony fishes and sharks. Their distribution is wide spread across tropical surface waters, with many hotspots in the biodiverse Indo-Pacific oceanic region. Whilst parrotfish are identified in tropical seagrass meadows and rocky shores, their primary habitat are coral reefs, which accommodate their diet perfectly for mutual benefit.
An Acquired Taste
The diet of most parrot fish is herbivorous. The availability of plant material in large masses in coral reefs is attached to the substratum. Feeding strategy and ecological implications vary between species. Scrapers and browsers such as the Princess parrotfish (Scarus taeniopterus) scrape the surface of the coral substrate to remove layers of macroalgae for nutrition. This prevents dense clumps of seaweed forming on the reef. Algae smother, chemically damage and outcompete coral. Reef ecosystems are a battleground between coral and seaweed for resources such as light, nutrient detritus and space. Without the help of their parrotfish guardians, coral loses the battle.
A study in the Caribbean and tropical reefs found that 40 to 70% of common seaweeds lead to coral bleaching and death of tissue when settled on the coral without being grazed. Removing the algae boosts recruitment by creating clean space for coral larvae to settle as the availability of reef substratum is a settlement cue. Checking seaweed overgrowth is how some parrot fish can act as cleaners and ecosystem engineers.
Other larger parrotfish species such as the Green humphead parrotfish are known as excavators. They graze and digest rock and coral, leaving behind visible scars on the coral reef. It may seem that this form of bioersion would damage the reef size but the clearing of dead coral increases the surface area of clean substratum space for even more coral larval settlement to occur. Excrement from the Green humphead is a sandy sediment that leads to the formation of islands by deposition. Because these humpheads can spend up to 90% of their day foraging the rock and coral to extract nutrition, it is no surprise that they excrete around 320 kilograms of sand a year.
The ability to chomp through hard rock and coral is no easy task and requires specific teeth adaptation in excavating parrotfish. The blunt-head parrotfish (Chlorurus microrhinos) has fluorapatite (Ca5(PO4)3F) biomineral teeth with a 7.3 GPa Vickers rating. This is almost as strong as martensite and around seven times stronger than stainless steel. The source of the strength is an interwoven crystalline, ‘chain-mail’ structure with nano- and microfibers. Tireless excavation of strong coral and rock is only possible due to the sheer power of these resistant biomineral structures which grow continuously from its beak-like structure.
Parrotfish as ecosystem engineers are crucial to the health and long-term survival of coral reefs. Scripps Institution of Oceanography analysed sediment core samples containing fish and coral fossils to create a 3,000-year record of Caribbean coral reefs. Data provided from this showed that historically when there are high populations of parrotfish the rate of coral growth increases. Fluctuations in population of parrotfish may even be more influential in coral reef health than climate change. In post-industrial times, we have seen an increased fishing pressure on reef fish and increased algal presence on reefs. Parrotfish have been shown to help reefs recover after natural disasters and this engineering property will also be helpful in restoring reefs from anthropogenic change and disaster in the future. Caribbean coral reefs generate $3 billion annually and have declined by over 50% since 1970. Now more, than ever do the vulnerable coral reefs need their parrotfish caretakers.
A key recommendation from the Global Coral Reef Monitoring Network was to cease all fishing of herbivorous reef fish immediately. The arguments presented are justified by rising fishing pressures and algal growth on coral in post-industrial times. Tropical reef areas are often targeted by fisheries due to their high species richness and density of fish. Overfishing parrotfish is usually sourced from small scale fisheries in less economic developed countries. Exploitative fisheries are outside the sphere of western influence and therefore not properly managed with ecological advisories. Measures are being to enforce legislation and red tape wherever possible. Barbuda for example has banned all parrotfish take from being sold and distributed through their ‘blue halo‘ initiative. This also prevents nets from being used in designated sanctuary areas.