The Stoplight Parrotfish: A rainbow of colors

On a sunny spring day, I tied up to the mooring on Hammerhead Hill on the North Wall of Grand Cayman. Looking down into the gin clear water, I could see the bottom, and fish moving around seventy feet below. I was diving with my kids, Jessica and Alexander, and we headed down the mooring, cameras in hand.
 

About halfway down, I noticed a lot of male and female parrotfish in mid-water; normally they are near the substrate, sculling along on their big pectoral fins, looking for the next piece of coral to chew on.

Jessica grunted at me in her mouthpiece and pointed. Several pairs of stoplight parrotfish were shooting up twenty feet, briefly touching belly to belly. There was a puff of released eggs and milt, and they would immediately head for the bottom. The process was repeated over and over again.

We moved away from the hill and swam out to the corner, where the current bounces up off the coral wall, and there are usually tons of creole fish feeding. Here again the stoplight males were chasing the females around the coral heads, and then suddenly shooting up in the water column to spawn.

Fertilization of the eggs is external, and appears quite haphazard. The fertilized eggs drift in the current for twenty four hours before hatching, and then the larvae are carried in the current to other islands and coral reefs where they settle as juveniles in shallow protected back reef habitat.

Parrotfish are one of the groups of reef fish that change sex. Most juvenile are born as females, and after two or three years, the females become male. Some begin life as males, called primary males. This is known as protogynous hermaphroditism. This unique ability allows sexes to adjust depending on population densities at the time.

The stoplight parrotfish is common throughout the Caribbean and tropical western Atlantic. Their name is derived from their coloration. The males are bright green with intricate yellow markings, and the females are a dull brown to bright red. Together they resemble a traffic signal light.

This is known as sexual dichromatism, where the males are brightly coloured, but the females typically are dull. 

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They are diurnal creatures, browsing along coral reefs from very shallow water to as deep as 150 feet. Seldom do they travel together; the large males are usually solitary, but groups of smaller females can be seen browsing together. They feed on algae growing on dead coral and on live coral tissue, by chewing off chunks with their powerful parrot-like beaks.

The coral is crushed in a specialized pharyngeal mill, two plates of teeth working against each other that crush the coral. The coral tissue and algae is digested and the resulting sand is excreted. In this way, parrotfish contribute significantly to the creation of sand in a coral reef environment.

Another unique feature of this family of fish is that at nighttime, they hide in coral crevices and caves, and secrete a mucous envelope that surrounds their body while they are sleeping. This gelatinous cocoon prevents their scent reaching the prowling nighttime predators such as moray eels, stingrays, and nurse sharks.

Daytime predators include barracuda and larger groupers such as the black grouper, yellowfin grouper and goliath grouper.

Along with other reef grazers such as surgeonfish, they keep the growth of algae under control. In other Caribbean islands, like my homeland of Jamaica, where there is a lot of spearfishing and widespread use of fish traps, the capture of these reef grazers leads to the increased growth of algae on dead coral. When this rate of extraction of reef grazers is combined with heavy silt loads on reefs from poor agricultural practices, and coastal development, near-shore reefs are severely traumatized. The reefs in Jamaica are particularly impacted by overfishing and siltation.

Parrotfish are a favorite as “fry fish” in Jamaica and are consumed in the Bahamas and in several Caribbean islands, as well as in Hawaii and in the western Pacific. The stoplight parrotfish is favored in the Caribbean because of its size and abundance. A big male may grow to three pounds, and live for ten years. Only the midnight parrotfish and the rainbow parrotfish grow larger in the Caribbean.

Here in the Cayman Islands there is little demand for parrotfish, and the focus on the diving industry as a non consumptive use of the reef and its creatures has protected most reef fish from extraction.

When we got back to the mooring line, the parrotfish were nowhere to be seen in the water column, but were browsing along the reef. We had been fortunate to witness the brief spawning act. There would have been water conditions that governed the sudden spawning; perhaps the tide, the water temperature, and the lunar cycle all play a part in controlling their reproduction.

Perhaps it would begin again the next day, as parrotfish spawn all year round.

It is our collective responsibility to conserve all marine creatures, and preserve the biodiversity of the planet. Fish responsibly, dive safely.
Guy Harvey PhD.

 

 

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