Parrotfish: Coral Reef Vegetarians

A rainbow parrotfish grazing on algae in the Caribbean
Photo source: CFMC, caribbeanfmc.com/fmp_reef_fish.html

Parrotfish are one of the most universally recognized reef fish, with their bright colors and beautiful varieties. If you've ever been snorkeling or SCUBA diving on a reef, you've probably easily spotted parrotfish swimming around, biting off pieces of coral - wait, biting coral? Isn't that bad for it? Aren't corals endangered?

Believe it or not, this "grazing" behavior is actually really good for coral reefs! Parrotfish are herbivores, which means that they eat only plant matter, not other fish, and when you see them biting at the coral, they are actually eating the algae that are growing there. This allows the living part of coral reefs, the polyps, to expand and grow, making the reef bigger and stronger. Without the parrotfish to remove the algae, the polyps would be smothered and eventually die! 

Parrotfish (along with snapper) for
sale in a Caribbean grocery store

Parrotfish are also important sources of food for many Caribbean islands. In the US Virgin Islands, the people of St. Croix especially love fishing and eating parrotfish. Managers must carefully assess the health of the parrotfish stock, due to this fishing demand.

On Leg 3 of our cruise, we are specifically targeting parrotfish larvae for the first time! We want to know where these fish are being spawned, retained, and/or transported, and what differences exist between the St. Thomas/St. John and St. Croix populations.

In the Virgin Islands, we mainly find two genera of parrotfish: Sparisoma and Scarus. Within these genera are more than a dozen different species, like midnight, rainbow, stoplight, queen, and princess parrotfish. Our taxonomists look at the fish's body shape, pigment patterns, and number of fin rays to determine which genus they find. Unfortunately we can't identify all these species at the larval stage with only a microscope (keep reading to find out how we can!). 

Taxonomists carefully collect the sample after a net tow and keep it on ice, then pick through the sample under a microscope, looking for fish larvae - live and on the ship! If we find any parrotfish larvae, they are identified to genus, measured, and then preserved based on the type of future analysis to be conducted. Some larvae are frozen in liquid nitrogen (about -340° F!) so that we can measure their stable carbon and nitrogen isotopes. Other larvae are preserved in refrigerated ethanol to keep their DNA pristine, which can be sequenced to identify the species of each fish, and what population it belongs to!

The two main genera in the Caribbean: Sparisoma (top), ~9mm SL, and Scarus (bottom), ~7mm SL.
Can you spot the differences between the two genera? (HINT: look at the pigments!)

So far, we've found over 300 parrotfish larvae in our samples! We can't wait to get back to the lab to see how many more we caught, and to discover more about the ecology and biology of these beautiful fish!