Here's a story of small things making big differences.
First, we have the long-spined sea urchin, Diadema antillarum. These little guys look like pincushions stuck full of porcupine spines. They are echinoderms, like starfish, distinguished by radial symmetry and a marine lifestyle. Their ancestors first appeared in the Cambrian period, 541 to 485 million years ago. They have no heads, and just a single hole on their undersides that serves as a mouth. They move around the ocean floor on little tube feet, which can propel them with astonishing speed when they feel threatened. They eat algae, vast amounts of algae.
Now, algae. Algae are not plants, though they are autotrophs that conduct photosynthesis. They are at least eukaryotes, organisms with membrane-bound organelles. The algae aren't really a true taxonomic grouping, because there are many different types and not all of them share a common ancestor. (Like slime molds and water molds, algae are now taxa without a kingdom. They just get a domain, Eukarya.) Green algaes do look like plants, and so we sometimes call them seaweed.
Algae are ubiquitous in the seas around the Bahamas. So are coral. For millenia (eons?), algae populations on coral reefs by animals that ate them. Parrotfish eat some algae off the coral, along with some of the coral, but sea urchins are the real stars. They're little vegetarian (algaetarian?) vacuum cleaners that spend their nights gorging on new algal growth on coral reefs, and their days hiding in cavities within the coral. It keeps the urchins happy and the coral clean and beautiful.
(Parrotfish start off life as females and then turn into males. Scientists call them sequential hermaphrodites.)
Third character: a pathogen. No one knows what kind, bacteria or virus or fungus. In any case, a pathogen is a disease-causing organism. In 1983 a pathogen entered the Atlantic through the Panama Canal, perhaps in the ballast water of a cargo ship. It was deadly to sea urchins and spread rapidly through the water. Within a year, some 97 percent of the Diadema from South America to the Bahamas had died.
The resulting algae growth on the coral was entirely predictable and unstoppable. Today Caribbean reefs look like shaggy green monsters.
Various organizations have been working to correct this situation. Some have been breeding sea urchins and restoring them to habitats. Others have been collecting urchins from the few places they still live and moving them to algae-covered reefs.
Blackbeard's Cruises and the Cat Ppalu have been conducting Diadema restoration trips for around a decade. I went on one of these trips with the Cat Ppalu back in 2005. It was hard work! We went out every night to free dive for urchins, brought them back to the boat, and then delivered them to their new homes that day. Urchins hide in daylight, which makes them nearly impossible to catch without seriously damaging them, so it has to be done by darkness, when they walk around the sandy bottom. The idea is to create a critical mass of adult urchins so that they're close enough to one another to breed successfully - they do that echinoderm thing of everyone spawning at once one very special night each year.
Reportedly these efforts have been paying off, and some reefs with restored urchins are looking cleaner. I'll get to see next week because I'm heading off to do another urchin restoration week on the Cat Ppalu.