Sea Urchins – Who Needs Them?

Swimmers, divers, and snorkelers are often overheard cursing the abominable long-spined sea urchin; particularly as they attempt to extract the barbed spines from a foot or hand. More times than I care to remember, I’ve painfully removed a dive skin that was pierced through and held fast by one of those long spines. Hapless waders, especially during the evening hours, repeatedly find the souls of their feet turned to pin cushions when they tromp through tidal pools, throwing caution to the wind.

So, who needs sea urchins and what is their purpose? Sea urchins are invertebrates belonging to the phylum Echinodermata, translated as spiny skinned animals. They are herbivorous and one of the most ancient forms of marine life, dating back some 225 million years. The fact that they feed on plants is the key to their importance in the chain of sea life.

In 1955, the first definitive study was conducted on the variety and density of life forms found on coral reefs. The conclusions showed that corals comprised only around 10% of the total mass while algae made up over 80%. Several forms of cryptic or hidden algae were found growing within the coral’s skeletons, within rock substrate and sediment bottoms, and on the surface of the reefs.

As has long been known, algae left unchecked can decimate a healthy reef, leaving a barren waste of dead coral. Herbivorous fish graze along reefs eating some of the more visible and easy-to-obtain algae, thus keeping those forms of algae in check. However, the job of ridding the reefs of the hidden and more detrimental algae falls to the sea urchins, particularly the long-spined species, Diadema.

This fact was never more evident than in the early 1980s when a pandemic swept through the Caribbean killing vast numbers of Diadema within a few years. Almost immediately, divers, fishermen, and scientists began noticing that the Caribbean coral reefs were smothering and dying under an unchecked growth of algae. Sadly, while we are just now beginning to see the return of a naturally evolving disease resistant Diadema, many of our coral reefs still suffer the effects of the lack of sea urchin grazing.

Bruce Purdy, a long-time diver and owner of Blackbeard‘s Cruises, the Aquacat, and the Cat Ppalu, has sailed the waters of the Bahamas and Exumas for several decades while showing passengers the wonders of the underwater world. Bruce studied coral reefs with The Marine Conservancy and NOAA and later became a RECON instructor so that he could teach his staff and clients reef monitoring techniques including checking for algal growth, water temperature, and performing fish surveys.

Not content with just monitoring the reefs, Bruce and his staff implemented a unique long-spined sea urchin reintroduction program. Clients participate in special liveaboard cruises where they collect Diadema from shallow waters in the evenings. The following day, the divers carry the specimens along where they are reintroduced to reefs threatened by algal overgrowth. Follow-up monitoring of those reefs is beginning to show some recovery. While there is a long way to go to bring the Caribbean reefs back to their once glorious and colorful state, one man with a vision has taken a giant step.

Becky Bauer is a scuba instructor and award-winning journalist covering the marine environment in the Caribbean. She is a contributing photographer to NOAA.