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Diadema, Caretaker of the Reefs By Martin Moe

The Great Plague In the spring of 1983 the coral reefs of the Florida Keys were quite different thanthey are today. Although they faced the pressures of high visitation, pollution andextensive fishing, absolute coral cover was stable at around 25 percent. This wasdown from the 50 percent of the early '70s, but live stony corals were still thedominant living structures of Keys reefs. On each coral head and in every rubblepatch in the Keys there lived at least a few purple­black sea urchins with long,pointed spines, known for skewering wetsuits and unwary hands. Then, over thecourse of a single week in July 1983, the urchins were all dead or dying. Diademaantillarum were suddenly only a memory.

In only one year, beginning in January 1983, the Diadema plague extended fromthe southern Caribbean north to Bermuda. As currents spread the pathogen, 92 to99 percent of all the billions of Diadema sea urchins in this vast 1.4­million­square­mile oceanic habitat died within 13 months. This was the most extensivemass mortality of any marine animal ever reported, and the species was suddenlyvery near extinction.

Diadema are (were) the keystone herbivores of the tropical western Atlantic reefs.They consume the macro algae that overgrow reefs, and their feeding activitycleans and scrapes the limestone substrates, stimulating settlement of coral andother invertebrate larvae. Without herbivores like Diadema, algae rapidlyovergrows corals, greatly reducing sunlight, trapping sediments and even wagingchemical warfare on coral tissues. Now, almost 30 years after the massive die­off,a limited return of the Diadema populations has occurred elsewhere in theCaribbean but not in the reefs of the Florida Keys. Ecological recovery of theKeys' coral reefs is simply not possible without the return of Diadema. When Diadema Return

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Ken Nedimyer and I, both marine biologists and members of the Florida KeysNational Marine Sanctuary Advisory Council, conducted a study to demonstratewhat would happen if Diadema returned to the reefs. In the fall of 2001, with thesupport and counsel of the Marine Sanctuary staff and the National UnderseaResearch Center (NURC) of the National Oceanic and AtmosphericAdministration (NOAA), we began work on a Diadema restoration project.

This project transferred at­risk juvenile urchins from rubble zones to deeper reefsto see if they could survive relocation and, if they did survive in adequatenumbers (about one urchin per square meter), whether they could change theecology of the reefs. We selected four small, 25­foot­deep patch reefs west ofPickles Reef near Key Largo. We introduced urchins to two of them — theexperimental reefs — and observed the other two as control reefs. NURCassessed the ecology of all four reefs before and one year after relocation of theDiadema urchins. The ecological effects of the relocated urchins on the twoexperimental reefs in the short space of one year were remarkable.

On the experimental reefs, stony coral cover increased 59 percent, while on thecontrol reefs stony coral cover declined 25 percent. The density of juvenile stonycoral colonies, an indicator of the success of coral reproduction, increased 151percent on the experimental reefs and only 55 percent on the control reefs.Crustose coralline algae, which promote settlement and survival of coral larvae,increased 160 percent on the experimental reefs and less than 1 percent on thecontrol reefs. Brown macro algae that fight with coral for light and spacedecreased on the experimental reefs by 45 percent and increased on the controlreefs by 31 percent.

This study empirically and emphatically demonstrated it is possible to restoreDiadema to a Florida Keys coral reef, and that restoration can reverse the declineof coral and decrease the macro algae that shroud the reefs. Culturing Diadema

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Martin Moe rears Diadema from planktonic larvae to juvenile urchins in his lab.The culmination of his work will be the reintroduction of these crucial

herbivores to Florida Keys coral reefs.

Diadema are still present on the reefs of the Keys but in such small numbers theyare not ecologically functional. A female Diadema can produce 15 million eggs ateach spawning, but she must be close to a male for reproduction to succeed. Thus,if enough Diadema can be maintained on a reef to keep it clear of macro algae,the coral would benefit greatly and the urchins could spawn effectively. All thatmust be done is to develop the technology for hatchery culture of Diadema andthen create a scientific program to establish and maintain functional populationson specific reefs. Surely this is not an impossible task, and justifiable if it canresult in the restoration and protection of the Keys' coral reef ecosystem, a truenational treasure. SpawningI kept a brood stock of 20 to 35 adult Diadema in a 200­gallon saltwater systemin my lab for more than four years. I found that simply placing adult urchins in acontainer of warm water stimulated them to spawn if they were ready. Thisprocess made it possible to spawn them whenever a rearing run was scheduled. Larvae Rearing

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Rearing Diadema larvae is more difficult than rearing the larvae of other seaurchins. Diadema larvae are among the smallest urchin larvae, and they have along planktonic life. Metamorphosis from a planktonic larva into juvenile urchinmay occur as early as 36 days but may take several months. The larvae requireslow­moving water similar to oceanic currents as well as open­ocean waterquality, making it difficult to maintain laboratory conditions necessary for theirgrowth and development. Successful culture to mature larvae and metamorphosisinto the early juvenile stage was routinely accomplished in my small laboratory;and although survival into stable, feeding juveniles was problematic, the basicspawning and larvae­rearing technology had been developed.

In the spring of 2011 the project entered its second phase and was transferred toDave Vaughan at the Mote Marine Laboratory's Tropical Research Lab (TRL) onSummerland Key. Vaughan is working with large­volume larvae­rearing tanks,successfully conducting post­metamorphosis feeding and producing many moreviable juveniles. Beginning with the work at Tom Capo's University of Miamilaboratory and continuing with my work at my small lab and Vaughan's work atMote TRL, the project is now moving ahead toward juvenile urchin behaviorstudies and research in the placement and survival of Diadema on reefs. I amconfident it will soon be possible to start the next phase of research: developingsuccessful methods of restoring and maintaining functional populations of thiscritical herbivore on the coral reefs of the Keys. Learn More

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This research was made possible by funding from the Protect Our Reefs specialtylicense plate program sponsored by Mote Marine Laboratory. With continuedfunding and support, Diadema may soon return to the reefs of the Florida Keys.You can support Diadema restoration by visiting www.mote.org to volunteer,purchase the specialty plate (Florida residents) or make a donation. Will Keysreefs ever return to the 50 percent coral cover seen in the 1970s? That might be astretch in our lifetimes, but with more Diadema on the reefs, coral restorationefforts by Ken Nedimyer's team and the water­quality improvements from sewerinstallations replacing septic tanks, scientists are fighting the good fight.

© Alert Diver — Winter 2012

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