And Then They Were Gone: The Urchins and the Reef
Martin A. Moe, Jr.

The great decline of the queen conch populations in the Florida Keys, the environmental and resource destruction caused by the extensive net fisheries, the decline of the great spiny lobster fishery, boating and environmental damage to the sea grass pastures, the decline of commercial food fish such as grouper and snapper--all these problems quickly capture the attention of fishery mangers and biologists, environmentalists, commercial and recreational fishermen, the dive and tourist industry, and even the public. A controversy develops on how best to fix the problem, and a determination to correct these problems arises. And this makes all the difference. Resources are studied and researched, the public is involved, conservation programs are developed, and progress is made on preservation of these important natural resources.

However, except for the attention of a few scientists, the almost total elimination of one organism absolutely essential to the health of our coral reefs has been largely ignored. A few studies have been accomplished that have demonstrated the great importance of the keystone herbivore, the Diadema sea urchin, that were lost in the great plague of 1983, but for the most part, the critical importance of restoration of Diadema to the reefs has not been recognized. These urchins have no commercial importance and their presence on the reefs was usually viewed as detrimental since their long sharp spines often caused injuries to reef explorers and complicated the activities of lobster divers. Once they were gone, however, and algae and coral disease decimated the reefs, it seems that only the reef ecologists have connected the loss of this keystone herbivore with the rapid decline of the tropical western Atlantic coral reefs.

For hundreds of thousands of years, since at least the dawn of humanity, the reefs of the tropical western Atlantic have evolved and developed within a complex ecological balance. The photosynthetic organisms, algae and corals, and the herbivores and carnivores that depend on the organic food base that these organisms create, exist together in dance of mindless competition. Every organism that shares this intensely competitive environment exploits every possible advantage that enhances survival of the individual and the species. This oceanic environment is not static, however, sea levels rose and fell over the millennia, the reefs expanded and contracted with the waters, and during periods of submergence great coral reefs formed the limestone foundations of the land that would become South Florida and the Florida Keys . And always, the ecological balance between the producers, the consumers, and the builders of the reefs maintained the delicate ecological balance that allowed the reefs to grow and build structure.

Despite the growth of civilization and the impacts of developing human populations, the reefs of the Florida Keys and the Caribbean thrived for hundreds of years while human populations exploded on the coastlines and islands, but then, suddenly, something changed. Within the geological blink of an eye, about 20 years, these reefs, those near human populations and those far from human impact, have precipitously declined. Coral cover on the Florida reef track has declined from over 70 percent in the 1960s and 70s to less than 10 percent today. Coral reefs throughout the world are in decline and none more so than the reefs of the tropical western Atlantic .

So what happened? Well, there are many factors implicated in the decline of tropical western Atlantic and worldwide coral reefs. Broadly, these factors are: increased nutrients, sedimentation, and turbidity from coastal development; direct impact from human visitation, over fishing, and destructive fishing methods; great ecological changes in reef organism diversity stemming from human exploitation and disease; and global warming (probably also anthropogenic) that raises surface seawater temperatures and so stresses corals that they release their symbiotic zooxanthellae algae (termed bleaching), weaken, and then die if the warming is severe and prolonged. The relative importance of these various factors vary with the environment and location of the reefs.

When coral tissue is stressed and dies from bleaching or disease and the stony limestone skeleton is exposed, this substrate is quickly colonized by algae. These algae, primarily fleshy macroagae, grow rapidly, trap sediments, and because of their rapid growth rates, out-compete corals for sunlight and substrate. Once dense macro algae growth is established bordering healthy coral growth, the coral tissue rapidly recedes from the point of interaction because the algae shades the coral and physically chafes and abrades the coral polyps, and some algae may even chemically attack coral tissue. So if algae can out-compete the coral, how could the coral reefs of the western Atlantic have become the magnificent and intricate living structures that they were only a few decades ago?

Through all the millennia, the long-spined Diadema sea urchins, and other herbivores, grazed the reefs and maintained the balance between coral and algae growth that allowed the corals to flourish and build the vast calcium carbonate structures of the reef. Among all the grazers, however, it was the long-spined sea urchin, Diadema antillarum, that was the keystone herbivore, the grazer that maintained the balance of growth and production between the coral and the algae. As all who dove the reefs before the early 80s well knows, there were immense populations of long-spined Diadema urchins on these reefs. Throughout this vast region the long-spined urchins were present in numbers of 2 to 20 urchins per square meter on the reefs and in the Florida Keys , 4 to 6 Diadema per square meter could easily be found on most reef formations. Small patch reefs could be easily identified from the surface by a mysterious white ring of exposed sediments that surrounded them. Research showed that these rings of exposed coal sand were caused by Diadema urchins moving off the reefs at night and feeding on the surrounding sediments and grass beds.

The urchins are gone now. Seagrass grows up to the edges and into the patch reefs and fleshy algae growth dominates the eroding limestone skeletons of ancient coral formations that were alive and vibrant only two decades ago. The complex ecological structure that built and sustained these reefs is rapidly disappearing and the ecological web of diverse organisms that inhabits the coral reef environment diminishes with every passing year.

The long-spined Diadema sea urchins of the tropical western Atlantic coral reefs died in 1983. Harilaos Lessios, A Senior Scientist at the Smithsonian Tropical Research Center , located on Barro Colorado Island at the Caribbean entrance to the Panama Canal , noticed in mid January of 1983 that the ubiquitous long-spined sea urchins found in immense numbers on reefs were in trouble. Just how serious this problem was would soon be very evident. The urchins became lethargic, did not retreat to shelter during the day, lost color and began to drop spines, and became easy prey for fish predators. Death followed quickly after the symptoms were first observed and within a few days all the Diadema on the reef were dead. The disease spread rapidly at a rate of about 1,250 miles per year to the east and nearly 1900 miles per year to the west. Soon the entire Caribbean was affected and within a year the disease spread rapidly through the Florida Keys and the Bahamas northward to Bermuda . It is estimated that 92 to 99.9 percent all the billions of Diadema sea urchins in this vast 1.35 million square miles of oceanic habitat died within 12 to 13 months. This was the most extensive mass mortality of any marine organism ever reported and the species was suddenly very near extinction. The rapidity and totality of the plague made it all but impossible to identify the causative organism. Two species of bacteria, however, Clostridium perfringens and C. sordelli were implicated since they caused the same symptoms followed by death when they were isolated from moribund urchins and injected into laboratory held Diadema.

The ecological impact of the loss of the Diadema urchins was soon apparent. In Jamaica, algae cover on the shallow reefs increased from 1% to as high as 95% within two years of the loss of the Diadema urchins, and at St. Croix, algal biomass increased by 27% within five days of the Diadema mortality and then algae biomass increased by 300 to 400% above the pre Diadema mortality levels (Lessios, 1988). Similar increases in algal biomass following the mortality were observed throughout the Caribbean and tropical western Atlantic reefs.

There is a great current discussion on the effects that anthropogenic nutrients might have on the coral reefs of the Keys and the Caribbean and the significance of these nutrients in the growth of algae. Without the presence of Diadema on the reefs, however, it is pure speculation to assume that reducing near shore nutrient levels will have any effect on algae growth on the offshore reefs. Any effects that anthropogenic nutrients might have on the reefs can be considered only when the reefs have their historical complement of Diadema urchins.

Nutrients that fueled the growth of these reefs have always been present. The great oceanic surface currents move through the tropical and temperate regions of the planet and accumulate life and nutrients along the way. As these warm waters cool at the Polar Regions and increase in density, they sink into the great oceanic depths. And in turn, the pressure from this vast quantity of descending water powers the deep sea currents of cold, dense, and nutrient rich water that flow slowly over the sea beds, and over centuries this water returns to tropical areas. Upwellings of this deep, cold water rich in nitrate and phosphate occur in many areas of the world. In the Florida Keys , for example, internal tidal bores of deep benthic waters on the reef slopes are estimated to deliver as much as 40 times the amount of nitrogen and phosphorus that are produced by near shore waste and storm water runoff ( Leichter, J. J. et. al., 2003, and Szmant, 2002). The photosynthetic organisms of the reefs, primarily phytoplankton, coral, and micro and macro algae, capture these nutrients and feed and sustain one of the most diverse ecosystems on Earth.

The large, pre-plague populations of Diadema that inhabited the reefs were efficient bioerroders of the limestone reef structure as well the keystone herbivores on the reefs. They created much of the coral sand that surrounds the reef and kept the reef substrates clear of the algae and sediment that inhibits the settlement and growth of juvenile corals. Algae and sediment now cover much of the reef structure so even when the remaining few corals successfully spawn in late summer and release millions of planula larvae, if there are only a few areas where the larvae can settle and grow, recruitment of new coral formations are few and far between. When Diadema populations are extremely high, maybe 15 or more individuals per square meter, their bioerosional activity may actually be detrimental to coral growth and settlement by undercutting coral heads and removal of newly settled juvenile corals. This same intense substrate cleaning and scraping activity at any level of population density, however, may perform a great service for the corals. Coral disease, bands of various bacteria and cyanobacteria (bluegreen algae) that expand in rings around a coral head can destroy a centuries old coral formation in few weeks to a few months, is the major cause of the loss of coral cover. Coral disease began to be noticed in the 1970s in the Caribbean and along the Florida reef track and became a very serious concern in the 1980s, just after the great Diadema mortality. Now correlation does not imply causality, but in sufficient numbers Diadema were very efficient cleaners and modifiers of reef substrate, did feed on coral tissue to some small degree, as well as on algae, and certainly could have functioned to clean diseased tissue from infected coral heads. And if this is so, then the loss of these urchins was a double disaster for the corals of the tropical western Atlantic .

It has been 20 years since the great urchin mortality. Diadema have not returned in large numbers and the coral reefs in this vast region have continued to decline. It was first thought that if at least small numbers of the Diadema antillarum species were able to survive the plague, then, because of the enormous reproductive capacity of the species (a single female may produce 10 to 20 million eggs at one spawning) these urchin populations would rapidly recover. This did not occur. Although there are encouraging signs of recovery of some Diadema populations in a few Caribbean areas, and some scattered small clusters of Diadema urchins along the reefs of the Florida Keys , this species is present primarily as small numbers of individuals scattered widely among the reefs.

The reasons for the lack of resurgence of the Diadema are not specifically known, but there are a number of possibilities. Despite their great reproductive potential, males and females must be quite close to each other, within a few feet, for fertilization to be successful. These urchins are broadcast spawners, like oysters. Release of sperm or eggs by one individual stimulates release of gametes by other nearby urchins. The eggs and sperm meet in the water column and the result is a fertilized egg that develops into a larva, which after 40 to50 days of floating and feeding in the plankton, settles out to become a tiny juvenile long-spined sea urchin. If the urchins are too far apart (which for the most part, they are), fertilization does not happen and the spawning effort is unproductive. Research has shown that there is an enormous amount of predation from fish and various invertebrates on juvenile sea urchins. Massive numbers of juvenile Diadema recruits are needed just to maintain a normal population density of these urchins. Considering the immense numbers of urchins present in the region before the plague, it is mind boggling to consider the numbers of Diadema larvae that must have been a major component of the plankton of the western Atlantic before the mortality. These immense numbers of larval Diadema are just not present today. The lack of an adequate number of Diadema larvae, coupled with the intense predation on the few juveniles that do survive their planktonic journey, is apparently the major constraint limiting the return of dense populations.

Dense populations of Diadema may also prepare the substrate for the successful settlement and survival of post larval urchins, may release a chemical that stimulates settlement of the post larvae, and may also provide protection to the juveniles under the spines of the adults. The strong tendency for Diadema to occur primarily in dense aggregations certainly indicates that some mechanisms; substrate preparation, post larval attraction, juvenile protection, and/or others, must function to create and maintain these dense clusters of adults.

Larval distribution and place of settlement of post-larval Diadema depends on the vagaries of oceanic currents and the interaction of oceanic and near shore water masses. Late stage larvae just before settlement may be broadly dispersed over great areas or concentrated in bands of plankton, and both of these conditions probably occur at various times over Florida reefs. Delivery of late larvae by favorable currents over a proper substrate will result in a population of juvenile Diadema. Post-larval Diadema do not settle and/or do not survive well in reef areas where algal growth on the rocky substrates is not removed by the scouring effects of wave action or by the activity of adult Diadema. A rocky reef surface cleared of most algal growth is apparently critical for post larval settlement of urchins (and corals).

The necessity of a suitable substrate that will allow post larval and juvenile Diadema to survive is demonstrated by the annual appearance in the late summer and fall of recruitment of juvenile urchins on the shallow rubble zones of the reef crests. These rubble zones are composed of the bones of past coral formations. Limestone rocks sized from pebbles to boulders compose shallow banks areas from 2 to 10 feet deep that extend for a half mile or more on the offshore crests of many reefs. This rock rubble is very unstable and is churned and rolled by the heavy seas that wash over these banks during the storms of winter, spring, and, of course, occasional tropical storms and hurricanes. The limestone rubble is tumbled and scraped by the heavy swells and this keeps the rocky surfaces relatively clean of algae and sediment. Thus hundreds to thousands of juvenile Diadema are able to settle and survive in this environment while very few are found widely scattered in the deeper reef areas only a few hundred yards from these rubble zones. Ironically, the very churning action of the loose rubble that prepares the substrate to receive the post larval Diadema destroys the 2 to 6 month old juveniles when the winter storms again visit these areas

After 20 years, even the limited return of the Diadema populations that has occurred in the Caribbean has not been seen along the Florida reefs. Predation on juvenile urchins may be considerably higher on Keys reefs, with their greater populations of fish, than on Caribbean reefs. The return of Diadema to Florida waters may not occur for decades, if ever, and by that time there will be little left of the glorious coral reefs of the Florida Keys . It may be possible, however, to aid the return of these urchins to the reefs and it is imperative that we at least research this possibility. Perhaps the first step would be to find out what would happen to a reef in the Florida Keys if a pre plague population of Diadema could be returned to the reef. And this first step has already been accomplished.

Ken Nedimyer, a marine life fisherman, and Martin Moe, a retired marine biologist, both members of the Florida Keys National Marine Sanctuary Advisory Council, were convinced that the loss of Diadema on the Florida reefs precipitated the drastic decline of these reefs and were determined to demonstrate what would happen if Diadema were returned to the reefs. They obtained a small grant from a NOAA reef restoration fund and began work on a Diadema restoration project with the support and counsel of the Sanctuary staff.

The project began in the fall of 2001 offshore of the Upper Keys. We wanted to explore the feasibility and ecological results of translocating juvenile long-spined sea urchins from areas with relatively high settlement and extensive winter mortality, the reef crest rubble zones, to nearby deeper water (about 25 feet, 7.5 m) patch reefs at densities approaching those on Florida reefs before the Diadema mortality. This project, involving just the straightforward transfer of at risk juveniles from the unstable rubble zones to stable deeper reefs, was designed to determine whether these juveniles could survive such movement and if they did survive in adequate numbers, could they change the ecology of the reefs.

Four patch reefs: two experimental and two controls, varying in size from about 44 to 96 sq. m were selected for the study. During the period from September 2001 to December 2001, 434 juvenile long-spined urchins were placed on experimental reef # 1 (96 sq. m), a total potential density of 4.5/m², and 262 were placed on experimental reef # 2 (88 sq. m), a potential density of 3.0/m². An additional 16 urchins were placed on reef # 2 on 10/23/02 bringing the total urchins placed on reef # 2 to 278, a potential density of 3.2/m². No Diadema urchins were placed on the control reefs. The translocated populations were evaluated for number and placement of surviving urchins 10 times on reef # 1, and 11 times on reef # 2 over various intervals during the period from September 8, 2001 to February 5, 2003 . NURC (NOAA’s National Undersea Research Center ) was contracted to perform a rapid habitat assessment of the four project reefs on 08/31/01 and 09/01/01 , before translocation of the urchins and again on 09/18/02 , about one year after translocation of the urchins to document the ecological changes that might occur on these reefs.

Initial survival after translocation of the juvenile Diadema urchins was very good. Survival rates for the juvenile urchins were 81 and 93 percent on experimental reefs #1 and #2 over the first month of the project. Survival declined to about 45 percent on both reefs after about three months and the slowly declined to about 20 to 25% after 17 months. The slow decline of the translocated Diadema population was due to steady predation on the urchins and lack of recruitment of enough juveniles to maintain the population.

Summary of survival and density of Diadema on the experimental reefs.

Experimental Reef # 1
27% survival after 17 months
Average density over the 17-month study, 1.6/sq. m
Final Density on 02/05/03 , 1.2/sq. m

Experimental Reef # 2
20% survival after 17 months
Average density over the 17-month study, 1.0/sq. m
Final density on 02/05/03 , 0.6/sq.m

Control Reef # 3
No placement of urchins on this reef

Control Reef #4
No placement of urchins on this reef

(Note: A small population of Diadema urchins, about 6 to 10 , was present on this reef before and during the study.)

Results of the ecological assessments

NURC carefully assessed the ecology of all four reefs before and after translocation of the Diadema urchins. The ecological effects of the translocated Diadema urchins on the two experimental reefs in the short space of one year were remarkable. Some of the most significant data developed from this project are summarized here and the entire study is posted on the Florida Keys National Marine Sanctuary web site. This data reports the major changes in benthic ecology between 08/31/01 and 09/18/02 as documented by the NURC assessments.

Percent total stony coral cover

Perhaps the most important statistic is the percent stony coral cover. This measures the actual extent of coral tissue recovery and also includes the amount of new coral tissue cover that may have developed from new settlement of juvenile corals.

Percent coral cover on experimental and control reefs before and after urchin placement.

Experimental reef # 1       Experimental reef # 2   Control Reef # 3   Control Reef # 4
Before       After                Before      After            Before    After             Before          After
14.00%      21.50%            5.50%      9.00%           6.25%     5.00%      12.00%       8.50%
54% increase                    64% increase               20% decrease         29% decrease

Loss of coral cover may be due to loss of coral tissue to disease or bleaching, or loss of coral tissue at the point of interaction with macro algae. This data show that over all coral cover increased significantly on the experimental reefs and decreased significantly on the control reefs. Whatever the dynamics of corals, algae, and urchins, this demonstrates that the presence of the urchins results in recovery of coral cover. And this is the bottom line for recovery of the coral reefs of the Keys.

Juvenile coral density

Total mean density (number per sq. m) of juvenile stony corals

Experimental reef # 1       Experimental reef # 2   Control Reef # 3   Control Reef # 4
Before       After                Before      After            Before    After        Before    After
6.57        13.14                5.77         17.47             5.93       11.06        7.21       8.81
100% increase                  203% increase             87% increase         22% increase

Although juvenile corals increased on both experimental and control reefs, the experimental reefs, with the translocated urchin populations, had a much greater increase. This indicates that the presence of the urchins changed the ecology of the experimental reefs to favor the settlement and/or survival of juvenile hard corals.

Percent crustose coralline algae

The presence of crustose coralline algae is very good for the reefs. Unlike foliose algae, crustose coralline algae coats the rock surfaces and presents a smooth, hard substrate free of foliose algae, sediment and algae turf. This is a substrate that attracts settlement and survival of juvenile stony corals. It has been shown that lettuce coral, A. agaricites, is stimulated to settle by the chemical secretions of coralline algae.

Experimental reef # 1       Experimental reef # 2   Control Reef # 3   Control Reef # 4
Before       After                Before      After            Before    After             Before    After
6.25%      18.50%             8.75%      19.50%       6.25%     9.75%       9.25%   6.75%
196% increase               123% increase             56% increase           27% decrease

Obviously the presence of the urchins stimulated growth of coralline algae on the experimental reefs as these algae increased three fold. Coralline algae increased a bit on control reef # 3, perhaps conditions favored it’s growth even without urchins present, or the transects in the assessment procedure cut across a point of stronger coralline algae growth in the second assessment. Control reef # 2 decreased, perhaps due to a loss of the urchins that were on that reef initially. It is obvious, however, that the presence of the urchins apparently contributed greatly to the expansion of this important substrate conditioning algae on the experimental reefs.

Brown foliose algae

Percent cover of brown foliose algae. This is the type of algae that competes directly with corals for space and light. It grows much faster than coral and diminishes coral cover where it occurs on the reefs. These brown algae are typically in the genera Tubinaria, Lobophora, Dictyota and Padina. (The green calcareous algae in the genus Halimeda also impact the reefs.)

Experimental reef # 1       Experimental reef # 2   Control Reef # 3   Control Reef # 4
Before       After                Before      After            Before    After        Before    After
11.00%     1.75%              9.00%        8.50%         6.00%     10.75%     3.00%   1.00%
84% decrease                    6% decrease               79% increase         67% decrease

The reduction of brown foliose algae on the experimental reefs, especially reef # 1, and the increase on control reef # 2 show without a doubt that the presence of the urchins greatly diminishes this competitive algae on the reefs. Its presence in low quantities on control reef # 4 only supports this conclusion because of the presence of low numbers of adult urchins on this reef before and during the study.

Percent total algae cover

The data for total algae cover showed little change on any reef during the course of the study. However, the figures for total algae include the data on crustose coralline algae, which changed considerably during the study. Removal of the crustose coralline algae data from the data on percent total algae cover on all four reefs shows the actual change that occurred in algae growth on all the reefs.

Experimental reef # 1       Experimental reef # 2   Control Reef # 3   Control Reef # 4
Before       After                Before      After            Before      After        Before    After
50.50%      33.25%            43.00%   34.25%      45.75%    43.75%    36.00%   35.25%
34% decrease                    20% decrease         4.4% decrease         2% decrease

So without the coralline algae included in the data for total algal cover, the control reefs remained essentially the same in percent algae cover while algae cover on experimental reef # 1 declined by about 34% and declined by on experimental reef # 2 by about 20%. Urchins live on algae, and other organic and inorganic matter removed from hard and soft substrates. When they were in abundance on small patch reefs, the reef had a white halo around it where urchins left the reef at night to feed on the grass beds around the reef. The high growing macro algae, such as Sargassum and Halimeda, that the urchins may not eat are eventually removed by the feeding activity of the urchins (bioerosion) on the substrates on which the algae grows. This activity gradually removes the existing macro algae growth and prevents new growth on the rocky substrates surrounding live coral growth.

Considerations on restoration of the long-spined sea urchin, Diadema antillarum to the reefs of Florida Keys .

The importance of restoration of Diadema to the Florida reefs cannot be overstated. The great importance of Diadema in the ecology of coral reefs was eloquently expressed by Ogden and Carpenter (1987), based on over 20 years of experiments and observations.

“Through direct effects on algal communities or indirect effects on other benthic reef organisms, grazing by Diadema is a major factor controlling the community structure of coral reefs. ….. Perhaps no other single species in the coral reef environment has such profound effects on the other organisms composing the reef community.”

Loss of this keystone herbivore has shifted the ecology of coral reefs from stony corals to dominance of macro algae and algal turf. Natural recovery of small pockets of Diadema populations in St. Croix , Jamaica , Belize , Dry Tortugas and other areas, and this project, and others, on artificial restoration of long-spined sea urchins has shown that when adequate populations of Diadema are restored to coral reef areas, this trend is reversed and coral settlement and growth once again become dominant on the reef. It is obvious that the restoration of Diadema to the coral reefs of the Florida Keys would be immeasurably beneficial to the ecology of the coral reefs and to the future economy of the Keys and all of South Florida .

Predation on juvenile and adult urchins is extensive and when favorable conditions of larval production and distribution do result in successful settlement, predation quickly begins to reduce their numbers. Despite the great fecundity of this species, the reproductive potential of the restricted, small populations of Diadema that do exist is not great enough to generate the incredibly vast numbers of larvae necessary to successfully create persistent pioneer populations on the algae dominated coral reefs, or to provide the number of recruits necessary to maintain and expand the small populations that do become established. Thus the lack of continuous settlement and survival apparently restricts successful colonization to areas with a low incidence of predation, and few of these Diadema survive to become large adults.

It is possible, however, that human effort can help restore this keystone herbivore to Florida Keys reefs. The immense fecundity of the adult female, 10 to 20 million eggs per spawn, is the key factor that can make the reproductive potential of numerous small, dense populations possibly great enough to enhance restoration of Diadema to the reefs.

It may be that in time Diadema will repopulate the reefs of the Keys naturally. But as we wait for this to occur, and it has already been two decades, our coral reefs continue to decline. If it is possible to enhance the recovery of Diadema on Florida reefs through human effort, it must be done soon.

There are two main pathways that should be followed that may aid restoration of Diadema to the reefs.

The first is the translocation of juvenile Diadema from areas where they are at high risk of mortality from storms and predation to small, complex reef areas. Also, concentration of widely scattered adult urchins to specific complex reef areas should enhance survival and reproductive success through aggregation of these larger individuals. We have demonstrated that the act of translocation causes little, if any, direct mortality. Areas in the lower Keys may be best since larvae originating there may have the best chance of settlement on Keys reefs. However, populations should be established on Upper Keys reefs as well since eddy currents will distribute larvae southward as well as northward. Continuous translocation of urchins to these reef locations would substitute for natural recruitment and maintain the populations on these selected reefs at densities that will transform these small reef areas from algal to coral dominance and provide a density of urchins that can reproduce successfully. The efficacy of this tactic will depend on the numbers of reef areas that can be maintained in this manner, the more the better. These areas will not only produce large numbers of Diadema larvae, but also serve as research areas for coral settlement and growth and, over time, perhaps as areas of high coral reproduction. Restoration and maintenance of Diadema on localized reef areas can be done with organized and directed volunteer effort and would entail relatively little expense. If successful, however, the benefit to the reefs from such projects could be enormous.

The second avenue is to work with hatchery techniques to produce larvae and juveniles from captive brood stock of adult Diadema. This process would be more costly but would have the advantage of controlled production with release in specific areas at specific times of large numbers of late larval and juvenile urchins. Tom Capo, Laboratory Manager of the Aplysia Resource Facility at the University of Miami , has already achieved hatchery production of juvenile Diadema. These laboratory reared Diadema have been released on Keys reefs in several experimental programs, but have not survived as well as wild collected juveniles. There are also difficulties in rearing larval Diadema past the final larval stages, through metamorphosis, and into the early juvenile stage. These difficulties, however, can be resolved though intensive research on specific rearing techniques for this species. Tom already reports that rearing through metamorphosis has been significantly improved.

There are two possibilities that hatchery production of Diadema present. The first is the liberation of vast numbers of late larvae, near to metamorphosis into juveniles, at reef areas favorable to settlement of post-larval urchins. Depending on many factors, this may or may not be a viable technique for urchin restoration, but if a facility for hatchery production of juveniles is established, then release of excess late larvae should certainly be attempted and researched. The second possibility, hatchery production of large numbers of Diadema juveniles competent to survive on natural reefs would be greatly advantageous to restoration efforts. These juveniles, which could be produced in large numbers, could be used to establish new populations of Diadema on the reefs and/or used to provide recruits for maintenance of populations already established.

It would not be necessary initially to build a large and expensive hatchery for research and production of competent Diadema juveniles. Several hatchery facilities adequate for research into the techniques of hatchery propagation of Diadema currently exist in the Keys and at the University of Miami . New Diadema research programs may be able to use existing facilities and new research programs could be funded independently or through these existing facilities. Once the techniques for production of competent juveniles are refined and hatchery reared juveniles are shown to survive on the reefs, then a hatchery capable of large-scale juvenile urchin production could be established.

There is little we can do locally to reverse or mitigate the effects of global warming or pollution from far off sources such as the rivers that empty in the Gulf of Mexico or wind blown African dust, but it may well be possible to greatly reduce the algal growth that is smothering our reefs through restoration of the long-spined sea urchin. The economic value of a successful restoration program can be measured by the value of our coral reefs to the economy of the Keys and South Florida . It may be, however, that for whatever reasons, efforts to restore Diadema to Florida reefs cannot succeed. The potential for restoration, however, is great enough, and the need for restoration of this herbivore so critical, that it is imperative that we make a strong effort to return Diadema to our reefs.

References

Leichter, J. J., H. L. Stewart and S. L. Miller. 2003. Episodic nutrient transport to Florida coral reefs. Limnol. Oceanogr., 48(4),1394-1407.

Lessios, H. A. 1988. Mass Mortality of Diadema antillarum in the Caribbean : What Have We Learned? Ann. Rev. Ecol. Syst. 19:371-93.

Nedimyer, K. and M. Moe. 2003. Techniques development for the reestablishment of the long-spined sea urchin, Diadema antillarum, on two small patch reefs in the Upper Florida Keys . http://www.fknms.nos.noaa.gov/research_monitoring/reports/diadema/diadema.html

Ogden, J. C. and R. C. Carpenter. 1987. Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes and Invertebrates ( South Florida ). Long-spined Black Sea Urchin. U. S. Fish and Wildlife Service. Biol. Rep. 82(11.77). U.S. Army Corps of Engineers, TR EL-82-4. 17 pp.