Synopsis of data from the Ken Nedimyer – Martin Moe Diadema sea urchin restoration demonstration project on two patch reefs of the Upper Keys



A project funded through FKNMS was begun in the fall of 2001 offshore of the Upper Keys to explore the feasibility and ecological results of translocating juvenile long-spined sea urchins, Diadema antillarum, 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 plague of the early 1980s. 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. 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) conducted 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.


The ecological effects of the translocated Diadema urchins on the two experimental reefs in the short space of one year were remarkable. Since the decline of the reefs began in the early 80s, this is the first time that human manipulation of the ecology of the reefs has reversed the decline of coral cover and decreased the growth of the macro algae that shroud the reefs. Although there is much more research that must be done, this study demonstrates the great potential for restoration of our coral reefs that Diadema sea urchins holds for the Florida Keys. Some of the most significant data developed from this project are summarized here and the entire study is posted on the FKNMS web site


First, there are some important basic considerations that must be taken into account in interpreting the data from this study.


  1. The urchin replacement study was done on reefs of two different ecotypes thus the ecological responses of the two experimental reefs were somewhat dissimilar. Experimental reefs 1 and 2 are quite different. #1 has high coral structures and # 2 is composed of low stony coral and soft coral growth. Control reefs # 3 was selected to be similar to experimental reef # 2 and control reef # 4 was selected to be similar to experimental reef # 1.
  2. Because of limited availability of urchins, fewer urchins were placed on experimental reef # 2 (272) than on experimental reef # 1 (434). Thus the density of urchins was higher on reef # 1 than on reef # 2 throughout the project. This difference in urchin density must also be considered when comparing the ecological response of the two experimental reefs.
  3. Control reef # 4 was much smaller than experimental reef # 1, 44 sq. m vs 96 sq. m, and control reef # 4 had a small population of adult urchins on the reef. They were present at the outset of the study and unfortunately this greatly reduced the value of the control to reef # 1. The presence of these urchins on this control reef has to be taken into account when making comparisons between the reefs. There were only about 0.1 urchins per sq. m on control reef # 4, as compared to almost 2.0 urchins on experimental reef # 1, but this was apparently enough to affect the cover of brown foliose algae, but evidently not enough to enhance the growth of crustose coralline algae.


The following data are the most important results of the year-long study. They have been excerpted from the final report of the Diadema restoration project.


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 study, 1.0/sq. m

Final density on 02/05/03, 0.6/sq. m


Changes in benthic ecology between 08/31/01 and 09/18/02 (NURC assessment)


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


The loss of coral cover may be due to loss of coral tissue to disease or loss of coral tissue at the point of interaction with macro algae. Coral tissue at the interface between coral and macro algae is lost very quickly. This shows 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 (no. 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 good, very good. 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 hard coral. It has been shown that lettuce coral, A. agaricites, is stimulated to settle by 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 this 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.


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 # 2 only supports this contention because of the presence of low numbers of urchins on this reef before and during the study.


Percent total algae cover


The data for total algae cover shows 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 growth macro algae on hard substrates that the urchins do not eat are 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.

The historical importance of Diadema on the reefs


There is a great current discussion on effects that increased nutrients have on the coral reefs of Keys. Without the presence of Diadema on the reefs, however, it is pure speculation to assume that reducing inshore nutrient levels will have any effect on algae growth on the offshore reefs. The effects of nutrients on the reefs can be considered only when the reefs have their historical complement of Diadema urchins. The two quotes below from researchers that have spent decades working on coral reefs and urchins well illustrate the importance of Diadema urchins to the coral reefs.

“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.”


Ogden and Carpenter (1987), based on over 20 years of experiments and observations



“It is possible to estimate past population sizes from DNA variability if the rate of DNA mutation is known. By estimating mutation rates between Diadema separated by the Isthmus of Panama, the expansion of Diadema populations is estimated at having occurred more than 100,000 years ago.  Thus the Atlantic reefs have evolved with abundant Diadema and any attempt at reef restoration should include them in high numbers.”



HA Lessios, MJ Garrido & BD Kessing – 2001


Both of us, Ken Nedimyer and Martin Moe, strongly believe that recovery of the Florida Keys coral reefs greatly depends upon restoration of Diadema sea urchins in numbers similar to those that existed on these reefs prior to 1983. We base this belief on the results of this study, the scientific literature on the biology and ecology of Diadema, and current observations in areas of the Caribbean where limited populations of Diadema have returned. We advocate continued research on the biology and ecology of Diadema on Florida reefs, programs that will establish effective spawning populations of Diadema, and research into hatchery techniques for reproduction and release of Diadema back to the reefs.



Martin Moe


222 Gulf View Drive

Islamorada, FL 33036


305 517-9085