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Diadema antillarum, the long-spined sea urchin, is an important reef resident whose presence or absence can change the very nature of a reef. Urchins are key grazers and bioeroders, and their long spines provide shelter and protection for small fish and invertebrates.

Urchins, along with large herbivorous fishes (F11),help “graze down” the turf algae and macroalgae that compete with corals for preciousDiadema Abundance Chart reef space.

  • When urchins are present • in optimal numbers,
    they help maintain a balance between coral and
    algal growth
  • When urchins are too few in number, algae can
    take over, sometimes transforming a vibrant coral
    reef into a dull, algae-dominated one.
  • When urchins are too numerous, they can scrape
    and erode and damage the reef framework.

In 1983, Mother Nature conducted a vast “natural experiment” that convincingly demonstrated the urchin’s importance in maintaining healthy coral reefs. That year, almost 98% of the Caribbean’s Diadema succumbed to disease.10 Due in part to this dramatic urchin decline,reefs in many areas subsequently shifted from coral dominance to macroalgal dominance.11 So snorkelers who once visited a colorful coral reef might now see one overgrown by a fuzz of fast-growing algae.

We recommend monitoring Diadema’s contribution to herbivory by tracking abundances. As with the herbivorous fishes (F11), measuring Diadema herbivory rates directly is not practical. The next best thing is this proxy indicator, expressed in terms of Diadema numbers per unit area.

A promising sign of Benchmark reef recovery would be a regional Diadema density of 1 urchin/m2.

  • Before the 1983 die-off 12, MAR Diadema densities
    ranged from 4 to 25 urchins/m2.
  • Years later (1998-2001),13 the regional average
    was still only 0.03 urchins/m2—well within Red Flag
    territory and only about one-fifth the Caribbean
  • More recent surveys (2004-2006)14 indicate
    some recovery: by 2006,the region as whole had
    achieved Target status.
Wide Average Status(1998-2001) Diadema Antillarum
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