Distribution, Abundance and Food of Sea Urchins on a Leeward Hawaiian Reef

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Echinometra mathaei was dominant, conprising 50-98% of the urchin populations. Its peak average density was 70.3 urchins.m-2 at 120m from shore on a calcareous pavement (1.5m deep) with scattered, small coral colonies. Although the distribution of all urchin species overlapped, all of the other species reached their peak densities shoreward of E. mathaei. The next most abundant urchin was E. oblonga (9.3.m-2 at 110m from shore) followed by Tripneustes gratilla (3.7.m-2 at 90m). Total urchin densities reached a peak average of 73.9.m-2 at 110m. Two Ulva species, two Pterocladia species, Grateloupia hawaiiana and Acanthophora spicifera (5-100cm tall on the rocks inshore) were the most abundant macroalgae. They became dramatically reduced in size and except for Pterocladia and Ulva, in abundance, from 80m offshore to the end of the transect (180m). Out of a total of 43 species of macroalgae and 71 species of microalgae recorded, 19 and 14 species respectively were found in the 70 urchin guts examined. Generally, urchins selected food in relationship to its abundance. There was evidence of a dependence upon drift algae.

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Bulletin of Marine Science, v. 45, issue 2, p. 539-549