The distribution, abundance and food of nine species of sea urchins along a 180-m transect on a leeward Hawaiian reef were studied in relation to algal distribution and habitat. Echinometra mathaei was dominant, comprising 50 to 98% of the urchin populations along the transect.
Its peak average density was 70.3 urchins·m−2 at 120 m from shore on a calcareous pavement (1.5 m 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 110 m from shore) followed by Tripneustes gratilla (3.7·m−2 at 90 m). Total urchin densities reached a peak average of 73.9·m−2
at 110 m. Two Ulva species, two Pterocladia species, Grateloupia hawaiiana and Acanthophora spicifera (5–100 cm 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 80 m offshore to the end of the transect. Out of a total of 43 species of macroalgae and 71 species of microalgae (excluding four species of crustose corallines) recorded on the transect, 19 and 14 species respectively were found in the 70 urchin guts examined. Generally,
urchins selected food in relationship to its abundance, and there was evidence of a dependence upon drift algae.
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