The first flank of Reef 21-187 is partly exposed to the prevailing wind and sea conditions found within the lagoon of the Great Barrier Reef. Manta tow surveys were conducted on an area that is shallow with no edge. The reef slope is generally shallow dropping off into deep water with no discernable bottom. The reef structure has many features including gullies, caves, bommies and overhangs.

The benthic community was dominated by rubble, but sand and coralline/turf algae were also common. Hard coral cover on the reef first flank was low composed mainly of Acropora species. The dominant coral lifeform was tabulate, but other forms such as sub-massive, foliose, branching and massive corals were also present in decreasing order of abundance. Coral bleaching was not present.

The fish abundance on the first flank of Reef 21-187 was moderate. Conspicuous groups of reef fishes included damselfish, parrotfish and fusiliers. No crown-of-thorns starfish were observed on the first flank of Reef 21-187 .