The back of Reef 21-187 is exposed to the prevailing wind and sea conditions found within the lagoon of the Great Barrier Reef. Manta tow surveys were conducted on the reef slope. The reef slope is generally vertical dropping off to a gently sloping reef base composed mainly of sand, rubble, hard coral and reef framework. The reef structure has many features including caves, overhangs, bommies and gullies.

The benthic community was dominated by coralline/turf algae, but hard coral, soft coral and sand were also common. Hard coral cover on the reef back was moderate with no one coral genus dominant. The dominant coral lifeform was branching, but other forms such as sub-massive, foliose, bottle-brush and massive corals were also present in decreasing order of abundance. Coral bleaching was not present.

The fish abundance on the back 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 back of Reef 21-187 .