Chagos Archipelago - Danger Island (BIDI16.5)

General benthic features

Close up, the substrate rather than the colonies is the subject: pink and mauve crustose coralline algae over consolidated pavement and rubble, blood-red encrusting patches, dark-green algal tufts, a glossy dark-brown ovoid consistent with a mantle-covered cowrie, and pale-tipped branching colonies at the margins. Non-coral cover is dominated by turf (28% of all points), with crustose coralline algae 5.9%.

Coral cover

• Coral cover: 46% — from imagery only, with no transect behind it. That value averages 13 hand-scored photographs, two of them surface frames of the cay; the 11 reef frames alone give 39%.
• This is a ".5" site: informal survey images were annotated in CoralNet, but no formal benthic transect was ever run here.
• The figure describes the frames that were photographed, not a surveyed reef, and it is not comparable with the transect-derived cover quoted at the numbered sites nearby — it must never be pooled or averaged with them.
• Image-based estimates in the method table are listed for comparison only.

Stressors

Published bleaching for this informal dive is 20% (Mayfield & Dempsey 2026, Oceans, suppl. S1; published, from 13 photographs). The imagery carries it. The single best frame: one Pocillopora-type colony strongly mottled, bone-white branch clusters interleaved with fully pigmented brown ones across the same colony — textbook patchy partial bleaching, unambiguous. A second frame is its complement, a large verrucose thicket where most of the colony has gone pale pink-cream with brown polyps still visible and one bone-white cluster below (e.g., Image 3).

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A third shows the tip-first pattern: a Pocillopora whose branch ends are white while the colony interior stays pigmented. In Pocillopora white branch ends are not a normal growth feature, which is what makes these three frames convincing where a pale Acropora tip would not be. Another frame adds a small, wholly bone-white branching colony standing among pigmented neighbors, and two more show pale patches on the upper surfaces of massive corals that are more likely partial mortality or old lesions than active bleaching. The point annotations independently confirm this read. Of 21 bleached points across the site, 11 (52%) fall in the two strongest bleaching frames, with 2 each in three other frames and 1 each in two more. Two methods, one conclusion: the bleaching at BIDI16.5 is real, spatially clustered, and concentrated in pocilloporids. Two annotation errors run in opposite directions, and both matter. First, 2 of the site's 21 bleached points sit on the two surface frames, photographs of the palm-fringed sand cay taken from the boat, 90% open water and sky, containing no coral whatsoever. Each was assigned one "bleached Acropora" point (one also picks up a "Porites"), almost certainly points landing on white breaking surf along the reef crest. That is 10% of the site's bleached signal generated by foam. Second, and pulling the other way, one frame contains a wholly white branching colony yet scores zero bleached points — 30 points simply did not land on it. Point counts on this site both over-call bleaching on blue-water frames and miss small whole-colony bleaching; do not treat the 30-point read as ground truth in either direction. One more frame deserves separate mention: a striking deep pink-magenta verrucose colony, compatible with a pink-pigmented Pocillopora, sitting on coralline pavement. Its color is recorded as colorful, cause unclear: pink and purple pigment in pocilloporids can be a colony's natural color or a stress response, and one frame cannot separate the two (the frame carries 2 bleached points in the comparison labels). Caution on those tips — in Acropora actively growing axial corallites are routinely pale, so tip pallor alone is not bleaching; the Pocillopora frames are the defensible evidence. Anderson's sampling sheet records the three colonies sampled here as healthy, one with some old mortality (see Dive record below). No crown-of-thorns starfish (COTS) or other corallivores were evident.

Fish and other fauna

Faunally, one frame has a large loose school of small slender silvery fish (fusilier- or silverside-type) threading the whole frame, another draws wrasses and damselfishes to its isolated bommie, and a sergeant major sits at the bottom of a third (e.g., Image 2).

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Conspicuous fauna and sessile invertebrates appear as sponges in 8 of the frames reviewed, anemones in 2, giant clams in 1.

Comparisons with other data

We found no other surveys at this site as of 2026. If you would like to share yours, please email us.

Dive record

This was Anderson's own short dive (the field sheet: maximum depth 4.8 m, 32 minutes, in at 12:35; a very shallow, high-light fringing reef, intensely surgy beneath the waves). Anderson sampled three Pocillopora damicornis colonies, confirmed by RFLP: CHPd13 (collected 12:50 at 2.5 m; a large colony covered in sand), CHPd14 (12:56, 2.5 m; healthy, dark pigmentation, short bulbous branches, some old mortality), and CHPd15 (13:00, 2.5 m; healthy, dark pigmentation, bush-like). All the other pocilloporids were present, but Anderson focused on P. damicornis; P. acuta was not present. Pocillopora acuta colonies from this archipelago were analyzed in Thomas et al. (2026, Global Ecology and Biogeography 35:e70305, https://doi.org/10.1111/geb.70305).

Other notes

The deep-magenta colony wedged among the branches is a stony coral: the corallite structure is stony, not octocorallian.