Chagos Archipelago - Great Chagos Bank West (BIEA24)

General benthic features

Porites leads at 7.6% of the benthos, 48% of all live coral present, then Pocillopora 3.1%, Acropora 0.9%, Ctenella 0.7% (the Chagos-endemic monotypic genus, not Cynarina), Favites and Astreopora 0.6% each, Platygyra 0.4%, Heliopora 0.3%, Leptastrea, Montipora, and Pavona 0.3% each, Turbinaria and Goniastrea 0.2% each, Pachyseris and Gardineroseris 0.1% each, and Galaxea <0.1%. The Heliopora record is Anderson's own determination: the field data are Heliopora, blue coral, determined by A. Mayfield, the expedition's coral-physiology lead. The corroborating evidence is that HELI reaches 93% cover on one transect, impossible for a solitary free-living mushroom coral, while HEFU, the Heliopora code, has only four records across all 7,029 GRE transect rows. It is labeled Heliopora here. The non-coral substrate is dominated by non-coral invertebrates at 25% — about one and a half times the live coral cover, and the largest single benthic category at this site — then bare substrate 15%, erect algae 14%, turf-with-sediment 12%, turf 8.6%, crustose coralline algae 8.1%, macroalgae 1.9%, and cyanobacteria 0.2%.

The dive opens on a wall and moves onto a terrace. Image 1 is a near-vertical pale carbonate face falling into deep blue: a brown ridged foliose plate and a yellow-green plating colony with fine even corallites on the lip, antipatharian whips and a bushy octocoral trailing off it, surgeonfish and white-tailed damselfishes streaming into open water.

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Image 13 is a clear wide benthic frame — a bommie of large, ruffled, pale yellow-green plates (foliose and scrolling) compatible with Turbinaria or Montipora folded over deeply pink- and magenta-crusted rock, a maroon foliose plate beside them, a yellow-green massive at the right edge and orange anthias across all of it. Image 5 is the broadest view of the bottom: pale tan massives and a dark honeycomb-corallite massive compatible with Favites or Goniastrea standing among foliose plates over rubble and turf-crusted pavement.

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Image 2 is the frame with the most live coral in it: a broad pale gray-tan massive dome compatible with Porites, knobby digitate colonies around it, feather stars, squirrelfish, and anthias.

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It sits between a large pink-cream lobate-columnar colony compatible with Porites and a golden-tan corymbose Acropora with brilliant white branch tips, over pavement carrying pink coralline crusts and inky black patches compatible with an excavating sponge.

Image 8 needs a specific note. It is a frame-filling macro of a foliose plate margin — red-brown tissue grading to teal-green, with conical, well-spaced corallites whose oral discs are pale white — compatible with Turbinaria.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 16%, from the Khaled bin Sultan Living Oceans Foundation's BIOT-1 in situ transect survey (the mean of all of this station's transects); the photographs illustrate the site. Algal cover is 45% of the benthos. Surveyed 16 March 2015 at Eagle Island on the western Great Chagos Bank (6.20961°S, 71.30426°E; fringing reef, leeward from its position on the northwest side of the Eagle Islands reef platform relative to the prevailing southeast trade winds (windward on Anderson's field sheet); station CTD temperature 29.3°C, salinity 34.3).
• Live hard coral 16% against algal cover 45% of the benthos, across 15 scleractinian genera plus the blue coral Heliopora.
• Transect mean 16 ± 2.9% SE (n = 9 transects; Living Oceans Foundation per-transect benthic data); the headline above is this all-transect mean, and the published station value, which weights each depth equally, is 13% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Image-based estimates (CoralNet, DINOv3, CoralSCOP, and Claude) are listed in the method table for comparison only; the transect figure is this site's cover value.

Stressors

Image 2 is also the site's condition frame — the dome carries pale gray patches and one bright pink-red patch with a smaller pink spot on its upper surface; no sharp-edged patch of freshly exposed skeleton is visible. Those pink spots read as a pigmentation response to injury or boring organisms; no disease is identified, and none can be diagnosed from an image. The tissue in Image 8 is pigmented: the white is the corallite mouths, not bare skeleton. That is worth recording because the point annotation in this page's photo-comparison table scored 16 of this frame's 30 points as bleached. It is reported here as seen, no bleaching percentage is derived from it, and the site's published bleached fraction is a separate number that no photograph revises. The same restraint applies to the brilliant white branch tips on the corymbose Acropora in Image 4, Image 11, and Image 19: white tips fit both normal growing axial corallites and early tip bleaching, they cannot be separated at this magnification, and they are reported as seen and not diagnosed.

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The frames show three things and no more: pale patches and pink spots on the massive dome in Image 2, pigmented tissue with white corallite mouths in the macro view of Image 8, and unresolved white branch tips in the closing moray sequence. No crown-of-thorns, no disease, and no storm rubble field appears anywhere in the set. No percentage bleached is estimated from these frames. Published bleaching is 21% (Mayfield & Dempsey 2026, suppl. S1), and the two colonies Anderson sampled here were 20% and 50% bleached (see Dive record below). The Living Oceans Foundation transects here recorded 0.2% cyanobacteria and no recently dead coral.

Fish and other fauna

A burst of frames works one overhang, and the emperor angelfish is the animal of the dive. Image 6 establishes the ledge: a ridge of brown digitate colonies over a maroon foliose plate, anthias, a dark-blue triggerfish, and an adult emperor angelfish at its base.

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The same adult carries the next frames, appearing in Image 9 and Image 18, and again in Image 5.

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Image 18 records a cleaning station mid-service: the angelfish hangs stationary with a small striped wrasse at its flank, over a dense stream of small golden fish compatible with apogonid cardinalfishes or sweepers filling the shadow under the ledge in Image 9 and Image 18.

The flash macros include two clear portraits. Image 3 and Image 7 are frame-filling close-ups of a single coral guard crab front-on among the pale nodular branches of its host colony: cream-white carapace and limbs boldly blotched and reticulated with crimson, stalked yellow-green eyes, jointed chelipeds tapering to orange-tipped dactyls — a red-spotted guard crab. Image 3 is the sharper of the two.

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The closing sequence is a moray. Image 4, Image 11, Image 16, and Image 19 show the same animal from progressively wider range — elongate head, small forward-set eye, tubular anterior nostril, jaw sloping back past the eye, dark ground densely peppered with white spots — compatible with a whitemouth moray. Image 12 is a blurred close-up of a mottled head with the mouth agape and pale lining showing, among pale cream nodular colonies; it is most consistent with the same animal, but the frame is too soft to settle that and it is not counted as a separate identification. Image 16 also carries the site's highest image dead-coral value.

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Conspicuous fauna and sessile invertebrates appear as sponges in 8 frames, sea cucumbers in 2, giant clams and anemones in 1 each.

Comparisons with other data

The XL Catlin Seaview Survey covered a nearby station (transect 36003) in 2015, 0.30 km from the pin; we could not confirm that it is the same reef. Within 5 km, one independent station, XL Catlin Seaview Survey at transect 36003, 0.30 km away, read 13% in 2015.

Dive record

Anderson sampled two pocilloporid colonies on this dive for a collaborator (the sampling sheet; Pocillopora sp., identity unsure for both): ChJo69 (collected 14:34 at 29.7 m; 15–20% bleached, with partial mortality; the bleaching part was sampled) and ChJo70 (14:54, 15.4 m; pale and sickly, 50% bleached; the bleached part was sampled). The notes record no Pocillopora damicornis, P. acuta, or Seriatopora at the site, on a fairly steep slope. 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

Fifteen habitat frames from the 16 March dive; close-ups of the colonies sampled are on file but are not shown here. The set is strongly bimodal — seven reef-scale frames followed by eight close and macro subjects, seven of the latter composed on two animals — and the camera changes job halfway through. Quality is good: the wide frames are sharp with natural color and good visibility, the strobe-lit macros are the sharpest images in the set, and Image 12 is the only genuinely soft frame. No quadrat, transect line, diver, or scale object appears in any of the fifteen frames. Anderson's field sheet records the aspect (LOF fellowship 2016 workbook: 'fringing reef', 'windward'); the expedition's 933-site table classes the station as 'intermediate' (fringing reef). The aspect shown, 'leeward', comes from the station's GPS position on the northwest side of the Eagle Islands reef platform relative to the prevailing southeast trade winds (about 164° off the upwind direction; the reef-edge reading agrees), which takes precedence over slope facing, lagoon shelter, imagery, and field labels. Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin on the northwest edge of the submerged spit that runs southwest from Eagle Island's southern tip, facing northwest (about 315°): windward for the northwest monsoon winds of the March survey and leeward for the southeast trades of April to November.