Chagos Archipelago - Great Chagos Bank West (BITB32)

Setting

The setting is a submerged lagoonal bank reef, windward from its position on the south side of the Three Brothers reef platform relative to the prevailing southeast trade winds (leeward on Anderson's field sheet), at 29.4°C and 34.4 salinity (station CTD).

General benthic features

The transect's genus breakdown is the most detailed account on this page. Thirty scleractinian genera were recorded, and this is a faviid/merulinid reef, not an Acropora reef. Porites 5.0%, Favia 4.2%, Pocillopora 3.9%, Platygyra 3.6%, Montipora 2.8%, Echinopora 2.1%, Favites 1.3%, and Acropora only 0.9%, then twenty-two more at or below 0.8% each: Montastrea 0.8%, Echinophyllia, Acanthastrea, and Pavona 0.5% each, Astreopora 0.4%, Symphyllia, Stylophora, Hydnophora, Gardineroseris, and Fungia 0.3% each, Leptastrea, Cyphastrea, and Goniastrea 0.2% each, Galaxea, Ctenella, Psammocora, Turbinaria, Leptoseris, and Coscinaraea 0.1% each, and Cantharellus, Mycedium, and Pachyseris <0.1% each. On the non-coral side the transect gives turf 15%, crustose coralline algae 13%, bare substrate 12%, invertebrates 10%, turf-plus-sediment 7.1%, erect algae 7.2%, and macroalgae 6.1%. With the nine lobster frames (see Fish and other fauna), the two giant-clam macros, and the colony macro, twelve of nineteen frames are composed on an animal or a single colony rather than on the reef. Several of those still show usable pavement and colonies around the animal. The reef those frames do show is a low-relief, high-diversity, small-colony bank. Image 5 is the best statement of it — a slope of dark massive domes, gray foliose plates, brown digitate and encrusting colonies, and mauve Pocillopora-type clumps packed shoulder to shoulder over purple-pink coralline pavement, with a checkerboard wrasse and a yellow sponge.

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Image 9 is the highest-cover frame — tan lobate Porites-type, pink-margined encrusting Montipora-type, corymbose Acropora, coarse-corallite faviids, and Pocillopora, scored 53% live.

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Image 4 and Image 8 are the best genus frames, each carrying a big tan knobby-columnar colony, a fine-valleyed meandroid brain, Astreopora-type domes, and a ridged plate in one view.

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Against these, Image 7, Image 17, Image 20, and Image 23 are genuinely thin — broad pale consolidated pavement and rubble carrying small, well-spaced colonies, with Moorish idols, and a parrotfish over them in Image 20 and a neon fusilier school streaming over (e.g., Image 17).

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The photographs corroborate the growth habits but not the names: fine-valleyed meandroid brains (e.g., Image 4, Image 8, Image 16), tan knobby-columnar and massive colonies compatible with Porites (e.g., Image 4, Image 8, Image 9), coarse-corallite domes compatible with Favia/Favites/Astreopora throughout, ridged plates (e.g., Image 4, Image 16), gray foliose sheets compatible with Echinopora/Pavona (e.g., Image 5), encrusting sheets compatible with Montipora, mauve clumps compatible with Pocillopora, and one branching colony compatible with Seriatopora/Stylophora (e.g., Image 1); plus calcified green algae, sponges, ascidians, and heavy pink coralline crust.

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The percentages above are transect identifications; the growth-form calls here are reads of photographs. Blue encrusting sponges recur in Image 3, Image 4, Image 12, and Image 19, and a sea star sits in the lower corner of Image 12.

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No survey apparatus appears in any frame — no tape, quadrat, slate, diver, or collecting gear — and Image 1 is the one specimen macro: a small olive-green branching colony with pale tips on coralline-crusted pavement, not resolvable to genus at this resolution and not called here (e.g., Image 1). Anderson's field sampling record shows that one Pocillopora acuta (CHPd21) and one Seriatopora hystrix (ChSe13) were sampled at this site, both target pocilloporid groups.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 30%, 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 48% of the benthos.
• Transect mean 30 ± 2.6% SE (n = 15 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 30% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Three depth strata were surveyed and they disagree: D2 37% coral, D3 27%, D4 28% — 30% is their mean, and the 27–37% span is the honest range of cover across the bank.
• 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

Anderson's published per-site bleached fraction for BITB32 is 7.6% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026 supplement). One entry is worth reading closely: the transect records Seriatopora at 0.0% even though a Seriatopora colony was collected at this site, which is itself an under-detection note about point-intercept survey at low abundance. The point labels in this page's photo-comparison table, listed for comparison only, place eight bleached points of 570: five fall on frames scored at or below 13% live, the only bleached-Acropora pair sits on Image 7, a wide view of dark maroon-black encrusting sponge sheets over pale sand that shows no Acropora, and another sits on Image 2, the giant-clam macro, where the pale shell margins are the brightest thing in frame. Image 1 is the only candidate frame for either sampled colony (the Pocillopora acuta or the Seriatopora) and cannot be resolved; the point annotation on it calls it Porites and Acropora, neither defensible.

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Qualitatively, structural loss is old and unremarkable and there is no acute damage anywhere: the pavement in Image 4, Image 12, Image 13, and Image 22 is maroon-and-pink coralline crust plated over old framework, ordinary turnover, and the only mortality worth a name is a maroon dead patch on the meandroid brain in Image 4.

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No crown-of-thorns starfish and no predation scars appear in any of the 19 frames. No disease lesions are visible in the frames, though Anderson's sampling sheet records the Pocillopora acuta sampled here as 5% bleached or dead and the one Seriatopora hystrix colony as possibly unhealthy (see Dive record below). The Living Oceans Foundation transects here recorded no cyanobacteria and no recently dead coral.

Fish and other fauna

Conspicuous fauna appear as sponges in 17 frames, sea cucumbers in 9, urchins in 6, giant clams in 2, and anemones in 1. BITB32 is the lobster dive, and that governs the whole photographic record. The camera finds two spiny lobsters (white-reticulated carapace, olive-green abdomen with white transverse banding, black-and-white ringed legs, pink antennular peduncles; not callable to species) walking in the open over pavement in broad daylight, and stays with them (e.g., Image 3). Nine of the nineteen edited frames are composed on lobsters — Image 13 and Image 15 are wide with both animals, Image 22 is closer, Image 3 is the best of them, Image 12 and Image 19 are from above with a mottled brown five-armed sea star below, and Image 4, Image 8, and Image 16 are on a single animal.

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Beyond the lobsters, Image 2 is a large live giant clam with its mantle fully open, olive-and-black patterned, wedged into coralline rock, and Image 14 appears to be a second, dead and coralline-overgrown clam shell, its fluted margins still legible under pink crust (e.g., Image 2, Image 14).

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Image 6 puts a requiem shark over the reef in open blue water; the dark-margined caudal and plain first dorsal are compatible with gray reef, but it is not callable from this frame (e.g., Image 6).

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Fish are otherwise sparse and small.

Comparisons with other data

Apart from the Living Oceans Foundation survey shown here, we found no other surveys at this site as of 2026, and no independent survey station lies within 5 km.

Dive record

Anderson's morning dive on 19 March 2015 entered the water at 08:11 and lasted 60 minutes, to a maximum depth of 29.6 m (the field sheet); the edited set keeps 19 stills, plus two movies. Anderson sampled five pocilloporid colonies on this dive (the sampling sheet): Pocillopora acuta CHPd21 (confirmed by RFLP; collected 08:16 at 28.7 m; a colony 5% bleached or dead, with a healthy and a bleached branch both sampled), Seriatopora hystrix ChSe13 (08:24, 26.4 m; the only colony seen at the site, large and spindly, possibly unhealthy), and, for a collaborator, Pocillopora sp. ChJo91 (08:40, 15.4 m; healthy, with a green fluorescent hue and some old mortality), ChJo92 (08:45, 15.2 m; healthy, candelabrum-shaped, next to a different Pocillopora species), and ChJo93 (08:55, 11.1 m; huge, candelabrum-shaped, healthy though relatively pale). Anderson's notes record a gentle current, clear water, a fairly steep slope where the reef bottoms out at 20 m (though corals were found deeper), and mating lobsters. 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

All 19 frames were reviewed — complete coverage. Two movies were shot mid-sequence. Close-ups of the colonies sampled (CHPd21, ChSe13) are on file but are not shown here. Anderson's field sheet records the aspect (LOF fellowship 2016 workbook: 'lagoonal bank reef', 'leeward'); the expedition's 933-site table classes the station as 'protected' (bank reef). The aspect shown, 'windward', comes from the station's GPS position on the south side of the Three Brothers reef platform relative to the prevailing southeast trade winds (about 42° 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) shows open water at the pin about 5 km east of South Brother, inside the Great Chagos Bank; no bottom or reef edge is visible, so the imagery cannot test the aspect.