Chagos Archipelago - Great Chagos Bank West (BITB33)

General benthic features

Twenty-seven scleractinian genera were recorded, and the assemblage is led only narrowly, by Pocillopora: Pocillopora 3.8% of the benthos, Porites 3.8%, Platygyra 3.7%, Acropora 2.7%, Favia 2.6%, Favites 1.0%, Montipora 0.9%, Ctenella 0.5% (the Chagos-endemic monotypic genus, not Cynarina), Pavona, Galaxea, Goniastrea, Cyphastrea, Oulophyllia, Psammocora, and Symphyllia 0.2% each, Pachyseris, Montastrea, Hydnophora, Stylophora, Astreopora, Echinopora, Leptastrea, and Cantharellus 0.1% each, and Leptoseris, Lobophyllia, Acanthastrea, and Fungia <0.1% each — with Seriatopora at exactly zero, matching Anderson's field sampling record ('no Seriatopora'). Bare substrate is 14%, and the depth structure explains it: the D4 transects are sand-dominated (233 and 210 sand points) while four of the six shallower D3 transects carry none (the other two carry 37 and 67). The 58% "algal cover" needs unpacking before it is read as degradation: it is crustose coralline algae 15% + turf 15% + turf-with-sediment 12% + erect algae 9.1% + macroalgae 7.8% + cyanobacteria 0.1% — only 17 percentage points of it are fleshy algae.

South Brother is a coral garden on a sand-and-pavement plain, and the imagery makes 21% look exactly right. Nothing here is continuous reef. Image 6, Image 11, Image 13, and Image 21 are the defining frames — white sand and pale consolidated pavement running to the horizon, with tan corymbose Acropora, small Porites domes and Pocillopora-type clumps on it as well-spaced, individually resolvable colonies, none touching. Image 8 is the emptiest: a low plain of small olive branching colonies over bare sand. Image 3 and Image 20 are the same ground with dark maroon-black encrusting sponge spread in sheets over the sand and pavement (Anderson's field sheet notes the site was “sponge dominated at 10 m”), and Image 17 — pink-and-maroon CCA over old dead massive skeletons, threaded with green calcified algae — is the clearest statement of old, non-acute structural loss here. Image 28 is the closest look at the algal fraction: dense dark green algal tufts spread across pavement between gold honeycomb-textured colonies and tan encrusting sheets, with dark ruffled fleshy macroalgae.

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The sightline in most of the wide frames runs flat into blue-gray with no structure breaking it.

Where coral does concentrate it is healthy and acroporid. Image 16 is a high-cover frame: two large gold-green tabular Acropora, over tan lobed Porites and small faviids, a Moorish idol behind.

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Image 22 is the best mixed frame — gold digitate Acropora, a large brown knobby-columnar Porites, and a pinkish-gray branching colony that is the site's clearest partial-mortality candidate, branch tips already carrying crustose coralline algae. Image 25 is a staghorn-and-corymbose Acropora thicket with a powder-blue surgeonfish in it.

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Image 4 is the signature image: gold-green and maroon-purple branching Acropora with a yellow transect tape running clean across the upper third of the frame. Image 7 and Image 28 are the two close benthic frames and carry the site's highest mask cover (35% and 32%) on encrusting and plating colonies the wide shots never resolve; Image 7 is a large tan encrusting and plating colony with a pale margin and scattered red encrusting sponge around it.

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From the photographs the Acropora shows in four habits — tabular plates, corymbose/digitate, gold-green arborescent staghorn, and the maroon-purple branching form in Image 4 — alongside Porites massive/domal, knobby-columnar and encrusting, Pocillopora-type verrucose clumps, faviid submassives, meandroid brains compatible with Platygyra, sheets compatible with Montipora/Pavona, and coarse honeycomb colonies compatible with Astreopora. Growth-form calls from photographs are given as "compatible with". Earlier, Image 12, Image 14, and Image 24 hold dense fusilier schools, the last banked over a broad lobate Porites dome on white sand.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 21%, 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 58% of the benthos. Surveyed 19 March 2015 at South Brother in the Three Brothers group (6.17930°S, 71.58126°E; lagoonal bank reef, submerged, leeward on Anderson's field sheet; dived to 22.3 m; station CTD temperature 29.5°C, salinity 34.4).
• Live hard coral 21% against algal cover 58%, bare substrate 14%, and non-coral invertebrates 6.7% of the benthos, taken on 10 transects across three depth strata (D2–D4).
• Transect mean 21 ± 3.9% SE (n = 10 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 19% (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

Several wide frames carry a flat blue-gray cast over pale pavement that makes fine condition calls unreliable at range. Mortality here is mostly old rather than acute (the only recent loss on record is 5% recent mortality on one colony on the sampling sheet; see below): CCA-plated dead massives in Image 17, and the pinkish-gray branching colony in Image 22 whose branch tips already carry coralline crust — the clearest partial-mortality candidate in the set. No frame in the 27 shows unambiguous whitening. The one genuine paling candidate is Image 13, a cream-yellow lobed massive standing against a dark maroon-black encrusting sponge sheet among normally pigmented neighbors; it is not callable from the frame and may equally be a naturally pale Porites. No crown-of-thorns appears in any of the 27 frames, no disease lesions, and no corallivore predation scars. No percentage bleached is derived from these photographs; this site's published per-site bleached fraction is a separate, authoritative number from CoralNet analysis of the expedition's survey images, trained on Anderson's point annotations (Mayfield & Dempsey 2026, suppl. S1), and no frame here confirms, qualifies, or revises it. Anderson's sampling sheet records three Pocillopora colonies sampled here at 10.4–20.4 m (ChJo94–ChJo96), all healthy, one with 5% recent mortality. Anderson's sheet records no P. damicornis or P. acuta at this site; these colonies were collected for a collaborator. Pocillopora acuta colonies from elsewhere in the archipelago were analyzed in Thomas et al. (2026, Global Ecology and Biogeography 35:e70305, https://doi.org/10.1111/geb.70305). The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

About 14 frames carry usable benthic content; five (e.g., Image 1, Image 2, Image 15, Image 19, Image 27) are water-column frames whose subject is a jack school or a shark; three (e.g., Image 5, Image 10, Image 18) are consecutive macros composed on a single lobster in shadow under a ledge, 11% of the set for one animal; three (e.g., Image 12, Image 14, Image 24) are fusilier frames with only incidental bottom.

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No scale reference appears in any benthic frame.

The fauna is dominated by one enormous trevally aggregation. Image 2, Image 19, and Image 27 show several hundred silvery, black-spotted, yellow-finned jacks filling the water column, not callable to species. Image 1, the opening frame, is a requiem shark in blue water — and it is NOT a whitetip: broad dark caudal margin, no white dorsal tip, so gray reef or a dusky-phase relative; not callable. Beyond those: Moorish idols in Image 11, Image 16, and Image 21, a sergeant-major-type damsel and dark surgeonfish in Image 21, blue damsels, anthias, and small planktivorous damselfishes in Image 6, Image 13, and Image 28, and an anemone with a resident fish beside a big honeycomb-textured colony in Image 6.

And the dive goes under a ledge. Image 5, Image 10, and Image 18 are three consecutive macros of a spiny lobster — boldly black-and-white banded legs, striped carapace, long pale antennae — beneath an overhang crusted in lime-yellow and red encrusting sponge, over coarse rubble, with bright turquoise water visible past the lip. Not callable to species. At least one animal, with a second set of antennae and legs entering the top-left of Image 5. Conspicuous fauna and sessile invertebrates appear as sponges in 15 frames, sea cucumbers in 7 frames, giant clams in 2 frames, urchins in 2 frames.

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.

Other notes

Twenty-seven habitat frames from the 19 March 2015 dive (Anderson's field sheet: in the water at 10:25 for 54 minutes, to 22.3 m). There are no sampled-colony close-ups, so the account rests entirely on the habitat set. Image 15 shows the jack school with a diver in it, carrying a white rectangular open frame, apparently a photoquadrat, on the reef at lower left — which confirms these are survey-time observations. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'lagoonal bank reef', 'leeward'); the expedition's 933-site table classes the station as 'protected' (bank reef). 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 4 km east-southeast of South Brother, inside the Great Chagos Bank; no bottom or reef edge is visible, so the imagery cannot test the aspect.