Chagos Archipelago - Great Chagos Bank West (BITB34)

General benthic features

Acropora was the most common coral genus on this station's transects (26% of the benthos), followed by Favia (8.9%), Stylophora (5.8%), Porites (3.7%), Gardineroseris (3.0%), and Pavona (2.0%); bare substrate covered 6.8%, behind crustose coralline algae (13%), the largest single living non-coral group (LOF transects; all algae together 28%). Among the algal classes, crustose coralline algae led (13% of the benthos), followed by turf (8.6%) and sediment-laden turf (3.6%). Where the reef is not plated it is foliose, and that is the site's signature (e.g., Image 4, Image 7, Image 16).

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Two frames show two striking colonies — single large mounded stands built entirely of tightly whorled purple-brown foliose plates, thousands of them, spiraling out from the center like stacked crockery and terminating in a clean pale growing margin (e.g., Image 8, Image 21).

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Both are compatible with a foliose Echinopora or Montipora (the transect records both, at 1.6% and 1.9%). Between the plates and the whorls the reef is a dense mosaic: the two highest-cover wide frames at the site show pavement invisible beneath corymbose and digitate Acropora, mauve staghorn, Platygyra-type brains, Astreopora-type domes, and a huge pink-brown knobby-lobed Porites (e.g., Image 19, Image 24); another frame runs a whole foreground of blunt gold-and-mauve digitate colonies compatible with Stylophora, the site's third-ranked genus at 5.8% (e.g., Image 32).

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One frame is the massive-coral frame — lobed Porites fingers, faviid domes, and purple-crusted submassives shoulder to shoulder, a diver hanging in the blue above (e.g., Image 27).

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Both target pocilloporid groups were sampled here (see Other notes). The imagery is consistent: four frames are close-up flash macros composed on a single pocilloporid colony, which is how a sampled colony gets documented. One is a compact verrucose colony whose upper branches and tips are cream-white with polyps still visible, while its interior and a neighboring colony of the same form are brown — partial paling of the upper surfaces (e.g., Image 6); another is a pale pink-and-cream colony between a Platygyra-type brain and a cerioid Favites-type sheet, with calcified green algae around it; a third a cream-yellow colony beneath a honeycomb massive; and a fourth a larger tan colony with pale extended polyps, ringed by green ascidians (e.g., Image 13, Image 17, Image 28).

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All four are compatible with the compact damicornis/acuta habit. Nothing in the photo set reads as Seriatopora.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Live hard coral measured on the in situ benthic transects of the LOF BIOT-1 expedition (the mean of all of this station's transects), against total algal cover (all algal groups) of 28% of the benthos.
• Transect mean 62 ± 4.0% SE (n = 18 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 61% (Mayfield & Dempsey 2026, Oceans, supplemental 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

Most of the colonies in the frames look healthy, but one pocilloporid shows partial paling of its upper branches (e.g., Image 6); published bleaching is 5.5% (Mayfield & Dempsey 2026, suppl. S1), and Anderson's sampling sheet records one huge, sickly Seriatopora hystrix and one 5–10% pale Pocillopora verrucosa among the colonies sampled; there is no survey apparatus anywhere. Structural loss is old: what dead framework there is has already been resurfaced in pink and mauve CCA — the pavement in three frames is almost entirely CCA-plated old framework. No COTS, no disease lesions, and no predation scars anywhere in the photo set. The Living Oceans Foundation transects here recorded 0.2% cyanobacteria and no recently dead coral.

Fish and other fauna

The standout macrofauna here is rays. Six frames are given to spotted eagle rays. One frame is the best — three rays in silhouette against open blue water, wings spread, whip tails trailing — while five others follow a single animal (in two of them with a remora on its back) planing low over a white sand flat, two more on the horizon of one of them (e.g., Image 5, Image 9, Image 18, Image 20).

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Another holds three trevally in silhouette with two further ray wings behind. Beyond the rays: three trevally, flanks stippled electric blue, over the plate reef in one frame; large schools of humpback red snapper in two frames; a sergeant-major school in another; a dense silverside or juvenile-fusilier cloud pouring over the reef in one more; blue-green chromis, parrotfish, dark surgeonfish, and butterflyfish throughout (e.g., Image 22, Image 25, Image 30, Image 31).

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The large dark oval silhouettes on the sand horizon of three of the ray frames are not callable — humphead wrasse and a large parrotfish are both compatible (e.g., Image 18, Image 20). Conspicuous fauna and sessile invertebrates appear as sponges in 5 frames and giant clams in 1 frame.

Comparisons with other data

Thriving Oceans surveyed a nearby station (GCB South Brother Lagoon) in 2015, 2018, and 2021, 0.30 km from the pin; we could not confirm that it is the same reef. Cover values from that work are not among the survey records shown on this page. Within 5 km, two independent stations recorded station-year means of 21–41% hard coral cover between 2006 and 2015; the nearest of these, XL Catlin Seaview Survey at transect 36004, 2.96 km away, read 21% in 2015.

Other notes

Anderson's sampling sheet records six target colonies sampled at this leeward fringing reef (CHPd22 and CHPd24 genotyped by RFLP, CHPd23 and CHPd25 not determined): Pocillopora acuta by RFLP (field call P. damicornis) CHPd22 (21.4 m, small, healthy); Pocillopora damicornis (field call; RFLP not determined) CHPd23 (19.4 m, small, no bleaching recorded); Pocillopora damicornis (confirmed by RFLP) CHPd24 (15.1 m, small, healthy); Seriatopora hystrix ChSe14 (20.4 m, huge, sickly) and ChSe15 (19.9 m, healthy). CHPd25 is recorded with dark coloration and no polyps extended (collected by a teammate). It also records one other pocilloporid sampled for a collaborator, Pocillopora verrucosa ChJo97 (21.5 m, 5–10% pale). 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). Note that the transect scored Seriatopora at zero — the genus was collected and observed but never fell under a transect point, which is exactly the kind of presence a cover survey cannot record. No tag, label, or marker is visible on any of the four pocilloporid macros (e.g., Image 6, Image 13, Image 17, Image 28), so it cannot be said which, if any, is the sampled colony CHPd22 or CHPd24. No transect tape, quadrat, slate, or collecting gear appears in any of the frames reviewed, although three frames show a diver (one in the water column, two photographing the eagle ray with a compact camera), and four are close-up macros composed on a colony rather than on the reef. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'fringing reef', 'leeward'); the expedition's 933-site table classes the station as 'protected' (fringing reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin off the northeastern end of South Brother, facing northeast (about 45°): side-on for the northwest monsoon winds of the March survey and side-on for the southeast trades of April to November, so the imagery reads this station as side-on rather than leeward; the aspect shown is the sheet's label.