Chagos Archipelago - Peros Banhos (BIPB73)

Setting

Quality is good — mostly well-exposed wide reef views in clear water at shallow to mid fore-reef depths, with a handful of shallow frames where the surface is visible (e.g., Image 41) and one deeper blue-water frame (e.g., Image 21).

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General benthic features

Against that, algal cover is a modest 23% and non-coral invertebrates hold 15% of the benthos — the largest non-coral, non-algal component of the substrate here, visible in the photographs as sponges, a resident anemone with its fish, and one possible giant clam (e.g., Image 20).

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Bare substrate is 13%, turf 11%, crustose coralline algae 4.8%, macroalgae 3.1%, erect algae 2.0%, and cyanobacteria 0.1%.

Seventeen scleractinian genera were recorded and the assemblage is carried by two of them: Acropora at 21% of the benthos (42% of live coral) and Porites at 16% (33%) — together about three-quarters of all coral on the reef. Montipora is a distant third at 2.8%, then Pocillopora 2.5%, Astreopora 1.7%, Pavona 0.9%, Echinopora and Psammocora 0.8% each, Ctenella 0.7% (the Chagos-endemic monotypic genus, not to be confused with Cynarina), Leptastrea 0.5%, Fungia and Stylophora 0.4% each, Goniastrea and Favia 0.3% each, and Favites, Lobophyllia, and Montastrea 0.1% each.

The depth structure is stated in full here, because the site-level averages describe no single stratum. Cover was taken on 11 transects in three depth strata, D2–D4. D2, the shallowest stratum, carries by far the highest cover of the three at 68% coral against only 23% algae, and it is essentially a two-genus reef: Acropora 35% (51% of the coral present) and Porites 18% (27%), with Pocillopora, Astreopora, Montipora, and Ctenella filling out the rest at 2.0–4.0% each (Ctenella reaching 2.0% of the D2 benthos, 2.9% of its coral). D3 drops sharply to 37% coral, the site's low point, and the genus dominance inverts — Porites leads at 16% (44% of the coral) over Acropora at 12% (33%) — with Montipora, Astreopora, Pocillopora, and Psammocora each in the low single digits. D4 partially recovers to 45% coral and Acropora regains the lead at 16% (36%), but the composition is not simply D2 repeated: Porites holds 13% (28%), while Echinopora and Montipora each reach 4.5% (10% of coral apiece) and Fungia appears at 2.0% — none of which feature meaningfully in the shallower strata. The site mean of 50% coral sits between D4 (45%) and D2 (68%) and matches no stratum, and the 21%/16% Acropora/Porites split is the average of a shallow reef that is essentially two genera and a deeper reef where a third and fourth genus (Echinopora, Fungia) become relevant. This depth-stratified breakdown is the most detailed account on this page and is the transect's own account of the reef.

The set opens on the site's signature structure. Image 1 and Image 46 show a large tiered, wedding-cake colony of stacked laminar whorls, mustard to olive-yellow, standing clear of white sand with encrusting and nodular growth banking its base and maroon coralline crusts in its shaded interstices, orange anthias and small planktivores streaming over it — a laminar plating growth form compatible with Echinopora or Montipora, though the frames alone cannot settle the genus.

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Both genera are on the transect list, Echinopora at 0.8% site-wide and 4.5% in D4.

Away from that structure the dominant impression is a dense, close-packed coral garden on pink and mauve coralline pavement. Corymbose, digitate, and bottlebrush Acropora in chartreuse, cream, gold, and tan fill the frames in Image 9, Image 10, Image 13, Image 19, Image 29, and Image 49; thin tabular acroporids from dinner-plate to meter-class, stacked in overlapping tiers, run through (e.g., Image 15, Image 16, Image 23, Image 27, Image 28, Image 43); lumpy and hemispherical massive Porites range from small heads to a boulder-sized dome filling most of Image 18; mauve and pink-brown pocilloporid clusters appear in Image 9, Image 13, Image 31, and Image 47; a scatter of meandroid brain colonies sits at bottom right of Image 5; and one mottled, mantle-like object set into the pavement in Image 20 may be a giant clam.

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Dense high-cover sections combining tabular and corymbose Acropora with large massive and faviid domes over pink CCA-encrusted framework dominate (e.g., Image 9, Image 13, Image 14, Image 20, Image 24, Image 31) — the photographic face of the mixed Acropora–Porites-plus-others assemblage the transects describe.

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Image 28 is a textbook fore-reef terracing structure: a large table coral shading a massive dome beneath it. Free-living and encrusting forms — small fungiids, foliose plates compatible with Montipora or Pavona — recur through (e.g., Image 17, Image 49).

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Sand and rubble are present but subordinate: white sand channels and patches floor (e.g., Image 1, Image 17, Image 24, Image 25, Image 28), and several frames catch the coral-garden-to-sand interface directly.

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Relief is moderate and built from coalesced bommies and pavement steps rather than from spur-and-groove. Image 32 is the useful counterpoint — a broad flat expanse of hard, pink coralline-crusted pavement carrying only scattered small coral recruits and one pale soft-coral colony — the photographic counterpart of the site's 13% bare substrate, and evidence that open low-relief surfaces occur between the coral thickets.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 50%, 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. Surveyed 18 April 2015 at Peros Banhos during the Khaled bin Sultan Living Oceans Foundation Global Reef Expedition (5.26732°S, 71.88862°E; a leeward fringing reef on the atoll's north rim, dived to 23.8 m; station CTD-cast midpoint 30.2°C, salinity 34.2). 6.4% of coral was bleached (CoralNet analysis of informal survey images; Mayfield & Dempsey 2026, Oceans).
• Transect mean 50 ± 5.0% SE (n = 11 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 50% (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.
• The photographs described here are not scored. The published bleached fraction, 6.4%, comes from the CoralNet analysis of the expedition's survey images (Mayfield & Dempsey 2026), and the image-based cover estimates are in the method table.
• Transect detail: 50% (formal transect surveys) · % bleached: 6.4% (CoralNet analysis of informal survey images).

Stressors

Bone-white and stark cream colonies are clearly present in this set and are recorded as seen. Image 5 holds a bright white patch bridging two otherwise pigmented plates on a massive colony. Image 2 shows a bone-white branching colony standing among fully pigmented neighbors in an otherwise dense garden. Image 37 carries a stark white wedge cutting across an otherwise olive-yellow massive coral. Image 43 holds white and cream tables and digitate clusters. Image 48 shows one large table coral that is almost entirely stark white with only a narrow band of pigmented brown tissue at one edge — an unambiguous, tissue-intact colony-scale event, not algae-covered mortality. Image 20 holds a group of unambiguously bone-white branching colonies against pigmented massives.

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Image 4 is the clearest single image of the phenomenon: one large table whose golden-brown pigmented margin grades inward to a bone-white center — partial bleaching within a single colony.

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Additional pale-cream tables in Image 8, Image 21, and Image 27 may be bleached or may simply be lightly pigmented tabular acroporids; that distinction cannot be settled photographically.

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No crown-of-thorns, no disease lesions, and no corallivore predation scars were observed in any of the 45 frames. Typical recent thermal stress: mean annual maximum of 4.4°C-weeks (Degree Heating Weeks, 2021-2025, NOAA CRW; Chagos virtual station). Note: this bleaching figure derives from automated image annotation (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). Anderson's sampling sheet logs scattered bleached colonies from mid-March, more through late April, and bleaching was most widespread in early May. Anderson's sampling sheet records five pocilloporid colonies sampled here for a collaborator (species by field call): Pocillopora sp. ChJo184 (18.5 m, large, healthy), ChJo187 (15.0 m, large, healthy, fairly pale), and ChJo188 (10.0 m, >90% bleached); Pocillopora verrucosa ChJo185 (20.5 m, 80% diseased or bleached; possible disease) and ChJo186 (14.5 m, large, healthy). 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). The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and 0.1% recently dead coral.

Fish and other fauna

Reef fauna is well represented and is the visible face of that 15% invertebrate cover. A resident sea anemone with yellow-tipped tentacles hosts both anemonefish and a crowd of three-spot dascyllus or domino damsels in Image 25 and Image 26.

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A honeycomb-patterned grouper is photographed twice at rest under a ledge, in Image 7 and Image 34 — two close-ups of the same animal, its flank a fine pale-outlined hexagonal reticulum, beneath an overhang crusted with red sponge and pale ascidians.

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A second and quite different fish, a heavy dark marbled grouper, hangs over sand in Image 12.

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Image 22 is a close frame of a large pink gastropod shell with a broadly flaring, algae-crusted lip, consistent with a conch.

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Powder-blue surgeonfish work the shallow garden in Image 2, Image 27, and Image 31; chevron and other butterflyfishes appear in Image 14; bannerfish in Image 18 and Image 21; angelfish in Image 18 and Image 31; a dark spotted snapper- or sweetlips-type fish in Image 30; yellow-and-blue fusilier-type schooling fish under the overhang in Image 17; clouds of anthias and chromis over the tiered colony in Image 46; and schooling fusiliers and damselfishes recur throughout.

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Comparisons with other data

Spalding et al. (2025) surveyed a nearby station (West Ile Bois Mangue), 0.44 km from the pin; we could not confirm that it is the same reef. Within 5 km, five Spalding et al. (2025) stations list undated values of 10–80% hard coral cover (values above 70% are best patches, not site means); the nearest, Spalding et al. (2025) at Peros Banhos - West Ile Bois Mangue, lagoon (N, 2-6 m), 0.44 km away, read 30%.

Other notes

Forty-five habitat frames from the 18 April dive; no sampled-colony images exist for this site, so the whole account is habitat. No quadrat, transect line, or scale reference appears in any frame, so no colony dimension or areal figure can be read from them. Aspect: the LOF habitat map has a cloud gap at the pin (deep ocean water 74 m north); the atoll rim here faces north, leeward of the southeast trades, which agrees with Anderson's field sheet (LOF fellowship 2016 workbook: 'fringing reef', 'leeward'); the expedition's 933-site table classes the station as 'exposed' (fringing reef).