Chagos Archipelago - Great Chagos Bank West (BIEA18)

General benthic features

The shallowest frames (e.g., Image 1, Image 14, Image 39, Image 44) show a low-relief consolidated pavement with broad pale carbonate and sand between coral patches: two large brown meandroid brain colonies compatible with Platygyra sit on bare pavement in Image 1 and again in Image 14, a golden branching Acropora alongside, red encrusting sponge and pink crustose coralline algae in the cracks. Image 39 and Image 44 are the honest wide views of this ground — small tabular and digitate Acropora, olive massive Porites, knobby pocilloporid heads, and dark encrusting patches scattered across pavement and rubble, with a good deal of open substrate between them. Image 10, Image 20, Image 34, Image 37, and Image 45 are the highest-cover wide frames in the set: close-packed olive digitate and lobate Porites, pale cream corymbose and tabular Acropora, brown pocilloporid knobs, pale lilac-gray encrusting and foliose sheets compatible with Montipora, ridged laminar plates, and pink coralline crusts binding it all.

11439441020343745

Orange anthias hang over these patches in most of them. Image 7, Image 29, and Image 42 hold the same community with pink-lilac branching colonies and conspicuous bright-green algal turf between the corals.

72942

From Image 11 onward the camera works a steeply dropping flank — Image 9, Image 12, Image 13, Image 16, Image 24, Image 26, Image 27, and Image 31 show dark encrusting and foliose framework hung with yellow gorgonian sea fans, pale cream tabular plates, crinoids, and clouds of orange anthias and small planktivores. Image 11 records the base: a pale sand plain running up against a reef margin carpeted in dense bright-green algal turf, with whip-like gorgonians standing off it. Image 39 holds a second, more distant elongate silhouette that is not callable from the frame.

11912131624262731

The benthos around the lobster has high cover: a large lavender-gray ridged plate directly overhead, brown digitate Porites clusters, brown meandroid colonies, bright green calcified alga, red and orange encrusting sponges, and a brown massive Porites (e.g., Image 43) carrying patches of partial mortality, mostly old and crusted. Image 33 records a single free-living mushroom coral (Fungia-type) on rubble beside a dark red-maroon encrusting mound.

4333

Porites was the most common coral genus on this station's transects (5.9% of the benthos), followed by Acropora (3.3%), Pocillopora (2.3%), Montipora (2.1%), Platygyra (1.5%), and Favites (1.4%); bare substrate covered 7.2%, behind other invertebrates (21%), the largest single living non-coral group (LOF transects; all algae together 51%). Among the algal classes, sediment-laden turf led (20% of the benthos), followed by erect algae (11%) and crustose coralline algae (9.4%).

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 51% of the benthos.
• The transects were run in three depth strata (D2 19%, D3 23%, and D4 20% coral).
• Transect mean 21 ± 2.2% SE (n = 20 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 21% (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.
• These products have different sampling footprints and denominators and should not be averaged; the transect value is the measurement.

Stressors

Pale and white-looking tissue appears in several frames, and the 30-point annotation of the analyzed subset scored a handful of bleached points — too few, and too dependent on where a single point falls, to carry any site-level meaning. At these ranges and under this lighting the photographs cannot reliably separate bleaching from recent mortality or from a naturally pale morph, so bleaching is reported here qualitatively only. Anderson's published per-site % bleached for BIEA18 is 12%; it comes from CoralNet analysis of the expedition's survey images (Mayfield & Dempsey 2026, suppl. S1), not from a field estimate. The clearest condition record in the frames is the partial mortality on the brown massive Porites at Image 43, mostly old and crusted. Image 1 shows about five small white star-shaped spots on one of the brain colonies, possibly feeding scars; nothing in the frame shows what made them, and the cause is unclear. Anderson's sampling sheet records five Pocillopora colonies sampled here at 11.2–25.2 m (ChJo51–ChJo55; species by field call): ChJo53 as 5% bleached, with its bleached part sampled, and the other four as healthy. 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 0.1% recently dead coral.

Fish and other fauna

Large mixed schools of dark surgeonfish and unicornfish (white-margined tails and visible keels) work the slope in Image 2, Image 3, Image 4, Image 5, Image 11, Image 12, Image 15, Image 16, Image 19, Image 28, Image 29, Image 31, Image 36, and Image 42; orange anthias carpet the wall in Image 2, Image 9, Image 24, Image 26, Image 27, Image 28, and Image 47; Image 2, Image 15, Image 24, and Image 28 are near-solid clouds of small planktivores; fusiliers cross the blue in Image 8, Image 10, and Image 13.

234515192836478

Bluefin trevally (unmistakable in Image 8 and Image 40) patrol the blue in fours and fives, with further jacks in Image 13 and Image 34; and there is an adult emperor angelfish in Image 22, a yellow-tailed sweetlips in Image 36, Moorish idol, butterflyfishes, a large hogfish-type wrasse, cleaner wrasse, damselfishes, and sergeant majors.

4022

Image 47 carries a pale, blunt-snouted shark, most consistent with a whitetip reef shark; the frame is backlit and at range.

Two invertebrate finds are worth pulling out. Image 23 is a tight aggregation of five or more long-spined urchins on open pavement, wedged in beside a knobby pocilloporid and a red encrusting sponge — given how much dead consolidated framework this site carries, a bioerosion signal worth recording.

23

And Image 17, Image 18, Image 21, Image 25, Image 35, Image 41, and Image 43 are seven frames of one painted spiny lobster sheltering under a ledge — ornate black-and-white marbled carapace, banded walking legs, long pale antennae. Image 21 is a frame-filling head study.

171821253541

Comparisons with other data

Apart from the Living Oceans Foundation survey shown here, we found no other surveys at this site as of 2026. If you would like to share yours, please email us.

Other notes

Forty-three habitat frames exist for BIEA18; there are no sampled-colony frames, so the account is entirely a habitat account. Roughly 30 frames carry real benthic information and about 11 (e.g., Image 2, Image 4, Image 5, Image 12, Image 15, Image 19, Image 28, Image 31, Image 36, and in part Image 8, Image 40) are water column, backlit silhouette, or open-blue fish frames with little or no substrate. Seven frames are a single pseudo-replicated lobster sequence over one ledge and describe that ledge, not the site. No transect line, quadrat, or scale object appears anywhere in the set.

Image 46 is a methodology frame: a gloved hand holding a stainless surgical rongeur at the substrate, beside two or three smooth pink globular masses that the annotator scored as sponge, with a green encrusting colony to the left and a pale meandroid brain above.

46