Indonesia - Komodo NP: Hot Rocks (Sangeang)

Setting

Anderson's log describes a thermally active site at the base of the "active, albeit dormant" Sangeang volcano; the Smithsonian Global Volcanism Program records an eruptive phase of Sangeang Api from July 2017 to June 2020, which spans these dives. Visibility was 5-10 m on the first dive. Descended into black sand though the reef structure was nearby. Sand was warm on account of volcanic activity. Hot water seeping from cracks. Worse visibility (5 m) and stronger current than first dive. Very dark at 17 m. Pea green soup-colored water.

General benthic features

A much older coral assemblage than at “Lava Flow” in the Bandas. Soft corals, stingrays, strange thermoclines, orangutan crab, Seriatopora caliendrum very common at site, young coral community growing on lava, Pocillopora acuta, Euphyllia, Plerogyra both common (e.g., Image 1).

1

Mostly gorgonians, though some Pachyseris. Some large, fluorescent green, foliaceous sheeting corals at 26-27 m. At 25 m, some areas dominated by corallimorphs. Pocillopora acuta relatively common in areas. Diploastrea, small Galaxea with attack tentacles, weird colonial anemones growing on stalks. Soft corals dominated at 13-14 m. Huge brain corals and some large Diploastrea. Really beautiful soft corals at 5-10 m (e.g., Image 3).

3

More Porites at 5 m (e.g., Image 4).

4

Soft corals dominant in many parts of shallows (e.g., Image 5).

5

On the second dive: Seriatopora caliendrum, stinging hydroids, Linckia sea stars, Stylophora pistillata, small massive poritids, and relatively high average live coral cover at 15 m. Fungia. Pocillopora acuta. Plerogyra and Euphyllia both common. Some small Acropora colonies in the shallows. Pocillopora verrucosa and Stylophora pistillata were also recorded, the latter common in the area.

Coral cover

• Reference: Claude CV point count (this site's photos)
• Coral cover: 9.1% — a blind Claude computer-vision point count of 120 of Anderson's edited photographs (20 points per photo; water, fish, and obstructions excluded). Other computer-vision methods are listed in the method table. Anderson's log puts average live coral cover at about 5% at 28-30 m.

Stressors

Neither dive log mentions bleaching, but one plating colony photographed on the second dive is stark white across its whole surface, with its ridged texture intact and no algal film (e.g., Image 37).

37

On 20 January, several brown branching colonies carry sections of stark-white branches with corallites visible and few or no polyps, recent mortality, cause not stated (e.g., Image 51, Image 52).

5152

No crown-of-thorns starfish (COTS) or other corallivores were evident.

Fish and other fauna

A yellow sea cucumber that Anderson knew from Raja Ampat was very common in places. Pygmy seahorses, one of which Anderson spent a long time with on the second dive. Large Bohar snapper. Nipple sea anemones also very common. Cool macro life in muck off reef. High cover and diversity of anemones. Many fish but mean size is very small. Yellow-green crinoids everywhere. Some large snapper at 30 m. Strange colonial anemones living on stalks, common at this site but uncommon at others. Only snapper for large fish though did see stingrays (as at first visit to site).

Comparisons with other data

We found no other surveys at this site as of 2026. Within 5 km, four WCS Indonesia via MERMAID stations recorded station-year means of 19–26% hard coral cover in 2020; the nearest, WCS Indonesia via MERMAID at Bubble Reef (Deep), 1.25 km away, read 26% in 2020.

Dive record

Two dives on 20 January 2019: 08:07-09:08 (61 min), maximum depth 30.5 m, 27.4°C (dive-computer average), 30% oxygen; and, as 'Hot Rocks-round 2', 14:42-15:44 (62 min), maximum depth 29.6 m, 27.6°C (dive-computer average), 30% oxygen.