Dataset: Counts of Rhizopsammia corals from islands throughout the Galapagos Islands between January 2024 and August 2025

Dives were made from vessels Valeska, Costa and Danubio Azul, under Chief Scientist Robert Lamb (University of Florida). We searched for modern-day corals while diving using a mixed-gas, closed-circuit rebreather (Hollis Prism 2) from the shallows to the mesophotic zone (0 to 60 meters (m)) in July and August 2024. At 0, 10, 20, 30, 40, 50, and 60 m, we recorded rock wall communities along twenty 2 m horizontal transects at 12 sites. The aim was to capture two transects per depth using a GoPro 11 camera with a video light. Due to camera failure by one of the two diver-held cameras at Cuatro Hermanos and Cabo Douglas, only one transect per depth was available for analysis at these sites. Care was taken to include the transect tape in the video for scale and maintain a consistent camera-to-subject angle. Temperature was recorded at the same depths as the video transects using a Shearwater Peregrine 3 wrist dive computer and analyzed with linear regression.

The analytical procedure for the video transects consisted of segmenting each entire video into non-overlapping, continuous screen shot images of the rock walls. The areas of the screen shot images were calculated using Image J software, and the total transect area was calculated as the sum of the areas of all the images (which was always less than the originally intended 40 square meters due to limited availability of high-quality screenshots). Rhizopsammia wellingtoni were recognized as solitary, black polyps (corallites) with distinct white septa or as larger aggregations of multiple corallites, which are referred to as colonies. Colonies were counted as aggregations of 2 or more corallites, while solitary individuals were scored as a single corallite separated by a minimum of 5 corallite diameters from other corallites. All the counts were made by J. Witman for consistency.