Two experiments were conducted. Experiment 1 was a preliminary test and consisted of 5 cups (5 different treatments, 1 cup per treatment, and no replicates). These had 0, 50, 100, 200, and 300 mg of CCA powder, respectively. Larvae in these cups were counted nondestructively after 24 hours and again after 48 hours. Experiment 2 consisted of 24 cups with 3 replicates each of 8 CCA powder amounts: 0, 25, 50, 75, 100, 150, 200, and 300 mg. The larvae were counted after 24 hours.

Plates of calcite (approximately 1-3 mm thick) covered in CCA were collected from the MOC aquarium tanks’ walls. These plates had full pink-colored CCA surface area coverage (taxonomy was not identified). Calcite plates were crushed into a paste with a mortar and pestle and allowed to dry in the shaded, open-air area until ca. 30 g of powder formed. Polypropylene specimen cups (140 ml) were used to hold 20 larvae each, and 2-inch diameter three-pointed ceramic stilts (Clay King DP2-Roselli stilts; SC, USA) without CCA were used to provide a settlement surface (Lillis et al., 2018; Aoki et al., 2024). The stilts placed in each cup were preconditioned in the aquarium tanks for over one month, but no natural CCA growth was observed on them during this period.

Each experimental “cup” container was filled with approximately 120 ml of FSW and the experimentally designated amount of powder. Cups were gently shaken to mix the powder, though the powder never fully went into solution and eventually precipitated out. Pocillopora damicornis larvae (n = 20 per cup) and a stilt were added to each cup. Larvae were one to two days old when the experiments started. These larvae were randomly distributed between sample cups and treatments. The containers were placed in custom racks gently floating on the surface, allowing them to maintain a constant position within the flow-through tank maintained at local seawater temperature.

All cups were scored after 24 hours by counting the number of larvae swimming or settled at each location: the lid, stilt, cup, or loose CCA chunk. Each cup was swirled gently to detach any sticky but not fully settled larvae from any surfaces. Then the cup’s lid was removed and visually examined under a UV light with a dissecting microscope to identify and count auto fluorescing larvae (NIGHTSEA; Hatfield, PA). Metamorphosed polyps were identified by having flattened and developed septa, which divided the polyp radially about the mouth.