Dataset: Results of an experiment on post-settlement survival for Porites corals in Palau

Adult massive Porites corals were collected from six sites in Palau immediately prior to their spawning season in April 2023 . Corals were either whole colonies or fragments of large colonies removed with a hammer and chisel (15 – 30 cm diameter). Collected corals were held in individual plastic containers (4.2 L; 17 x 13 x 19 cm) in ambient unfiltered flow-through seawater tanks at the Palau International Coral Reef Center (PICRC). At sunset, water levels in flow-through tanks were lowered to ~15 cm so that each colony was isolated in its individual container, maintaining gametes separate until fertilization. Sperm was collected directly from male colonies using a large plastic pipette during release, and eggs were collected by pipette or by scooping from the surface with a clean petri dish. One hour after fertilization began, embryos underwent a series of dilutions with 5 µm filtered seawater (FSW) and gently split into multiple containers filled with 5 µm FSW to dilute any remaining sperm and prevent polyspermy. We pooled gametes from 4 males and one female. Three days post fertilization, larvae were settled on glass microscope slides with crushed crustose coralline algae used as a settlement cue (CCA, multiple types combined). There were 22.8 ± 1.4 individuals per slide (mean ± SE). Slides were randomly distributed among two temperature treatments (30° C, 33° C) and two light levels (ambient, high) in a 2-way crossed design. Each treatment consisted of one water tank (50 x 37 x 30 cm) (n=18 slides per treatment) with constantly-circulating FSW (5 μm). Partial water changes (~30%) were conducted every two days. Water temperature was controlled using aquarium heaters, and light levels were held constant using 165W LED aquarium lights (ARKNOAH) suspended over each tank (12:12 light:dark cycle). Glass slides with attached settlers were placed in plastic trays in a horizontal orientation with the most densely-populated side of the slide facing up. Slides were elevated off the bottom of the tank ~10 cm by a PVC rack. Temperature and light level in each treatment were logged using Hobo pendants (Onset, Wareham, USA). Immediately after settlement and every two days throughout the experiment, slides were photographed individually using a Nikon D850 camera with a macro lens. Living settlers were counted in each image to track settler survival over time.

In order to determine carryover effects of delayed settlement for massive Porites, a subset of larvae from the post-settlement mortality experiment was held in FSW (0.5 μm) for an additional 8 days (i.e., until 11 dpf) before being presented with a substrate to settle on or any settlement cues. Partial water changes (~50%) were conducted every two days to prevent formation of a biofilm, which can induce settlement. These delayed-settlement larvae were allowed to settle on glass slides as described above, resulting in 70.3 ± 13.8 settlers per slide. Slides were distributed between 30° C and 33° C temperature treatments, both with ambient light, such that an approximately equal number of settlers were in each treatment (n=5 slides per treatment). Images were recorded as described above. Images of settlers on each of 5 randomly-selected slides from the control treatment (30° C, ambient light) for both prompt (4 dpf) and delayed-settlement individuals (12 dpf) were used to quantify size at settlement. Selected individuals were settled apart from any other individual, so that the size was not influenced by crowding. The slide width was used a size reference, and settler diameter was measured using the straight-line tool in ImageJ.