We conducted an experiment to characterize the PLD of massive Porites larvae in April 2023. Adult massive Porites corals were collected from six sites in Palau immediately prior to their spawning season. 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.

Larvae from two bulk crosses were pooled for this experiment. Each cross included sperm from 3 males and eggs from one female (same males, different female in each cross). 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. Three-dpf larvae (n=20 individuals three days post fertilization) were added to clear plastic culture dishes (~200 mL) with ambient seawater (ASW) at three different filtrations: unfiltered, 5 μm filtered, or 0.5 μm filtered (n=10 per treatment). No additional settlement cues were provided.Culture dishes were maintained in flow-through ASW tanks at PICRC, and ~80% water changes were conducted every 2 days.

Temperature and light experienced by larvae during the experiment were recorded using Hobo pendant loggers in an identical culture dish in the same seawater tank.

Immediately prior to water changes, all larvae and settlers in each culture dish were counted. Settlement was defined as a larva attaching itself to the bottom of the dish, remaining attached after mechanical disturbance (i.e., gently shaking the dish), and visually beginning to undergo metamorphosis.