Dataset: Counts of individuals and cell division for 100 Plexaura homomalla embryos from fertilization to 24 hours after fertilization from adult corals collected in Round Bay, St. John, USVI in July of 2019

Methodology: 

Eleven female and four male branches of Plexaura homomalla were collected from adult colonies on an octocoral-dominated reef in Round Bay, St. John, USVI on July 14–15, 2019. All branches were about 15 cm long. Colonies were determined to be gravid in the field by cutting a 5-cm piece diagonally and looking for spermaries or eggs. Colonies were transported to the Virgin Islands Environmental Research Station and maintained in a sea table.

We monitored for signs of spawning for 5 hr starting at sunset starting one night after the full moon from July 17–23, 2019. Eggs and sperm were collected together with 60- mL syringes as they were released from the colonies and were stored in water from the tank for 30 min to ensure fertilization. We refer to the time at which gametes were collected as t = 0. On the second night of spawning, 100 eggs were placed in an isolated container and the number of cells was counted in triplicate by two different observers. Counts were taken every 0.5 hr from 2.5 to 7 hr after fertilization, hourly until 10 h after fertilization, and at 15, 18, and 24 hr after fertilization.

Instruments: 
dissecting microscope, hand lens