Forty Strongylocentrotus purpuratus were collected from kelp forests between 2.2 – 8.8 meters from Carpinteria Reef in the Santa Barbara Channel, California in June 2022. Before spawning, individuals were held at 13°C for 4 weeks at UC Davis Bodega Marine Laboratory and were fed 20 grams of Urchinomics Inc. kelp-based pelleted food every other day.

A homogeneous pool of fertilized embryos were assigned to temperature treatments. Spawning was induced by injecting 1–2 mL of 0.5 M KCl. Female urchins were placed upside down over 250 mL beakers filled with filtered to 0.1 µm seawater (FSW) at ambient temperature. Eggs were allowed to collect at the bottom of beakers. Sperm was collected dry from the aboral surface and placed on ice in 2.0 mL microcentrifuge tubes until use. Gamete quality was visually inspected with eggs assessed for shape, color, and uniform size and sperm for motility. Eggs from four females were pooled and rinsed before being fertilized by sperm from four males. Approximately equal proportions of gametes from each individual within each sex were represented. Successful fertilization was determined by 95% of eggs having a fertilization envelope with no polyspermy. Fertilized eggs were rinsed three times with FSW to remove excess sperm to prevent polyspermy. Approximately 2800 embryos (concentration of ~5.5 embryos mL^-1) were placed in each 500 mL glass bottles filled with FSW and placed on roller tables to suspend larvae (Karelitz et al. 2020).

Roller agitation tables were constructed to provide water movement for larval sea urchin cultures. Rollers on free moving axles kept larval culture bottles rotating to prevent larvae from settling to the bottom. Bottles were partially submerged in water baths heated or chilled using Heater/Chillers (TECO US Inc. TK-500) to maintain target temperatures. Embryos were initially stocked in bottles with ambient temperature (16°C) FSW to minimize temperature shock but immediately placed into water baths to allow embryos to acclimate to target temperatures. Each of the 24 temperature x food treatments had four replicate bottles for a total of 96 bottles. Larvae were fed daily starting at 4 days post-fertilization (when larvae were observed to be competent to feed) and water changes and feeding were conducted every 2 days.

Larval samples were collected 4, 9, and 15 days post-fertilization and fixed in 10% PBS buffered formalin for morphometric analyses. Samples were collected before feeding to minimize influence of algae on stomach measurements. Fixed larvae were then photographed using an Olympus BX41 compound microscope with a Nikon 3400 DSLR camera at 4X magnification and postoral arm length, body length, stomach length/height were measured in 20 randomly selected larvae from each sample using ImageJ, version 1.53t (Schneider et al. 2012). Metamorphic competency was determined when >50% of larvae exhibited tube feet and adhered to the bottle.