We collected adult sand dollars (D. excentricus) from Semiahmoo Bay, WA, in July 2017 and maintained them in 14°C continuous flowing seawater at the Shannon Point Marine Center. We induced twelve individuals to spawn by injecting 1-mL of 0.5-M KCl into the coelom following methods outlined by Strathmann (1987).  We then collected and mixed concentrated gametes of four males and four females for fertilization. We added five drops of sperm to 500-mL of filtered seawater and 5-mL of eggs. We placed the fertilized eggs in 12°C incubator and bubbled them with ambient pCO2 condition for 12-hrs before dividing the embryos into pCO2 treatment conditions before gastrulation.

We reared D. excentricus larvae (2 individuals mL-1) at 12°C in eight 3-L jars that were individually bubbled with CO2 to achieve four replicates of ambient (400ppm) and acidic (1500ppm) pCO2 conditions. Daily we gave each larval jar a water change from pre-equilibrated ambient and acidic water and fed the larvae D. tertiolecta (6,000 cells ml-1).

We measured morphological characteristics of larvae from each rearing jar when larvae were in the 4- and 6-arm stages following diagrams in Chan et al. (2011) and Reitzel et al. (2004). Larvae were relaxed in 7% magnesium chloride, fixed in 3% formalin, imaged using a stereomicroscope equipped with a camera (Leica MC170 HD and Leica Application Suite, Leica, Wetzlar, Germany), and measured with ImageJ software. 4-arm larvae were fixed on 7/24/17 and 6-arm larvae were fixed on 7/28/18. The following morphological characteristics were measured: length of postoral arms (PO), anterolateral arms (AL), length of body midline, body width, and longitudinal length of stomach.

This dataset includes unprocessed data.