Larval morphometrics of Strongylocentrotus purpuratus under ocean warming and food availability scenarios

Trends in survival, morphometric patterns, and time to metamorphic competency across temperature and food availability treatments were measured in S. purpuratus larvae.

Larvae were spawned from adult purple urchins collected in Santa Barbara, CA. Effects of temperature and food availability were tested using a 6 x 4 factorial experimental design with six temperature treatments (10, 12, 14, 16, 18, 20°C) and four concentrations of Rhodomonas sp. microalgae (500, 2500, 5000, 10,000 cells mL-1) for a total of 24 treatment combinations. Food treatments represented algal concentrations from starvation to ad libitum conditions. Temperature and food treatments were based on historical observations of temperature trends and chlorophyll abundance. 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 and 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.