Reproductive responses of purple sea urchin (Strongylocentrotus purpuratus) populations to environmental conditions across a coastal depth gradient

Ecological influences on reproduction in sea urchins, particularly photoperiod, have been studied extensively. Effects of proximate environmental factors (temperature, water motion, food) on reproduction of purple urchins, Strongylocentrotus purpuratus (Stimpson, 1857), were sampled monthly for 2 yrs for intertidal, and subtidal populations at the same latitude in the point Loma kelp forest, San diego, California, USA. gonad index (GI) was highest at 8 m, intermediate in the intertidal, and lowest at 18 m. GI increased across sites from spring through autumn and decreased sharply from late autumn through early winter. Stomach index (SI) peaked in summer and in autumn-winter. Increases in SI followed temporal peaks in drift plant abundance, but preceded increases in GI. Temperatures ranged from 11-22 °C, varying inversely with depth. Water movements were highest at deeper sites. Spawning coincided with high wave periods. Abundance of drift macroalgal food generally increased from deep to shallow sites, with rare peaks in abundance of intertidal drift. Algal food quantity appears to regulate reproductive output, not timing. C:N ratio of drift kelp was generally higher, and nitrogen correspondingly lower, at warmer temperatures or shallower depths; nitrogen was lower in drift than in live kelp. gonadal growth, development, and spawning occurred earlier at deeper subtidal sites with faster flows, lower temperatures, and higher food quality, despite low drift abundance. gonad production was highest intertidally despite high population density, prolonged high temperatures > 17 °C limiting gametogenesis, and only sporadic availability of drift. Naturally co-occurring abiotic and biotic factors combine to affect reproduction in S. purpuratus.