The rock scallop (also known as “donkey thorny oyster,” “spiny oyster,” and “thorny oyster”), Spondylus calcifer, is the largest member of any Panamic Province Spondylus and has played important economic, political, and cultural roles in coastal communities of the Eastern Tropical Pacific for thousands of years. Despite its importance, knowledge of its biology is scant. We assessed seasonal variations in shell and adductor muscle growth, longevity, reproductive age and period, and population sex ratios in the upper Gulf of California, Mexico, the northernmost area within the geographic distribution of this species. Information on shell growth and age was obtained via the use of stable oxygen isotope profiles of shell aragonite cross-referenced with mark recapture data. Spondylus calcifer forms white growth bands during winter and spring months. Shell growth accelerates during warmer months and diminishes during colder months. Likewise, the adductor muscle increases in size and weight during colder months, affecting fishermen's distribution of fishing effort. These seasonal variations in growth are likely a result of energetic shifts related to resource allocation pre and post reproduction, which takes place in July-August as water temperatures reach 28°C to 30°C. The species reaches sexual maturity between 2.5–4 y of age and can live to at least 12 y. Overall sex ratios consisted of 1:1 male:female, a population structure in accordance with previous reports for the southern Gulf of California. We discuss our findings in the context of management and conservation of the species.
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Vol. 27 • No. 4