Age and growth of Glycymeris longior (Sowerby, 1832) clam at the southern edge of its distribution (Argentine Sea)

Even though Glycymeris longior is a clam widely distributed in the SW Atlantic Ocean, little is known about its biology and life history. The present study assessed the periodicity of the internal growth increments of G. longior using thin shell sections. Each internal growth increment was composed of two alternating bands: a translucent band (light-coloured when viewed with transmitted light) and an opaque band (dark-coloured). Annual formation for each pair of bands was demonstrated. The formation of the annual growth increments was synchronous among individuals. Growth was determined from live clams collected at El Sótano, Argentine Sea (age range = 29 to 69 years). According to the growth model, G. longior grows fast during the first 5 years of life and then growth becomes slower in later years; individuals reached 50% and 90% of maximum size at 5 and 13 years of age, respectively. High variability was found in shell height for the first 10 years: differences up to 5–7 mm among individuals were registered for the first 2 years of age, and up to 11 mm between the ages of 3 and 9 years. The growth performance index phi-prime (φ′) and the index of growth performance (P) of G. longior were compared with those of other Glycymeris species. Our results indicate that G. longior is a slow-growing species with a long lifespan (maximum longevity = 69 years).