Temporal variation in life-history traits of the clam Tivela mactroides (Bivalvia: Veneridae): Density-dependent processes in sandy beaches

Temporal variation in the structure and dynamics of a population of Tivela mactroides was examined over two periods (2003–2004 and 2007–2008) in the southern part of Caraguatatuba Bay, southeastern Brazil. During the first period from January 2003 to October 2004, sampling was conducted monthly. Sampling in the second period was performed in the summer (January and February 2007 and 2008) and winter (July and August 2007 and 2008). The von Bertalanffy growth function was applied to estimate growth parameters for both periods from length-frequency distributions. Production was determined using the mass-specific growth rate method. Results indicated that the mean abundance (±SE) of T. mactroides varied sharply between the two periods, with an increase of almost 150 times from 2003 to 2004 (8.67·102 ± 0.83·102 ind m−1) to 2007–2008 (1.25·105 ± 3.33·104 ind m−1). The higher abundance in the second period was related to successful recruitment events. While the mean biomass and the production were higher in the second (5.43 ± 0.87 kg AFDM m−1 and 7.89 kg AFDM m−1 yr−1) than in the first period (0.18 ± 0.02 kg AFDM m−1 and 0.18 kg AFDM m−1 yr−1), lower values of shell length, curvature parameter, asymptotic length of the VBGF, and the growth index phi-prime in 2007–2008 (17.57 ± 1.35 mm; K = 0.40 yr−1; L∞ = 38.60 mm, ϕ′ = 2.78) than in 2003–2004 (26.21 ± 1.21 mm; K = 1.00 yr−1; L∞ = 40.75 mm, ϕ′ = 3.22) were related to a strongly density-dependent growth process in the second period. The oscillation in growth observed in the second, but not in the first period also indicates a process of density-dependent growth. These sharp temporal variations in population parameters of T. mactroides suggest the occurrence of density-dependent processes, and reinforce the importance of these processes in structuring sandy-beach populations.