Inducing the spawning of bivalves in aquaculture is a crucial step for seed production. Several methods, including thermal, chemical, physical and/or hormonal stimulation, are used depending on the species and the conditions. Because it is low-cost and easy to apply, thermal manipulation is the most widely used method in hatchery settings to induce spawning in bivalves. Thermal manipulation mimics natural season changes that trigger spawning in the wild. Under laboratory conditions, it involves exposing bivalves to a sudden increase or fluctuation in water temperature. Based on previous results, here we evaluated the use of temperature manipulation in conjunction with the addition of male gonad extract as a non-invasive technique to induce the spawning of the yellow clam, A. mactroides. In addition, we assessed the feasibility of using freshly collected wild clams versus wild individuals that were conditioned in the laboratory for 30 and 45 days.
Adult clams (length ≥ 50 mm) were collected in Cassino beach, Rio Grande, Brazil, on October 2, 2023. In the laboratory, clams were kept buried in experimental units (EU; Ø = 15 cm x 10 cm of sand over a 200 µm mesh) placed in a tank with 360 L of filtered (1 µm) and dechlorinated seawater. Mean (± SD) temperature was 18.1 °C (± 1.3). A microalgal concentrate containing 70% Isochrysis galbana and 30% Chaetoceros muelleri (MixFresh, AlgaSul, Rio Grande, Brazil) was supplied daily to maintain a concentration of 14 x 104 cells.mL-1. One day after the clams were collected in the wild (Day 1) and at the end of the conditioning periods (Days 30 and 45), spawning induction was attempted. Thirty-six EUs were used: four replicates for each time point (Days 1, 30 and 45) and three shock temperatures (from 18.1 °C to 23.0 °C, 26.0 °C and 29.0 °C).
Spawnings were observed at all temperatures and conditioning days. Two-way ANOVA indicated no significant effect of the interaction between temperature and conditioning days (P = 0.690). However, both shock temperature and conditioning period had significant effects on the number of eggs released (P = 0.001 and P = 0.044, respectively). The number of eggs was significantly higher in the 26 °C treatments, both on Day 1 and Day 45, which did not differ from each other.
Using freshly collected wild clams and wild individuals conditioned in the laboratory for 45 days and exposing them to a temperature shock of 18.1°C to 26.0°C, spawns ranging from 6,200 x 103 to 8,600 x 103 eggs were obtained. Although this was not the objective of the study, larvae were observed in all combinations of temperature and conditioning period, with the highest number (2,300 x 103) estimated for the treatments at 26°C. These results contribute to determining the temperature range that, combined with the sperm solution, stimulates spawning in A. mactroides, minimizing the need to sacrifice broodstock to obtain larvae and optimizing hatchery protocols for this important bivalve species.