Evaluation of spawning induction techniques for white clam Amarilladesma mactroides (Reeve, 1854)
The yellow clam is a marine bivalve distributed on the Brazilian South Atlantic beaches whose population has been negatively impacted in the last decades by overfishing, events of massive mortality, climate change and anthropogenic activities. One solution for this problem is to control the production of juvenile and adult clams in laboratory. Therefore, there is a need to establish a protocol for the maintenance and the gonadal maturation of the yellow clam in captivity. Thus, this dissertation evaluated non-invasive techniques of spawning induction of the yellow clam. Specimens with a minimum length of 50mm were collected on the beaches of Rio Grande and São José do Norte, southern Brazil, and after were selected and measured (length, height and weight). A series of spawning induction experiments, with or without broodstock conditioning, were carried out. In the experiments with conditioning (experiments I, II and III), the clams were submitted to spawning induction through temperature manipulation with an exposure time of 60 to 120 minutes. In the experiments without conditioning (IV, V and VI), the clams were kept in the laboratory for 45 min and subsequently exposed to different treatments (temperature manipulation and/or addition of sperm extract, increased salinity, alkaline pH, addition of peroxide hydrogen) for periods of 60 to 210 minutes. In the case of a spawning event occurring, the number of oocytes, zygotes and D-larvae were estimated. The gonad stage and oocyte size were evaluated through histology. The data was tested for the premises necessary to use analysis of variance (ANOVA), and when these were not met the premises, data transformation was performed. Analyses were performed at the level of significance of 0.05. There were no spawning events in the experiments I, II and III. The histological analyses indicated that the clams in these experiments were not mature, but the diameter of the oocytes in experiment II increased significantly after conditioning. In the experiments without conditioning significant differences in the dimensions of the oocytes were also detected. The clams in experiment IV were more mature than those in experiments V and VI. On the other hand, there were spawning events in experiments IV, V and VI and there were differences in the size of the oocytes, indicating that the clams in experiment IV were more mature than those in experiment V and VI. However, only in experiments IV and V the presence of D larvae was observed. Broodstock survival rates in experiments I, II, and III were greater than 70%, and in experiments IV, V and VI the rates were greater than 80%, with the exception of experiment VI, in which the treatments involved hydrogen peroxide. These results show that there was an increase in the oocyte size of the yellow clams that were raised in laboratory-conditions from experiment II, but the duration of the conditioning period was not enough for the full maturation of the gametes. In the experiments without conditioning, spawning may have occurred due to the presence of free mature oocytes and sperm in the lumen of the follicle. Further studies are necessary to assess the individual factors that promoted the spawning induction of the yellow clam, and the influence of photoperiod.