The area occupied by the serotonergic innervation field encircling gonad acini varied according to the gonadal stages (active phase, ripe phase, partially spawned phase, spent phase).
When the clams have released all their gametes, a significant decrease is noted between the ripe phase (stage 2 ) or the partially spawned phase (stage 3) and the spent phase (stage 3, P < 0.001, Fig.
As the spent phase was reached, almost all mature oocytes have been expelled, and gonadal tissues began to disrupt.
When surf clams have completed spawning (stage four, spent phase) and almost all mature gametes have been expelled, leaving nearly empty, shrunken, and lower-mass gonad, numerous serotonin-containing fibers encircle the acini, as also reported for the scallop Placopecten magellanicus of the same gonadal stage (Croll et al., 1995).
This may be partly due to the great variations in serotonin concentrations between individual specimens and the small sample size for the fourth phase (spent phase) of the gametogenic cycle.