Fig. 5: Schematic figure illustrating the life cycle of Mauritanian lucinid bivalves associated with horizontally transmitted symbionts.

Egg masses are deposited by attaching them to seagrass blades while they are still connected to the mother’s mucus tube. The larvae inside the egg masses are aposymbiotic and undergo development via a trochophore (ref. [11]; ~8 days after egg mass release) and a veliger larval stage (ref. [11]; ~12 days after egg mass formation) before they hatch as mature pediveliger larvae. Gill development is initiated upon hatching and the sulfur-oxidizing endosymbionts colonize the settled juvenile. Environmental bacteria are shown in pink, blue, yellow, and brown. The specific sulfur-oxidizing symbiont is depicted in red. Broken lines indicate that we could not verify the presence of a membrane that separates the larvae inside the egg mass although such a barrier has been reported from other lucinid larvae [13]. Not shown to scale.