Embryonic development and endosymbiont transmission mode in the symbiotic clam Lucinoma aequizonata (Bivalvia : Lucinidae)

Lucinoma aequizonata is a large lucinid clam which lives in reducing mud ar ound 500 m deep. Adults harbor intracellular chemoautotrophic sulfur-oxidiz ing bacteria in specialized gill cells called bacteriocytes. The embryonic and early larval development of L. aequizonata is described by using light and scanning electron microscopy. Gametes were obtained by injection of 0.2 ml of 4 mM serotonin solution in seawater into the posterior adductor musc le. The oocytes, 200 mu m in diameter, are surrounded by a glycoprotein cap sule which gives to the egg a total diameter of 500 mu m. The development w hich occurs at 10 degrees C is slow. The first polar body is detected 2.5 h after contact between sperm and oocytes (T-0 + 2.5 h), and the first cleav age begins 10 h later (T-0 +12.5 h). The following successive cleavages pro duce a nonciliated morula, then a ciliated gastrula which begins to rotate within the egg-capsule at T-0 + 4.5 days. At this stage, the first shell pe llicle appears on the dorsal side of the embryo. At T-0 + 8 days, the troch ophore larvae develop discrete ciliary bands which constitute the prototroc h. Typical straight-hinge veligers, D-shaped larvae, hatch from the egg-cap sule 12 days after fertilization. The newly hatched larvae are 240 mu m in length and 200 mu m in height, and the straight hinge 150 mu m long. To elu cidate the symbiont transmission mode, two symbiont-specific primers were d esigned and used in amplifications by PCR. This primer set was unsuccessful in amplifying symbiont DNA targets from mature gonads, spawned oocytes, eg gs, and veligers whereas successful amplifications were obtained from symbi ont-containing gill tissues. These data rule out the vertical transmission mode and strongly suggest that the symbionts are environmentally transmitte d to the new host generation in L. aequizonata as for all tropical lucinids examined to date.