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

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Abstract
Lucinoma aequizonata is a large lucinid clam which lives in reducing mud around 500 m deep. Adults harbor intracellular chemoautotrophic sulfur-oxidizing 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 0.2ml of 4 mM serotonin solution in seawater into the posterior adductor muscle. The oocytes, 200 μm in diameter, are surrounded by a glycoprotein capsule which gives to the egg a total diameter of 500μm. The development which occurs at 10°C is slow. The first polar body is detected 2.5 h after contact between sperm and oocytes (To+2.5 h), and the first cleavage begins 10 h later (To+12.5 h). The following successive cleavages produce a nonciliated morula, then a ciliated gastrula which begins to rotate within the egg-capsule at To+4.5 days. At this stage, the first shell pellicle appears on the dorsal side of the embryo. At T0+8 days, the trochophore larvae develop discrete ciliary bands which constitute the prototroch. Typical straight-hinge veligers, D-shaped larvae, hatch from the egg-capsule 12 days after fertilization. The newly hatched larvae are 240 μm in length and 200μm in height, and the straight hinge 150μm long. To elucidate the symbiont transmission mode, two symbiont-specific primers were designed and used in amplifications by PCR. This primer set was unsuccessful in amplifying symbiont DNA targets from mature gonads, spawned oocytes, eggs, and veligers whereas successful amplifications were obtained from symbiont-containing gill tissues. These data rule out the vertical transmission mode and strongly suggest that the symbionts are environmentally transmitted to the new host generation in L. aequizonata as for all tropical lucinids examined to date.