Mya arenaria, Linnaeus, 1758

Results View in CoL

Spawning of Mya arenaria in the White Sea occurs in summer – early autumn with two peaks: in late June-early July and late August. The diameter of fertilized oocytes is 73 to 76 µm. Selection of the first polar body occurs within 10–15 min after fertilization; during 5– 10 min after that the second polar body stands out ( Fig. 1A View Fig ). Within 30 min after fertilization the first polar lobe begins to form, then the first cleavage furrow is forming ( Fig. 1B View Fig ). In 1 hour after fertilization, the formation of two blastomeres occurs ( Fig. 1C View Fig ). The second division takes place within 1.5 hours after fertilization, however the formation of the second polar lobe is not observed. After several successive divisions a blastula consists of 32 blastomeres ( Fig. 1D View Fig ). With further development the blastula forms cilia, and the larva shifts to active swimming.

Larva stays at the blastula stage for a relatively long time: from 4.5 to 15 hours after fertilization. Prototroch cilia are formed only at the 10th hour of development, while at the 12th hour apical tuft appears and photo- and the gravitational taxes begin to work. Caudal tuft is absent in this species ( Fig. 1E View Fig ).

Gastrulation of M. arenaria larvae starts in 15–16 hours after fertilization and continues for 1.5–2 hours. Formation of the shell gland begins a little earlier than invagination of archenteron. These processes lead to formation of conchostoma stage ( Fig. 1F View Fig ), which lasts for 10–12 hours.

Approximately a day after fertilization, the eversion of shell gland starts and larva moves to the trochophora stage. The shell begins to form on the dorsal side of the larva ( Fig. 1G View Fig ). At this stage of development the shell is a single twolobed structure. Archenteron is greatly lengthened and bent, velum and digestive system begin to form.

About 1.5 days after fertilization the process leading to the formation of early veliger begins. By this time larva had formed digestive system, however the rear gut has no opening and the larva does not feed. The size of larvae at the early veliger stage is 85–90 µm.

Two days after fertilization the formation of velum ends, larva has a D-shaped shell and an anterior adductor ( Fig. 2A View Fig ). The larva passes to the stage of veliger; its size reaches 110–120 µm. The larva actively moves due to beating of the velum cilia and is able to control its movement by the apical tuft. At this stage the digestive system is fully formed, larva begins to feed. Digestive system of M. arenaria is asymmetric. Digestive gland is shifted to the right, and the esophagus empties into the stomach on the left side. The digestive gland is rich of lipid inclusions and colored in orange or brown.

Veliger of M. arenaria has a well-developed muscular system consisting of muscles-retractors of velum, dorsal muscles, pre- and postanal muscles. Thus, larva can pull the velum quickly, and actively move its body inside the shell by pre- and postanal muscles.

On the 15th-17th day of development, when shell reaches a size of 220 µm, larvae begin to form foot, posterior adductor, rudiments of gills and statocyst. The larva passes to the stage of pediveliger ( Fig. 2B View Fig ). At the size of 230–240 µm larva has posterior adductor and foot. The gill filaments increase. The intestine becomes longer, but in general the digestive system remains almost unchanged. At the stage of pediveliger the byssal gland begins to function.

The larva of M. arenaria at the stage of straight hinge has a shell length 130–135 µm. The shell is rounded with a slightly pointed anterior end, umbos are not formed ( Fig. 3A View Fig ). Star and radial zones are distinct. The hinge edge is narrow, with small projection and a recess on each valve ( Fig. 4A View Fig ).

When the shell size is 170 µm, formation of umbos starts ( Fig. 3B View Fig ). Fully formed umbos were reported in larvae with a size of 200 µm. The proximal edge of the shell is elongated and pointed, the distal and ventral edges are roundish. Ridges and flanges are formed on a periphery of the hinge (one ridge and one flange on each valve) ( Fig. 4B View Fig ).

Veliger at the size 240 µm has the rounded shell with distinct umbos ( Fig. 3C View Fig ). The anterior shoulder is longer. The anterior edge of the shell is somewhat pointed. At the posterior part of provinculum the ligament begins to form ( Fig. 4C View Fig ). The hinge edge has the tooth-like projection and a recess on each valve.

At the stage of 270–280 µm larva has a fully formed larval shell with a slightly pointed anterior edge ( Fig. 3D View Fig ). The umbos are separated and fully formed, anterior shoulder is longer than posterior. Around the perimeter of the shell a mantle line is clearly visible. The ligament continues to evolve; the number of projections on the hinge edges does not change ( Fig. 4D View Fig ).

Before metamorphosis (shell size 300–310 µm), the shell acquires a more rounded form, the shoulders become almost equal ( Fig.3E View Fig ). Growing edge of prodissoconch bears clear concentric striations.Tooth-like projections of the hinge become more complex, but their number remain unchanged ( Fig. 4E View Fig ).