To observe the first growth ring on the shell's ventral margin and hinge plate dorsal margin of the ark shell from Sendai Bay, we cut a thin cross-section throughout the SH axis of the shell around 40 mm in SL.
In Sendai Bay, the peak of the ark shell spawning season is estimated to be in August, based on the seasonal change in gonad development (Fisheries Agency 1977).
infinity]]) and mean SLs at successive ages were largest for the ark shell population from Nanao Bay, followed by the population from Sendai Bay and then the cultured individuals from Mutsu Bay (Table 1, Fig.
Summer is the peak spawning season for the ark shell in Sendai Bay (Fisheries Agency 1977), and at this time the seawater temperature is at the warmest level of the year (Fig.
Thus, formation of the growth ring of the ark shell seems to be accompanied by the cessation of shell growth in summer.
The overlap of modes in the size distribution of the ark shell was slight for Nanao Bay during the 1980s (Machida 1988), which made the size distribution method successful.
This result illustrates how the shell sectioning and staining method for observing internal growth rings avoids underestimation of ark shell age in Sendai Bay, especially for specimens older than 10 y.
The developing stage of the subcrenated ark shell occurred from April to July (Fig.
We examined annual changes in CF index in the subcrenated ark shell (Fig.
Our histological observations revealed that gametogenesis of the subcrenated ark shell was characterized by a unimodal cycle in relation to water temperature, and that the spawning period of this species in Ariake Bay is from July to August.
In previous reports, the spawning period of the subcrenated ark shell has been determined to be June to October in Ariake Bay (Tanaka 1954) and from June to December in Kasaoka Bay in the Seto Inland Sea (Ting et al.
Our results revealed that the subcrenated ark shell accumulated glycogen mainly in the foot, which is therefore a suitable tissue for use as an indicator of nutritional composition.
In the subcrenated ark shell, the glycogen content remained high from winter to early spring.
Sexual maturation in the attached juvenile stage of the ursine ark shell, Scapharca ursus, in the innermost area of Ariake Sound, Japan.