玄界灘の砂浜海岸におけるナミノコガイの成長と分布様式

抄録

Growth pattern of the wedge clam Latona cuneata (Linnaeus) (Donacidae, Veneroida, Bivalvia) was surveyed on a sandy beach in Mitoma, Fukuoka City, Japan by monthly sampling during 16 months. Juveniles recruited to the intertidal area from July to August with around 3.2 mm shell length (SL), which exhibited rapid growth till the next November except during winter (December–February) and summer (July–August). In the second winter, their mean SL was 20 mm. In March, clams with 18–27 mm SL were divided into three cohorts that had overwintered 2–4 times. Large clams with SL>28 mm that had overwintered five times were rarely found, suggesting that in L. cuneata mortality before the fifth winter is high. Clam distribution on the beach was surveyed by quadrat sampling along a 50-m line toward the seashore in May, July and November. L. cuneata changed its distribution pattern with topographic changes of the beach; however, all clams concentrated in the foreshore area (retention zone). Clams aggregated at a high density in May, but showed dispersed polymodal patterns with a low density in July and November. Two types of growth lines were categorized on L. cuneata shells, one deeply notched (Line A) and one with a hyaline margin (Line B). SL of three generations divided by the number of lines on L. cuneata collected in June roughly agreed with that of cohorts from the June population. SL at growth lines also agreed with cohort analysis results from December (with Line A) and August populations (with Line B). Thus, the growth lines were indicative of overwintering and reproduction in summer.

Contents of supplementary materials: Photographs of study animals and study site.