Vertical Changes in Density, Size Structure and Shell Shape of the Bivalve Lasaea Undulata Within Intertidal Mussel Beds

Abstract

Vertical changes in density, size structure and shell shape of Lasaea undulata (Bivalvia: Galeommatoidea) inhabiting intertidal mussel beds were studied at Wakayama Prefecture, Japan. At the study site, where two mytilids Septifer virgatus and Hormomya mutabilis formed vertically contiguous mussel beds, Lasaea density decreased downshore within the S. virgatus bed of the upper and mid zones. Few individuals were found within the H. mutabilis bed of the lower zone. Multiple linear regression analysis revealed that shore height was the only factor which showed a significant, positive correlation with density. Size structure differed between the upper and lower parts of the S. virgatus bed, suggesting that reproductive periods or growth rates of L. undulata differed with shore height. Prodissoconch length and shell shape were also different among shore heights. Number, size and shell shape of juveniles brooded within parents indicated that this species releases its young as non-planktotrophic, crawl-away juveniles. There was no significant difference in density of recruits between artificial mussel clumps of S. virgatus and H. mutabilis within enclosure cages, and no specific association was detected between L. undulata and either of the two mussel species.