MECHANICS OF THE LIGAMENT IN THE BIVALVE SPISULA SOLIDISSIMA IN RELATION TO MODE OF LIFE

¹ Department of Zoology, University of Glasgow, Glasgow, W. 2, Scotland and Osborn Zoological Laboratory, Yale University, New Haven, Conn.

1. The mechanical characteristics of the ligament in Spisula solidissima have been investigated and, for a range of sizes, hysteresis curves prepared and opening moments calculated.

2. In this species, the opening moment of the ligament can be related satisfactorily to the volume of the clam, and a mean value per unit volume of 608.8 g. mm. per ml. was determined.

3. The mean angle of gape in life of the shell valves of S. solidissima is 8.6 degrees, and the mean maximum gape (under no load) is 22.1 degrees.

4. Compared with the structurally similar ligament of Mya arenaria, that of S. solidissima is about 3.5 times more powerful (in terms of opening moments), and also more "efficient" (as measured by the "damping" of its elasticity in hysteresis loops). Clearly the ligament in Spisula provides the most important antagonist to the contraction of the adductor muscles, while in Mya water pressures generated in the mantle cavity are important.

5. These mechanical differences reflect the modes of life of the two clams: Spisula is one of the so-called "surf-clams," actively maintaining itself just within an unstable substrate; Mya is a sedentary "deep-burrower," almost immobile in adult life.

6. A phyletic explanation is suggested for conditions in the deep-burrowing mactrid Lutraria lutraria, where the ligament is about 1.5 times more powerful than that of Mya, despite their similarities in external form and habit. Lutraria is probably derived from a mactrid resembling Spisula solidissima and having a powerful ligament.