The dynamics of burrowing inEnsis(Bivalvia)

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Abstract
The digging activity of Ensis arcuatus shows six stages, together termed the 'digging cycle', which are repeated cyclically and are similar to those of other burrowing bivalves. A digging cycle involves the integration of pedal protraction and retraction with the opening and closing of the valves, much of the musculature of the body playing a part in each cycle. Extension and probing of the foot involves only the intrinsic pedal musculature which generates low pressures (10 cm) in the pedal haemocoele. The hinged shell acts as the basis of a fluid/muscle system which allows the strength of adduction to be used in digging and consists of two separate fluid filled chambers, the haemocoele and the mantle cavity. Adduction of the valves generates high pressures (100 cm) in each equally and simultaneously. In the haemocoele this pressure gives rise to the characteristic bulbous form of the foot which ensures a secure anchorage so that at pedal retraction the shell is drawn down. In the mantle cavity the pressure produces powerful jets of water which assist movement of the shell by loosening the adjacent sand. Estimates of the muscle tensions indicate that the pressures recorded may be obtained at tensions of not more than 2 Kg/cm$^{2}$.

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