The dynamics of burrowing inEnsis(Bivalvia)

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 .

Localized fluidization burrowing mechanics ofEnsis directus

“…Trueman measured ~10N as the maximum pulling force that E. directus can exert to pull its valves into soil (Trueman, 1967). Using a blunt body with a size and shape similar to that of E. directus, pressed into the animal's habitat soil on a mud flat in Gloucester, MA, we measured that 10N of force should enable E. directus to submerge to approximately 1-2cm.…”

“…Numerous soft-bodied organisms that live in particulate substrates saturated with a pore fluid use a two-anchor system to burrow: one section of the animal expands to form an anchor while another section contracts and extends to progress forward in the burrow; once extension is exhausted, the roles of each section are reversed (Dorgan et al, 2005;Fager, 1964;Holland and Dean, 1977;Jung, 2010b;Shin et al, 2002;Stanley, 1969;Trueman, 1966a;Trueman, 1966b;Trueman, 1967;Trueman, 1975). In this paper, we show that the Atlantic razor clam (Ensis directus Conrad 1843), which burrows via the two-anchor method, uses motions of its valves to create a pocket of fluidized substrate around its body to reduce drag forces and burrowing energy expenditure.…”

“…Ensis directus burrows by using a series of valve and foot motions to draw itself into the substrate (Fig.1). We estimate an upper bound of the mechanical energy per depth for the animal to advance its valves downwards by adapting pedal strength, valve displacement, hinge stiffness and mantle cavity pressure measurements from Trueman (Trueman, 1967) and summing the expended mechanical energy and attained displacements for each burrowing motion: valve uplift (0.05J, 0.5cm), valve contraction (0.07J, 20deg) and valve penetration (0.20J, -2.0cm), for a total of 0.21Jcm…”

The diversity of hydrostatic skeletons

“…In addition to the examples outlined above, hydraulic mechanisms are also employed in the foot of burrowing bivalves (Trueman, 1967;Trueman, 1968a;Trueman, 1975;Trueman et al, 1966) and some snails (Trueman, 1968b), as a mechanism to extend the legs of spiders (Parry and Brown, 1959), in the ligula (copulatory organ) of some octopuses (Thompson and Voight, 2003), and in the burrowing of many worm-like animals that lack segmentation, such as the lugworm Arenicola (Trueman, 1966;Seymour, 1970) or in priapulid worms that use the proboscis for burrowing (Hammond, 1970).…”

The growth and dynamics of Ensis directus in the near-shore Dutch coastal zone of the North Sea

“…nor by other local bivalves. 62 E. directus can live in these dynamic sedimentary conditions because it can rapidly retract 63 itself deep in the sediment (Drew 1907;Trueman, 1967). Hence, accurate density estimates 64 are often hampered by sampling difficulties.…”