Viscoelasticity and slow locomotion allow Leptosynapta clarki, an infaunal, soft-bodied vermiform sea cucumber to burrow into stiff sediment

The burrowing sea cucumber, Leptosynapta clarki, has to face the challenge of burrowing into stiff, unyielding sediment with a soft body lacking tube feet. Observations in cryolite revealed that a combination of whole body peristalsis and tentacular crawling facilitated the elongation and burrowing of L. clarki. These sea cucumbers were able to extend their bodies nearly three times their minimum length as measured in a laboratory setting, using time-lapse video. Individual L. clarki whose tentacles were removed were only able to extend twice their minimum length and could not burrow while those with intact tentacles extended 3.4 times their minimum length. This indicates that the oral tentacles are required for extensive elongation and burrowing. Scanning electron microscopy and polarized light microscopy showed circumferential muscle fibers in the interambulacrum region, longitudinal fibers in the ambulacra, and calcareous ossicles within the epidermis. Tensile tests were performed parallel to the anterior-posterior axis on sections of intact, cylindrical body wall (obtained by removing the anterior and posterior ends from anesthetized animals and leaving the viscera in place) and on isolated longitudinal strips of body wall. The samples were pulled at extension rates of either 2mm/min, which was comparable to the extension rate achieved by L. clarki, or 50mm/min. Neither stiffness nor strength of cut strips of body wall was affected by the rate of extension. In contrast, samples of cylindrical body wall were 3 times stiffer, 4 times stronger, and 4 times tougher when pulled at the faster rate. It is possible that the viscoelastic elements in the body wall or in the viscera of L. clarki might have been disrupted when it was cut. Alternatively, different catch connective tissue states might be responsible for the responses of the tissues. Regardless, this result indicates that the whole L. clarki has viscoelastic properties, which, along with slow locomotion, enable the organism to extend to a remarkable degree with very little force