Burrowing by small polychaetes – mechanics, behavior, and muscle structure ofCapitellasp.

Abstract

Worms of different sizes extend burrows through muddy sediments by fracture, applying dorso-ventral forces that are amplified at the crack tip. Smaller worms displace sediments less than larger worms and therefore are limited in how much force they can apply to burrow walls. We hypothesized that small worms would exhibit a transition in burrowing mechanics, specifically a lower limit in body size for the ability to burrow by fracture, corresponding with an ontogenetic transition in muscle morphology. Kinematics of burrowing in a mud analog, external morphology and muscle arrangement were examined in juveniles and adults of the small polychaete Capitella sp. We found that it moves by peristalsis, and no obvious differences were observed among worms of different sizes; even very small juveniles were able to burrow through a clear mud analog by fracture. Interestingly, we found that in addition to longitudinal and circular muscles needed for peristaltic movements, left-and right-handed helical muscles wrap around the thorax of worms of all sizes. We suggest that in small worms helical muscles may function to supplement forces generated by longitudinal muscles and to maintain hydrostatic pressure, enabling higher forces to be exerted on the crack wall. Further research is needed, however, to determine whether surficial sediments inhabited by small worms fail by fracture or plastically deform under forces of the magnitudes applied by Capitella sp.

First evidence for temporary and permanent adhesive systems in the stalked barnacle cyprid, Octolasmis angulata

“…The cyprids (n = 6) were relaxed in 8% magnesium chloride515253 for 1 h, washed in 0.45 μm filtered sea water and then fixed with 2.5% glutaraldehyde (sea water base) for 1 h. The cyprids were dehydrated through an ascending series of ethanol, critical point dried, mounted on aluminium stubs and sputter-coated with gold. The coated cyprids were viewed using a Hitachi SU8000 field-emission scanning electron microscope operating from 5 to 15 kV.…”

“…One, three and five-day-old cyprids (each samples, n = 4) were relaxed with 8% magnesium chloride515253 for 1 h and washed thoroughly with 0.45 μm filtered 33 ppt sea water. The cyprids were then fixed with modified Karnovsky (2% paraformaldehyde and 2.5% double-distilled glutaraldehyde buffered with 0.1 M sodium cacodylate containing 3% sucrose, pH 7.4) overnight at 4 °C.…”

Strategies of burrowing in soft muddy sediments by diverse polychaetes

“…Interestingly, in both Scalibregma inflatum and Capitella sp., worms evert their pharynges while burrowing but do not extend them anteriorly beyond the head (cf. Grill & Dorgan ), suggesting that in these species pharynx eversion does not drive crack growth as observed in Alitta virens (Dorgan et al. ).…”

“…Applying enough force to burrow walls to extend a burrow by fracture is more difficult for smaller worms (Che & Dorgan ), but even very small worms (e.g., Capitella sp.) can extend burrows by fracture (Grill & Dorgan ). Our data suggest that although small worms are capable of burrowing by fracture, the range of sediment responses may include plastic deformation.…”

Tunnel-tube and Fourier methods for measuring three-dimensional medium reaction force in burrowing animals

“…Trueman (1967, 1968) used an impedance pneumograph to measure pressure changes in wet sand caused by the burrowing patterns of freely moving aquatic invertebrates. More recently, photoelastic stress techniques (Full et al, 1995) have been used to study the biomechanics of burrowing behaviors in marine invertebrates (Che and Dorgan, 2010;Dorgan, 2015;Grill and Dorgan, 2015;Murphy and Dorgan, 2011). O' Reilly and colleagues (1997) studied the burrowing locomotion of caecilians using implanted air-filled catheters to measure pleuroperitoneal pressure together with fore-aft force measured by a vertically oriented force platform.…”