Material properties of Didemnum vexillum and prediction of tendril fragmentation

The colonial tunicate Didemnum vexillum has recently invaded the North American coast and has the potential to cause economic and ecological damage. One potential mechanism for adult D. vexillum colonies to disperse is fragmentation and subsequent reattachment to another substrate. To understand the life history and ecology of D. vexillum and obtain a first-order estimate of dispersal potential via fragmentation, (1) the basic material properties of D. vexillum sampled from two locations in southern New England were measured in two growth forms (i.e., encrusting and tendril forms); (2) summer calcium concentration was measured as a proxy for spicule densities; and (3) the environmental stress factor (ESF) of tendrils under various hydrodynamic circumstances (i.e., water velocities from 0.01 to 20 m s−1) was determined. Results show that D. vexillum colonies have high tensile strengths compared to other colonial tunicates. Colonies with higher calcium concentration tended to have greater elastic modulus (resistance to deformation). Calculations of ESF suggest that tidal currents are insufficient to cause fragmentation; however, wave-generated (in rare circumstances) and mechanical forces from vessels and trawl equipment may cause fragmentation.