“Hooking” and “Sheeting:” strategies utilized by Haminoea vesicula to maintain stability on different substrates

Abstract

Many marine gastropods face the challenge of adhering to substrates under adverse flow conditions. Some species live on both solid and sedimentary substrates, although how these substrate generalists resist detachment from materials with fundamentally different mechanical properties is not well studied. We used field observations and flow tank experiments to compare the capabilities and tactics of Haminoea vesicula, an opisthobranch gastropod, when exposed to destabilizing flow on its native sand and eelgrass. Velocities of outgoing and incoming spring tides in the tidal creeks and over and within eelgrass beds where H. vesicula live at False Bay, San Juan Island, WA ranged from 1 to 30 cm/s. In a flow tank, snails’ resistance to current depended on substrate and orientation. On eelgrass, snails moving into flow maintained their position at velocities exceeding 40 cm/s, but were more vulnerable when facing away or sideways to flow (falling off at mean velocities of 25 and 32 cm/s respectively). In the latter orientation, snails would often rotate into flow by hooking onto the leading edge of eelgrass with the anterior edge of the cephalic shield. In this position, snails resisted flows exceeding 40 cm/s. Snails crawling on sediment and exposed to turbulent flow drew sediment-encrusted mucus sheets dorsally, completely ensheathing their bodies. “Sheeting” significantly increased snails’ effective weight and ability to resist flow. On sand, snails ensheathed in sediment were least stable in the sideways orientation compared to those facing into or away from flow (mean velocities of 13 vs. 20 and 23 cm/s). Experiments on sandpaper indicated that snails were destabilized on sediment due to failure of adhesion of the sedimentary layers beneath them rather than their attachment to the sand.