Is the burrowing performance of a sandy beach surfing gastropod limiting for its macroscale distribution?

Abstract

The size-specific burrowing capacity of Olivella semistriata, an extremely abundant surfing gastropod of exposed sandy beaches in the tropical East Pacific, was investigated in terms of sediment grain size. For all investigated sand classes, there was a significant increase in burial time with size. Burrowing was fastest in sand with grain size between 150 and 355 μm, as well as in native sediment (median grain size 209 μm) and in field conditions (median grain size 223 μm). Values of the burrowing rate index (BRI) were found to be between 3 and 7, rating the burrowing capacity of O. semistriata to be fast to very fast. Data from previous qualitative and quantitative sampling campaigns were used to identify the macroscale (i.e., between beaches) distribution of O. semistriata in terms of sediment grain size and swash conditions. Swash period was shorter than burial time, excluding this as a factor limiting the distribution. Swash standstill time (the time between uprush and backwash), however, was just long enough on the beaches, where O. semistriata was present to allow for securing firm anchorage. On reflective beaches, the swash standstill time is as short as 1 or 2 s, denying O. semistriata the time to burry itself before being swept away by the backwash. As such, swash standstill time is advocated as a valuable part of the swash exclusion hypothesis. A survey of the available literature on the burrowing of surfers shows that mole crabs are by far the fastest burrowers and the only surfers that burrow sufficiently fast to withstand the extremely short swash standstill time on reflective beaches. Burrowing ability of surfing gastropods is found in the same range as surfing bivalves, both being insufficient to cope with coarse sediment on steep beaches. Finally, we suggest that neither burial time nor BRI, yet rather the minimal burial time—the time needed to anchor securely in a certain sand at a given swash velocity—should be used to judge the limitations of burial in terms of sediment and swash conditions.