BURROWING PERFORMANCE OF PACIFIC SAND CRABS (EMERITA ANALOGA) IN SUBSTRATA OF VARIOUS SIZES
Stacy Schkoda, Joseph Gamez, Austin Xu, Nathan Vega, Jordan Abney, William Hoese, Erin Misty Paig-Tran.
California State University, Fullerton, Fullerton, CA.
Pacific sand crabs (Emerita analoga) rely on burrowing to maintain position within the swash zone and to avoid exposure to predators, yet must remain near the surface to respire and filter-feed. We hypothesized burrowing efficiency is related to 1) body size to substrata ratio, and 2) penetrability of the substratum (a measure of how displaceable sand grains are). We collected sand from 6 tidal depths and measured grain size composition within each aliquot. We measured burrowing performance (the ability to burrow) and time to burrow (body lengths/second) in 6 substrata categories ranging from fine grain (0.05 – 0.3mm) to shell (> 4mm), in N = 15 small (8mm – 15mm) and N = 17 large (longer than 15mm) crabs using high-speed video (600 FPS). We recorded a minimum of three burrowing trials per size class for each substratum category, and each crab was used once (N = 32 separate burrowing trials). We used particle image velocimetry (PIV) to test whether crabs fluidized the substrata while burrowing; however, only large crabs were capable of this. Burrowing performance was significantly different between small and large crabs across all substrata (p < 0.001); time to burrow (bl/s) was different among substratum (p = 0.008). Small crabs’ burrowing performance peaked in fine sediment (0.3 – 0.7mm), while larger crabs peaked in mid-range size sediment (0.7 – 1mm). Substratum penetrability was significantly different among all substrata types (p < 0.001). Large crabs are true burrowing generalists, while small crabs are constrained to smaller grain sizes.