Hydrodynamic role of substrate attachment in shaping the suspension-feeding current created by the marine gastropod Crepidula fornicata

As in lamellibranch bivalves, individuals of the common Atlantic slippersnail Crepidula fornicata beat cilia on their gill filaments to produce a suspension-feeding current. Having only one shell and no siphons with which to direct water flow, however, individuals of C. fornicata must adhere to a solid substrate to facilitate normal feeding. Thus, what hydrodynamic role does substrate attachment play in producing, regulating, and directing the suspension-feeding current for this species? Here, a combined particle image velocimetry and computational fluid dynamics study was conducted to address this question. Three findings were obtained: (1) Juveniles of C. fornicata (shell length 6.0-10.6 mm) whose foot was attached to a solid surface generated a strong, fan-like exhalant current and an almost equally strong, convergent inhalant current, both being spatially well extended; (2) juveniles of C. fornicata that were prevented from adhering to any surface also generated a strong, fan-like exhalant current but a much weaker and spatially limited inhalant current; and (3) whether or not they were attached to a solid surface, juveniles of C. fornicata had almost the same performance or system characteristics of the ciliary water pump, including the relationship between flow pressure rise Delta p across the ciliary zone and volume flow rate Q, pump resistance Delta p/Q, and pressure coefficient for laminar flow C-p,C-l. These results indicate that the primary hydrodynamic effect of substrate attachment in C. fornicata is to form a complete inhalant chamber with a narrowed opening, such that negative flow pressure develops in the inhalant chamber, and a strong, convergent, spatially well-extended inhalant current is generated to effectively bring in food particles from farther distances and to reduce refiltration of the outflowing water. Finally, ecological trade-offs are discussed regarding the two distinct shell configuration strategies: (1) that of the single-shelled C. fornicata, with only a naturally formed exhalant chamber and opening but not a morphologically defined inhalant chamber and opening, and (2) that of two-shelled bivalves, with naturally formed exhalant and inhalant chambers.