Recruitment of encrusting benthic invertebrates in boundary‐layer flows: A deep‐water experiment on Cross Seamount

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Abstract
A 48‐d field experiment on recruitment of metazoan larvae and other foraminiferal propagules of deep‐water, encrusting invertebrates was conducted on the summit of Cross Seamount (central North Pacific, 410‐m depth). Experimental substrata were circular flat plates with differing thicknesses and surface characteristics, designed so that larval responses to the plate boundary‐layer flow could be evaluated relative to other factors, such as substratum composition and texture. Flume studies indicated that rectangular plates with thin edges (1.5 mm) develop a boundary layer that thickens gradually downstream from the upstream edge, while plates with thick edges (10.0 mm) develop a separation eddy extending 2–3 cm downstream from the leading edge. Dissolution patterns of alabaster disks deployed on Cross Seamount indicated that similar flow patterns developed over plates at the field site. Recruitment of organisms (mostly benthic foraminifers) onto the thin plates was significantly lower very near the edges than near the centers of the plates. Recruits on the thick plates were most abundant 2–3 cm in from the edges of the plates, where the attachment point of the separation eddy was expected to occur most frequently. These results suggest that larval settlement may be a function of very small‐scale variations in the boundary‐layer flow, reflecting, for example, larval supply to the plate, larval retention on the plate surface, and active larval responses to the flow regime over the plates. Several taxa recruited exclusively onto thick plates covered with finely powdered ferromanganese crust, however, suggesting active selection for substratum composition or texture.