Figure 5.

Proposed trade-off between penis form and flow environment. (a) In quiet water, the 25% longer penis of a protected-shore barnacle relative to its exposed-shore counterpart (figure 2a) may increase the reachable area (the area within which to find mates) by as much as 90% (compare the area encompassed by the dotted line with the shaded area). (b) In high flow, a small increase in the ability of the stouter exposed-shore penis form to resist bending due to drag would enable the penis of an exposed-shore barnacle to extend across more streamlines and yield an effective reachable area (shaded region) greater than that of a similar-sized quiet water barnacle with a penis more prone to downstream deflection due to drag (area bounded by the dotted line).