Fig 1.
Crab and wave ecotype L. saxatilis showing measurements of the shell.
A) Typical “crab” ecotype with large size, thick shell, wide aperture, and an elongated spire, sampled at left (boulder) end of the transect shown in Fig 2. B) Typical “wave” ecotype with small size, thin shell, wide aperture and compressed spire, sampled ~10 m before the right (cliff) end of the transect in Fig 2). C) Shell measurements (aperture view) used in this study: Outer aperture area (OA, colored orange), inner aperture area (IA, colored yellow), Length 1 (L1), Width 1 (W1), Length 2 (L2), Width 2 (W2). D) Projection of the shell area (SA, colored gray) from the spire view.
Fig 2.
Resistance to water flow along the wave exposure transect.
Schematic view showing the southern bay of the island of Ramsö (N 58°49'27", E 11°03'46"). The sampling position of each snail is represented by a circle filled with the colour indicating the snail's maximum resistance to water flow in the experimental pipe. Each colour is bounded by two free stream water velocities, except dark blue that shows maximum resistance < 0.75 m.s-1, and red that shows minimum resistance > 2.69 m.s-1. The sampling positions of the snails that did not attach to the flume pipe are marked by empty circles. Relative density of Fucus seaweeds is illustrated by green circles, wider circles representing larger amounts of Fucus (on a relative scale from 0 to 1) in an area of 4 m radius around the point. The Bézier path used for assigning a position to each snail in the clinal fit is represented by a dashed white path. A black arrow marks the transition from boulder to cliff habitat, a green arrow marks disappearance of Fucus sp.
Fig 3.
Variation in morphology along the transect.
Shell shapes 1 and 2 (S1 and S2, as shown in Fig 1) estimate shell globosity and lateral compression, respectively, and foot area (FA), outer and inner-aperture areas (OA, IA, see Fig 1) were scaled on the shell area (SA) to represent relative areas (RFA, ROA and RIA, respectively). Lines are the best fitting sigmoid functions describing the clines in the traits, with y values on the left of the graph. S1 filled purple squares, S2 empty brown circles, RFA filled red circles, ROA empty green squares, RIA empty black diamond. The best fitting cline for the squared flow resistance (SFR) is plotted as a bold black curve, with y values on the right of the graph. A black arrow marks the position of the transition from boulder to cliff habitat and a green arrow indicates the disappearance of Fucus sp.
Table 1.
Correlations between residuals of morphological traits and residuals of squared maximum flow speed resistance.
Fig 4.
Boxplots of the phenotypes residuals vs the squared flow resistance residuals.
Residuals of the three phenotypes with significant association with the snails' flow speed resistance are plotted against the residuals of squared maximum free stream velocity resisted. Boxes encompass 50% of the data points for each flow speed category. The median and mean values for each category are represented respectively as a thick horizontal bar and a filled diamond. The whiskers extend to the most extreme data point within 1.5 times the length of the box away from the box. Black regression lines illustrate the positive relationship between the trait residuals and flow resistance residuals. Small amounts of random variation were added in the x dimension to facilitate the visualization of all data points.
Fig 5.
Resistance to water speed in snails reared in a common garden.
Number of snails that resisted various maximum water speeds (free stream velocity). Note that the x-axis is not a linear scale. A) Ten months old adults born and raised in the laboratory from parents sampled at each end of a crab-wave transect (island of Ramsökalv). B) Two week old juveniles born and raised in the laboratory from parents sampled at each end of a crab-wave transect (island of Saltö).