Argopecten purpuratus (Clark, 1965)

Serb, Jeanne M., Alejandrino, Alvin, Otárola-Castillo, Erik & Adams, Dean C., 2011, Morphological convergence of shell shape in distantly related scallop species (Mollusca: Pectinidae), Zoological Journal of the Linnean Society 163 (2), pp. 571-584 : 577-578

Argopecten purpuratus 0.0001 0.0001 0.0001 0 0.092344 0.078985 0.082532 found is and species species, between shape Argopecten irradians 0.0001 0.0001 0 0.040458 0.066356 0.058188 0.060339 between shell in Amusium’

‘ balloti 0.0683 NS 0 0.097673 0.120306 0.065465 0.067704 0.047089 difference). significant

differences of shape NS (, not pairwise Amusium pleuronectes 0 0.025271 0.101694 0.126472 0.071253 0.073565 0.0546 amount the diagonal of as the assessment purpuratus sentis septemradiatus treated is above found in are. bold Statistical pleuronectes balloti ’ irradians behringiana . Table 2 Amusium ‘ Amusium Argopecten Argopecten Chlamys Caribachlamys Pseudamussium distance Euclidean are) permutations values Significant

When morphological disparity was examined, we found that the long-distance swimming species ( Am. pleuronectes and ‘Am.’ balloti ) exhibited considerably less within-species morphological variation as compared to the remaining species ( Table 3). Further, statistical comparisons of disparity revealed that in all cases, these differences were statistically significant ( Table 3). These analyses revealed that the degree of morphological variation in the two longdistance swimming species was significantly less than what was observed in the species from other behavioural groups. Amongst the remaining five species, P. septemradiatus had the smallest within-species disparity ( Table 3).

Visualizing shell shape variation using principal component analysis reflected the statistical findings above. The first principal component axis (PC1) described 57% of the total variation, and described a pattern of nearly continual variation from free-living species to byssal attaching species to long-distance swimmers ( Fig. 3 ). The shell shape of P. septemradiatus occupies the morphospace between byssal attaching and long-distance swimming species ( Fig. 3 ). The behavioural groups could be distinguished on the basis of shell shape along PC1; however, there was also significant separation between species within behavioural groups when viewed along both PC1 and PC2. In particular, both free-living species formed distinct clouds in morphospace, as did both byssal attaching species ( Fig. 3 ). Importantly, the one notable exception to this pattern was the longdistance swimmers Am. pleuronectes and ‘Am.’ balloti , which overlapped considerably in morphospace ( Fig. 3 ). This finding was consistent with the statistical analyses described above ( Table 3). In addition to the separation between species, differences in disparity were also evident amongst species. In particular, both Am. pleuronectes and ‘Am.’ balloti displayed considerably less variability as compared to the remaining species.

Morphologically, the continuum of shell shape variation represented an anatomical gradient from smooth-shelled species to species exhibiting distinct ribs/ ridges on their shells. This pattern is evident when comparing 3D surface scans of representative individuals along PC1 ( Fig. 3 ). In addition to alteration in shell sculpturing, there was a clear change in auricle size, shape, and symmetry. Smooth-shelled species exhibited a relative reduction in auricle size as compared to other behavioural groups and these auricles were relatively more symmetric and created a flared edge to the hinge line. On the other extreme of shell shape, byssal-attaching species exhibited a relative elongation of their anterior auricle, resulting in a straight, asymmetric hinge along the anteroposterior axis. Interestingly, these anatomical Differences in disparity measures between Am. pleuronectes and ‘Am.’ balloti versus all non-long-distance swimmers are displayed, along with their probabilities (based on a permutation procedure using 9999 iterations: see Material and methods).

Significant values based on sequential Bonferroni adjustment are shown in bold.

changes correspond closely to functional differences of the shell. Mechanistically, the longer anterior auricle adds support to the byssal attachment site and prevents overturning of the animal ( Stanley, 1970) ( Fig. 3A ). When compared to byssal attaching ( Ch. behringiana , Ca. sentis ) or free-living species ( Ar. irradians , Ar. purpuratus ), the long-distance swimming species ( Am. pleuronectes , ‘ Am.’ balloti ) exhibit a unique shape with larger umbonal angles, which increases the shell area perpendicular to the direction of movement (i.e. ‘aspect ratio’). This circular shape of the shell disk results in an increased lift/drag ratio when compared to byssally attached forms ( Stanley, 1970; Gould, 1971).

Figures

Figure 3. Principal component plot of shell shape variation for specimens used in this study. The first three principal component (PC) axes explain 75.8% of the total variation in shell shape (PC1 = 57.3%; PC2 = 11.6%; PC3 = 6.9%). (A) Shape variation along PC1 and PC2 with photographs of representative individuals for each species. Scale bars = 1 cm. (B) Shape variation along PC1 and PC3. Species are designated as: Amusium pleuronectes (blue squares), ‘Amusium’ balloti (blue circles), Argopecten irradians (red squares), Argopecten purpuratus (red circles), Chlamys behringiana (green triangles), Caribachlamys sentis (green diamonds), Pseudamussium septemradiatus (white inverted triangles).