Asymmetry in Structural Defenses: Insights Into Selective Predation in the Wild

Abstract

Assessment of geographical patterns in fluctuating asymmetry (small, random differences between sides of bilateral characters) among populations shows promise as a tool to resolve the relative biomechanical importance of traits, in addition to being a possible indicator of habitat quality. We used 115 endemic freshwater populations of threespine stickleback (Gasterosteus aculeatus) from Haida Gwaii (Queen Charlotte Islands), British Columbia, Canada, to explore the degree of concordance between geographical variation of asymmetry in a predator defense structure (bony lateral plates) and geographical variation in several indirect measures of predation regime as well as several abiotic habitat variables. We found a geographical cline in the population frequency of lateral plate asymmetries, with reduced asymmetry in the southern clear-water regions of the archipelago characterized by long reaction distances and greater chance of capture by predators, and elevated asymmetry in the northern stained-water regions with poor visibility and low chances of capture. Lateral plate asymmetry was strongly correlated with expression of several defensive armor traits, including total plate numbers among populations, mean cross-sectional diameter of stickleback with the dorsal and pelvic spines erect, and mean degree of overlap between the plates and spine supports. There were no significant correlations between frequency of asymmetric fish and any of our abiotic habitat variables. Stickleback with structural plate asymmetries had fewer trout-induced scars than symmetric fish in the significant majority of populations, and there was a decrease in structural plate asymmetry with age in stained-water habitats, suggesting that trout predators may be selectively removing asymmetric fish in some lakes. This study provides evidence that geographical variation in developmental stability of threespine stickleback, as seen in the frequencies of asymmetry, reflects differences among populations in the importance of structural defenses to fitness rather than differences in habitat quality, and that asymmetry may be a target of selection by predators in wild populations.

Morphometric and meristic variation in Galician threespine stickleback populations, northwest Spain

“…There is in particular a great deal of variation within and among populations in the elements of the defensive complex, which includes the dorsal and pelvic spines, pelvic girdle, and associated lateral plates (reviewed in Wootton 1984, Bell & Foster 1994. Reimchen (1983) demonstrated that these characters form a functional unit that resists deflection of the spines by predatory vertebrates; asymmetry in these bones may be a target by predators in wild populations (Bergstrom & Reimchen 2003).…”

Selection on the morphology–physiology‐performance nexus: Lessons from freshwater stickleback morphs

“…These included standard length (SL) and the area covered by nonstructural plates (i.e., the lateral plates not buttressing the dorsal and pelvic spines). Note that only the first 20 nonstructural plates were measured because of difficulties with the identification of small caudal plates, typically associated with the keel region—for a detailed description of lateral plate identification and numbering, see Bergstrom and Reimchen (2003). …”

Geographical and Temporal FA Variation in Threespine Stickleback Populations from Galicia (NW Spain)

“…To assess the variation in FA, five meristic and one morphometric bilateral characters were selected. Most of these have been used previously in studies of FA in G. aculeatus (Moodie and Moodie, 1996;Hatfield, 1997;Mazzi and Bakker, 2001;Hermida et al, 2002;Bergstrom and Reimchen, 2003). The meristic traits included the number of lateral plates, lower gillrakers, pectoral fin rays, upper gillrakers and lateral forks on the ascending process of the pelvic plate.…”

“…high FA) correlates with low phenotypic quality and, therefore, is associated with reduced fitness (Møller, 1997;Perfectti and Camacho, 1999;Bergstrom and Reimchen, 2003). Inverse associations between asymmetry and fitness have been reported (Møller, 1997), owing to competitive inferiority of asymmetric phenotypes or elevated levels of parasitic infection (Bonn et al, 1996;Møller, 1996;Reimchen, 1997).…”

“…Spanish stickleback populations are limited to a few rivers, and most of these are restricted to the northwestern region of the Iberian Peninsula (Ferna´ndez et al, 2000;Doadrio, 2001). Morphological variation in this fish has been extensively studied (reviews in Wootton, 1984;Bell and Foster, 1994), including the study of FA among various morphological traits (Hatfield, 1997;Mazzi and Bakker, 2001;Bergstrom and Reimchen, 2003;Mazzi et al, 2004). Several studies have found asymmetries in the number of lateral plates in sticklebacks (Hagen, 1973;Hagen and Gilbertson, 1973;Moodie and Moodie, 1996), leading to various interpretations of their functional implications.…”