Can spatial sorting associated with spawning migration explain evolution of body size and vertebral number in Anguilla eels?

Spatial sorting is a process that can contribute to microevolutionary change by assembling phenotypes through space, owing to nonrandom dispersal. Here we first build upon and develop the “neutral” version of the spatial sorting hypothesis by arguing that in systems that are not characterized by repeated range expansions, the evolutionary effects of variation in dispersal capacity and assortative mating might not be independent of but interact with natural selection. In addition to generating assortative mating, variation in dispersal capacity together with spatial and temporal variation in quality of spawning area is likely to influence both reproductive success and survival of spawning migrating individuals, and this will contribute to the evolution of dispersal‐enhancing traits. Next, we use a comparative approach to examine whether differences in spawning migration distance among 18 species of freshwater Anguilla eels have evolved in tandem with two dispersal‐favoring traits. In our analyses, we use information on spawning migration distance, body length, and vertebral number that was obtained from the literature, and a published whole mitochondrial DNA‐based phylogeny. Results from comparative analysis of independent contrasts showed that macroevolutionary shifts in body length throughout the phylogeny have been associated with concomitant shifts in spawning migration. Shifts in migration distance were not associated with shifts in number of vertebrae. These findings are consistent with the hypothesis that spatial sorting has contributed to the evolution of more elongated bodies in species with longer spawning migration distances, or resulted in evolution of longer migration distances in species with larger body size. This novel demonstration is important in that it expands the list of ecological settings and hierarchical levels of biological organization for which the spatial sorting hypothesis seems to have predictive power.     

Exploring Jordan's rule in Pacific three‐spined stickleback Gasterosteus aculeatus

Coastal marine Gasterosteus aculeatus were captured from seven locations along the Pacific coast of North America, ranging across 21·8° latitude to test Jordan's rule, i.e. that vertebral number should increase with increasing latitude for related populations of fish. Vertebral number significantly increased with increasing latitude for both total and caudal vertebral number. Increasing length with latitude (sensu Bergmann's rule) was also supported, but the predictions for Jordan's rule held when controlling for standard length. Pleomerism was weakly evidenced. Gasterosteus aculeatus exhibited sexual dimorphism for Jordan's rule, with both sexes having more vertebrae at higher latitudes, but only males showing a positive association between latitude and the ratio of caudal to abdominal vertebrae. The number of dorsal‐ and anal‐fin rays and basals increased with increasing latitude, while pectoral‐fin ray number decreased. This study reinforces the association between phenotypic variation and environmental variation in marine populations of G. aculeatus.     

Variation in Vertebral Number in Galaxiid Fishes,How Fishes Swim and a Possible Reason for Pleomerism

The number of vertebrae in fishes is widely variable, with this variation having connections to phyletic position, geography, various environmental factors such as temperature and salinity, biome occupied and life history pattern. Variation is sometimes a response to environment, sometimes explicitly adaptive and probably often both. Swimming mode is likely to be influenced by body flexibility, which in turn is influenced by vertebral counts. Since vertebral number is fixed early in ontogeny, there is a predictive element in the choice of vertebral number during development that affects later adaptiveness. Pleomerism, the relationship between vertebral number and body size across the diversity of fishes, may be driven by the square/cube relationships between length, cross-sectional area and volume. Pleomerism in diadromous galaxiid fishes probably reflects adaptive advantages achieved during marine juvenile life and in non-diadromous species may reflect the size at which mid-water, shoaling juveniles become to benthic, cryptic, within-substrate behaviours.     

Dissociation of somatic growth from segmentation drives gigantism in snakes

Body size is significantly correlated with number of vertebrae (pleomerism) in multiple vertebrate lineages, indicating that change in number of body segments produced during somitogenesis is an important factor in evolutionary change in body size, but the role of segmentation in the evolution of extreme sizes, including gigantism, has not been examined. We explored the relationship between body size and vertebral count in basal snakes that exhibit gigantism. Boids, pythonids and the typhlopid genera, Typhlops and Rhinotyphlops, possess a positive relationship between body size and vertebral count, confirming the importance of pleomerism; however, giant taxa possessed fewer than expected vertebrae, indicating that a separate process underlies the evolution of gigantism in snakes. The lack of correlation between body size and vertebral number in giant taxa demonstrates dissociation of segment production in early development from somatic growth during maturation, indicating that gigantism is achieved by modifying development at a different stage from that normally selected for changes in body size.     

Evolutionary history of elongation and maximum body length in moray eels (Anguilliformes: Muraenidae)

Body shape and size are important axes of organismal diversification. The elongate body form has evolved repeatedly in disparate vertebrate clades, and is associated with a variety of maximum body lengths. We used a time‐calibrated phylogeny for 40 species of moray eels to analyse the evolution of elongation and the morphological mechanisms underlying variation in body shape and maximum body length. We find that body elongation in morays evolves independently of elongation of the vertebral column. In contrast, maximum body length evolves by a different mechanism: through region‐specific increases in vertebral number, elongation of individual vertebral centra, and postembryonic somatic growth. We reconstruct an ancestral moray eel and provide evidence for accelerated morphological evolution in three highly elongate species that are associated with a burrowing lifestyle. We compare these patterns with those described for other vertebrates, and show that body shape and body length may evolve independently of each other and (in the case of shape) of the vertebral column. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 861–875.     

Jordan's and other ecogeographical rules, and the vertebral number in fishes

Aim  To explore variation in the number of vertebrae in fishes in the context of Jordan's rule and other ecogeographical rules.
Location  Global.
Methods  The study is based on literature review.
Results  The number of vertebrae varies very widely across the diversity of fishes. Jordan's rule states that vertebral number increases with latitude, and this is widely attributed to ambient temperatures during ontogeny of individual fishes. However, the number of vertebrae may depend on both the ontogenetic environment and inheritance. Diverse other aspects of fish development and ecology are suggested as influencing vertebral number, including fish size, phyletic position, body shape and swimming mode.
Main conclusions  The number of different factors that influence the number of vertebrae in fishes makes for highly complex patterns of variation, and means that unravelling causes is difficult. The question needs to be addressed at the population/species/species group scale; moreover, the lack of discrimination between environmental and inherited causes of variation adds to the complexity.     

Vertebral number in adders, Vipera berus: direct and indirect effects on growth

Given the importance of body size, and thereby growth rate, for many reproductive parameters in snakes, morphological traits conferring an advantage in terms of growth may be important targets of selection. Studies have demonstrated effects of vertebral number of growth rate in garter snakes. In this study effects of total number of body vertebrae and of number of abnormal body vertebrae (obtained by counting number of ventral scutes and number of abnormal scutes) on growth rate in free-ranging male and female adders, Vipera berus (L.), are examined by calculating directional performance gradients (estimating linear effects) and stabilizing performance gradients (estimating curvilinear effects). After controlling for body size (SVL) female adders demonstrated a significant positive directional gradient for vertebral number, and a significant interaction between body size and vertebral number, showing that females with more vertebrae have higher size-specific growth rates, and that this effect is strongest among small, fast growing individuals. Females also showed a weak stabilizing effect of abnormal vertebrae. Males, on the other hand, showed a positive directional gradient for number of abnormal vertebrae, whereas no effect of vertebral number was observed. Indirect effects of the same variables were evaluated by use of path analysis. Generally, indirect effects were weak and did not substantially increase the amount of explained variance in growth rate. Field data showed that the correlation between vertebral number and growth rate in females was stronger in years with higher overall growth rate. To evaluate whether vertebral number and food availability show an interactive effect I used captive born juvenile adders in an experiment with two different food levels. The experiment confirmed the field data. No relationship between vertebral number and growth was observed in the low food level group, whereas in the high food level group a significant positive correlation was demonstrated. Finally, the heritability of vertebral number was examined using a mother-offspring regression and a full-sib analysis. The estimated heritabilities were 0.30 and 0.39, respectively. From these results it is concluded that both vertebral number and abnormal vertebral number may significantly affect growth in adders, but that this effect may differ between sexes and among years.