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We examined genetic and genomic evidence for latitudinal and climate-associated clines in both native and introduced species in the northwest (NW) Atlantic, from North Carolina, United States to Labrador, Canada. This region spans a 16 latitudinal range and a strong environmental gradient, with an abrupt transition on the Eastern Scotian Shelf (Fig. 1). Previous genetic studies over this range suggest a broad north-south clinal pattern within individual species (table S1), and the potential for adaptive divergence (5), but whether these patterns recur across taxa and at what ecological scale remains unknown.

Download File Stable 6 cline lines for satellit...

Our model predicts a northward shift (224.6 96.6 km, means SD) in the distribution of all species and population structures (Fig. 4, A and B), mirroring shifts already observed in the northeast Atlantic (17). In the NW Atlantic, the asymmetrical influence of changing climatic conditions leads to more pronounced distributional shifts and changes suitable habitat availability, on average, in northern subpopulations (Fig. 4), where seasonal thermal minima largely define suitable habitat, consistent with our observations of environmental relationships with the genetic clinal structure (Fig. 3). Predicted distributional shifts were particularly pronounced in northern populations of lobster, green crab, and shrimp (Fig. 4A), with average shifts of 359 km (115). Only cod demonstrated a significantly higher northern shift in the southern population, echoing fluctuations in abundance across their range (27, 28) and significant reductions in the availability of suitable habitat (Fig. 4C). For northern shrimp, declines in landings over the last decade (29) are consistent with predictions of declining habitat availability and a shift to higher latitudes over the coming decades. Predicted northern distributional shifts in lobster, green crab, and scallop were associated with habitat expansions in the northern part of their respective ranges, whereas southern ranges were not associated with expanded habitat, except for green crab (Fig. 4). Predicted species distributional changes will also influence the position and shape of the multispecies genetic cline, suggesting variation in genetic structure, and could potentially offer a sensitive indicator for ecological change in warming oceans.

Bergmann's rule states that organisms at higher latitudes should be larger and thicker than those closer to the equator to better conserve heat, and Allen's rule states that they will have shorter and thicker limbs at higher latitudes. Alternative explanations for latitudinal size clines include plant productivity and seasonality. The rules generally hold in endotherms, but in insects different species within the same genus can respond to latitude in unpredictable ways. We present the first biogeographical analysis of these rules in stick insects (order Phasmatodea), using four European species. Their long and thin bauplan makes the Phasmatodea ideal for ecomorphological studies of body length, which could identify the evolutionary drivers of their remarkable size range (including the world's longest insects). Using preserved specimens from collections across Europe; body segment and limb measurements were taken for both genders of the species Bacillus rossius, Clonopsis gallica, Leptynia attenuata, and Pijnackeria hispanica. Lengths and volumetric features were compared to latitude as well as annual mean temperature, net primary productivity, and annual growing degree days, using weighted linear regressions and ANOVA analyses. At lower latitudes/higher temperatures, B. rossius and L. attenuata had longer limbs [Allen clines] and were larger bodied and/or longer [converse-Bergmann clines], while the other species did not show latitudinal clines per se. This matches what was predicted based on closely related insects and the presence of large Phasmatodea in the tropics, but violates the temperature-size rule. Most variation in size could be attributed to temperature, but untested factors could also play a role Whether these ecogeographic rules hold true for tropical Phasmatodea and whether genetics or environment play are more important in determining adult length are topics for future research. 041b061a72


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