Horseshoe crab body size cline

Aim Adult body size often exhibits patterns across large-scale environmental gradients, creating ecogeographic clines. However, the form of body size clines varies across taxonomic groups, with linear and non-linear patterns in body size observed in nature. Non-linear body size clines have received less study, and questions remain about how environmental gradients interact to produce non-linear clines. We examined the body size of the American horseshoe crab (Limulus polyphemus), a widely distributed marine arthropod, and evaluated the hypothesis that temperature and active season length can interact multiplicatively to result in a dome-shaped distribution. Location Fourteen states in the United States of America and three Mexican states, representing the entire geographic range of the species. Methods We compiled environmental data and body size measurements from more than 49,000 individual horseshoe crabs. For each location, we extracted from the literature or calculated from raw data the mean male prosoma width and the mean female prosoma width. We applied a General Additive Modeling (GAM) approach to characterize the body size cline, test a hypothesis regarding temperature and season length, and explore evidence for the influence of additional environmental factors. Results Model results indicate temperature and season length could act multiplicatively to produce dome-shaped clines, and these findings align with and quantify previous anecdotal reports of a strong dome-shaped body size cline across latitude for horseshoe crabs. Main conclusions Active season length appears to become relatively more influential on horseshoe crab body size in the northern part of their range, while temperature effects per se appear to dominate in southern latitudes. For horseshoe crabs, the pattern of size variation is consistent with the predictions of Optimal Resource Allocation models, but more study is needed to elucidate mechanistic underpinnings. Considering climate change projections, results from our study suggest future shifts in horseshoe crab body sizes.

研究目的：成体体型常随大规模环境梯度呈现规律性分布模式，进而形成生态地理渐变群（ecogeographic clines）。不过不同类群的体型渐变群形式存在差异，自然界中已观测到线性与非线性两类体型变化模式。目前针对非线性体型渐变群的研究相对较少，关于环境梯度如何相互作用以形成此类渐变群的问题仍有待解答。我们以分布广泛的海洋节肢动物美洲鲎（Limulus polyphemus）为研究对象，验证"温度与活动季长度可通过相乘作用形成穹顶状分布格局"这一假说。
研究区域：涵盖美国14个州与墨西哥3个州，覆盖该物种的完整地理分布范围。
研究方法：我们整合了超过49000只美洲鲎的环境数据与体型测量数据。针对每个采样点，我们从文献中提取或基于原始数据计算得到雄性头胸甲平均宽度与雌性头胸甲平均宽度。我们采用广义可加模型（General Additive Modeling, GAM）刻画体型渐变群特征，验证温度与活动季长度相关假说，并探究其他环境因子的影响证据。
研究结果：模型结果表明，温度与活动季长度可通过相乘作用形成穹顶状体型渐变群，该发现与此前关于美洲鲎沿纬度存在显著穹顶状体型渐变群的零散报道一致，并对此进行了量化。
主要结论：在美洲鲎分布范围的北部区域，活动季长度对其体型的相对影响更为显著；而在南部纬度区域，温度本身的影响占主导地位。美洲鲎的体型变异模式符合最优资源分配模型（Optimal Resource Allocation models）的预测，但仍需更多研究阐明其内在机制。结合气候变化预测结果，本研究结果表明美洲鲎的体型未来或将发生改变。