Functional divergence of scorpion pedipalps: Musculoskeletal specialization toward opposing performance optima.

Abstract When selective pressures for different functions act simultaneously to shape structures, Evolutionary trade-offs can arise. These trade-offs are believed to constrain adaptive responses, changing trajectories of evolutionary diversification, and leading to a high degree of covariation among traits toward a single performance optimum. We studied two species representing the morphological extremes in the chela (pincer) shape of scorpions. Across scorpions, chelae show significant morphological diversity associated with ecology, and their performance is subject to a force/velocity trade-off. We built a biomechanical model integrating synchrotron microtomographies, performance, and muscle architecture data. Our findings reveal different arrangements of muscles and structural elements towards two functional optima of performance: closing force is optimized in the short-fingered species through mechanical advantage of levers and muscles, as well as increased sarcomere length. In the slender-chela species closing velocity is optimized, e.g. by means of muscle orientations favouring early acceleration. Although other functional demands may be at play, one design seems optimized for snatching prey, and the other for prey crushing. These divergent optima, driven by trade-offs, may have had profound impacts on the trophic biology of scorpions, and their venom evolution. Description of the data and file structure These datasets and scripts were used to model the chela closing performance in scorpions Files and variables File: Datasets1.zip Description: This is a collection of datasets: "Dataset1" includes data of performance and sarcomere length in the two analyzed species of scorpions "datasets_Hottentotta_Coordinates" includes all the coordinates of the landmarks placed per muscle in Hottentotta chela "datasets_Scorpio_ Coordinate" includes all the coordinates of the landmarks placed per muscle in Scorpio chela "datasets_Hottentotta_output_per_bundle" includes all the output of the model for Hottentotta chela "datasets_Scorpio_output_per_bundle" includes all the output of the model for Hottentotta chela Details on : Dataset1.xlsx this dataset includes data of performance and sarcomere length in the two analyzed species of scorpions Description: Variables - the column L and R represent left and righ chela - MaxL (Maximum velocity Left chela in mm/s) - MaxR (Maximum velocity Right chela in mm/s) - AverageMax (Average between the maximum veloctiy Left and the Maximum velocity right chela in mm/s) - maxvelocity (Max velocity calculated form frame 1 to the end of the closing event in mm/s) - meanvelocity (Mean velocity calculated form frame 1 to the end of the closing event in mm/s) - maxaccel (Max acceleration calculated form frame 1 to the end of the closing event in mm/s2) - meanaccel (Mean acceleration calculated form frame 1 to the end of the closing event in mm/s2) - meandecel (Mean deceleration calculated form frame 1 to the end of the closing event in mm/s2) - MaxLAng (Max angular velocity Left chela rad/s) - MaxRang (Max angular velocity Right chela rad/s) - Average maxAng (Average between the maximum angular veloctiy Left and the Maximum angular velocity right chela in rad/s) - maxangvel (Max angular velocity calculated form frame 1 to the end of the closing event in rad/s) - meanangvel (Max angular velocity calculated form frame 1 to the end of the closing event in rad/s) - duration (duration in s of the closing event) - anglemaxangvelocity (Angle in radians where the maximum velocity is reached during closing event) - startangle ( Initial opening angle of the chela in radians) - ProsL (Prosoma Length mm) - ProsW (Prosoma width mm) - MesoL (Mesosoma Length mm) - 4thMesoW ( 4th mesosoma tergite width mm) -1stMetaW (1st Metasoma segment width mm) -MetaL (Metasoma Length excluding telson mm) -TelsonL (Telson length mm) - FemL (Pedipalp Femur Length mm) - PatL ( Pedipalp patella Length mm) - ManusH (Chela manus Height mm) - ManusW (Chela manus Width mm) - ManusL (Chela manus Length mm) - Movfing (Movable finger Length mm) - Mech.Adv (Mechanical advantage ) - Biteforce (Biteforce in N) - IsoSize_Vector (Isosize vector to size-correct performance and morphological variables) - Sarcomere - L ( average sarcomere Length in the manus irrespective of position)# 摘要 当不同功能的选择压力共同作用塑造生物结构时，便会产生进化权衡（Evolutionary trade-offs）。这类权衡被认为会限制适应性响应，改变进化分化的轨迹，并导致性状间朝着单一性能最优方向产生高度的协变。 本研究选取两种代表蝎子螯（chela，钳状结构）形态极端的物种展开分析。在蝎子类群中，螯的形态多样性与其生态位密切相关，且其性能受到力-速度权衡的约束。我们构建了整合同步辐射显微断层扫描（synchrotron microtomographies）、运动性能与肌肉架构数据的生物力学模型（biomechanical model）。 研究结果揭示了朝向两类性能最优解的肌肉与结构元素排布差异：短指物种通过杠杆与肌肉的机械优势，以及更长的肌节（sarcomere）长度优化了闭合力量；而细螯物种则通过利于早期加速的肌肉取向等方式，优化了闭合速度。尽管可能存在其他功能需求，但一种设计似乎专为攫取猎物优化，另一种则专为碾碎猎物优化。这类由权衡驱动的分化最优解，可能对蝎子的营养生物学（trophic biology）及其毒液演化产生了深远影响。 ## 数据与文件结构说明 本数据集与脚本用于模拟蝎子螯的闭合性能。 ### 文件与变量说明 1. **文件：Datasets1.zip** 本压缩包包含以下数据集： - "Dataset1"：包含两种受试蝎子物种的运动性能与肌节长度数据 - "datasets_Hottentotta_Coordinates"：包含霍屯都蝎属（Hottentotta）螯部各肌肉的地标坐标数据 - "datasets_Scorpio_Coordinate"：包含蝎属（Scorpio）螯部各肌肉的地标坐标数据 - "datasets_Hottentotta_output_per_bundle"：包含霍屯都蝎属螯部模型的每一束肌肉输出结果 - "datasets_Scorpio_output_per_bundle"：包含霍屯都蝎属螯部模型的每一束肌肉输出结果 2. **Dataset1.xlsx 详情** 该数据集包含两种受试蝎子物种的运动性能与肌节长度数据，各变量说明如下： - `L`、`R`：分别代表左、右螯 - `MaxL`：左螯最大闭合速度（单位：mm/s） - `MaxR`：右螯最大闭合速度（单位：mm/s） - `AverageMax`：左、右螯最大闭合速度的平均值（单位：mm/s） - `maxvelocity`：闭合事件全程（从第1帧至结束）的最大闭合速度（单位：mm/s） - `meanvelocity`：闭合事件全程的平均闭合速度（单位：mm/s） - `maxaccel`：闭合事件全程的最大加速度（单位：mm/s²） - `meanaccel`：闭合事件全程的平均加速度（单位：mm/s²） - `meandecel`：闭合事件全程的平均减速度（单位：mm/s²） - `MaxLAng`：左螯最大角速度（单位：rad/s） - `MaxRang`：右螯最大角速度（单位：rad/s） - `Average maxAng`：左、右螯最大角速度的平均值（单位：rad/s） - `maxangvel`：闭合事件全程的最大角速度（单位：rad/s） - `meanangvel`：闭合事件全程的平均角速度（单位：rad/s） - `duration`：闭合事件持续时长（单位：s） - `anglemaxangvelocity`：闭合过程中达到最大速度时对应的角度（单位：rad） - `startangle`：螯初始张开角度（单位：rad） - `ProsL`：头胸部（Prosoma）长度（单位：mm） - `ProsW`：头胸部宽度（单位：mm） - `MesoL`：中体部（Mesosoma）长度（单位：mm） - `4thMesoW`：第4中体背板宽度（单位：mm） - `1stMetaW`：第1后体节宽度（单位：mm） - `MetaL`：后体部（Metasoma）长度（不含尾节，单位：mm） - `TelsonL`：尾节长度（单位：mm） - `FemL`：须肢股节（Pedipalp Femur）长度（单位：mm） - `PatL`：须肢膝节（Pedipalp patella）长度（单位：mm） - `ManusH`：螯掌部高度（单位：mm） - `ManusW`：螯掌部宽度（单位：mm） - `ManusL`：螯掌部长度（单位：mm） - `Movfing`：可动指长度（单位：mm） - `Mech.Adv`：机械优势 - `Biteforce`：咬合力（单位：N） - `IsoSize_Vector`：用于对性能与形态变量进行尺寸校正的等尺寸向量 - `Sarcomere` - `L`：螯掌部平均肌节长度（不考虑位置差异，单位：mm）