Crushers and graspers: diversification of a musculoskeletal system toward two opposite functional 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: datasets.zip

Description: This is a collection of datasets:

"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

File: Dataset1.xlsx

this dataset includes data of performance and sarcomere length in different 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)