DOI: 10.5061/dryad.15dv41nvp
title: Snaps of a tiny amphipod push the boundary of ultrafast, repeatable movement

description: Surprisingly, the fastest motions are not produced by large animals or
 robots. Rather, small organisms or structures, including cnidarian
 stinging cells, fungal shooting spores, and mandible strikes of ants,
 termites, and spiders hold the world acceleration records. These
 diverse systems share common features: they rapidly convert potential
 energy - stored in deformed material or fluid - into kinetic energy when a
 latch is released. However, the fastest and smallest known movements often
 cannot be used multiple times, because mechanical components are broken or
 ejected. Furthermore, some of these systems must overcome the added
 challenge of moving in water, where high density and viscosity constrain
 acceleration at small sizes. Here we report the kinematics of repeatable,
 ultrafast snaps by tiny marine amphipods (Dulichiella cf. appendiculata).
 Males use their enlarged major claw, which exceeds 30% of body mass, to
 snap a 1 mm-long dactyl with a diameter equivalent to a human hair (184
 µm). The claw snaps closed extremely rapidly, averaging 93 µs, 17 m s-1
 and 2.4 x 105 m s-2. These snaps are among the smallest and fastest-moving
 of any documented repeatable movement and are sufficiently fast to operate
 in the inertial hydrodynamic regime (Re &gt;10,000). They generate
 audible pops and rapid water jets, which occasionally yield cavitation,
 and may be used for defense. These amphipod snaps push the boundaries of
 acceleration and size for repeatable movements, particularly in water, and
 exemplify how new biomechanical insights can arise from unassuming
 animals.
description: Videos for kinematic analyses were recorded at 300,000 frames
 s<sup>−1</sup> (256x128 pixel resolution, 2.33 μs shutter
 duration, FASTCAM SA-Z type 2100K-M-64GB, Photron, San Diego, CA,
 USA).  See Usage notes below and the Current Biology
 online supplement for our detailed methods. 
description: General notes: High speed video files are saved in .avi format.
 Their names begin with the video collection date, followed by “_Amphipod_”
 then the individual ID (the letter A or B with a 1 or 2 digit number),
 followed by another underscore and the video number for that individual.
 Some file names have additional letters and numbers following this that
 are not pertinent to this study. For instance,
 “071018_Amphipod_A2_4_DAQ12” refers to the fourth video collected for
 individual A2 on July 10, 2018. Steps throughout our analyses preserve
 Individual ID and video numbers.  Dryad submission
 contents: 1.    Snap_Example_071218_Amphipod_A11_2.avi:
 Example video of a gnathopod snap. This video was filmed at 300,000 fps.
 For our analysis, the strike parameters were quantified from frame 6 to
 frame 30. This video has been cropped and bit shifted to aid in
 visualization.  2.  
  Cavitation_Example_071118_Amphipod_A5_1.avi: Example video of a gnathopod
 snap that generates a cavitation bubble. The bubble is first visible on
 frame 25 and completes initial collapse on frame 35, followed by addition
 bubble rebound. This video was filmed at 300,000 fps and plays back at
 30fps. This video has been cropped and bit shifted to aid in
 visualization 3.  
  Amphipod_kinematics_ALL_FINAL_wIDs.csv contains the kinematics for all 60
 strikes in the dataset. Details about each column heading are as follows:
 ID_supplemental_table1: The first column “ID_supplemental_table1” has the
 ID number (1-16) arbitrarily given to each individual in Supplemental
 Table 1 in the Current Biology online supplement; ID: Individual specimen
 identifier; Video: Name of each individual video file. Usually the date
 collected [underscore] Amphipod [underscore] ID [underscore] video number;
 bitShift: indicate the value by which the video sequence was bit-shifted
 prior to digitization (a value of zero indicates that there was no
 bit-shift applied); firstFrame: number of frame that marks the beginning
 of the strike; endFrame: number of the frame that marks the end of the
 strike; dactyl_px: dactyl length in pixels determined from the video
 sequence; propodus_px: propodus length in pixels determined from the video
 sequence; num_strikes_analyzed: the number of strikes in total from that
 individual amphipod (including the current entry);    Date_vid_collected:
 date that the video sequence was collected;    Scale_by: indicates whether
 dactyl length or propodus height as measured under the microscope was used
 to scale the video sequence; Dactyl_length_for_scale_mm: if applicable,
 the dactyl length in millimeters measured from scope images of the
 individual to be used to scale the video sequence;
 Propodus_length_for_scale_mm: if applicable, the propodus height in
 millimeters measured from scope images of the individual to be used to
 scale the video sequence; mean_total_dry_mass_mg: the mean total dry mass
 of the amphipod reported in milligrams. A mean is reported because three
 measurements were made and averaged;    mean_claw_dry_mass_mg: the mean
 total dry mass of dactyl and propodus reported in milligrams. A mean is
 reported because three measurements were made and averaged;
 mean_propodus_dry_mass_mg: the mean total dry mass of propodus after
 removal of the dactyl is reported in milligrams. A mean is reported
 because three measurements were made and averaged;
 mean_dactyl_dry_mass_mg: the mean total dry mass of dactyl is reported in
 milligrams. This value was calculated as the difference in the mean total
 claw mass and mean propodus mass for that individual;
 vid_dactyl_length_mm: the length of the dactyl reported in millimeters for
 that individual. This is either the length of the dactyl measured directly
 on the individual under a microscope and reported as
 "Dactyl_length_for_scale_mm" or it is calculated from
 "Propodus_length_for_scale_mm" using the lengths of the propodus
 and dactyl in pixels; calculated_vidScale_ppmm: the scale used to
 calibrate the video in pixels per millimeter (ppmm). This scale is
 determined using the known dimensions of the dactyl or propodus from the
 microscope and their lengths in pixels measured from the video;
 duration_frame: the duration of the strike in number of frames;
 duration_s: the duration of the strike in seconds; angle_t1_deg: the angle
 between the line on the dactyl and the line on the propodus at the start
 of the strike in degrees (see full methods in paper for details);
 angle_t2_deg: the angle between the line on the dactyl and the line on the
 propodus at the end of the strike in degrees (see full methods in paper
 for details); rotation_deg: change in angle in degrees between the start
 and end of the strike; rotation_rad: change in angle in radians between
 the start and end of the strike; average_rot_v_dps: the average rotational
 velocity over the course of the strike in degrees per second (dps);
 average_rot_a_dps2: the average rotational acceleration over the course of
 the strike in degrees per second squared (dps2); average_rot_v_rps: the
 average rotational velocity over the course of the strike in radians per
 second (rps); average_rot_a_rps2: the average rotational acceleration over
 the course of the strike in radians per second squared (rps2);
 average_speed_mmps: the average linear speed over the course of the strike
 in millimeters per second; average_speed_mps: the average linear speed
 over the course of the strike in meters per second; average_at_mmps2: the
 average linear acceleration over the course of the strike in millimeters
 per second; average_at_mps2: the average linear acceleration over the
 course of the strike in meters per second.  4.  
  Digitize_amphipod_videos_inFiji_FINAL.ijm is a macro script for Fiji that
 was used to automate the digitization of videos for our analyses. With a
 video open, run the script. It will prompt you to choose a location for
 the output and then walk you through the process to digitize the first and
 last frame of the strike, as well as to measure the scale, dactyl and
 propodus lengths.  5.    final_dig_files.zip contains
 the digitizing output for all 60 strikes in our dataset. Additionally, the
 output subfolder contains output from the kinematic analyses performed in
 R (see below). There are 60 files pertaining to the kinematic output for
 each video in the dataset (Amphipod_kinematics_[video name]_final.csv),
 the master output file for all 60 videos
 (Amphipod_kinematics_ALL_FINAL.csv) and a clean summary of kinematics by
 individual used to create the supplemental table
 (Amphipod_kinematics_SUPPLEMENTAL_TABLE_FINAL.csv). Information about
 column headers can be found in the file "Digitization Files READ
 ME.txt". 6.    metadata.zip contains metadata for
 individuals (e.g., masses, dactyl lengths) and videos (e.g.,  date,
 framerate, name of the person who digitized, etc). Individual summary data
 can be found in : Amphipod_Individual_SUMMARY_dryad.csv and video data can
 be found in Amphipod_Diglog_dryad.csv. These files serve as additional
 inputs in the R script below. Information about column headers can be
 found in the file "Metadata READ ME.txt".
  <br> 7.  
  Amphipod_Kinematics_from_Digitizing_Rscript_FINAL.R computes and
 summarizes kinematics using the digitization files and metadata above as
 inputs.  8.    Comparative_dataset_FINAL.csv: Dataset
 used to create the comparative plot in Figure 1. See supplemental methods
 for more details. Details about each column heading are as follows:
 PLOTCODE: code used to facilitate plotting in R; MASSKG: mass of actuated
 structure in kilogram; MASS_g: mass of actuated structure in grams;
 ACCELMpSsqd: acceleration in meters per second squared; Citation: citation
 for papers where information for mass and acceleration were taken from or
 derived. <br>