Data from: Collective movement increases initial accuracy and path efficiency in talitrid amphipod orientation

Description

Notes

Methods

Experiments were conducted between July - August, in 2021 and 2023. Animals were collected from a 50 meter stretch of sandy beach in Seattle, WA (47.66, -122.43). All animals were collected within ~20 minutes of each other and used in experiments within 30 minutes of collection. The trials were conducted between 8:00 am and 1:00 pm while air temperatures ranged from 14°C to 26°C. The arenas consisted of a circular, clear plastic dish (diameter 27 cm, depth of 3.1 cm), held 43 cm over the ground by PVC pipes. The cameras used to film all trials were the Apexcam M80 Air Action Camera. The temperature was recorded at the beginning of each trial by using the My Location section of the iOS Weather app (iPhone 14, iOS version 16.5.1). Cloud cover was categorized as either "cloudy" or "sunny" where cloudy days were defined as days where more than 50% of the sky had cloud cover and the sun was obscured. In addition, there was a nearby forest fire from August 14th-17th 2021, so trials conducted during those four days were classified as "cloudy" due to the poor air quality and low visibility.

Animals were released in experimental arenas either alone (hereafter "GS1" or "individuals"), or in groups of size five ("GS5") or ten ("GS10").

The arenas consisted of a circular, clear plastic dish (27 cm diameter), surrounded by white, opaque paper to obscure any landscape cues along the horizon. At the beginning of each trial, the arenas were leveled and rotated randomly relative to the beach. A camera underneath the arena filmed the animals' movement through the bottom of the dish. The camera was always placed in the same position at the center of the arena and in the same orientation relative to the arena to identify whether the sandhoppers were orientating to any features of the experimental set up itself. Before the animals were introduced to the arena, the direction of the shortest distance to the wrack line was indicated to the camera, so that the animal's movement relative to the beach could be calculated.

The arenas were placed two meters landward of the wrack line to motivate the amphipods to move seawards. On macrotidal beaches, such as the study site, talitrid amphipods rarely travel above the wrack line- this zone has the same risks of predation and desiccation as below the wrack line but with few of the potential benefits from foraging. The amphipods would likely only enter this area through passive displacement, such as a storm, or a navigational mistake. Therefore, talitrid amphipods who find themselves landward of the wrack line are strongly motivated to return to the optimal burrowing zone.

Before each trial began, the animals were allowed to acclimate to the arena for two minutes in a transparent cup (6.5 cm diameter) placed at the center of the arena. After that time had elapsed, the cup was lifted from a randomized direction, and the arena was covered with a translucent lid.

A total of 119 trials (n=35 for GS1, n=38 for GS5, n=46 for GS10) were digitized using the software DLTdv8a. The path of each animal was digitized from the lifting of the cup to the moment the animal reached the arena's edge:

The Time at which the Trial Started: The lifting of the acclimation cup was the start of the trial. It was determined that the cup was "lifted" when there was a significant shift in cup location between frames and/or it was clear that individuals were able to travel under the rim of the cup. Occasionally the exact frame where the cup was lifted was difficult to determine, so a general error of + 3 frames (0.1 seconds) should be assumed for all calculations.

The Time at which the Animal Started Moving: The start of movement was determined as the moment when an individual began to clearly move in relation to the arena. For example, slight jostling of the arena itself, especially while placing the lid after lifting the cup, did not count as movement of an individual. In cases where an individual was bumped by the lifting of the cup or jumped once at the very start of the trial and then proceeded to hold still for more than ~8 frames, this was not deemed the start of their movement and the start was determined to be their next movement. Occasionally the exact frame where movement began was difficult to determine, so a general error of + 3 frames (0.1 seconds) should be assumed for all calculations.

Digitization Location: During digitization, the digitizer always clicked on the same part of the individual's body: the center of the head when visible. If the center of the head could not be determined, the next closest possible location was selected. In general, human accuracy during digitization should be assumed to be on the order of ~0.1mm (which was the average distance found between repeated attempts to select the center of the arena).

 The Time at which the Trial Ended: The trial ended whenever any part of an individual's body touched the edge of the arena or could have conceivably touched the arena. If an individual passed behind a column, it was decided that the animal would be counted as having reached the edge on the frame where they first were lost from view of the camera. Occasionally the exact frame where the animal reached the edge was difficult to determine, so a general error of + 3 frames (0.1 seconds) should be assumed for all calculations.

Interpolating Movement if the Animal was not Visible: Occasionally animals would be lost from view during digitization, for example, due to movement that was too fast to be captured by the camera. Whenever an animal was not visible, their movement during this period was interpolated to be constant, straight line movement between the bounding visible locations using the function na.approx from the package zoo (version 1.8.12).

Direction of the Wrack Line and Data Transformation: The direction of the wrack line was indicated to the camera by placing an index card into the arena with an arrow pointing towards the shortest path to the wrack line. This was digitized by selecting a point along the edge of the arena that was along the line of the indicative arrow. From there, all data was translated such that the center of the arena was at x=0, y=0, and rotated such that the direction of the wrack line was at an angle of 0^o compared to the center of the arena.

After an outlier analysis, animals that took longer than 18 s to complete the trial were excluded from all further analyses. Only trials on clear weather conditions were used in the permutation analysis.