Poster Q1 - Mechanosensation of Semiterrestrial Crabs

Introduction: Semiterrestrial crabs have developed adaptations to overcome the challenges of living inland. They dig burrows in the sand close to a body of water for rest and shelter. This lifestyle, though, exposes them to an abundance of mechanical cues resulting from the compressive forces of their burrow walls and forces resulting from fluid flow over the sand surface, among others. The crabs also have predators that walk on the top of the sand when the crabs are in their burrows. Mechanosensory systems in crabs may have a significant role in predator-prey relationships, as they can potentially detect pressure waves produced by other creatures, such as birds walking on the ground above them. The detection of these mechanical cues is critical for the survival of these animals and would provide an evolutionary survival advantage over time. The objective of our study is to investigate how crabs perceive and respond to predator/prey signals in their immediate environment by analyzing the behavioral reactions of semiterrestrial crabs to mechanical stimulation.

Materials and

Methods: We investigate Ocypode quadrata, the Atlantic Ghost Crab (Figure 1 (a)), as these crabs dig burrows into the sand and have predators that walk on the sand surface, including herons and seagulls. To understand the response of the crabs, we built a transparent test enclosure partially filled with sand to mimic the crab’s natural habitat. The enclosure features an ultrasonic transducer as a source of mechanical stimulation, a sensor to measure the vibrations, a function generator and amplifier to condition the signals, a remotely triggered control and data acquisition unit, and a Wi-Fi camera to capture the crab response. To introduce the vibrations into the enclosure, the transducer was directly placed in the sand. First, we characterized the propagation of the ultrasonic wave into sand by collecting the sensor readings at various distances from the ultrasonic transducer, generating the vibrations. Second, we tested the crabs to ensure that they were able to detect vibrations by first tapping the enclosure walls and observing a change in behavior. Third, we created a stimulation signal using a train of square waves to drive the ultrasonic function generator. We captured the responses of crabs using a Wi-Fi camera looking into the enclosure, and we analyzed the resulting videos. The setup is shown in Figure 1 (b). There were 6 trials with crabs being tested multiple times, with all of the trials starting with the crabs inside their burrows.

Results, Conclusions, and Discussions: The ultrasonic system that we designed successfully produced vibrations in sand, which were detectable across the enclosure from the source. When subjected to ultrasonic stimulation, the Ocypode quadrata displayed avoidant behavior. The crabs were observed taking shelter in their burrows during stimulation. In some of the trials, the crabs were observed peeking out of their burrows in between pulses but remained still in their burrows throughout the duration of the trials. In some cases, the crabs were observed exploring their surroundings after peeking out but eventually taking shelter in their burrows. The results of all trials show that the crabs spent an average of 9 minutes and 26 seconds in their burrows and 4 minutes and 53 seconds outside of their burrows. Furthermore, the results of all trials indicated that the majority of detected motion occurred in the latter half of the experiment when the stimulation pulses were more spaced out in time (Figure 1 (c)). The results of our investigation have significant implications in various domains, such as comprehending the ecological obstacles that burrowing creatures encounter, studying predator/prey dynamics, and exploring mechano-transduction.

Acknowledgements (Optional): This work is co-financed by Fundação para a Ciência e a Tecnologia (Portuguese Foundation for Science and Technology) through the Carnegie Mellon Portugal Program under the fellowship SFRH/BD/150824/2021.