Mirror self-recognition in ghost crab (Ocypode quadrata)

Mirror self-recognition has been examined primarily in vertebrate species, largely through the use of a mirror mark test. Recently, however, there has been growing interest in the notion that all animals likely need some form of self-representation to successfully interact with their environment (Kohda et al. in Plos Biol https://doi.org/10.1371/journal.pbio.3001529, 2022; de Waal in Plos Biol https://doi.org/10.1371/journal.pbio.3000112, 2019). Our knowledge of this trait in invertebrate species is particularly limited, while there are several species potentially well suited for mirror mark tests. To better understand this trait and its distribution, the current research examines mirror self-recognition in ghost crab (Ocypode quadrata) using a visual mirror mark test. In an initial phase, animals were exposed to a flat oriented mirror for an extended period. In a subsequent testing phase, subjects were either (1) marked (green sticker) or sham marked (clear sticker) and exposed to a mirror, (2) marked and placed in a tank without a mirror, or (3) marked and shown video of a marked conspecific. Results showed that marked, relative to sham marked, animals were significantly more likely to remove marks when exposed to mirrors. Findings also suggests this behavior was not driven by the perception of mirror images as conspecifics, or issues related to mark placement. These data are interpreted in support of a rudimentary form of self-awareness in this species. Discussions highlight potential cognitive mechanisms that facilitate this type of self-representation and the need for additional research that informs about more sophisticated forms of self-awareness.