Beneath the surface: correlates of solitary, paired, and group living in a cryptic burrowing ghost shrimp Trypaea australiensis Dana, 1852 (Decapoda: Axiidea: Calianassidae)

Marine organisms, particularly invertebrates, are relatively understudied regarding social structure and behaviour, especially those that are cryptic. While empirically challenging to investigate, these species can provide novel insights into the evolution of sociality given their unique ecology. Our aim was to quantify in-situ the social structure and its underlying correlates in the Australian burrowing shrimp, Trypaea australiensis Dana, 1852 (Callianassidae). We show that T. australiensis exhibits intraspecific variation in social structure, being most often found in solitary arrangements, frequently in pairs (including homosexual and heterosexual arrangements), and sometimes in groups. The majority of shrimp in pairs and groups were found in heterosexual pairings or groups of one male and multiple females. Variation in social structure was correlated with variation in body size characteristics, seasonality, and site. These findings demonstrate that a wider array of social structures are exhibited by this species than previously expected, but the obvious limitations associated with sampling a cryptic burrowing species in situ mean that further investigations under controlled conditions, including laboratory behavioural manipulations, will be needed to confirm the current findings. The use of resin casts to understand burrow morphology would be important for understanding the causes of intraspecific variation in this and other cryptic species.