Host selection and preferences of coral symbiotic crab Tetralia rubridactyla

• The study revealed the condition underlying host selection by the crab.
• Shelter is the priority function for the crab when it comes to habitation.
• The coral crab showed highly divergent preferences on test objects.
• Acropora corals were the preferred shelters for the symbiotic crab.
• The crab expressed neither preference nor fidelity on specific Acropora coral.

Coral symbiotic crabs provide considerable benefits to their host corals. A comprehensive understanding of the association between these crabs and their hosts could help clarify the relationship, interaction, and importance of symbionts with coral reefs as hosts. In this study, the coral symbiotic crab Tetralia rubridactyla was test for host preference and fidelity. Five oceanic objects were provided to the crabs: common host corals (Acropora hyacinthus and A. digitifera), uncommon host corals (Pocillopora damicornis and Stylophora pistillata), and dead coral skeletons. The crabs were collected from the 2 source host corals A. hyacinthus and A. digitifera and subjected to an experiment comprising 7 treatments. Each treatment included 2 stages of no-choice and choice conditions to estimate the expected selection frequencies. The results revealed that the crabs chose any available object under the no-choice condition, and exhibited various preferences under the choice condition. Moreover, T. rubridactyla exhibited significantly higher frequencies to inhabit Acropora corals (p < 0.01, χ2 test), than dead coral skeletons and uncommon host corals. In all the treatments, the preferences of the crabs from the 2 source hosts were similar. Present results demonstrated T. rubridactyla host selection conditioning as follows: (1) Under the no-choice condition, inhabit any choice object for shelter; (2) under the choice condition, if without a common host, randomly inhabit any uncommon choice object as a host; and (3) under the choice condition, if a common host is available, selecting the common host is the first priority because it could provide food and space. This study revealed that T. rubridactyla express neither fidelity nor preference between A. hyacinthus and A. digitifera. Thus, these results also suggested that the distribution of T. rubridactyla on Acropora corals in the reef is affected by an abundance of corals rather than the preferences of coral species.