Onset of symbiosis between Tetralia crabs and Acropora corals revealing the smallest host habitat size

Abstract

Coral reef ecosystems rely on complex networks of mutu alistic associations (Gates and Ainsworth 2011). Some mutu alistic relationships contribute to the survival and protection of young coral recruits, which are critical stages for coral reef community dynamics and resilience. Sym­ biotic crabs of the genus Tetralia specifically associate with acroporid corals. They are known to be beneficial to juvenile coral stages by providing cleaning (Stewart et al. 2006) and defense services (Rouzé et al. 2014). However, the onset of this coral­crab symbiosis is still unknown. Here, we report for the first time the onset of the coral-crab association in Acropora tenuis . We collected and cross­fer tilized egg­sperm bundles from A. tenuis on the 20 th June 2008 and reared planula larvae until settlement on pre­con ditioned tiles at Sesoko Station, Okinawa, Japan. Newly set tled polyps were transplanted to their parental habitat near Sesoko Island on the 2 nd July 2008 and moni­ tored for presence of exosymbiotic crabs at one, two, four (Fig. 1a), eight (Fig. 1b) and thirteen (Fig. 1c) months after settlement. Symbiotic crabs were not observed until thir­ teen months, indicating that the onset of the coral­crab mutualistic asso ciation was ini tiated more than eight months after coral settlement. At this time, the inhabited coral hosts were 1.5 - 2.2 cm in diameter (between 53 and 163 polyps per colony), with up to twenty branchlets