Onset of Symbiosis Between Tetralia Crabs and Acropora Corals Revealing the Smallest Host Habitat Size

Galaxea, Journal of Coral Reef Studies 21: 1-2（2019）

Photogallery

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

Héloïse ROUZÉ, Frederic SINNIGER, and Saki HARII

Tropical Biosphere Research Center, University of the Ryukyus, Motobu, 3422 Sesoko, Motobu, Okinawa 905-0227, Japan

* Corresponding author: Héloïse Rouzé E-mail: [email protected]

Communicated by Michio Hidaka (Biology Editor)

Keywords coral, crab, symbiosis, ontogeny

Coral reef ecosystems rely on complex networks of mutualistic associations (Gates and Ainsworth 2011). Some mutualistic 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 coral-crab symbiosis is still unknown. Here, we report for the first time the onset of the coral- crab association in Acropora tenuis. We collected and cross-fertilized egg-sperm bundles from A. tenuis on the 20th June 2008 and reared planula larvae until settlement on pre-conditioned tiles at Sesoko Station, Okinawa, Japan. Newly settled polyps were transplanted to their parental habitat near Sesoko Island on the 2nd 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 coral-crab Fig. 1 a Acropora tenuis juveniles transplanted to the reef after 4 months (October 2008), b after 8 months (February mutualistic association was initiated more than eight 2009) and c after 13 months (July 2009) with the occurrence of months after coral settlement. At this time, the inhabited Tetralia cf. nigrolineata crab (blue arrow). Scale bars represent coral hosts were 1.5-2.2 cm in diameter (between 53 and 2 mm. 163 polyps per colony), with up to twenty branchlets 2 Rouzé et al.: Onset of coral-crab symbiosis

(diameter of unitary distal branches <2 mm; four colonies measured; Fig. 1c). This represents the smallest host habitat size for Tetralia crabs (carapace width ~3 mm) in which narrow spaces between coral branchlets likely provide a perfectly fitting shelter protected from predation. Out of 18 juvenile corals observed, eight colonies were inhabited by crabs. Most of the young corals hosted a single crab while one hosted two crabs. By deciphering the onset of the coral-crab partnership for additional species of the sensitive genus Acropora, better protective strategies and restoration programs could be specifically implemented to enhance coral survival during the vulnerable phase not associated with crabs (e.g. rearing the corals under controlled protective conditions until they reach a size suitable for crab colonization).

Acknowledgments We thank Sesoko Station staff for field assistance. We also thank Dr. E. Howells for her comments on the manuscript. This study was supported by JSPS Grants-in-Aid for JSPS Research Fellow (16F16780) to HR and for Scientific Research (C) (No. 24570030) and (A) (No. 16H02490) to SH.

References Gates RD, Ainsworth TD (2011) The nature and taxonomic composition of coral symbiomes as drivers of performance limits in scleractinian corals. J Exp Mar Bio Ecol 408: 94-101 Rouzé H, Lecellier G, Mills SC, Planes S, Berteaux-Lecellier V, Stewart H (2014) Juvenile Trapezia spp. crabs can increase juvenile host coral survival by protection from predation. Mar Ecol Prog Ser 515: 151-159 Stewart HL, Holbrook SJ, Schmitt RJ, Brooks AJ (2006) Symbiotic crabs maintain coral health by clearing sediments. Coral Reefs 25: 609-615