REPORT Endosymbiotic copepods may feed on zooxanthellae from their coral host, Pocillopora damicornis Y.-R. Cheng • C.-F. Dai Received: 15 April 2009 / Accepted: 15 October 2009 / Published online: 5 November 2009 Ó Springer-Verlag 2009 Abstract The Xarifiidae is one of the most common families of endosymbiotic copepods that live in close association with scleractinian corals. Previous studies on xarifiids primarily focused on their taxonomy and mor- phology, while their influence on corals is still unknown. In this study, we collected a total of 1,579 individuals belonging to 6 species of xarifiids from 360 colonies of Pocillopora damicornis at Nanwan Bay, southern Taiwan from July 2007 to May 2008. Furthermore, using optical and electron microscopic observations, we examined the gut contents of Xarifia fissilis, the most abundant species of the Xarifiidae that we collected. We found that the gut of X. fissilis was characterized by a reddish-brown color due to the presence of numerous unicellular algae with diam- eters of 5–10 lm. TEM observations indicated that the unicellular algae possessed typical characteristics of Sym- biodinium including a peripheral chloroplast, stalked pyr- enoids, starch sheaths, mesokaryotic nuclei, amphiesmas, an accumulation body, and mitochondria. After starving the isolated X. fissilis in the light and dark (light intensity: 140 lmol photon m -2 s -1 ; photoperiod: 12 h light/12 h dark) for 2 weeks, fluorescence was clearly visible in its gut and fecal pellets under fluorescent microscopic obser- vations. The cultivation experiment supports the hypothesis that the unicellular algae were beneficial to the survival of X. fissilis under light conditions, possibly through trans- ferring photosynthates to the hosts. These results suggest that X. fissilis may consume and retain unicellular algae for further photosynthesis. Keywords Endosymbiotic copepods Á Coral parasites Á Xarifia Á Zooxanthellae Á Pocillopora damicornis Introduction Coral colonies provide space and resources for many symbiotic organisms, thus allowing greater diversity of symbiotic modes of life than many other types of host (Humes 1985a). More than 350 species of copepods have been found living in various degree of association with hard and soft corals (Humes 1985a, 1994). Among these associates, the taxon Xarifiidae, which includes 86 species, contains the most common copepods inhabiting the gas- trovascular cavities of scleractinian polyps (Humes 1985b; Cheng et al. 2007). Previous studies on xarifiids primarily focused on their taxonomy and morphology, and very few studies have been done on the relationship between xarifiids and their host corals (Dojiri 1988; Cheng and Dai 2009). Dojiri (1988) reported the gall-inhabiting copepods and the possible scenario for development of galls on coral hosts. Cheng and Dai (2009) reported the infection process of a parasitic copepod, Xarifia obesa, with Stylophora pistillata. There- fore, the resources that xarifiids use from their host corals are still unknown. Many copepods are opportunistic feed- ers, and they feed on many species of zooplankton and phytoplankton (Turner 1991). Humes (1985a) speculated that endosymbiotic copepods feed on the mucus released by corals. Dojiri (1988) also suggested that the gall- inducing copepod, Isomolgus desmotes, utilizes mucus as its food source. Since there are numerous zooxanthellae Communicated by Biology Editor Dr. Clay Cook Y.-R. Cheng Á C.-F. Dai (&) Institute of Oceanography, National Taiwan University, 1 Roosevelt Road, Sect. 4, Taipei 106, Taiwan e-mail: corallab@ntu.edu.tw 123 Coral Reefs (2010) 29:13–18 DOI 10.1007/s00338-009-0559-8