RESEARCH ARTICLE Mauricio Rodriguez-Lanetty Elisha M. Wood-Charlson Æ Lea L. Hollingsworth Dave A. Krupp Æ Virginia M. Weis Temporal and spatial infection dynamics indicate recognition events in the early hours of a dinoﬂagellate/coral symbiosis Received: 20 July 2005 / Accepted: 10 November 2005 / Published online: 21 February 2006 Ó Springer-Verlag 2006 Abstract The obligate symbiotic relationship between dinoﬂagellates, Symbiodinium spp. and reef building corals is re-established each host generation. The soli- tary coral Fungia scutaria Lamarck 1801 harbors a single algal strain, Symbiodinium ITS2 type C1f (homologous strain) during adulthood. Previous studies have shown that distinct algal ITS2 types in clade C correlate with F. scutaria—Symbiodinium speciﬁcity during the onset of symbiosis in the larval stage. The present study examined the early speciﬁcity events in the onset of symbiosis between F. scutaria larvae and Symbiodinium spp., by looking at the temporal and spatial infection dynamics of larvae challenged with diﬀerent symbiont types. The results show that speci- ﬁcity at the onset of symbiosis was mediated by rec- ognition events during the initial symbiont—host physical contact before phagocytosis, and by sub- sequent cellular events after the symbionts were incor- porated into host cells. Moreover, homologous and heterologous Symbiodinium sp. strains did not exhibit the same pattern of localization within larvae. When larvae were infected with homologous symbionts (C1f), 70% of the total acquired algae were found in the equatorial area of the larvae, between the oral and aboral ends, 21 h after inoculation. In contrast, no spatial diﬀerence in algal localization was observed in larvae infected with heterologous symbionts. This result provides evidence of functional diﬀerences among gastrodermal cells, during development of the larvae. The cells in the larval equator function as nutritive phagocytes, and also appear to function as a region of enhanced symbiont acquisition in F. scutaria. Introduction Mutualistic endosymbiosis between two organisms in- cludes a stage where the larger host ﬁrst acquires its smaller symbiont. Symbionts may be transmitted verti- cally where the symbiont is passed directly from host parent to oﬀspring, or horizontally where host sexual progeny must acquire symbionts from the environment (Douglas 1994). In horizontally transmitted associa- tions, the onset of symbiosis has been shown for several well-studied symbioses, such as squid/luminous bacteria and legume/nitrogen ﬁxing bacteria mutualisms, to in- clude a complex series of steps, recently referred to by Nyholm and McFall-Ngai (2004) as ‘‘the winnowing’’. These steps range from molecular signaling to inter- microbe ecological interactions, all of which are neces- sary but none of which are suﬃcient alone to establish a successful, speciﬁc symbiosis (e.g., Nyholm and McFall- Ngai 2004; Somers et al. 2004). Moreover, this series of speciﬁcity mechanisms occurs over a considerable time, starting minutes after ﬁrst contact and extending from weeks to months into host development and maturity. The onset of symbiosis in the highly diverse array of cnidarian/algal associations has been studied in a broad range of partnerships. These include symbioses between (1) the freshwater hydroid Hydra viridis and a chlorophyte (Muscatine et al. 1975, reviewed by Jolley and Smith 1980), (2) the scyphozoan Cassiopeia xamachana and the dinoﬂagellate Symbiodinium spp. Communicated by J.P. Grassle, New Brunswick M. Rodriguez-Lanetty (&) Æ Elisha M. Wood-Charlson V. M. Weis Department of Zoology, Oregon State University, Corvallis, OR, 97331, USA E-mail: rodrigm@science.oregonstate.edu Tel.: +1-541-7374358 Fax: +1-541-7370501 L. L. Hollingsworth Æ D. A. Krupp Department of Natural Sciences, Windward Community College, Kane’ohe, HI, 96744, USA Present address: M. Rodriguez-Lanetty Centre for Marine Studies, University of Queensland, 4072, St. Lucia, QLD, Australia Marine Biology (2006) 149: 713–719 DOI 10.1007/s00227-006-0272-x