REPORT Zohar Pasternak Æ Bernd Blasius Æ Avigdor Abelson Yair Achituv Host-finding behaviour and navigation capabilities of symbiotic zooxanthellae Received: 28 August 2005 / Accepted: 12 December 2005 Ó Springer-Verlag 2006 Abstract Past studies have shown that the initiation of symbiosis between the Red-Sea soft coral Heteroxenia fuscescens and its symbiotic dinoflagellates occurs due to the chemical attraction of the motile algal cells to sub- stances emanating from the coral polyps. However, the resulting swimming patterns of zooxanthellae have not been previously studied. This work examined algal swimming behaviour, host location and navigation capabilities under four conditions: (1) still water, (2) in still water with waterborne host attractants, (3) in flowing water, and (4) in flow with host attractants. Algae were capable of actively and effectively locating their host in still water as well as in flow. When in water containing host attractants, swimming became slower, motion patterns straighter and the direction of motion was mainly towards the host—even if this meant advancing upstream against flow velocities of up to 0.5 mm s 1 . Coral-algae encounter probability decreased the further downstream of the host algae were located, probably due to diffusion of the chemical signal. The results show how the chemoreceptive zooxanthellae modify their swimming pattern, direction, velocity, circuity and turning rate to accommodate efficient navigation in changing environmental conditions. Keywords Symbiosis Æ Algal acquisition Æ Coral reef Æ Chemotaxis Æ Flow Introduction Zooxanthellae—symbiotic dinoflagellates of the genus Symbiodinium—inhabit a wide range of aquatic hosts (Trench 1993), most notably anthozoan cnidarians. The photosynthetic zooxanthellae provide the host with en- ergy rich food, while the host provides inorganic carbon and other nutrients to the algae (Muscatine 1990); this symbiosis is essential for the existence and well being of oligotrophic tropical reef ecosystems (Levinton 1995). Acquisition of dinoflagellates in marine cnidarians, i.e., the initial infection event during which a new sexually produced offspring acquires its first complement of zooxanthellae, occurs via two main pathways: vertically (a ‘‘closed system’’), in which new coral polyps are provided with algal cells by maternal inheritance, or horizontally (an ‘‘open system’’), in which they acquire algal cells from the ambient environment (Trench 1987; Douglas 1994). Open systems are found in 85% of cnidarian species hosting zooxanthellae (Harrison and Wallace 1990; Richmond 1997), the main advantage presumably being the ability to form partnerships that are best adapted to environmental conditions in the host’s eventual habitat (Buddemeier and Fautin 1993). Algal acquisition from the environment may take place at the embryonic stage (e.g., the scyphozoan Linuche unguiculata; Montgomery and Kremer 1995), the plan- ula larval stage (e.g., the scleractinian coral Fungia scu- taria; Krupp 1983) or the metamorphosed polyp stage (e.g., the soft coral Heteroxenia fuscescens; Benayahu et al. 1989a). Several studies have established that azooxanthellate juvenile coral polyps (and adult anemones) are rapidly colonized by zooxanthellae in both the field and the laboratory, even when the algal cells are in low ambient concentration (Schwarz et al. 1999; Coffroth et al. 2001; Kinzie et al. 2001; Weis et al. 2001). H. fuscescens, the Communicated by Biology Editor H.R. Lasker Z. Pasternak Æ Y. Achituv Faculty of Life Sciences, Bar-Ilan University, 52900 Ramat-Gan, Israel A. Abelson Æ Z. Pasternak (&) Institute for Nature Conservation Research, Tel-Aviv University, 69978 Tel-Aviv, Israel E-mail: zpast@yahoo.com Tel.: +972-3-5318570 Fax: +972-3-5351824 B. Blasius Institute of Physics, Potsdam University, Am Neuen Palais 10, 14469 Potsdam, Germany Coral Reefs (2006) DOI 10.1007/s00338-005-0085-2