Marine Biology (2003) 142: 1055–1063 DOI 10.1007/s00227-003-1035-6 T. Yacobovitch Æ V. M. Weis Æ Y. Benayahu Development and survivorship of zooxanthellate and azooxanthellate primary polyps of the soft coral Heteroxenia fuscescens: laboratory and field comparisons Received: 26 February 2002 / Accepted: 17 January 2003 / Published online: 14 March 2003 Ó Springer-Verlag 2003 Abstract The zooxanthellate Red Sea soft coral Hete- other seasons, thus strengthening existing evidence that roxenia fuscescens releases aposymbiotic planulae nearly the summer is the most favorable season for the breeding all year round, with higher numbers in the summer than and development of H. fuscescens sexual progeny. in other seasons. After metamorphosis, primary polyps Comparisons between laboratory-reared zooxanthellate become infected with symbiotic dinoflagellates (zoo- primary polyps and naturally settled animals in the field xanthellae), derived from the ambient seawater. This showed a higher survivorship in the laboratory, possibly study compares aspects of development and survivor- due to unfavorable conditions commonly occurring in ship of metamorphic stages of H. fuscescens in relation the field. Our findings indicate that H. fuscescens rep- to onset of infection by the algal symbionts in the lab- resents an appropriate model system for study of ac- oratory and the field. We revealed no distinct differences quisition of algae by aposymbiotic offspring and the in the timing and sequence of morphogenetic events impact of the process on their development and surviv- during metamorphosis between zooxanthellate and orship. azooxanthellate primary polyps in the laboratory. In the field, the polyps exhibited higher developmental syn- chronization during the first days after metamorphosis Introduction compared to the laboratory-reared ones, probably as result of a rapid and synchronized onset of metamor- Various studies have addressed the question of the de- phosis in the former. Later, there were no distinct velopmental stage at which sexually produced offspring differences in these parameters between the laboratory- of symbiotic hosts acquire their dinoflagellate algal reared zooxanthellate polyps and those in the field. symbionts (zooxanthellae) (e.g. Muscatine 1974; Fitt Although the symbiotic state did not appear to affect 1984; Trench 1987; Douglas 1998). Trench (1987) sug- developmental sequence or timing, it did increase host gested two modes of symbiont acquisition by sexual survivorship. In the laboratory, the survivorship of progeny: direct transmission via the eggs or brooded zooxanthellate primary polyps was significantly higher larvae (also known as vertical or maternal inheritance) compared to azooxanthellate ones. Therefore, the sym- or by post-larval stages from the ambient environment biotic state appears to be ultimately important in overall (also known also as the horizontal or open system). survivorship. The survivorship of zooxanthellate pri- Direct transmission of symbionts has been documented mary polyps developed from planulae released during in cnidarian species: classes Hydrozoa (Campbell 1990), the summer months was higher compared to those from Scyphozoa (Montgomery and Kremer 1995) and An- thozoa (Glynn et al. 1991; Benayahu et al. 1992; Hirose et al. 2000). Acquisition from the ambient environment Communicated by O. Kinne, Oldendorf/Luhe is far more common than maternal inheritance, and occurs in stony corals (Harrison and Wallace 1990; & T. Yacobovitch Æ Y. Benayahu ( ) Shlesinger and Loya 1991; Schwarz et al. 1999) and soft Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, 69978 Tel Aviv, Israel corals (Kinzie 1974; Benayahu et al. 1989a; Achituv et al. 1992; Benayahu 1997; Coffroth et al. 2001). Eggs E-mail: [email protected] Tel.: +972-3-6409090 broadcast by the vast majority of species of both coral Fax: +972-3-6409403 groups lack algal symbionts upon release (azooxanthel- late), whereas brooded planulae are mostly symbiotic V. M. Weis Department of Zoology, Oregon State University, (zooxanthellate) (Babcock and Heyward 1986; Harrison 3029 Cordley Hall, Corvallis, OR 97331, USA and Wallace 1990; Benayahu et al. 1992; Benayahu and 1056 Schleyer 1998). Consequently, horizontal acquisition of determine whether algal acquisition is required for nor- symbionts is far more common than vertical acquisition. mal early development of the juvenile host. Examination Open and closed symbioses offer different evolution- of these processes provides a point of departure for ary scenarios in regard to their influence on zooxanthella study of the entire complex process of symbiosis onset. diversity (Rowan 1998). In contrast to closed symbioses, an open system should facilitate polymorphic symbioses by providing members of a host species with many op- Materials and methods portunities to obtain different symbionts (Rowan and Knowlton 1995; Douglas 1998). Recently, it has been Collection of planulae shown that initial zooxanthella acquisition by newly Mature colonies of Heteroxenia fuscescens (>36 mm in diameter, settled polyps of gorgonians was non-selective and did see Achituv and Benayahu 1990) were sampled randomly from the not reflect the adult host specificity, which appeared over coral reef (3–8 m deep) across from the Interuniversity Institute of time (Coffroth et al. 2001). Experimental manipulations Eilat (IUI) over a 22-month period (November 1998–August 2000). In the laboratory, these colonies were placed in containers with of cnidarian sexual offspring in the field and the labo- )1 ratory are still needed in order to answer open questions running seawater, at a flow rate of 2 l min . Prior to sunset, each colony was transferred to a separate aerated aquarium, and ex- concerning the significance of the mode of symbiont amined the following morning for the presence of planulae (see also acquisition in the processes in which the coral host and Ben-David-Zaslow and Benayahu 1996). For each colony, released its algal partners are engaged. In choosing a system to planulae were counted and placed into 250-ml PVC containers investigate these aspects, the best strategy is undoubt- filled with Millipore-filtered (0.22 lm) seawater (FSW), to avoid bacterial contamination. Planulation was monitored for three to edly to study an association with an open system (sensu five successive nights using 10–15 colonies per sampling month. Trench 1987), in which both azooxanthellate and zoo- The average number of planulae (±SD) released per colony per xanthellate early ontogenetic stages exist. Therefore, night in the different months was calculated. The calculation of experiments to date on the dynamics of initiation of each monthly average was based on colonies that produced planulae during at least one of the successive monitoring nights, algal-cnidarian symbioses have been largely confined to including zero results from these colonies when applicable (i.e. associations displaying an open system of symbiont n-value for each month=number of colonies that produced plan- transmission (see Montgomery and Kremer 1995; Sch- ulae at least one night each month·number of monitoring nights). warz et al. 1999; Weis et al. 2001). The planulae and their respective parental colonies were trans- The intriguing questions related to onset of infection ported to Tel Aviv for further experiments. Availability of planulae and the resulting primary polyps throughout the study period of early developmental stages of a cnidarian host by dictated the number of replications of each experiment, in both the zooxanthellae led us to launch a comprehensive study of laboratory and the field (see below). the cascade of events that occur during this process. For this purpose, we chose the early developmental stages of the zooxanthellate Red Sea soft coral Heteroxenia Development of primary polyps in the laboratory and in the field fuscescens. The main nutritional source of this species is from uptake of dissolved organic material (DOM) and We conducted a series of experiments to examine several aspects of utilization of its symbiotic algal photosynthesis products development in metamorphic stages of H. fuscescens. For this (Schlichter 1982). H. fuscescens is a hermaphroditic soft purpose, we reared zooxanthellate and azooxanthellate primary polyps in the laboratory and zooxanthellate primary polyps in the coral that broods planulae (Benayahu et al. 1989a; field. A scoring system was developed, corresponding to the dif- Benayahu 1991). These planulae lack zooxanthellae, ferent morphogenetic stages of the primary polyps, which were also unlike brooded planulae of other soft corals, which al- visualized by scanning electron microscope (see below). The polyps ready harbor symbiotic algae upon release (Benayahu were monitored under a dissecting microscope, and the timing and sequence of morphogenetic events of the primary polyps were et al. 1989b, 1992; Benayahu and Schleyer 1998). The routinely recorded and scored accordingly. This enabled us to release of H. fuscescens planulae can be observed nearly compare the morphogenetic events of the three types of polyps at all year round, with highest rates occurring in summer different ages. The number of surviving primary polyps at different (Ben-David-Zaslow et al. 1999). In the field, these ages was quantified as the ratio between the number of counted planulae settled 5–8 h after release and then underwent primary polyps and the initial number of planulae for each of the experiments detailed below. metamorphosis into