Reference: Biol. Hull. 196: 70-79. (February, 1999) Late Larval Development and Onset of Symbiosis in the Scleractinian Coral Fungia scutaria JODI A. SCHWARZ 1 , DAVE A. KRUPP 2 , AND VIRGINIA M. WEIS 1 '* 1 Department of Zoologv, Oregon State University, Comillis. Oregon 97331. and 2 Department of Natural Sciences, Windward Community College, Kaneohe, Hawaii 96744 Abstract. Many corals that harbor symbiotic algae (zoo-xanthellae) produce offspring that initially lack zooxanthel-lae. This study examined late larval development and the acquisition of zooxanthellae in the scleractinian coral Fun-gin scutaria, which produces planula larvae that lack zoo-xanthellae. Larvae reared under laboratory conditions de-veloped the ability to feed 3 days after fertilization; feeding behavior was stimulated by homogenized Artemia. Larvae began to settle and metamorphose 5 days after fertilization. In laboratory experiments, larvae acquired experimentally added zooxanthellae by ingesting them while feeding. Zooxanthellae entered the gastric cavity and were phagocy-tosed by endodermal cells. As early as 1 h after feeding, zooxanthellae were observed in both endodermal and ecto-dermal cells. Larvae were able to form an association with three genetically distinct strains of zooxanthellae. Both zooxanthellate and azooxanthellate larvae underwent meta-morphosis, and azooxanthellate polyps were able to acquire zooxanthellae from the environment. Preliminary evidence suggests that the onset of symbiosis may influence larval development; in one study symbiotic larvae settled earlier than aposymbiotic larvae. Protein profiles of eggs and larvae throughout development revealed a putative yolk protein doublet that was abundant in eggs and 1 -day-old larvae and was absent by day 6. This study is the first to examine the onset of symbiosis between a motile cnidarian host and its algal symbiont. Introduction The life history of symbiotic associations between organ-isms necessarily includes a stage during which a new gen-Received 30 April 1998; accepted 25 September 1998. * To whom correspondence should be addressed. E-mail: weisv@bcc. orst.edu eration of hosts first acquires its symbionts (Douglas, 1994). Symbionts may be acquired either vertically, whereby the symbiont is transmitted directly from parent to offspring, or horizontally, whereby the offspring must acquire symbionts from the environment (Trench, 1987). Vertical transmission ensures that offspring are provided with a complement of symbionts, whereas horizontal transmission is more uncer-tain; environmental variability may prevent contact between symbiont and host, resulting in the failure of the host to become infected by its symbiont. Many members of the phylum Cnidaria (such as corals, sea anemones, and jellyfish) harbor intracellular photosyn-thetic dinorlagellates (Symbiodinium spp.) in a mutually beneficial symbiotic association. The dinoflagellates, also known as zooxanthellae, contribute to host nutrition by translocating photosynthetically fixed carbon, while the hosts provide the zooxanthellae with nutrients and a pro-tected, high-light environment. Many cnidarian host species are obligately symbiotic with zooxanthellae, thus vertical transmission might be predicted to be the dominant mode of symbiont transmission. However, this is not the case, at least in scleractinian corals. Most scleractinian coral species spawn gametes that are azooxanthellate (i.e.. lack zooxan-thellae) (Fadlallah. 1983; Babcock and Heyward, 1986; Harrison and Wallace, 1990; Richmond and Hunter, 1990; Richmond, 1997). The gametes are fertilized within the water column and develop into azooxanthellate planula larvae that must acquire zooxanthellae at some stage of their development (Trench, 1987). Offsetting the uncertainty of infection via horizontal transmission is the benefit that acquisition of symbionts from the environment might allow the host to form an association with genetically distinct symbionts that are adapted to local conditions. Rowan and Knowlton (1995) found that the corals Montastraea faveolata and M. annn-70
BioStor
