Reference: Biol. Bull. 173: 367-376. (October, 1987) THE GREEN HYDRA SYMBIOSIS: ANALYSIS OF A FIELD POPULATION G. MULLER-PARKER* AND R. L. PARDY School of Biological Sciences. University of Nebraska, Lincoln. Nebraska 68588-0118 ABSTRACT Green hydra were abundant on the alga I 'aucheria taylorii in a shallow woodland stream near Lincoln, Nebraska, from March to June 1985. Green hydra were also found in low numbers throughout the winter. The algal-animal biomass characteris-tics of field populations of green hydra are compared to those of cultures established from the field populations and maintained under defined laboratory conditions. Al-though of similar protein biomass, freshly collected hydra contained greater numbers of symbiotic algae than did cultured hydra. Algae in field hydra were larger and con-tained more chlorophyll than algae in cultured hydra. Field populations of green hydra were highly productive; 16 ;ug C-h~' -mg hydra protein' 1 were fixed by the endosymbiotic algae at an irradiance of 28 ^E m~ 2 s" ' . INTRODUCTION The green hydra-Chlorella association has been intensively studied for two de-cades. The association involves C/ilore/la-\ikQ algae living within the digestive cells of the freshwater polyp Hydra viridissima. It is one of a group of algal-invertebrate associations called phycozoans (Pardy, 1983) to emphasize their algal-animal com-posite nature. Much is known about the metabolic interdependency of the bionts, the recognition processes whereby hydra acquire algae, and some of the regulatory processes which stabilize the association. Comprehensive reviews concerning the as-sociation may be found in Cook ( 1 980, 1981,1983). The majority of experimental studies have used laboratory cultures of green hy-dra. These hydra have been cultured under defined conditions with respect to temper-ature, light intensity, and feeding and maintenance schedules. Such stringent culture methods yield populations of green hydra of uniform size and age distributions, with stable densities of symbiotic algae. Although these laboratory cultures of green hydra are useful for certain experimental studies, they may bear little resemblance to green hydra living in ponds and streams. While a variety of ecological studies (Welch and Loomis, 1924; Miller, 1936; Bryden, 1952; Carrick, 1956; Cuker and Mozley, 1981; Ribi el al, 1985) have attempted to describe the seasonal distribution and abundance of non-symbiotic hydra, similar studies of green hydra are lacking. Although green hydra have been collected from a variety of habitats [standing water (Whitney, 1907; Lashley, 1915), a swamp (Carrick, 1956), a river (McAuley, 1984), and roadside ditches (Forrest, 1959)], none of these studies have described the algal-animal bio-mass characteristics of freshly collected hydra. In this paper we analyze the biomass parameters of symbiotic algae in field popu-lations of green hydra, and compare these parameters to those of cultures established Received 13 April 1987; accepted 22 July 1987. * Present address: University of Maryland, C.E.E.S., Chesapeake Biological Laboratory, Solomons, Maryland 20688-0038. 367
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