Gemmules of Ephydatia muelleri, exposed to anoxic conditions (nitrogen) in sealed glass ampules, generally showed no differences from oxic controls in hatchability or hatching rate over exposure periods of 1 to 112 days. Unlike controls, hatching was totally inhibited under anoxia at +20?C. Our results directly demonstrate that this species, and probably many freshwater sponges, can survive seasonal anoxia in the gemmule stage. High gemmule survival under nitrogen also permits them to be stored and transported without regard for external temperature. These results are consistent with existence of an ametabolic state under anoxia, but much more detailed work is needed to prove this conjecture. Additional key words: Spongillidae, respiration, cold-tolerance, dormancy, hatching Many aquatic invertebrates survive seasonally unfavorable environmental conditions by production of specialized dormant stages, usually encapsulated eggs or embryos (Hochachka & Guppy 1987). In the case of planktonic forms, these stages are usually negatively buoyant, accumulate on and in sediments, and thus form an egg or seed bank with potential long-term viability (Marcus et al. 1994). Immotile benthic invertebrates, such as sponges and bryozoans, typically use non-embryonic stages in a similar survival strategy. Here undifferentiated parental cells, rich in nutrient reserves, aggregate and are encapsulated in a continuous collagen capsule of varying complexity, constituting the gemmules of sponges and statoblasts of bryozoans. In the case of freshwater sponges, a major portion of parental biomass is thereby packaged in myriad, small, resistant gemmules attached more-or-less tightly to either the substratum, the parental skeleton, or both (Simpson & Fell 1974). Recent work on gemmule physiology has dealt with desiccation resistance (Fell 1987; Fell & Bazer 1990), cold hardiness (Fell & Fell 1987; Barbeau et al. 1989; Fell & Levasseur 1991; Ungemach et al. 1997), the characteristics of true diapause (Fell 1990, 1995), and the detailed energetics of the hatching process (Loomis et al. 1996). One component of gemmule survival frequently suggested, but not previously investigated, is an ability to withstand seasonal anoxia. Gemmules attached to exposed rock surfaces in ponds or pools of a Author for correspondence. E-mail: inhr@musicb.mcgill.ca b Present address: Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, P.E.I., C1A 4P3, Canada slow streams are often covered in layers of decaying leaf litter for most of winter. Under such circumstances they would likely be exposed to hypoxic and anoxic conditions for periods of weeks or months before spring flush-out. Gemmules of sponges that grow on macrophytes fall to the lake or pond floor during late fall die-back of their substrate and are thus exposed to the organic-rich, anoxic sediment surface for months (Frost et al. 1982; Frost 1991). In both cases, new sponges arise at the onset of spring mixing and reoxygenation of the microhabitat, and it seems likely that this repopulation is due to hatching of gemmules that have survived months of reduced oxygen availability. Repeated observations that gemmules collected for laboratory study survive for weeks or months in closed jars or vials with decaying parental tissues or in high densities in small amounts of water, which are almost certainly anoxic, under refrigeration at 0-4?C also suggest an ability to survive without oxygen (Fell 1993, 1994). Because over-winter survival under anoxia would appear to be of such importance to many sponge populations in nature, and because this phenomenon may provide important clues to the physiological processes controlling dormancy in this group, we decided to examine gemmule survival experimentally. We tested the gemmulehatching success of a locally abundant freshwater sponge in the absence of oxygen over an ecologically significant time scale.
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Open Access
Feb 1, 2004
D R Gray 
