Deep Diapause and the Influence of Low Temperature on the Hatching of the Gemmules of Spongilla lacustris (L.) and Eunapius fragilis (Leidy)

Abstract

Yellow, thin-capsuled gemmules of Spongilla lacustris undergo deep diapause. This is the first demonstration of deep diapause in gemmules of this species. Exposure to low temperature promotes emergence from diapause. Evidence suggesting that low temperature may be a necessary condition for breaking deep diapause in sponge gemmules is strongest for Eunapius fragilis. Gemmules stored at 30 C for 3 weeks and then maintained continuously at 20? C for 27 weeks did not hatch, whereas gemmules exposed to 3? C for 18 weeks before they were placed at 20? C hatched within 4 days. Finally, green, thin-capsuled gemmules of Spongilla lacustris and gemmules of Eunapius fragilis eventually hatch at 3? C, but only after a period of several months and only under uncrowded conditions. Additional key words: sponges, dormancy Many freshwater and some estuarine/marine sponges produce dormant propagules called gemmules. Each gemmule consists of a mass of cells enclosed by a collagenous capsule that in many cases contains siliceous spicules. Frequently gemmules are the only form of the sponge that persists during adverse environmental conditions. With the return of favorable conditions, the gemmules hatch and give rise to active sponges (Simpson & Fell 1974; Simpson 1984; Fell 1993; Pronzato & Manconi 1994). Gemmule dormancy is of two general types, quiescence and diapause. Quiescence is imposed by unfavorable environmental conditions, including low temperature, and ends when favorable conditions recur (Rasmont 1954, 1962; Mukai 1980). On the other hand, diapause is imposed by an endogenous mechanism even in the presence of favorable environmental conditions. The breaking of diapause often requires or is at least accelerated by exposure to low temperature and may be followed by a period of gemmule quiescence (Rasmont 1955, 1962; Mukai 1980; Fell 1987a). Although there have been only few studies of sponge dormancy, it appears that the depth of gemmule diapause is highly variable both among and within species. Gemmules undergoing shallow diapause hatch rapidly at warm temperatures (around 20? C) after a few days exposure to low temperature (around 5? C) and may even hatch more slowly at warm temperatures without exposure to cold, whereas gemmules in deep diapause require one or more months of cold treatment before they are capable of hatching at warm temperatures (Fell 1993). Low temperature was believed to keep nondiapausing gemmules and gemmules that have emerged from diapause in a quiescent state more or less indefinitely (Rasmont 1963; Fell 1987a). This may be true for gemmules stored under crowded conditions; however, the gemmules of some sponges do hatch eventually at low temperature under field and/or laboratory conditions (Simpson & Gilbert 1973; Barbeau et al. 1989; Fell 1994). The gemmules of Spongilla lacustris are of two types. In thick-capsuled gemmules, the brown, threelayered capsule contains spicules and a single micropyle. The cells possess few, if any, symbiotic zoochlorellae and appear white. In thin-capsuled gemmules, the single-layered capsule features multiple micropyles and no spicules (Gilbert & Simpson 1976). These gemmules range in color from bright green to golden yellow to white, reflecting both the abundance of green symbiotic algae (Williamson 1979) and the amount of yellow pigment in the capsule. Thick-capsuled gemmules of Spongilla lascustris are produced in small numbers by a few individuals during the summer, in the basal regions of encrusting specimens or along the axes of thick branches. Thin-capsuled gemmules are formed in large numbers throughout the body of the sponge during the fall; and all of the individuals within a population gemmulate at this time (Simpson & Gilbert 1973; Gilbert & Simpson 1976). Some evidence suggests that the two gemmule types of Spongilla lacustris may undergo different types of dormancy. Whereas green, thin-capsuled gemmules are quiescent, This content downloaded from 157.55.39.162 on Thu, 11 Aug 2016 05:21:04 UTC All use subject to http://about.jstor.org/terms