Freshwater Biology (1995) 33, 485^95 Long-term dynamics of a freshwater sponge population ROBERTO PRONZATO Istituto di Zoologia dell'University, via Baibi 5, 1-16126 Genova, Italy RENATA MANCONI Istituto di Zoologia dell'Universita, via Muroni 25, 1-07100 Sassari, Italy SUMMARY 1. Quantitative field data were collected from a Sardinian population of the sponge Ephydafia fluviatilis from 1987 to 1993 in order to study its life style. 2. Growth of sponges was high during the annual cycle, and showed few changes in 6 years. Gemmulation was very rapid and highly efficient. Life spans of many specimens extended up to 5 years. 3. The population was destroyed by one flood during our study period. The persistence of gemmules on substrata suggested that dispersal was low in our study population. Recolonization started 7 months after the flood and re-established a new population, different from the previous one in terms of spatial structure and specimen size. Introduction The population structure and dynamics of sponges exhibit an extremely slow rate of change in stable environments such as marine rocky littoral habitats (Stone, 1970; Reiswig, 1973; Dayton, 1979; Wulff, 1991; Pansini & Pronzato, 1990). In habitats subject to notable seasonal variations of physieoehemieal parameters, cyclic phenomena are evident in the life style of sponges (Fell, 1974,1978; Elvin, 1976). Severely fluctu- ating habitats, such as lagoons or estuaries, are associ- ated with extinction and recolonization phenomena (Fell, Lewandrowsky & Lovice, 1979; Fell & Lewand- rowsky, 1981; Gaino etal., 1992). In freshwaters, Porifera have developed adaptive strategies, at morphological, physiological and behavi- oural levels. These strategies can be observed as repetitive processes in their cyclic activity, but quantit- ative data are few (Van de Vyver & Willenz, 1975; Frost & Williamson, 1980; Frost, de Nagy & Gilbert, 1982; Rader & Winget, 1985; Pronzato & Manconi, 1991;Corrieroe(fl/., 1994). Ephydatia fluviatilis (L., 1758) is the most common freshwater sponge; its plasticity is related to adaptabil- ity of its Ufe cycle to different climates. This has led © 1995 Blackwell Science Ltd to a world-wide distribution and to its presence in a wide variety of freshwater habitats (Penney & Racek, 1968; Pronzato & Manconi, 1994). The aim of this paper is to describe the following traits of E. fluviatilis: (i) the persistence of sponges and gemmules on substrata, (ii) the speed of germination, growth and gemmulation, (iii) the long-term stability of life-cycle rhythms, (iv) the population structure and its development, before and after a catastrophic event. Study area and methods The population of sponges from Rio Bunnari (north- west Sardinia, 40M2'N, 8°34'E) was studied from 1987 to 1993 as a part of a wider investigation of spongillids on Mediterranean islands (Manconi & Pronzato, 1994). At the sampling site, the stream (width 2 m) had a rocky bed that was completely covered with substrata suitable for sponges, such as pebbles, cobbles, and boulders (Fig. 1); sand, silt and organic detritus were also present. Flow rate was quite regular through the year (0.2-0.5 m s''), the stream being supplied by perennial springs and an artificial lake. Depth ranged 485