Fig. 3
Spongia fluviatilis Linnaeus, 1759: 1348.
Material examined (4 specimens, Table 1)
SWEDEN – Stockholm • 3 specs; Helgarsviken; 59.289° N, 18.7032° E; 2 Sep. 2017; Raquel Pereira leg.; UPSZMC 188229, 188231, 188233 • 1 spec.; Stavsnäs vinterhamn; 59.2889° N, 18.7062° E; 31 Dec. 2021; Raquel Pereira and Jesper Svedberg leg.; UPSZMC 189246
HABITUS. Colour from pale to brownish, encrusting (Fig. 3A).
SPICULAE. Megascleres and gemmuloscleres. Megascleres thin, smooth and acute oxeas (Fig. 3B), 250 μm (60–360 μm) long (Fig. 3C), 7.5 μm (2–15 μm) wide. Very few gemmuloscleres found in the collected specimens; birotulated, often a spine on the shaft, 20 µm long and 25 μm wide.
This species was only found in the Baltic, but was missing from the bona fide freshwater samples.
Five specimens from inland waters tentatively identified as E. fluviatilis (based on megasclers only, lacking gemmulae) was reassigned to Eunapius fragilis based on bar-coding sequences and a re-scrutiny of spiculae, so there is a possibility that some previously reported specimens of this (in Sweden) rare species may be misidentifications. It is quite possible, however, that it is present at greater depth at some of the sites, and thus missed in our study – the specimens from the Baltic were all from greater depths. We note, though, that Arndt (1932) only listed specimens from seven sites, all but one site outside (south of) the present study area, so the lack of observations may reflect a rare occurrence.