The Effects of Current and Cell Size on Epiphytism of Synedra parasitica var. Parasitica on Surirella robusta var. Splendida

Factors affecting the association of Synedra parasitica and Surirella robusta var. splendida in an epipelic habitat were investigated by manipulation of natural populations in the field and a quantitative assessment of the locale in which they co-existed. Scanning electron and light microscopy were used to assess the habit of the attachment to Surirella by Synedra. Synedra attaches firmly, but elastically, to the girdle band. Specimens were observed individually, in zig-zag arrays, or in extended rosettes. Relative to other habitats, epiphytism was most prevalent in swift current with flocculent substrate. Synedra epiphytes per Surirella individuals decreased from 6 cells/Surirella to 2 cells/ Surirella in populations kept isolated from current in the field. Numbers of Surirella cells epiphytized and Synedra cells/Surirella in current-influenced sites were almost double those of sites not influenced by current. Synedra loads were correlated with Surirella size (P < 0.05, r = 0.370), indicating that size may affect Synedra densities on Surirella. However, populations of S. robusta var. splendida did not differ significantly with respect to size at surveyed sites (P > 0.50). Surirella densities were greatest in quiet sites. Synedra parasitica was otherwise absent in habitats where Surirella robusta var. splendida was not found (e.g., plankton, submerged macrophytes). Synedra was able to survive in the epipelic habitat by way of its association with Surirella, which is well-adapted to an epipelic existence.