Influence of prey competition with Noctiluca scintillans on the grazing rate of marine oligotrich ciliate Strombidium sp.

Marine oligotrich ciliates play an important role in transferring carbon from the microbial loop to the traditional food web. To the authors’ knowledge, the grazing rates of oligotrich ciliates were observed under single species of predator on each prey under different concentrations. The competitive relationship between two predators on the same prey is still unknown. Here, oligotrich ciliate Strombidium sp. (Strom) and Noctiluca scintillans (NS) were provided as predators on Tetraselmis chui (Ply) and 7 different combinations of culture experiments, including 1) Strom + Ply, 2) NS + Ply, 3) Ply, 4) Strom, 5) NS, 6) Strom + NS and 7) Strom + NS + Ply, were proceeded, in order to understand if i) Strom and NS could eat each other, ii) their growth rates would be affected by each other, and iii) the behavior of prey competition should result in higher demand for food, i.e., higher grazing rate.

The growth rates of oligotrich ciliates Strombidium sp. showed no significant difference in 1) Strom + Ply and Strom + NS + Ply, indicating that the growth rate of Strom was not affected under the competition of NS with prey supply and in 2) Strom and Strom + NS, demonstrating that even without prey supply there was no predatory behavior of NS on Strom, vice versa, or Strom cannot get enough energy from NS for higher growth rates.

The growth rate of Noctiluca scintillans in combination of 1) Strom + NS was lower than that in NS (0.009 vs. 0.017), indicating that NS was attacked by Strom without any food supply and in combination of 2) NS + Ply and Strom + NS + Ply were different (0.077 vs. 0.063), implying that NS may attack NS or compete for the same prey. However, there was no significant difference between the growth rates.

The specific grazing rate in Strom + NS + Ply (0.56 d^-1) was larger than the sum in Strom + Ply and NS + Ply, where the former was 0.25 d^-1 and the latter was 0.11 d^-1, suggesting the higher demand for prey under multiple predator culture, i.e., the grazing rate of Strom on Ply increased apparently from 87 cells Strom^-1 d^-1 to 112 cells Strom^-1 d^-1 with the existence of NS.