Trends on the distribution of ciliates in the open Pacific Ocean

The distribution of planktonic ciliates, aloricate (naked) and loricate (tintinnid) was investigated in the open waters of the Oyashio and Kuroshio Currents, Philippine, Sulu, Celebes and South China Seas, and the western and central equatorial Pacific. The abundance of nauplii and post-naupliar copepods as potential predators was estimated. In average, the tintinnids represented 10-20% of the abundance of aloricate ciliates (50-200 cells L−1). One hundred and two species of 37 genera of tintinnids were recorded. As a general trend, the highest species richness was found in moderate oligotrophic waters. Photographic records of some taxa of interest and unidentified specimens were reported. In the subarctic waters of the Oyashio Current, a few tintinnid species showed high abundance fluctuations that may be controlled by the copepods. During the summer the species of Parafavella with longer loricae predominated in parallel to the increase of the copepodite abundance. In warm open waters, the success of a ciliate species could depend on its anti-grazing strategy. Eutintinnus apertus attached to a spine-bearing diatom was the most ubiquitous species. Tintinnids may be subjected to lower predation pressure than aloricate ciliates. The increase of the lorica length or the association with diatoms may be an anti-grazing strategy, which determines the success of one tintinnid taxon versus other congeneric species. Although the aloricate ciliates would be less affected than tintinnids by the reduction of food availability under oligotrophic conditions, the ciliate populations as a general trend seem to be controlled by the predators (top-down) rather than by the availability of food resources (bottom-up).