Factors Controlling the Abundance and Size Distribution of the Phototrophic Ciliate Myrionecta rubra in Open Waters of the North Atlantic DAVID J. S. MONTAGNES, a JOHN ALLEN, b LOUISE BROWN, b,1 CELIA BULIT, c RUSSELL DAVIDSON, b CARLOS DI ´ AZ-A ´ VALOS, d SOPHIE FIELDING, b MIKE HEATH, e NAOMI P. HOLLIDAY, b JENS RASMUSSEN, e RICHARD SANDERS, b JOANNA J. WANIEK b,2 and DAVID WILSON a a School of Biological Sciences, University of Liverpool, Biosciences Building, Crown Street, Liverpool L69 7ZB, United Kingdom, and b National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, United Kingdom, and c Departamento El Hombre y su Ambiente, Universidad Auto´noma Metropolitana-Xochimilco, Calzada del Hueso 1100, 04960 Me´xico DF, Me´xico, and d Instituto de Investigaciones en Matema´ticas Aplicadas y Sistemas, Universidad Nacional Auto´noma de Me´xico, Apartado Postal 20-726, 01000 Me´xico DF, Me´xico, and e FRS Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen, AB11 9BD, United Kingdom ABSTRACT. Myrionecta rubra, a ubiquitous planktonic ciliate, has received much attention due to its wide distribution, occurrence as a red tide organism, and unusual cryptophyte endosymbiont. Although well studied in coastal waters, M. rubra is poorly examined in the open ocean. In the Irminger Basin, North Atlantic, the abundance of M. rubra was 0–5 cells/ml, which is low compared with that found in coastal areas. Distinct patchiness (100 km) was revealed by geostatistical analysis. Multiple regression indicated there was little rela- tionship between M. rubra abundance and a number of environmental factors, with the exception of temperature and phytoplankton biomass, which influenced abundance in the spring. We also improve on studies that indicate distinct size classes of M. rubra; we sta- tistically recognise four significantly distinct width classes (5–16, 12–23, 18–27, 21–33 mm), which decrease in abundance with increasing size. A multinomial logistic regression revealed the main variable correlated with this size distribution was ambient nitrate concentration. Finally, we propose a hypothesis for the distribution of sizes, involving nutrients, feeding, and dividing of the endosymbiont. Key Words. Cell size, Mesodinium rubrum, microzooplankton, plankton, protist distribution. M YRIONECTA rubra Jankowski, 1976 ( 5 Mesodinium rub- rum; Krainer and Foissner 1990) is a common, cosmopol- itan planktonic ciliate, in coastal inlets and shelf seas (Lindholm 1985). The ciliate hosts a cryptophyte endosymbiont (Hansen and Fenchel 2006; Hibberd 1977; Taylor, Blackbourn, and Blackbo- urn 1971) and acts essentially as a phototroph, with considerable migratory and photosynthetic abilities (Crawford 1989), but M. rubra also consumes cryptophytes and may sequester the prey chloroplast (Gustafson et al. 2000; Johnson and Stoecker 2005; Yih et al. 2004). This protist has generated considerable interest as it forms blooms and may be a link to upper trophic levels, through consumption by copepod nauplii (Irigoien et al. 2003) and fish larvae (Figueiredo, Nash, and Montagnes 2005). In fact M. rubra can represent close to 100% of the chlorophyll at times in some waters (Montagnes, Poulton, and Shammon 1999). Thus, it may be an important component of marine planktonic food webs. Although there are numerous studies on the abundance and distribution of M. rubra in coastal waters, there are no rigorous assessments of its distribution in open ocean waters, and to de- termine its ubiquitous nature and dispersal patterns, large scale distributional data are needed. Here, using geostatistical analysis, we examine the temporal and spatial distribution of M. rubra in one large region of the open North Atlantic, which is now under rigorous scrutiny (see ‘‘Materials and Methods’’). Then, taking advantage of the extensive physical and biological data set simul- taneously collected (e.g. depth; temperature; light; salinity; nutri- ents; flagellates o4 mm, as a measure of potential cryptophytes prey; total phytoplankton), we rigorously assess using multiple regression if the observed distributional patterns can be ac- counted for. We also examine an unusual autecological attribute of M. rubra that lacks rigorous analysis: this ciliate, which based on morphol- ogy we consider to be a single species, displays a large range of, apparently, distinctive sizes. Previous studies have split it into two to four size classes (e.g. Dale and Burkill 1982; Leakey, Burkill, and Sleigh 1993; Lindholm 1978, 1985; Montagnes et al. 1999). However, to date there has been no systematic evaluation of these classes nor are the factors that control their distribution known, although grazing pressure (Witek 1998), season (Montagnes and Lynn 1989), and depth (Rychert 2004) have been proposed to in- fluence size. Thus, in this work, we first provide a quantitative discrimination of the size classes, by applying peak-fitting tech- niques to our extensive data set. We then specifically examine the factors that potentially control size distribution (i.e. those men- tioned above), using multiple regression methods. Finally, we speculate on the causes of their occurrence, proposing a nutrient driven mechanism that results in our observed four distinct size classes. MATERIALS AND METHODS Hydrology and ecology of the Irminger Basin. Extensive research has recently been conducted in the Irminger Basin, be- tween Greenland and the Reykjanes Ridge, southwest of Iceland (Fig. 1). The physical oceanography of this region and its control on autotrophic protists have been examined in detail (Holliday et al. 2005; Waniek and Holliday 2005), and there has been an in- depth analysis of the spatial demography of the key zooplankter Calanus finmarchicus, which grazes protists, in the region (Heath et al. 2008). Thus, this large region of the North Atlantic is be- coming a recognised study area for general ecological processes associated with protists. In short, the circulation in this area is dominated by the Irminger Current, a branch of the North Atlantic current that en- ters the area in the east and flows north along the west side of the Reykjanes Ridge. The Irminger Current turns west as it reaches the Iceland Shelf, and then south again, joining the East Greenland Current. The water in the Central Irminger basin is fresher than the Irminger Current, and originates in the Labrador Sea. Corresponding Author: D. Montagnes, School of Biological Scien- ces, University of Liverpool, Biosciences Building, Crown Street, Liv- erpool L69 7ZB, UK—Telephone number: 144 151 795 4515; e-mail: dmontag@liv.ac.uk 1 Present address: School of Geography and Geosciences, University of St. Andrews, Irvine Building, St. Andrews, Fife KY16 9AL, Scotland. 2 Present address: Institut fu ¨r Ostseeforschung Warnemu ¨ 15, D18119 Rostock, Germany 457 J. Eukaryot. Microbiol., 55(5), 2008 pp. 457–465 r 2008 The Author(s) Journal compilation r 2008 by the International Society of Protistologists DOI: 10.1111/j.1550-7408.2008.00344.x