**1. Introduction**

Foraminifera are one of the most abundant and diverse heterotrophic protists in the oceans consisting of a major group of calcareous marine microplankton [1]. Due to their great abundance and their good fossilization potential, foraminifera are commonly used for bio-ecostratigraphic [2–7], paleoceanographic/paleoclimatic [8–19], and/or paleobiogeographic [20–24] studies. Particularly, planktonic foraminifera are the most frequently applied microfossil group in this manner because they have an excellent fossil record with global distribution, high abundance in sedimentary archives, and further present an enhanced sensitivity to varying sea surface conditions [25,26]. The geographic ranges and abundance of these organisms can also provide valuable quantitative and qualitative proxy

data through the estimation of several paleoceanographic and paleoclimatic indices for reconstructing paleoenvironments [27–29]. Both their abundance and shape-size-related changes are strongly related to surface ocean physico-chemical properties, most notably temperature, but also nutrient and oxygen availability, water column stratification, salinity, turbidity, and carbonate saturation [8,25,30–38]. Elucidation of the factors governing their distribution, and processes involved in their ontogenetic development are therefore essential for reconstructing paleoceanographic conditions.

Compared to the large body of knowledge on the taxonomy, physiology, and ecology of planktonic foraminiferal species, their spatial distribution and size response to hydroclimate remains poorly studied, especially for the oceanic sub-basins and/or marginal seas, which are often more responsive to paleoceanographic and paleoclimatic changes than global oceans. Moreover, as a significant constituent of microzooplankton, they are key components of pelagic food webs and the main predators of phytoplankton in (sub)tropical oligotrophic waters regulating thus the carbon flux in such environments [39–43]. Although at a global scale, their abundance follows the overall pattern of primary productivity (PP; [44]), at a regional scale this relationship is weaker possibly due to the omnivorous diet of the planktonic foraminifera in the marine food web, and phase shifts in the production of phytoplankton and zooplankton [1]. Species abundance varies with season, water mass, and water depth [36]. Both the highest horizontal and vertical separation of species are recorded from temperate to subtropical waters, owing to a wider diversity of meso-scale local hydrographic features and biotic variables, which make their distribution patchy on temporal and spatial scales [45,46].

Based on the species-specific ecological tolerance limits of modern planktonic foraminifera [1,47], the potential reduction in abundance is related to their departure from optimum conditions [48], and the subsequent size-related and/or weight-related changes on the planktonic fauna [49], since planktonic foraminifera must compensate for their greater shell weight to maintain buoyancy [50]. Changes in size can be attributed to different processes (volume or surface area dependent) linked to the ecology of each species [51–53]. Particularly, isometric features increase with the cube of linear dimensions, while surface area-related features increase merely with the square of linear dimensions. Consequently, surface area-dependent processes, such as feeding, respiration, and skeletal support of foraminifera have to keep up with volume and weight changes.

The main goal of this study is to evaluate and further quantify the spatial and sizerelated distribution of modern planktonic foraminifera from the central Mediterranean Sea. In our attempt to better understand the paleoecological and paleobiogeographical significance of size variability in planktonic foraminiferal fauna of Adriatic and Ionian basins, the relative importance of environmental factors controlling rates, magnitudes, spatial scale, and biotic change are also examined. This allows for the establishment of a detailed reference record for the central Mediterranean in terms of planktonic foraminiferal ecosystem functioning (including species composition, diversity, and size variability), providing, on the one hand, a solid eco-morphological microplankton response to environmental conditions for the study area, and facilitating, on the other hand, paleoceanographic correlations at a local (e.g., eastern Mediterranean—Aegean and Levantine basins; [24]) and a global, e.g., [37], scale.
