*Article* **Latitudinal Differentiation among Modern Planktonic Foraminiferal Populations of Central Mediterranean: Species–Specific Distribution Patterns and Size Variability**

**George Kontakiotis 1,\* , Eirini Efstathiou <sup>1</sup> , Stergios D. Zarkogiannis <sup>2</sup> , Evangelia Besiou <sup>1</sup> and Assimina Antonarakou <sup>2</sup>**



**Citation:** Kontakiotis, G.; Efstathiou, E.; Zarkogiannis, S.D.; Besiou, E.; Antonarakou, A. Latitudinal Differentiation among Modern Planktonic Foraminiferal Populations of Central Mediterranean: Species –Specific Distribution Patterns and Size Variability. *J. Mar. Sci. Eng.* **2021**, *9*, 551. https://doi.org/10.3390/ jmse9050551

Academic Editor: Wonho Yih

Received: 28 April 2021 Accepted: 18 May 2021 Published: 20 May 2021

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**Abstract:** Studies of the spatial distribution and size of modern planktonic foraminifera are still lacking in the Mediterranean Sea. In this study, 17 core-top sediments collected from a north-south transect along the central Mediterranean have been analyzed for planktonic foraminiferal content, in terms of their distributional pattern and intraspecific size variability. Among the analyzed planktonic foraminiferal species, *Globigerina bulloides* and *Globigerinoides ruber* (w) were the most abundant, presenting an antagonistic behavior and an overall decreasing trend in their average size values from Adriatic to Ionian sub-basins. Intraspecific differences have been also documented for *G. ruber* (w), with the dominant sensu stricto morphotype to present generally higher frequencies and more constant shell sizes than sensu lato. The greater size variability of the latter is possibly related to its adaptation in particular hydrographic conditions based on its depth habitat preference and ecological characteristics to reach the (sub)optimum growth conditions. The rest of the species occur in minor percentages and show on average 11% increase with decreasing latitude characterized by distinct species-specific size variations along the transect. Our results show that the relationship between planktonic foraminifera shell size and abundance or sea surface temperature are either absent or weaker than previously reported for other regions and that in central Mediterranean assemblages' size may be mainly related to nutrient availability. Besides the environmental parameters (sea surface temperature, primary productivity, water depth, stratification), the possible hidden cryptic diversity, still lingers to be consistently determined, could give a better understanding of the geographic and morphological differentiation within the Mediterranean planktonic populations.

**Keywords:** planktonic foraminiferal biogeography; surface sediments; morphometrics; shell size; environmental biomonitoring; ecological optimum conditions; primary productivity; depth habitat preference; cryptic speciation; central Mediterranean hydrodynamics
