*5.3. The Possible Role of a Hidden Biological Diversity in Mediterranean Assemblages*

Numerous studies have shown that many planktonic foraminiferal species display multiple genotypes, which are genetically independent but morphologically similar [142–144]. These cryptic species may also have different geographical distributions exhibiting possible morphological gradients with latitude [145] or longitude [107], occupy different niches [142], and display different relationships between abundance, size patterns, and environmental parameters [146–148]. Among the identified species, variable relationships have been discovered between their cryptic species and depth-, and/or environmental (SST, stratification, productivity)-related specializations whose abundance, shell size, and porosity vary with latitude. For instance, the size-abundance-environmental relationship has been documented for species with cryptic diversity, namely *O. universa* [131,146,149–151], *G. inflata* [152], *G. ruber* [144,146,153], *G. siphonifera* [148,154,155], *G. truncatulinoides* [156,157], *G. bulloides* [144,146,158,159] and *N. pachyderma* [160–164]. For most of them, both spinose and non-spinose species, it is premature to invoke cryptic diversity and ecophenotypy relationships due to the lack of large-scale inter-basin analysis within the Mediterranean Sea. However, some of them, especially those presenting the greatest variability in shell size ranges (*G. truncatulinoides, G. inflata,* and *O. universa*) could constitute good candidates for better explaining the observed geographic differentiation in the study area. Therefore, we interpret the documented latitudinal divergent trends among the fauna as the consequence of a hidden biological diversity with different mean sizes within these taxa, adapted to particular hydrographic conditions. We further note that there may be additional undiscovered genotypes in different lineages, especially in small and neglected species [165], possibly due to their relatively low number of specimens that have been surveyed so far.

#### *5.4. Potential Depth Preferred Regulating Mechanism*

We did not find any relationship between population shell size and abundance, caution thus against the concept of intraspecific size variation in parallel with population abundances, in accordance with previous studies [38,166]. In the present study, species richness which previously has been positively correlated with assemblage size [37] is found constant along our transect, indicating that the population size variation cannot be explained by assemblage changes. The relatively constant number of 13 species identified in the majority of the studied core-top samples is by ~40% lower than that of the low productivity oceanic gyre centers (>20 species; [167–169]), but similar to that of the marginal seas [22,89,170]. The slightly higher number in species richness reported here, compared to the number of 10 that characterizes the ultra-oligotrophic regions (e.g., Levantine Sea [20], Red Sea [171]), results mainly from the presence of cold- and deep-water dwellers that are associated with enhanced seasonal primary productivity [109,172]. The mechanism that could explain the expected ecological optimum pattern of simultaneous large sizes and high abundances involves higher feeding frequency (higher nutrient availability) leading to higher individual growth and finally to higher population growth [130]. However, it also implies that populations in different productive regimes have different generation times, with the more generations at optimum conditions to lead in higher abundance in the sediment, but relative to other populations of the same species, and not relative to the local assemblage [136]. In the local basin-wide abundance, nutrient availability is the same for all co-occurring species. Both satellite and in situ productivity data measured across the central Mediterranean reveal an increasing N-S oligotrophy gradient, mainly due to the limiting Chl-a concentrations from the Adriatic to Ionian Sea [115]. As would be expected, it is not followed by a relevant decrease in abundance for several species (e.g., *G. ruber, G. siphonifera, G. glutinata*), which means that the above mechanism cannot be applied here, since the primary production pattern of the region cannot explain sufficiently the observed decrease in mean foraminiferal size with latitude for the most abundant species either the opposite trend for the rest of the species in accordance also with their abundances. To understand the observed trends, we need to consider the effects of vertical instability on planktonic assemblages. The mesotrophic to oligotrophic character of the central Mediterranean basin is reflected by surface-dwelling oligotrophic and deeper eutrophic faunas, both of which are growing outside their ecologically optimum ranges (eastern and western Mediterranean respectively). In this setting, the overall assemblage is thus characterized by many species with almost overlapping shell sizes (Table 2). The species with a constant depth habitat such as *G. ruber* s.s, *T. quinqueloba,* and *T. trilobus* obviously are within optimal conditions and therefore present the lowest size variability. On the contrary, the species presenting a more variable depth habitat (e.g., *G. bulloides, G. ruber* s.l., *O. universa*) in their attempt to find plentiful food should be adapted to greater depths, representing sub-optimal ecological conditions for their survival.

#### **6. Conclusions**

In the present work, we studied the abundance and size distribution of recent planktonic foraminiferal populations derived by 17 surface sediment samples spanning from the mesotrophic Adriatic to the oligotrophic Ionian Sea. The fauna consists of subtropical species, mostly symbiont-bearing spinose species indicative of the mesotrophic-tooligotrophic nature of the study area. Even though the most abundant species *G. bulloides* and *G. ruber* (w) show an antagonistic distributional pattern, both present the opposite trend compared to the rest of the species with their average size decreasing with latitude. The recent fluctuations in the relative abundance along with morphospecies-specific shell size trends of the dominating species may reflect the current adjustment to ongoing sea surface warming and a decrease in primary productivity in certain depth levels along the N-S transect. Overall, our findings are consistent with previous studies, which suggest that sea surface temperature, depth habitat, and food availability are the main controlling factors for their latitudinal distributional differentiation in the central Mediterranean. Moreover, shell

size variation in planktonic foraminiferal species analyzed cannot be consistently predicted by the environment, with a hidden biological diversity with different mean sizes within these taxa to be possibly appeared, as an adaptation to ecological (sub)optimum conditions.

**Author Contributions:** Conceptualization, G.K., S.D.Z.; methodology, G.K., S.D.Z.; software, G.K., S.D.Z., E.E.; validation, G.K., S.D.Z., E.E.; formal analysis, G.K., S.D.Z., E.E.; investigation, G.K., S.D.Z., E.E., E.B., A.A.; resources, G.K.; data curation, G.K., S.D.Z., E.E., A.A.; writing—Original draft preparation, G.K.; writing—Review and editing, G.K., S.D.Z., E.E., E.B., A.A.; visualization, G.K., S.D.Z.; supervision, A.A.; project administration, G.K., A.A.; funding acquisition, G.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data that support the findings of this study are available from the corresponding author upon reasonable request.

**Acknowledgments:** The authors are grateful to Jurgen Möbius for providing part of the study material. The constructive and thorough reviews of two anonymous reviewers are warmly acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

