Anastasios Varkoulis

Hierarchical ecological filters structure occupancy, abundance, and body size in co-occurring marine grazers, but these responses are rarely evaluated jointly despite reflecting distinct ecological processes. Here, we applied a hypothesis-driven modeling framework to disentangle how environmental, habitat, and biotic drivers shape these response levels in 3 benthic grazers ( Rhyssoplax olivacea , Paracentrotus lividus , Arbacia lixula ) inhabiting shallow Mediterranean boulderfields. Using a 2-part modeling approach for occupancy and conditional abundance, coupled with quantile GAMs for size structure and joint species distribution models, we explicitly test how ecological filters operate across organizational levels. Occupancy patterns were primarily structured by shared environmental and assemblage-level processes, resulting in strong co-occurrence among species. In contrast, abundance responses diverged among species and were best explained by combined effects of habitat configuration, environmental gradients, and grazer context, indicating species-specific population regulation after establishment. Body size exhibited the most complex and heterogeneous responses, with strong quantile-specific effects revealing size-dependent filtering linked to habitat structure, disturbance, and biotic associations. Residual co-variation among species persisted after accounting for environmental predictors, suggesting shared microhabitat use and unmeasured ecological processes. Overall, our results demonstrate a clear hierarchy of ecological controls, with increasing complexity from occurrence to population density and demographic structure. Integrating multiple response variables reveals mechanisms that remain undetected when focusing on a single dimension, providing a more complete understanding of how benthic assemblages respond to environmental heterogeneity.

Extreme weather events (EWEs) are intensifying in frequency and severity, posing complex pulse and press disturbances to coastal ecosystems. While each disturbance type has been studied in isolation, empirical data on their combined effects under realistic field conditions are virtually absent. Here, we examine how compound pulse–press disturbances shape the successional dynamics of taxonomic and functional beta diversity in free-living marine nematodes, a group well suited for detecting community reassembly due to their short generation times, ecological sensitivity, and trait diversity. Following two EWEs in central Greece, we monitored eight sites at 0-, 6-, and 12-months post-disturbance. Beta diversity was partitioned into turnover and nestedness components using presence–absence and abundance data. Taxonomic beta diversity was shaped predominantly by turnover and responded strongly to disturbance intensity and time, whereas functional beta diversity showed subtle responses, likely reflecting high trait redundancy. Early stages reflected spatially structured legacies, mid-stages showed deterministic environmental filtering, and late stages revealed decoupling between taxonomic and functional trajectories. This study provides the first comprehensive empirical evidence of how compound pulse–press disturbances drive divergent successional dynamics in marine sediments, and highlights nematodes as powerful yet underutilized model organisms of post-disturbance reassembly.

The Mediterranean Sea is characterized as a biodiversity and climate change hotspot, highlighting the importance of monitoring and conserving a variety of habitats. Posidonia oceanica, Cymodocea nodosa and Pinna nobilis are ecosystem engineers, offering suitable environments for a plethora of species, but they are classified as threatened or endangered. Well-sorted fine sand also has ecological value but is susceptible to anthropogenic pressure regarding changes in organic matter. Here, we examined temporal patterns in meiofaunal community structure in these habitats, which are possibly linked to temporal changes in abiotic conditions. Sediments of the C. nodosa habitat and the P. nobilis habitat exhibited the highest meiofaunal abundance in both 0-5 and 5-10 cm sediment layers, while the bare sand habitat showed the highest values for all diversity indices for the upper sediment layer. Taxon richness was the highest in the C. nodosa habitat; no difference was observed when P. nobilis was included. Temporal changes in meiofaunal and nematode abundance were observed only for the P. nobilis and the C. nodosa habitats. Ordination (non-metric multidimensional scaling) and hierarchical clustering grouped P. nobilis and C. nodosa habitat samples for both sediment layers, while the P. oceanica habitat samples were separated from the bare sand habitat samples in the upper sediment layer. Organic carbon was the most important abiotic factor influencing meiofaunal communities in both sediment layers. This study highlights that meiofauna can successfully be used for habitat monitoring, as shown by differences in diversity and abundance among habitat types.

Rhyssoplax olivacea is a common mediterranean chiton that exhibits great geographic distribution characterized by variability in its abiotic parameters. Using morphometric measurements of the valves and radula, as well as the element composition of its tooth types from individuals sampled from five different regions across the Hellenic seas, intraspecific differences were examined. The relationship between the abiotic factors and elemental composition was also investigated. Hierarchical clustering on principal components (HCPC) was employed separately on the radular and valve characteristics to determine whether these traits can distinguish regions in the form of clusters, while canonical correspondence analysis (CCA) with ANOVA testing were used to examine the effect of temperature, depth and salinity on these features. Both datasets resulted in three clusters; however, investigation of the radula appeared to better distinguish populations among the examined regions, differentiating Kymi and Pagasitikos. The morphometrics of the valves distinguished the North Aegean Sea (Chalkidiki) from the other regions. The CCA reported that the depth, minimum temperature and average salinity influenced the elemental composition of the radular teeth, while the depth and maximum temperature explained variation regarding the valve morphometrics.

Rhyssoplax olivacea is the most common Mediterranean polyplacophoran species; however, no information exists regarding the functionality of its skeleton. The present study describes allometric trends related to its valves and radula and examines their chemical composition using specimens from the Aegean Sea, eastern Mediterranean Sea. Differences in valve width and thickness were found among all three valves; in particular, the intermediate valve had a significantly shorter length compared to the two terminal ones. The intermediate valve exhibited different trends for valve length to valve width and valve length to valve thickness compared to the terminal valves. However, all valve morphometrics to body length appeared to adhere to a shared trend. The radula to body length exhibited positive allometry. Regarding the elemental composition, all three valves appeared to have similar elemental compositions; however, the element concentrations in the radula differed with the tooth type. Iron was always the dominant element, with the highest values reported for the major lateral tooth (83% wt%). This study provides valuable insights into the different aspects of the skeleton of R. olivacea, enabling future research to focus on the skeletal functionality from evolutionary and ecological perspectives.

The reproductive biology of the invasive blue crab Callinectes sapidus was studied in the Thermaikos Gulf (Northwest Aegean Sea, Eastern Mediterranean). In the two-year survey, 5698 (2897♂/2801♀) crabs were caught with the use of fyke nets. Total sex ratio (♂/♀) reached equality (1.03:1). The female blue crab exhibited a protracted reproductive period. Mature and ovigerous females exhibit short migratory movements from estuarine and inshore waters, where the population mostly congregates (0–3 m), and move to slightly deeper waters (1–3 m) up to 9 m for spawning. A total of 340 ovigerous females were caught. Their number varied both spatially and temporally; they were observed for a 7-month period (April to October) with a clear peak in July–August and at a 3 m depth gradient corresponding to ≈60% of the total number of ovigerous females caught in both years. Size at first sexual maturity (CW₅₀) was estimated at 113.1 mm CW. Average fecundity was ≈790,000 eggs. Experimental trawling showed that inshore waters (<1 m) in the estuaries serve as nursery areas for juveniles. Defining the spatiotemporal and bathymetrical distribution of ovigerous females in any invaded coastal habitat could be considered key information for the implementation of a management policy for the species.

The reproductive biology of the invasive blue crab Callinectes sapidus was studied in the Thermaikos Gulf (Northwest Aegean Sea, Eastern Mediterranean). In the two-year survey 5,698 (2,897♂/ 2,801♀) crabs were caught with the use of fyke nets. Total sex ratio (♂/♀) reached equality (1.03:1). The female blue crab exhibited a protracted reproductive period. Mature and ovigerous females exhibit short migratory movements from estuarine and inshore waters, where the population mostly congregates (0-3 m), and move to slightly deeper waters (1-3 m) up to 9 m for spawning. A total of 340 ovigerous females were caught. Their number varied both spatially and temporally; they were observed for a 7-month period (April to October) with a clear peak in July - August and at a 3 m depth gradient corresponding to ≈60% of the total number of ovigerous females caught in both years. Size at first sexual maturity (CW50) was estimated at 113.1 mm CW. Average fecundity was ≈790,000 eggs. Experimental trawling showed that inshore waters (<1 m) in the estuaries serve as nursery areas for juveniles. Defining the spatiotemporal and bathymetrical distribution of ovigerous females in any invaded coastal habitat could be considered key information for the implementation of a management policy for the species.

Sea cucumbers have become a highly valuable fishery product, and therefore the determination of their proximate compositions and fatty acid profiles are useful indicators of their physiological status and nutritional value. Thus, various raw and processed products, such as raw, dried, frozen, boiled, and salted Holothuria polii and Holothuria tubulosa that were collected from the Aegean Sea were analyzed. Although there were some notable differences in the proximate and fatty acid profiles between the two species and among the various processing forms, both H. polii and H. tubulosa had a high nutritional value. They were rich in proteins (up to 68.5% in the dried form), and although their lipid fraction was low (0.3–3.9%), it was characterized by very high levels of arachidonic acid (19.1–30.9% of total fatty acids) and eicosapentaenoic acid (8.5–15.7) that play vital roles in human growth, development, and health. In addition, their n-3/n-6 fatty acid ratios were within the recommended ranges, while their low Atherogenic and Thrombogenic indices and high Hypocholesterolemic index denote their high protective role against coronary artery disease. The provided information sheds light on the high nutritional value of this important marine fishery resource and provides valuable information for its preservation and processing. This information could also help policy makers, stakeholders, and the public to recognize the importance of this valuable fishery resource for human nutrition and to adopt preventative measures toward its sustainable exploitation.