Background The Indo-Pacific sea urchin Diadema setosum has invaded the Mediterranean Sea and has spread along many locations in the southeastern part of the basin, where established populations exist on the shallow subtidal rocky shore. Diadema setosum is a ubiquitous species, of particular ecological importance due to the high levels of grazing pressure it imposes on benthic communities. Its biology, however, is not adequately studied, especially along its introduced range of distribution. The present study examines the population status of D. setosum outside its native range, in the Dodecanese island complex, south Aegean Sea. Thirty-four stations located across 16 islands were surveyed by scientific SCUBA-diving (up to a depth of 10 m) in December 2019 and June-July 2020. Samplings included: (i) visual census along transects to estimate relative abundance and population density, and (ii) random collection of specimens from densely populated stations to assess biometry and reproductive condition (histological examination of gonads) of D. setosum. Results Diadema setosum was found in 21 out of the 34 surveyed stations. The species had sparse populations of well-hidden individuals in rocky crevices, but with dense localized patches in Agathonisi, Leros, Kalymnos, Pserimos, Symi, Alimia and Chalki islands. In those seven islands, mean population density was 2.5 ± 1.48 individuals m−2. Diadema setosum had denser populations in shallower depths but larger dimensions in deeper; these results suggest segregated density and size patterns along a depth gradient. The size structure, according to the size frequency distribution of the test diameter, was unimodal with a fitted mode at 4.0–4.5 and 6.5–7.0 cm in shallow and deep populations, respectively. The examined morphometric relationships followed negative allometry, as previously suggested for the species within its native range of distribution, and test diameter appeared to be a good predictor of biomass. Diadema setosum specimens had immature gonads in winter and mature in summer, suggesting a synchronous reproductive pattern. These results conform to previous data from temperate populations of the species. Conclusions Differences in local environmental conditions, e.g. hydrodynamics and habitat type, together with biotic interactions, e.g. recruitment and competition, probably shape D. setosum population in the south Aegean distributional range. The establishment of D. setosum has severe implications on benthic communities and local sea urchin populations demanding management measures to prevent the forecasted further expansion of this invasive species.

In the Mediterranean Sea, the species Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis often coexist, occupying different subareas of the same habitat. The mechanical and chemical properties of their calcitic skeletons are affected both by their microstructural morphology and chemical composition. The present study describes the main morphologic features and the possible temporal differences in elemental composition of the test and spines of the three species, while also determining the molar ratio of each element of their crystalline phase. Scanning electron microscopy showed major differences in the ultrastructure of the spines, while minor differences in the test were also noticed. More specifically, the spines of all three sea urchins possess wedges, however A. lixula exhibits bridges connecting each wedge, while barbs are observed in the wedges of S. granularis. The spines of P. lividus are devoid of both microstructures. Secondary tubercles are absent in the test of A. lixula, while the tests and spines of all three species are characterized by different superficial stereom. Energy dispersive x-ray spectroscopy detected that Ca, Mg, S, Na and Cl were present in all specimen. Mg and Mg/Ca showed significant differences between species both in test and spines with S. granularis having the highest concentration. The spines of P. lividus exhibited lowest values between all species. Differences between spines and test were observed in all elements for P. lividus except S. A. lixula exhibited different concentrations between test and spines for Ca, Mg and Mg/Ca, whereas S. granularis for Mg, Cl and Mg/Ca. Finally, temporal differences for Ca were observed in the test of P. lividus and the spines of S. granularis, for Mg in test of S. granularis, for S in the spines of A. lixula and the test and spine of S. granularis, for Na in the test of P. lividus and A. lixula and for Cl and Mg/Ca in the test P. lividus. Powder X-ray diffractometry determined that, out of all three species, the spines of P. lividus contained the least Mg, while the test of the same species exhibited higher Mg concentration compared to A. lixula and S. granularis. The current study, although not labeling the specimens attempts to estimate potential time-related elemental differences among other results. These may occur due to changes in abiotic factors, probably water temperature, salinity and/or pH. Divergence in food preference and food availability may also play a key role in possible temporal differences the skeletons of these species

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