Voulgaris K et al. (2021), Mechanical defensive adaptations of three Medit...

ECB-ART-50586

Ecol Evol 2021 Dec 01;1124:17734-17743. doi: 10.1002/ece3.8247.

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Mechanical defensive adaptations of three Mediterranean sea urchin species.

Voulgaris K , Varkoulis A , Zaoutsos S , Stratakis A , Vafidis D .

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In the Mediterranean, Paracentrotus lividus and Sphaerechinus granularis are important drivers of benthic ecosystems, often coexisting in sublittoral communities. However, the introduction of the invasive diadematoid Diadema setosum, which utilizes venomous spines, may affect these communities. To describe the mechanical properties of the test and spines of these three species, specimens were collected in winter of 2019 from the sublittoral zone of the Dodecanese island complex, southeastern Aegean Sea. This region serves as a gateway for invasive species to the Mediterranean Sea. Crushing test was conducted on live individuals, while 3-point bending test was used to estimate spine stiffness. Porosity and mineralogy of the test and spine, thickness of the test, and breaking length of the spine were measured and compared, while the microstructural architecture was also determined. The test of S. granularis was the most robust (194.35 ± 59.59 N), while the spines of D. setosum (4.76 ± 2.13 GPa) exhibited highest flexibility. Increased porosity and thickness of the test were related to increased robustness, whereas increased flexibility of the spine was attributed to high porosity, indicating that porosity in the skeleton plays a key role in preventing fracture. The spines of S. granularis exhibited highest length after fracture % (71.54 ± 5.5%). D. setosum exhibited higher values of Mg concentration in the test (10%) compared with the spines (4%). For the first time, the mineralogy of an invasive species is compared with its native counterpart, while a comparison of the mechanical properties of different species of the same ecosystem also takes place. This study highlights different ways, in which sea urchins utilize their skeleton and showcases the ecological significance of these adaptations, one of which is the different ways of utilization of the skeleton for defensive purposes, while the other is the ability of D. setosum to decrease the Mg % of its skeleton degrading its mechanical properties, without compromising its defense, by depending on venomous bearing spines. This enables this species to occupy not only tropical habitats, where it is indigenous, but also temperate like the eastern Mediterranean, which it has recently invaded.

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	FIGURE 1. SEM micrographs of ambital interambulacral plates. (a). External surface of (i) Diadema setosum (scale bar‐mm, ×15), (ii) Paracentrotus lividus, and (iii) Sphaerechinus granularis (scale bar‐500 μm, ×30). Notice that the plate of D. setosum possesses a bigger primary tubercle and less secondary ones. Images in circles present the porous stereom of each species (scale bar‐50 μm, ×500). (b). Cross section of the same plates. (i) D. setosum, (ii) P. lividus, and S. granularis (scale bar‐50 μm, ×400). (c). Inner surface of the plates. (i) D. setosum, (ii) P. lividus, and (iii) S. granularis (scale bar‐50 μm, ×400). Vectors indicate the mamelon in each species
	FIGURE 2. SEM micrographs of the spines of the three species. (a, d, g) External view of Diadema setosum (scale bar—200 μm, ×65), Paracentrotus lividus, and Sphaerechinus granularis (scale bar—100 μm, ×140). (b, e, h) Longitudinal section of D. setosum (scale bar—100 μm, ×100), P. lividus (scale bar—100 μm, ×150), and S. granularis (scale bar—100 μm, ×140). (c, f, i) Cross section of D. setosum (scale bar—200 μm, ×85), P. lividus (scale bar—100 μm, ×130), and S. granularis (scale bar—200 μm, ×85). V, verticillation; L, lumen
	FIGURE 3. Error plots of (a): load (N), thickness (mm), and porosity (%) for the tests and (b) of Young's modulus (GPa), breaking length (%), and porosity (%) for the spines of the three species. All data are means, ±2 SE; n = 10. * Indicates the groups with no statistical differences
	FIGURE 4. Stress–strain diagram of one representative spine per species after a three‐point bending test. The spines of Diadema setosum and most of the spines of Paracentrotus lividus exhibited linear stress increase and fracture after maximum input. On the contrary, most of the spines of Sphaerechinus granularis exhibited slopes indicating microfractures before the main fracture
	FIGURE 5. XRD spectra of the test and spine of Diadema setosum. Notice the three different types of peaks corresponding to different types of Mg‐ calcite, according to the Mg concentration in the crystal

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