Effects of seawater chemistry (Mg2+/Ca2+ ratio) and diet on the skeletal Mg/Ca ratio in the common sea urchin Paracentrotus lividus

Tooth Mg/Ca ratios and Aristotle's lantern morphometrics reflect trophic types in echinoids

Several studies have inferred the ecological significance regarding the morphometrics of Aristotle's lantern and the mechanical properties of magnesium in echinoid teeth. This study attempts to combine these aspects, connecting them to the trophic habits of three native and an invasive echinoid in the Eastern Mediterranean Sea. Spatiotemporal data from the central and southern Aegean Sea were obtained, regarding the relative size of lanterns and demi-pyramids of Arbacia lixula, Paracentrotus lividus, Sphaerechinus granularis, and the invasive echinoid Diadema setosum and the Mg/Ca ratios of four zones on the tooth cross-section. Since environmental factors affect the examined factors, data for temperature, salinity, and concentration of chlorophyll-a were included in a principal component analysis. A. lixula and P. lividus presented intraspecific differences in the relative size of the lantern and demi-pyramid, while S. granularis and D. setosum exhibited variation in the elongation index. Differences in the Mg/Ca ratios were observed for all species although in different zones. Temperature appears to be related to all Mg/Ca zones except for the stone part, while the elongation index showed an inverse trend to all other morphometric parameters. The results of the PCA for the four species on the spatiotemporal level exhibited a distinction of individuals with season but not species, except for A. lixula, an omnivore with a carnivorous tendency, which was clearly separated from the herbivorous species. Using hierarchical clustering on the principal components it was evident that the three native species occupy different clusters, but when D. setosum was added, it shared the same cluster with S. granularis. This might infer similar feeding preferences, specifically for coralline algae, which might lead to a swift in the ecological equilibrium in regions, where D. setosum is found, either by affecting habitat type, or by restricting the distribution of S. granularis as was previously observed with Diadema africanum.

Skeletal Mg content in common echinoderm species from Deception and Livingston Islands (South Shetland Islands, Antarctica) in the context of global change

Echinoderms with high levels of magnesium (Mg) in their skeletons may be especially sensitive to ocean acidification, as the solubility of calcite increases with its Mg content. However, other structural characteristics and environmental/biological factors may affect skeletal solubility. To better understand which factors can influence skeletal mineralogy, we analyzed the Mg content of Antarctic echinoderms from Deception Island, an active volcano with reduced pH and relatively warm water temperatures, and Livingston Island. We found significant interclass and inter- and intraspecific differences in the Mg content, with asteroids exhibiting the highest levels, followed by ophiuroids and echinoids. Specimens exposed to hydrothermal fluids showed lower Mg levels, which may indicate local environmental effects. These patterns suggest that environmental factors such as seawater Mg2+/Ca2+ ratio and temperature may influence the Mg content of some echinoderms and affect their susceptibility to future environmental changes.

A Devonian crinoid with a diamond microlattice

Owing to their remarkable physical properties, cellular structures, such as triply periodic minimal surfaces (TPMS), have multidisciplinary and multifunctional applications. Although these structures are observed in nature, examples of TPMS with large length scales in living organisms are exceedingly rare. Recently, microstructure reminiscent of the diamond-type TPMS was documented in the skeleton of the modern knobby starfish Protoreaster nodosus. Here we report a similar microlattice in a 385 Myr old crinoid Haplocrinites, which pushes back the origins of this highly ordered microstructure in echinoderms into the Devonian. Despite the low Mg2+/Ca2+ ratio of the 'calcite' Devonian sea, the skeleton of these crinoids has high-Mg content, which indicates strong biological control over biomineralogy. We suggest that such an optimization of trabecular arrangement additionally enriched in magnesium, which enhances the mechanical properties, might have evolved in these crinoids in response to increased predation pressure during the Middle Palaeozoic Marine Revolution. This discovery illustrates the remarkable ability of echinoderms, through the process of evolutionary optimization, to form a lightweight, stiff and damage-tolerant skeleton, which serves as an inspiration for biomimetic materials.

Modeling of the Elements Ca2+, Mg2+ and Si in the Sediments and the Body Walls of Sea Cucumbers in the Tropical Seagrass Meadows

The interrelationship of the minerals calcium (Ca2+), magnesium (Mg2+) and silicon (Si) in the sediments and in the body walls of four tropical sea cucumber species was explored by modeling the concentrations of these minerals. The elemental concentrations of Ca2+, Mg2+ and Si were measured in the body walls and in the ambient sediments occupied by the sea cucumbers Holothuria scabra, H. leucospilota, H. atra and Bohadschia marmorata. The results indicate that the concentrations of Ca2+ and Mg2+ in the body walls of the four sea cucumber species are significantly different from each other, indicating a varying degree of biomineralization across sea cucumber taxa. In contrast, only B. marmorata showed a significant difference in the concentration of Si when compared to the rest of the species tested. Further analysis using linear mixed models revealed that the Ca2+, Mg2+ and Si concentrations in the body walls of the tested sea cucumber species are associated with the sediment concentrations of the same elements. The relatively high concentrations of Ca2+ and Mg2+ in the sediments indicate that these minerals are sufficiently high in sea cucumbers to support their biomineralization. The relationship between the Mg/Ca ratio in the body walls of the sea cucumbers and minerals in the sediments revealed that Si was the only mineral that was not correlated with the Mg/Ca ratio. Predicting the relationship of the elements Ca2+, Mg2+ and Si between the sediments and the body walls of sea cucumbers may be complex due to the various factors that influence the metabolism and biomineralization in sea cucumbers.

Effects of seawater Mg2+ /Ca2+ ratio and diet on the biomineralization and growth of sea urchins and the relevance of fossil echinoderms to paleoenvironmental reconstructions

It has been argued that skeletal Mg/Ca ratio in echinoderms is mostly governed by Mg²⁺ and Ca²⁺ concentrations in the ambient seawater. Accordingly, well-preserved fossil echinoderms were used to reconstruct Phanerozoic seawater Mg²⁺ /Ca²⁺ ratio. However, Mg/Ca ratio in echinoderm skeleton can be affected by a number of environmental and physiological factors, the effects of which are still poorly understood. Notably, experimental data supporting the applicability of echinoderms in paleoenvironmental reconstructions remain limited. Here, we investigated the effect of ambient Mg²⁺ /Ca²⁺ seawater ratio and diet on skeletal Mg/Ca ratio and growth rate in two echinoid species (Psammechinus miliaris and Prionocidaris baculosa). Sea urchins were tagged with manganese and then cultured in different Mg²⁺ /Ca²⁺ conditions to simulate fluctuations in the Mg²⁺ /Ca²⁺ seawater ratios in the Phanerozoic. Simultaneously, they were fed on a diet containing different amounts of magnesium. Our results show that the skeletal Mg/Ca ratio in both species varied not only between ossicle types but also between different types of stereom within a single ossicle. Importantly, the skeletal Mg/Ca ratio in both species decreased proportionally with decreasing seawater Mg²⁺ /Ca²⁺ ratio. However, sea urchins feeding on Mg-enriched diet produced a skeleton with a higher Mg/Ca ratio. We also found that although incubation in lower ambient Mg²⁺ /Ca²⁺ ratio did not affect echinoid respiration rates, it led to a decrease or inhibition of their growth. Overall, these results demonstrate that although skeletal Mg/Ca ratios in echinoderms can be largely determined by seawater chemistry, the type of diet may also influence skeletal geochemistry, which imposes constraints on the application of fossil echinoderms as a reliable proxy. The accuracy of paleoseawater Mg²⁺ /Ca²⁺ calculations is further limited by the fact that Mg partition coefficients vary significantly at different scales (between species, specimens feeding on different types of food, different ossicle types, and stereom types within a single ossicle).

Chemical Composition and Microstructural Morphology of Spines and Tests of Three Common Sea Urchins Species of the Sublittoral Zone of the Mediterranean Sea

Simple Summary

Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis play a key role in many sublittoral biocommunities of the Mediterranean Sea. However, their skeletons seem to differ, both morphologically and in chemical composition. Thus, the skeletal elements display different properties, which are affected not only by the environment, but also by the vital effect of each species. We studied the microstructural morphology and crystalline phase of the test and spines, while also examining the effect of time on their elemental composition. Results showed morphologic differences among the three species both in spines and tests. They also seem to respond differently to possible time-related changes. The mineralogical composition of P. lividus appears to be quite different compare to the other two species. The results of the present study may contribute to a better understanding of the skeletal properties of these species, this being especially useful in predicting the effects of ocean acidification. More specifically, since the skeleton plays a key role for the survival of sea urchins in general, a potential change in any skeletal structure, either morphologically or chemically, may affect these organisms directly while also affecting their ecosystem indirectly.

Abstract

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