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The Mediterranean sea urchin Paracentrotus lividus Lamark, 1816 is a resource as food and model organism for scientific research. Increasing market demand, commercial fisheries and destructing harvesting methods produced a dramatic reduction of natural stocks along the Mediterranean coasts. The techniques for in vitro fertilization and growth of larvae and adults are well known but various bottlenecks still hinder the industrial maturity of its aquaculture. In particular, the transition from planktonic larvae to benthic juveniles, as well as their correct development and survival, still represent critical phases, since larval and post-larval experiences are the primary determinant of juvenile fitness. Thus, techniques improving survival and settlement rates in the first two months after fertilization could considerably improve the effectiveness of echinoculture practices. This study was conducted to determine, for the first time, the effect of strictly benthic diatoms on P. lividus post-larval performances, as compared to other micro- and macro-algae. In addition, three larval dietary treatments were applied, to confirm previous findings on the efficacy of algae and formulated diets. In particular, survival and ability to produce competent larvae for settlement were tested in groups subjected to three algal diets (Isochrisis galbana, Rhodomonas baltica and Dunaliella tertiolecta), all integrated with a formulated food. Furthermore, six post-larval feeding regimes, i.e., the benthic diatoms Cocconeis scutellum parva, Diploneis sp., Navicula incerta, Cylindrotheca closterium and Nanofrustulum shiloi, compared with Ulva rigida along with a negative control (absence of food), were tested on larvae deriving from the most effective treatment. Our results confirmed the efficacy of Dunaliella tertiolecta integrated with a formulated food, consistently producing about 60% survival. Furthermore, the most adhesive benthic diatom, Cocconeis scutellum parva, sustained settlement (about 63%) and good survival in the first two months of life of sea urchins.

In marinas and harbours, the accumulation of pollutants in sediments, combined with poor exchange of water with the open sea, poses a major environmental threat. The presence of photosynthetic organisms and the related oxygen production, however, may alleviate the negative effects of environmental contamination on heterotrophic organisms, enhancing their physiological defences. Furthermore, possible transgenerational buffer effects may increase the ability of natural populations to face environmental stress. Here we tested the occurrence of transgenerational effects on larvae of the sea urchin Paracentrotus lividus, whose parents were exposed, during the gametogenesis, to contaminated sediments subject to two temporal patterns of water re-suspension events and normal- (90%) vs. super-saturated (200%) levels of O2. The study site was Bagnoli-Coroglio (Gulf of Naples, southern Tyrrhenian Sea), a historically polluted brownfield and Site of National Interest for which environmental restoration options are currently under exploration. Larvae from different adult populations were significantly, although not linearly, affected by the interaction of all factors to which parents were exposed, at both 24h and 48h post fertilization. Specifically, the exposure of larvae to elutriates from contaminated sediments determined a developmental delay, a reduction in size and an increased percentage of abnormalities in all larval populations independently of their parental exposure. On the contrary, larvae from parents exposed to contaminated sediments, when reared in clean filtered sea water, succeeded in developing until the echinopluteus stage after 48h, with size and abundance comparable to those of larvae from control parents. Pre-exposure of parents to contaminated sediments did not successfully buffer the negative effects of elutriates on their offspring, and no positive effects of ‘super-saturated’ levels of O2 in response to contaminants were observed, suggesting that the Bagnoli-Coroglio area is currently not suitable for the re-stocking or re-introduction of this species.

Diatoms are unicellular algae playing a key role as photosynthetic organisms in the world’s ocean food webs. The chemical ecology of planktonic diatoms is well documented, but few studies have reported on the effects of benthic diatoms on their consumers, also due to difficulties in the collection, quantification and massive culturing of benthic species. Here for the first time we investigate the effects of feeding on two abundantly occurring benthic diatoms, Nanofrustulum shiloi and Cylindrotheca closterium, isolated from the leaves of the seagrass Posidonia oceanica, on the sea urchin Paracentrotus lividus. Adult P. lividus were fed for one month on diets of either one of the two diatoms and on the green alga Ulva rigida, used as a feeding control. By combining morphological, metabolomic and de novo transcriptomic approaches, we demonstrate toxigenic effect on embryos generated by females fed with these benthic diatoms. Furthermore, chemical analysis reveal the presence of polyunsaturated aldehydes only for N. shiloi, and a high production of other oxylipins (cytotoxic compounds on their grazers and on cancer cell lines) for both diatoms, including some additional peaks not correlated to the canonic oxylipins commonly observed in planktonic diatoms. These findings open new perspectives in the study of diatom secondary metabolites influencing their grazers.