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Atmosphere, earth and water compose the environment. The presence of heavy metals in the environment has grown because of their large employment in some industrial and agricultural activities. Although these metals are terrestrial products, they flow into the sea through effluents and sewage or are directly discharged from industries placed on the seawater front. It should be considered that metals concentrations vary widely according to different seawater latitudes and depths and can be strongly influenced by fresh water discharges from heavily polluted rivers. In this review recent studies on heavy metal pollution in marine ecosystems and their organisms will be presented. Metal speciation, bioaccumulation in biota, as well as abiotic and biotic factors affecting their bioavailability will be reviewed. Moreover, the use of bioindicator organisms for the biomonitoring of heavy metal toxicity and their ecological effects will be defined. Many marine invertebrate species fulfill the following criteria: Sensitivity to a wide range of chemicals (especially to heavy metals), cost-effectiveness for repeatable tests, readily interpretable biological consequences of pollution. Among the most important marine invertebrates used as bioindicators, the sea urchin embryo is one of the most suitable, especially to assess metal/heavy metal pollution.

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In recent years, researches about the defense strategies induced by cadmium stress have greatly increased, invading several fields of scientific research. Mechanisms of cadmium-induced toxicity continue to be of interest for researchers given its ubiquitous nature and environmental distribution, where it often plays the role of pollutant for numerous organisms. The presence in the environment of this heavy metal has been constantly increasing because of its large employment in several industrial and agricultural activities. Cadmium does not have any biological role and, since it cannot be degraded by living organisms, it is irreversibly accumulated into cells, interacting with cellular components and molecular targets. Cadmium is one of the most studied heavy metal inductors of stress and a potent modulator of several processes such as apoptosis, autophagy, reactive oxygen species, protein kinase and phosphatase, mitochondrial function, metallothioneins, and heat-shock proteins. Sea urchins (adults, gametes, embryos, and larvae) offer an optimal opportunity to investigate the possible adaptive response of cells exposed to cadmium, since these cells are known to accumulate contaminants. In this review, we will examine several responses to stress induced by cadmium in different sea urchin species, with a focus on Paracentrotus lividus embryos. The sea urchin embryo represents a suitable system, as it is not subjected to legislation on animal welfare and can be easily used for toxicological studies and as a bioindicator of environmental pollution. Recently, it has been included into the guidelines for the use and interpretation of assays to monitor autophagy.