Towards sea cucumbers as a new model in embryo-larval bioassays: Holothuria tubulosa as test species for the assessment of marine pollution

Sea cucumbers are widely distributed deposit-feeders that represent an important component of benthic communities worldwide. These echinoderms were recently proposed as candidates in embryo bioassays to provide a new tool in the toxicity assessment of pollutants in marine water and sediments. The aim of this study is to evaluate the usefulness of a new species, Holothuria tubulosa (Gmelin, 1788), as a model organism for sensitive embryo bioassays, defining the acceptability of controls, minimum sample size, embryo density and salinity range. Cadmium (Cd), copper (Cu), lead (Pb), mercury (Hg), sodium dodecyl sulphate (SDS) and 4-n-nonylphenol (NP) were used as reference toxicants to assess specific embryotoxicity endpoints. Sea cucumber sensitivity to marine sediment elutriates were finally assessed by comparing their responsiveness in tandem with that of routinely employed sea urchin embryos. The results showed an acceptability threshold of 10% (abnormal embryos), a minimum sample size of 200 embryos, an embryo density of 200 embryos/mL and an optimal salinity range of 36–37‰. The sensitivity to the environmental pollutants and matrices tested revealed values (expressed as EC50) comparable with those of embryos belonging to other marine invertebrates commonly used in bioassays, indicating that this species has a good level of responsiveness. A specific integrative toxicity index (ITI) was applied, combining the frequency of developmental anomalies and weighting their severity. ITI data demonstrated good discrimination of sample toxicity, with a dose-dependent increase of teratogenic effects for all the tested substances, indicating H. tubulosa as a promising species for future assessments of marine pollution.