First occurrence and toxic effects of Prymnesium parvum on northeastern Mediterranean harpacticoid copepod species

The ichthyotoxic haptophyte Prymnesium parvum f. patelliferum is reported for the first time in the coastal marine waters of the Mediterranean Sea, isolated from its northern Levantine Basin at a high salinity of 38 ppt. The culture was grown with F/20 medium under 90 μmolm^-2 s^-1 light intensity at 20 °C. The toxic effects of this isolate were evaluated across a concentration gradient (0.25 × 10³ to 20 × 10³ cells ml^-1, which rose up to 175 × 10³ cells ml^-1 at the end of the experiment), using two harpacticoid copepod species, Tisbe perplexa, and Sarsamphiascus parvus. Results were compared against both starved individuals and those fed by the non-toxic diatom Chaetoceros socialis as control. Adults of both copepod species exhibited high sensitivity to P. parvum at concentrations ≥2.5 × 10³ cells ml^-1, with 100% mortality occurring within 3-5 days for T. perplexa and 9-22 days for S. parvus. Notably, starved copepods survived longer than those exposed to P. parvum, confirming its direct toxicity. While the lowest concentration (0.25 × 10³ cells ml^-1) was not immediately lethal, it significantly suppressed reproductive success. In the low-density treatment (0.25 × 10³ cells ml^-1), P. parvum populations grew naturally from their initial baseline, inducing 100% copepod mortality at substantially lower final cell densities than those observed in the high-density treatments (10 × 10³-20 × 10³ cells ml^-1). This may indicate a cumulative toxic effect, where prolonged exposure to moderate algal densities over an extended duration is equally lethal as short-term exposure to acute densities. These findings underscore the potential ecological threat this species poses to Mediterranean food webs and mariculture.