Cultivation of the marine calanoid copepod Acartia tonsa (Dana) as alternative feed in aquaculture: evaluation of reproductive performance (fecundity and larval survival) and gene expression in different stress conditions

The calanoid copepod Acartia tonsa has been recommended as a marine organism for aquaculture and ecotoxicological tests due to its wide distribution, short life cycle and high productivity. Marine fish larvae in cultures require a large number of organisms as the first-feeding larvae. Eggs stored under cold conditions that delay hatching could ensure sufficient quantities of biological materials for aquaculture. In the current study early-spawned eggs were stored at 3°C (±1) up to 240 days and their hatching success was evaluated on a monthly basis. Our results showed that the percentage of hatching success for eggs stored for 30 days was >80% and decreased by about 8% for every 20 days of storage, up to 120 days. A further increase of time in cold storage brought about a significant reduction, in statistical term, of hatching success compared with the control group (43.69 ± 22.19%). Almost 50% of eggs hatched or died during the cold storage period, with more than 80% lost after periods longer than 150 days. Nanoparticles compounds are widely used in industries and have been massively introduced in the environment. Here we report the effect of three different chemical forms of NiCl2 (as reference) and nickel nanoparticles (NiNPs) and Quantum dot nanoparticles (QDs), on the reproduction of A. tonsa. The behavior of NiNPs was analyzed with different techniques and with two protocols. Acute and semichronic tests, conducted exposing A. tonsa eggs to NiCl2 concentrations from 0.025 to 0.63 mg L?1, showed EC50 of 0.164 and 0.039 mg L?1, respectively. Acute tests with QDs (concentration tested from 0.15 to 1.5 nM) showed an increased naupliar mortality in response to QD treatment, exhibiting an EC50 of 0.7 nM. Overall, A. tonsa is more sensitive to NiCl2 than NiNPs with EC50 being one order of magnitude higher for NiNPs. Finally, we exposed adult copepods for chronic test (4 days) to NiCl2 and NiNPs (chronic exposure) to study the effect on fecundity in terms of daily egg production and naupliar viability. Egg production is not affected by either form of nickel, whereas egg viability is significantly reduced by 0.025 mg L?1NiCl2 and by 8.5 mg L?1NiNPs. At NiNP concentration below the acute EC50 (17 mg L?1) only 9% of eggs hatched after 4 days. Chronic test showed no negative effect on egg production, except on the last two days at the highest QD concentration (2.5 nM). No significant reduction of the percentage of egg hatching success was recorded during the exposure. Interestingly, the percentage of naupliar mortality (>82%) observed in the semichronic test at the nominal concentration of 10 mg L?1 NiNPs corresponding to almost 0.10 mg L?1 of dissolved Ni, was similar to that recorded at the same Ni salt concentration. Electron microscopicalanalyses revealed that A. tonsa adults ingest NiNPs and excrete them through fecal pellets. Overall, these results suggest that species unable to swim along the water column, and early hatched copepods, could be more exposed to toxic effects of NPs which tend to aggregate and settle in seawater. To the best of our knowledge, this is the first study investigating the toxicity of two different forms of Ni and QDs on the reproductive physiology of the copepod. Transcriptome sequencing and gene expression analysis open new perspective in studying copepod physiology and allow to select organisms with a good reproductive fitness. The comparative sequencing of A. tonsa between control and toxic groups has made a database about the gene variations involved with some pathways, which might potentially reflect stress response exposed to toxicants. In the Lipid biosynthesis pathway, there were no significant differentially gene regulation under slightly toxic level exposure. This conclusion was also supported by results of fecundity of A. tonsa, which were also not significantly effected by toxic exposure. This is the first study which investigate on the transcriptome and gene expression of A. tonsa copepod.