Juvenile diploid and triploid female European plaice × male European flounder hybrids are larger than both parental species, but have poor vertebral health

We explore the effects of triploidy and hybridisation with the European flounder ( Platichthys flesus ) on European plaice ( Pleuronectes platessa ). Single families of diploid and triploid plaice, diploid flounder, and diploid and triploid female plaice × male flounder hybrids were assessed for body size, vertebral deformities and meristics, and sidedness at metamorphosis (eyes on the left or right-side) at 1 year of age. Wild caught flounder (range, 0.3–2.0 kg) were assessed to determine a baseline for the vertebral endpoints. In the diploids, the hybrids (mean 136 g) were 58% heavier than plaice and 151% heavier than flounder. However, the hybrid (81%) and the flounder (88%) had ~4.5 times more fish with one or more deformed vertebrae than plaice (14%). The farmed diploid flounder also had twice as many deformed individuals than in wild flounder (44%). Triploidy was induced using a cold shock deemed to be ≥ 95% efficient. It had no significant effect on weight or vertebral deformities in either plaice or hybrids. However, the median number of vertebrae was more like plaice (43) than flounder (35) in the triploid (41) compared to diploid (40) hybrid. In contrast, although 99% of plaice were right-sided, and 32% of the diploid flounder were left-sided, left sidedness was almost absent in both hybrids (1–2%). Left sided diploid flounder were significantly 15% heavier than their right-sided counterparts, but sidedness had no impact on meristic counts or vertebral health. In conclusion, i) hybrids were larger than the parental species but had poorer vertebral health, ii) ploidy had no impact on juvenile plaice or hybrid culture traits, and iii) farmed diploid flounder had poor vertebral health. Further work should expand on the single families used here and include later life stages when the fish reach reproductive/market size.