Thermotolerance of Greenshell™ mussel spat (Perna canaliculus) improved by prickly pear (Opuntia ficus indica) treatments

Cultured Greenshell™ mussels (Perna canaliculus) are exposed to biotic and abiotic stresses, all of which can negatively impact growth, fecundity, and survival. These impacts are especially pronounced in sensitive larval and spat stages and may result in substantial losses when spat are transferred from hatchery to grow-out farms. Immersion in solutions of plant extracts have been shown to improve stress tolerance in certain aquatic species and may offer new husbandry and market access opportunities, e.g., more amenable to organic farming practices. Here the efficacy of immersing P. canaliculus spat in ‘high’ or ‘low’ concentrations of prickly pear cactus (Opuntia ficus indica) solutions was examined. Solutions were made from either dry cactus leaf powder i.e., 4.5 (low) or 45 (high) g L⁻¹ or cactus fruit extract (4 μl or 40 μl L⁻¹) and mussels immersed for 1 h to stimulate potential defence mechanisms within mussels prior to acute thermal challenge. Following a subsequent 1 h thermal shock, the predicted 50% survival temperature (LT₅₀) was raised following pre-treatment with O. ficus indica solutions. However, higher resilience to thermal stress was evident in P. canaliculus spat immersed in low concentration solutions of O. ficus indica powder (LT₅₀ = 32.3 °C) and extracts (LT₅₀ = 32.4 °C) compared to spat in immersed in higher concentrations (i.e., LT₅₀ = 31.8 °C ‘Powder’ and 31.4 °C ‘Extract’ respectively) and to naïve controls (LT₅₀ = 30.7 °C). The results provide evidence for the potential for bioactive plant extracts to augment physiological resilience in a shellfish species. While internal biochemical validation is required, the straightforward exposure protocols applied here would be readily transferrable into commercial hatchery culture methods for P. canaliculus.

Keywords: Mussel; Aquaculture; Live transport; Spat; Heat stress; Thermotolerance; Mortality; Opuntia ficus indica; Heat shock protein induction