Effects of animal size and nutritional status on the RNA/DNA ratio in different tissues of the green-lipped mussel Perna viridis

This study aimed to examine the responses of RNA/DNA ratio in Perna viridis under different nutritional status via both field and laboratory studies, and hence evaluate the usefulness of this ratio as a rapid growth biomarker in the mussels. First, the effects of size (small: 30–40 mm; medium: 40–50 mm; large: >50 mm) and tissue type (adductor muscle, foot, gill and hepatopancreas) on the RNA/DNA ratio were investigated in P. viridis collected from three different sites with different degrees of eutrophication in Hong Kong waters. Across all sizes, the mussels collected from a fairly ‘eutrophic’ mariculture zone had significantly higher RNA/DNA ratios in their gills than those from the other two relatively clean sites. The RNA/DNA ratio in small mussels was generally higher than in medium and large individuals, though such a size effect significantly interacted with tissue type and site. Second, we conducted a 10-day comparative laboratory study to elucidate the influence of starvation and feeding on the RNA/DNA ratio in the mussels. We observed that both hepatopancreas and foot muscle generally exhibited significant and rapid response to such a short-term starvation or food addition. The present results confirmed that the RNA/DNA ratio in P. viridis is a sensitive biomarker to gauge their growth and general health condition in accordance with food availability and/or eutrophication condition.