Adaptation of gill-palp ratio by mussels after transplantation to culture plots with different seston conditions

Transplantation of bivalves between culture areas or systems that vary in environmental conditions is an important tool for growers to increase aquaculture productivity, but it sometimes leads to adverse effects on growth and survival. Bivalves filter ambient water including seston. Particles are retained on the gills and transported to the palps, where edible particles are selected for further digestion and where the rest is excreted as pseudofaeces. Both quantity and quality of suspended particles in the water affect efficiency of this process. As a consequence of transplantations, shellfish are forced to spend energy on adapting the gills and palps. This has implications for the energy available for growth and reproduction and might ultimately reduce yields. Here, we investigate, using gill-to-palp ratio as proxy, if adaptations in the feeding apparatus after transplantation of mussels to a culture area with higher seston quantity but lower quality, can explain an observed reduction in yield. We hypothesize that when mussels are transplanted to an environment with higher seston concentrations, (1) the gill-to-palp ratio becomes smaller, because mussels will grow larger palps to process the filtered material, while growth of gills will be reduced to reduce the amount of seston filtered and (2) condition of mussels will remain lower than reference levels during adaptation period. Three different mussel cohorts that were transported from the Oosterschelde (OS, NL) to the Wadden Sea (WS, NL) were monitored. Wadden Sea is more turbid (~2.4 × Total Particulate Matter OS), and is higher in chlorophyll-a concentration (~1.6 × Chl-a OS), but lower in food quality (~0.65 × ratio Chl-a:TPM OS). Gill-to-palp ratios and condition indices were measured from transplanted mussel cohorts and from reference populations over a period of three months. Results reveal that the gill-to-palp ratio between OS and WS is indeed significantly different (on average respectively 3.4 and 2.3), and that transplanted mussels show a remarkable physiological plasticity by adapting the gill-to-palp ratio within 1 month after transplant to match reference levels. They achieve this by a relative increase in the growth rate of the palps, while the growth rate of the gills remain constant. Condition index of transplanted mussels were not lower than reference levels with an equivalent increase. It is more likely that observed lower yields of transplanted mussels are related to factors that affect survival, because mussel adjustment to feeding conditions seems to occur quickly and mussel condition swiftly follows the reference population trajectory.