This study investigated the effect of extruded feed physical pellet quality (PPQ) on the physiological response of spotted seabass (Lateolabrax maculatus). Two batches of pellets with the same formula but contrasting PPQ were produced by extrusion. The contrast in PPQ was created by applying two levels of water (14% versus 20%) into the feed mixture prior to extrusion. This resulted in a low-water addition treatment (LW) with a short pellet hydration time versus a high-water addition treatment (HW) with a long pellet hydration time. Fish (average weight, 159 g) were fed to satiation twice daily for 48 days. Faeces were collected during the experimental period for digestibility measurements. At the end of the experiment, chyme was collected from the stomach and intestine at 0.5, 2, 4, 6, 8, 10, 12, 16 and 24 h after the last meal during which fish were also fed to satiation. Chyme dry matter, crude protein, and inert marker (Y2O3) content were analysed for the calculation of gastrointestinal evacuation, kinetics of digestion and water fluxes. A significantly larger stomach water influx led to a lower stomach DM content at 2 h postprandial in the LW-pellets compared to the HW-pellets. During the last meal, fish fed LW-pellets consumed more feed and had a higher plasma alpha-melanocyte stimulating hormone concentration at postprandial 2 h. On a fresh basis, LW-pellets generated a faster stomach digesta evacuation, while on a DM basis stomach emptying was similar for both treatments. Fish fed LW-pellets had a lower feed conversion ratio, a higher protein efficiency ratio, hepatosomatic index and postprandial plasma glucose level than fish fed HW-pellets. Nevertheless, PPQ did not statistically influence on the stomach digestion kinetics, faecal digestibility values and feed intake during the experimental period. Our results show that the PPQ can alter stomach water fluxes and digesta evacuation rate in spotted seabass. Additionally, it is suggested that PPQ, particularly hydration time, can affect fish's feed intake regulation and metabolic responses.
