Determining gut transit rates in gilthead seabream larvae fed microdiets

In this study we assessed an easy methodology for testing gut filling dynamics in early fish larvae fed on microdiets by means of image analysis. As model for this study we used gilthead seabream Sparus aurata, a species whose feeding physiology during the larval stage is well-known. Ingestion and food transit assays were carried out at different ages between 9 and 27 days after hatching (dph). Gut content was examined by image analysis in 10 larvae sampled respectively at 0, 20, 40, 60, and 120 min after the first morning meal. Gut fullness level was determined measuring the pigmented area within the digestive cavity. In each sampling point and larvae, relative percentage of shadowed area respect to the total area of intestinal cavity was considered as fullness percentage, assuming that intestinal area and volume are highly correlated. Data obtained were adjusted to an exponential model with maximum asymptotic: C = Cmax(1−e(−bt)), where C = % fullness; Cmax is the asymptote (theoretical percentage of maximum fullness), t = sampling time (min) and b is a regression parameter indicating the slope of the filling curve. Ingestion rate (I) was calculated as I = Cmax∗b. Ingestion rate was considered as Filling rate taken that the determined data are percentages instead of a number of prey items as usual. Filling rate was expressed as % of total gut volume per time unit. The food transit time (time required to fill completely the gut) was calculated as 100/I. Our results demonstrate that the methodology based in the image analysis of the shadowed area by food within the gut is a useful tool to build saturation curves as a function of time in small early larvae, using early seabream larvae as model. Our results also showed that although food intake increases with age the filling rate of digestive tract is practically constant in the range of age assessed. In summary, feeding rates and gut transit time can be estimated by image analysis in early fish larvae while the body still remains transparent. This methodology is particularly useful for advancing in the feeding behaviour of larvae fed on microdiets.