Intestinal Function of the Stomachless Fish, Ballan Wrasse (Labrus bergylta)

Abstract

Little is known about the digestion of nutrients in stomachless (agastric) fishes with short intestines, such as wrasse. This study describes the digestion, absorption and evacuation rates in ballan wrasse (Labrus bergylta) fed either dry or pre-soaked diets. Ballan wrasse juveniles received either dry or pre-soaked diets containing inert markers to determine intestinal evacuation rates over a 4-14 h period after ingestion. The digestive tract evacuation was completed in 12-14 h, and was not affected by dietary moisture level. Within the different segments of the intestine, 90% of the digesta moved from the foregut to the midgut after 4-8 h, and over 7% of the digesta had arrived in the hindgut 4 h after feeding. The foregut was a major site of digestion, where 86% of carbohydrates, 74% of proteins, and 50% of lipids, were absorbed from the diet. The absorption of carbohydrates and lipids increased until the hindgut (98 and 80%, respectively). Protein absorption continued along the length of the intestine reaching 90% absorption at the hindgut. Protein and carbohydrate absorption was slightly higher in the foregut for fish fed dry versus moist diets, but this difference disappeared in the midgut. The moisture levels of diet had no effect on the digesta moisture levels, expression of metabolic genes, the levels of nutrients in plasma, or intestinal health. Nine aquaporins were found to be expressed in the wrasse intestinal tissue. Vertebrate aquaporins are involved in water transport from the intestinal lumen into the body and may play a role in maintaining stable moisture level of the digesta in the intestine of ballan wrasse regardless of the initial dietary moisture levels.

Nutrient sensing; transcriptomic response and regulation of gut motility in an agastric vertebrate

“…Eviscerated intestines (5 – 9 cm length) were immediately immersed in Ringer’s solution according to Rønnestad et al (2000) with some modifications (in mM: NaCl, 129; KCl, 2.5; MgCl 2 , 0.47; CaCl 2 , 1.5; NaHCO 3 , 20.2; and NaH 2 PO 4 , 0.42). The intestines were administered a bolus of one of the five diets (IL, HL, IP, HP, and CL were described in section Nutrient bolus ) or one plastic bead (2 – 3 mm diameter and 20 – 30 mg weight) to mimic ingestion of a meal of 0.1% body weight (Le et al, 2019b). The prepared intestines were rapidly mounted in individual glass tubes containing 25 mL of Ringer’s solution at 14°C, aerated with 95% O 2 and 5% CO 2 .…”

“…Thus, the activity of the intestines was recorded for a total of 14 hours after a 20-min.-acclimatation period to examine the effects of the different nutrients on intestinal motility. The 14-hour duration was chosen for Experiment 1 according to the in vivo passage rate of ballan wrasse juveniles, in which ingested feed took 10 – 14 h to pass through the alimentary tract (Le et al, 2019b). A time-lapse series of images of intestines was captured during the experiment using a camera (Nikon DS-Fi3) with a macro lens (Nikon, AF Micro-Nikkor 60mm f/2.8D), at a resolution of 1024×768 pixels.…”

“…The bulbous is a morphologically distinct feature of the anterior gut of the ballan wrasse, with an expanded diameter relative to more posterior regions [Figure 1 in Le et al (2019b)]. Here we defined the time of emptying the bulbous (bulbous emptying time - BET) as the time at which Segment 1 propelled the chyme to the next section and reduced its average diameter to its minimum value.…”

“…In this study, we defined the four ballan wrasse intestine segments as the average ratios of segment length / total intestine length for Segment 1 (bulbous), Segment 2, Segment 3, and Segment 4 (hindgut), i.e. 0.39, 0.23, 0.23, and 0.15, respectively, according to the morphological description by Le et al (2019b) (on Figure 1) and Le et al (2019a). The average width of Segment 1 (AW1) in each frame in the 14-hour video was calculated as the mean value of diameters at the pixels covering a length of 39% of the anterior intestine.…”

“…The indigestible CL was used to evaluate the stretching effects on gut activity compared to the control and to separate the effects of stretching to that of nutrients in the HL and HP groups. The intestines fed 0.1% body weight, according to Le et al (2019b) of one of three nutrient bolus types (HL, HP, or CL) and empty intestines were incubated in Ringer’s solution for one hour before tissue samples for transcriptomic analysis were collected. Segment 1 was selected for transcriptomic analysis because it is the main site for digestion (Le et al, 2019b).…”