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FIGURE 3 | Length of the total intestine and esophagus of leptocephalus larvae of the European eel (Anguilla anguilla) plotted versus larval standard length.

Figure 3 | Length of the total intestine and esophagus of leptocephalus larvae of the European eel (Anguilla anguilla) plotted versus larval standard length.

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FIGURE 1 | Sampling locations of European eel (Anguilla anguilla) leptocephalus larvae in the Sargasso Sea. This map was created using BatchGeo (07.04.2015, https://batchgeo.com/). (A) West, (B) North east, (C) Mid-east, and (D) Southeast.
FIGURE 1 | Sampling locations of European eel (Anguilla anguilla) leptocephalus larvae in the Sargasso Sea. This map was created using BatchGeo (07.04.2015, https://batchgeo.com/). (A) West, (B) North east, (C) Mid-east, and (D) Southeast.
Larvae were measured live after catch and after fixation. The * shows larvae used for histology/electron microscopy. TABLE 1 | External measurements (standard length, SL) of sampled leptocephalus larvae of the European eel (Anguilla anguilla) in the Sargasso Sea.
Larvae were measured live after catch and after fixation. The * shows larvae used for histology/electron microscopy. TABLE 1 | External measurements (standard length, SL) of sampled leptocephalus larvae of the European eel (Anguilla anguilla) in the Sargasso Sea.
FIGURE 2 | External appearance of the digestive tract of leptocephalus larvae of the European eel (Anguilla anguilla) (L10, SL 11.17) showing the difference between the two areas of the digestive tract, the esophagus and the intestine, and the transition zone between these.
FIGURE 2 | External appearance of the digestive tract of leptocephalus larvae of the European eel (Anguilla anguilla) (L10, SL 11.17) showing the difference between the two areas of the digestive tract, the esophagus and the intestine, and the transition zone between these.
FIGURE 4 | Presence of food-like particles in the intestine of three leptocephalus larvae of the European eel (Anguilla anguilla). (A) L9 (SL 10.95 mm), 4.0x. (B) L10 (SL 11.17 mm), 4.0x. (C) L1 (SL 6.97 mm), 5.0x.
FIGURE 4 | Presence of food-like particles in the intestine of three leptocephalus larvae of the European eel (Anguilla anguilla). (A) L9 (SL 10.95 mm), 4.0x. (B) L10 (SL 11.17 mm), 4.0x. (C) L1 (SL 6.97 mm), 5.0x.
FIGURE 5 | The mouth/pharynx of the European eel (Anguilla anguilla) leptocephalus larvae. (A) Longitudinal section of the larval head, illustrating the mouth/pharynx and esophagus (Eso) where the stratified squamous epithelium (E) covered a layer of striated muscle (Mu), visible from behind the eyes. The cartilage structure of the larval jaw bones (Mc, q, r, t) and the anterior part of the glycosaminoglycan (GAG) layer is also visible. (B) Higher magnification of same larval section as in (A), with more details of the epithelium structure. The striated squamous epithelium (E) covered a layer of striated muscle (Mu), visible from behind the eyes. The cartilage structure of the larval jaw bones (Me, q, r, t) and the anterior part of the glycosaminoglycan (GAG) layer is also visible. (C) Overview of sectional area of pharynx. Cross section of eel larval head, through the heart (behind the eyes). The squared dotted lines illustrate the esophagus shown in (D). (D) Close up of esophagus/pharynx, showing the rough, squamous epithelium (E) and the striated muscle (Mu). Abbreviations: E, epithelium; Eso, esophagus; GAG, glycosaminoglycan; H, heart; h, hyomandibulare; Lu, lumen; Mc, Meckel’s cartilage; Mu, striated muscle; q, quadratum; r, rostrum; t, trabeculum cranii.
FIGURE 5 | The mouth/pharynx of the European eel (Anguilla anguilla) leptocephalus larvae. (A) Longitudinal section of the larval head, illustrating the mouth/pharynx and esophagus (Eso) where the stratified squamous epithelium (E) covered a layer of striated muscle (Mu), visible from behind the eyes. The cartilage structure of the larval jaw bones (Mc, q, r, t) and the anterior part of the glycosaminoglycan (GAG) layer is also visible. (B) Higher magnification of same larval section as in (A), with more details of the epithelium structure. The striated squamous epithelium (E) covered a layer of striated muscle (Mu), visible from behind the eyes. The cartilage structure of the larval jaw bones (Me, q, r, t) and the anterior part of the glycosaminoglycan (GAG) layer is also visible. (C) Overview of sectional area of pharynx. Cross section of eel larval head, through the heart (behind the eyes). The squared dotted lines illustrate the esophagus shown in (D). (D) Close up of esophagus/pharynx, showing the rough, squamous epithelium (E) and the striated muscle (Mu). Abbreviations: E, epithelium; Eso, esophagus; GAG, glycosaminoglycan; H, heart; h, hyomandibulare; Lu, lumen; Mc, Meckel’s cartilage; Mu, striated muscle; q, quadratum; r, rostrum; t, trabeculum cranii.
FIGURE 6 | Cross sections of the anterior esophagus (Eso) of the largest sampled leptocephalus larvae of the European eel (Anguilla anguilla) (L21, SL = 23.33), demonstrating the rapid changes in morphology that occurs along the anterior esophagus tissue. (A) Close up of the beginning of pharynx/esophagus, with the circular and longitudinal striated skeletal muscle layers (Mu) surrounding the rough, squamous surface epithelium (E), mucus cells (MC), and the beginning of a lamina propria (LP). (B) Section through the “head kidney” with glomerulus, Bowman’s space and nephric tubules (K), 200 zm further behind (A), with the multilayered striated muscle (Mu) and a rough surface epithelium. (C) Overview 600 j1m behind (A). (D) Close up of the larva in (C), where the surface epithelium is now ciliated (*), the fibroelastic lamina propria (LP) is well developed, and the multilayered striated muscle (Mu) is organized into circular and longitudinal muscle bundles. (E) Overview 800 jm behind (A), with a larger lumen (Lu), ciliated surface epithelium (arrow), the lamina propria appeared thinner (vertical double line bar), and the striated muscle bundles are thinner and more fragmented. Abbreviations: *, ciliated surface epithelium; E, Epithelium; Eso, esophagus; GAG, glycosaminoglycan; K, head kidney (6B) or kidney/nephric ducts (6E); LP, lamina propria; Lu, lumen; Mu, striated muscle.
FIGURE 6 | Cross sections of the anterior esophagus (Eso) of the largest sampled leptocephalus larvae of the European eel (Anguilla anguilla) (L21, SL = 23.33), demonstrating the rapid changes in morphology that occurs along the anterior esophagus tissue. (A) Close up of the beginning of pharynx/esophagus, with the circular and longitudinal striated skeletal muscle layers (Mu) surrounding the rough, squamous surface epithelium (E), mucus cells (MC), and the beginning of a lamina propria (LP). (B) Section through the “head kidney” with glomerulus, Bowman’s space and nephric tubules (K), 200 zm further behind (A), with the multilayered striated muscle (Mu) and a rough surface epithelium. (C) Overview 600 j1m behind (A). (D) Close up of the larva in (C), where the surface epithelium is now ciliated (*), the fibroelastic lamina propria (LP) is well developed, and the multilayered striated muscle (Mu) is organized into circular and longitudinal muscle bundles. (E) Overview 800 jm behind (A), with a larger lumen (Lu), ciliated surface epithelium (arrow), the lamina propria appeared thinner (vertical double line bar), and the striated muscle bundles are thinner and more fragmented. Abbreviations: *, ciliated surface epithelium; E, Epithelium; Eso, esophagus; GAG, glycosaminoglycan; K, head kidney (6B) or kidney/nephric ducts (6E); LP, lamina propria; Lu, lumen; Mu, striated muscle.
FIGURE 7 | Middle part of esophagus (A) and transition area between the esophagus and intestine of leptocephalus larvae of the European eel (B) (Anguilla anguilla (L21, SL = 23.33). (A) The esophagus with a thick surface epithelium (E), striated muscularis externa (Mu), and serosa (Se). The liver (L) is also present. (B) The liver is present, esophagus (Eso), kidneys (K), and the gall bladder (GB). The glycosaminoglycan layer (GAG) is visible above the digestive tract. Abbreviations: E, epithelium: Eso, esophagus; GAG, glycosaminoglycan; GB, gall bladder; K, kidney/nephric ducts; L, liver; LP, lamina propria; Lu, lumen; Mu, striated muscle; Se, serosa.
FIGURE 7 | Middle part of esophagus (A) and transition area between the esophagus and intestine of leptocephalus larvae of the European eel (B) (Anguilla anguilla (L21, SL = 23.33). (A) The esophagus with a thick surface epithelium (E), striated muscularis externa (Mu), and serosa (Se). The liver (L) is also present. (B) The liver is present, esophagus (Eso), kidneys (K), and the gall bladder (GB). The glycosaminoglycan layer (GAG) is visible above the digestive tract. Abbreviations: E, epithelium: Eso, esophagus; GAG, glycosaminoglycan; GB, gall bladder; K, kidney/nephric ducts; L, liver; LP, lamina propria; Lu, lumen; Mu, striated muscle; Se, serosa.
FIGURE 8 | Transmission electron micrograph of the liver in a European eel leptocephalus larva (Anguilla anguilla). The hepatocytes were characterized by a nucleus (Nu), numerous mitochondria (Mi), dense structures of smooth and rough endoplasmic reticulum (ER), and the perisinusoidal space (space of Disse; *) was observer between the hepatocytes and a sinusoid (Si). Abbreviations: *, space of Disse; ER, endoplasmic reticulum; Mi, mitochondria; Si, sinusoid.
FIGURE 8 | Transmission electron micrograph of the liver in a European eel leptocephalus larva (Anguilla anguilla). The hepatocytes were characterized by a nucleus (Nu), numerous mitochondria (Mi), dense structures of smooth and rough endoplasmic reticulum (ER), and the perisinusoidal space (space of Disse; *) was observer between the hepatocytes and a sinusoid (Si). Abbreviations: *, space of Disse; ER, endoplasmic reticulum; Mi, mitochondria; Si, sinusoid.
FIGURE 9 | Transverse sections of the middle intestine (A) and the hindgut/anus (B) of a European eel larva (Anguilla anguilla) (L21, SL = 23.33). (A) The lumen (Lu) is wide, and the mucosa is folded in several villi, with an absorptive epithelium of oblong columnar cells with a microvilli brush border (BB). The dorsal part had a thinner intestinal wall, where some scattered cilia were observed. Some absorptive vacuoles were observed especially in the apical part of the enterocytes (Va). The kidney/nephric tubules were united to form a urinary tract (UT). (B) The thick intestinal wall of the hindgut/anus, consisting of columnar epithelium and the microvilli brush border. There was a very distinct area of ciliated (Ci) cells in the dorsal part of the gut epithelium. The urine bladder (UB) is visible dorsally to the intestine. The “submucosal” part of the hindgut was very vascular, and several blood vessels (BV) were visible. Abbreviations: BB, microvilli brush border; BV, blood vessels; Ci, ciliated cells; Lu, lumen; Nu, nucleus; UB, urine bladder; UT, urinary tract; Va, vacuoles.
FIGURE 9 | Transverse sections of the middle intestine (A) and the hindgut/anus (B) of a European eel larva (Anguilla anguilla) (L21, SL = 23.33). (A) The lumen (Lu) is wide, and the mucosa is folded in several villi, with an absorptive epithelium of oblong columnar cells with a microvilli brush border (BB). The dorsal part had a thinner intestinal wall, where some scattered cilia were observed. Some absorptive vacuoles were observed especially in the apical part of the enterocytes (Va). The kidney/nephric tubules were united to form a urinary tract (UT). (B) The thick intestinal wall of the hindgut/anus, consisting of columnar epithelium and the microvilli brush border. There was a very distinct area of ciliated (Ci) cells in the dorsal part of the gut epithelium. The urine bladder (UB) is visible dorsally to the intestine. The “submucosal” part of the hindgut was very vascular, and several blood vessels (BV) were visible. Abbreviations: BB, microvilli brush border; BV, blood vessels; Ci, ciliated cells; Lu, lumen; Nu, nucleus; UB, urine bladder; UT, urinary tract; Va, vacuoles.
FIGURE 10 | Transmission electron micrograph of enterocytes from a European eel larva (Anguilla anguilla) (L21, SL = 23.33). (A,B) The apical part of the columnar enterocytes in the epithelium had large nuclei (Nu) with regular alignment in the basal part of the cells, and microvilli (MV) covered the luminal side of the epithelial cells. The enterocytes were characterized by oblong mitochondria (Mi) and much endoplasmic reticulum (ER), and some absorptive vacuoles (Va) were observed in the cytoplasm. Nutrient-like transport could be seen in between the intestinal epithelial cells (*). (C) The dorsal part of the intestinal epithelium, where some cells were ciliated (Ci). Abbreviations: (*), lipid-like transport; Ci, cilia; ER, endoplasmic reticulum; Lu, lumen; Mi, mitochondria; Mv, microvilli; Nu, nucleus; Va, vacuole.
FIGURE 10 | Transmission electron micrograph of enterocytes from a European eel larva (Anguilla anguilla) (L21, SL = 23.33). (A,B) The apical part of the columnar enterocytes in the epithelium had large nuclei (Nu) with regular alignment in the basal part of the cells, and microvilli (MV) covered the luminal side of the epithelial cells. The enterocytes were characterized by oblong mitochondria (Mi) and much endoplasmic reticulum (ER), and some absorptive vacuoles (Va) were observed in the cytoplasm. Nutrient-like transport could be seen in between the intestinal epithelial cells (*). (C) The dorsal part of the intestinal epithelium, where some cells were ciliated (Ci). Abbreviations: (*), lipid-like transport; Ci, cilia; ER, endoplasmic reticulum; Lu, lumen; Mi, mitochondria; Mv, microvilli; Nu, nucleus; Va, vacuole.
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