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Long-term rearing of Japanese eel larvae using a liquid-type diet: food intake, survival and growth
Abstract
The efficiency of a new liquid-type diet for long-term rearing of Japanese eel larvae until metamorphosis was examined, as was the effect of diet viscosity on diet intake and on the survival and growth of early larvae. The highest intake of the experimental diet by 6- and 9-day post-hatch (dph) larvae occurred at viscosities of 20–50 mPa·s, much lower than the viscosity of the currently used slurry-type diet (ca. 2900 mPa·s). Long-term feeding trials for 259 days (n = 4) showed that overall survival rates of larvae (37–59%) fed a liquid-type diet with lower viscosity (40–680 mPa·s) were 2 to 3.4 times higher than the survival rates of larvae fed the slurry-type diet (11–25%). Because nutrients were diluted in the liquid-type diet, the growth of larvae fed this diet was slower after about 200 days and metamorphosis was delayed. However, the yield of glass eels was 1.1 to 3.2 times higher in larvae fed the liquid than the slurry diet. These findings suggest that feeding the liquid-type diet can result in the mass production of glass eels by ensuring high growth, survival and metamorphosis rates.
... The low survival rate of newly hatched larvae in lab is likely due to nutritional deficiencies in their artificial diets. Despite efforts to include shark egg yolk, Antarctic krill, and rotifer paste, additional nutritional improvements are still necessary (Tanaka et al., 2003;Okamura et al., 2013;Liu et al., 2017;Okamura et al., 2019). Pousão-Ferreira et al. (1999) successfully used gilthead seabream (Sparus aurata) eggs to feed S. aurata larvae, determining nutritional requirements during the larval rearing based on egg composition. ...
... The digestive system function in fish larvae depends on both genetically pre-programmed and extrinsically influenced factors (Politis et al., 2018). The expression of several selected pancreatic enzyme genes indicated that A. japonica larvae acquire full function by the onset of exogenous feeding at 8-12 dph (Okamura et al., 2019). However, other digestive enzyme genes and nutrient transporters in A. japonica are not well studied, and few studies address food digestion and nutrient absorption during the preleptocephali stage. ...
The starter diet for Japanese eel (Anguilla japonica) has always been a difficult problem for the realization of total artificial reproduction. Therefore, this research analyzed the nutritional composition of artificially fertilized eggs, and transcriptome of samples from early hatchlings of fry to better understand nutrients requirements. The composition of crude lipid and crude protein in fertilized eggs was 7.24% ± 0.32% and 10.56% ± 0.41%, respectively. Seven kinds of essential amino acids (EAA) were detected but took a comparable lower content (3.19%) than other marine fish eggs. We randomly assembled 265.74 million clean reads and identified 1751 differentially expressed genes (DEGs) (P < 0.01) from pre-leptocephalus larvae. A total of 23 KEGG pathways related to the digestive and metabolic system were detected. Genes related to the secretion pathway of saliva, pancreatic juice and other digestive juices were significantly changed. Transcriptome analysis showed that as larvae aged, glycolytic metabolism and the transcription level of hexokinase (HK) increased significantly (day 0 to 12). This study will facilitate future studies on the nutrition of A. japonica larvae and other biological traits to reproductive research.
... As such, the present experiment was designed to identify the appropriate food amount that should be used to feed European eel larvae with the aim of improving feeding success and growth without compromising healthy larvae-bacteria interactions and larval wellbeing. For this, two liquid-type food amounts were tested: Low-0.5 mL food/L water (as in [9]) and High-1.5 mL food/L water (as in [18]) to evaluate the effects on feeding success, growth, and survival of European eel larvae. Moreover, the bacterial community composition of water and food, as well as the changes in larval bacterial community over time, were studied. ...
... At the end of day 9 (post-hatch), the larvae were transferred to replicated 8 L Kreisel tanks (n = 6) at a density of~180 larvae/L and randomly connected to one of two separate but identical RAS units (three Kreisel tanks per RAS), which were allocated to one of the two experimental treatments (Low and High). Considered the control group, the Low treatment received 0.5 mL food/L of water, as previously described for the European eel by [9], while the High treatment received 1.5 mL food/L of water, as previously described for the Japanese eel by [18]. Each RAS was composed of an upper sump reservoir of 370 L, which housed an 80 L wet/dry trickle filter filled with RK bio-elements (240 m 2 surface area or 0.12 m 2 per L), a lower sump reservoir (260 L) and a protein skimmer (Aquamedic 5000 single 6.0, Bissendorf, Germany). ...
Production of European eel offspring has become a reality, but liquid diets during larval culture hold new challenges. This study focused on increasing food amounts without compromising well-being or healthy larvae-bacteria interactions. First-feeding larvae were fed two food amounts (Low = 0.5 mL food/L water vs. High = 1.5 mL food/L water) until 30 days post-hatch (dph). Results indicated that ~75% of larvae ingested the diet in both treatments, but upregulation of a stress/repair-related gene (hsp90) on 25 and 30 dph indicated nutritional inadequacy. Larvae fed a High amount of food were 3.68% bigger, while larvae in the Low-food group showed 45.2% lower gut fullness and upregulated expression of the gene encoding the “hunger hormone” ghrelin (ghrl), indicating signs of starvation. The High-food group larvae exhibited a healthier bacteriome with a higher abundance of potentially beneficial orders (Lactobacillales and Bacillales), whereas the Low-food group showed more potentially harmful orders (Vibrionales, Rhodobacterales, and Alteromonadales). While survival was initially lower in the High-food group, both treatments had comparable survival by the end of the experiment. In conclusion, feeding European eel larvae with High food amounts seemed beneficial, supported by increased gut fullness, reduced ghrl expression (no starvation), enhanced growth, and the presence of a healthier bacteriome.
... The optimum dietary level of the highly unsaturated fatty acid, arachidonic acid, for Japanese eel larvae was also recently determined (Shahkar et al. 2016). Another improvement was found by diluting the slurry-type diet with water to make it easier for the larvae to ingest Okamura et al. 2019). Research showed that starting to feed the larvae at 5 days after hatching was advantageous compared to initial feeding at 7 or 8 days ). ...
... But enzymes would likely be unable to quickly digest all components of objects such as fecal pellets, so the assimilation process may be designed to absorb the easily digestible or absorbable components and then continue to ingest more food particles if they are available. Feeding of leptocephali on artificial diets suggests that growth is fastest when they feed on a semi-liquid diet Okamura et al. 2019), so soft easily digested and assimilated materials may be the target of what they consume. For example, when larvacean houses are observed in leptocephalus gut contents, they are usually accompanied by amorphous material, so it is not clear whether the target of the larvae was ingestion of the house or ingestion of other aggregates associated with the house. ...
Leptocephalus larvae have transparent bodies with tubular intestines that usually lack identifiable food items when they are collected, so mystery has surrounded efforts to determine what they feed on. Artificially spawned and reared first-feeding larvae were found to be highly selective in what they would eat, but they would consume rotifers and eventually ate specially formulated diets that contained shark egg yolk. Gut content studies on wild-caught leptocephali in the Atlantic and Pacific observed marine snow-associated materials such as discarded appendicularian houses, zooplankton fecal pellets, protists, and amorphous materials, and DNA sequencing indicated that the gut contents contain materials originating from a wide range of microorganisms and food web zooplankton species that were likely consumed in marine snow. Isotopic studies found a low trophic position of leptocephali and inter-taxa and geographic signature differences. Behavioral studies with leptocephali and the characteristics and size-scaling of the teeth are also consistent with feeding on marine snow-related particles. The feeding strategy of leptocephali appears to be based on consuming types of marine snow that contain nutritious and easily assimilated carbohydrates, fatty acids, and other materials that facilitate rapid conversion to glycosaminoglycans and tissues for energy storage and growth.
... The optimum dietary level of the highly unsaturated fatty acid, arachidonic acid, for Japanese eel larvae was also recently determined (Shahkar et al. 2016). Another improvement was found by diluting the slurry-type diet with water to make it easier for the larvae to ingest Okamura et al. 2019). Research showed that starting to feed the larvae at 5 days after hatching was advantageous compared to initial feeding at 7 or 8 days ). ...
... But enzymes would likely be unable to quickly digest all components of objects such as fecal pellets, so the assimilation process may be designed to absorb the easily digestible or absorbable components and then continue to ingest more food particles if they are available. Feeding of leptocephali on artificial diets suggests that growth is fastest when they feed on a semi-liquid diet Okamura et al. 2019), so soft easily digested and assimilated materials may be the target of what they consume. For example, when larvacean houses are observed in leptocephalus gut contents, they are usually accompanied by amorphous material, so it is not clear whether the target of the larvae was ingestion of the house or ingestion of other aggregates associated with the house. ...
... l −1 , which was consistent with the typical method for rearing eel larvae (10−50 ind. l −1 ) (Okamura et al. 2009a(Okamura et al. , 2019. The tank was supplied with 50% diluted seawater (salinity = 17.5) at a constant water temperature of 23°C and a flow rate of 1.5 l min −1 ; 50% seawater is advantageous for the growth and survival of eel larvae because of the lower energy cost of osmoregulation (Okamura et al. 2009b). ...
Even though reared leptocephalus larvae of the Japanese eel Anguilla japonica have a high incidence of notochord deformities (>60%), the cause is unknown. We performed histological examinations of the notochord and associated organs in reared larvae to better understand the process causing notochord deformation in eel larvae. In deformed larvae, unknown tissue thickening was discovered near the notochord sheath. Azan staining revealed that these tissue thickenings are most likely collagen fibers within fibrous connective tissue. This was almost identical to the connective tissue found in the primordium of the vertebral body around the notochord sheath in properly metamorphosing larvae. Furthermore, the amount of the thyroid hormone triiodothyronine (T3) was significantly higher in deformed larvae than in normal larvae, indicating that notochord deformity is probably linked to metamorphosis despite the immature stage of growth. We suggest that the aberrant growth of connective tissue surrounding the notochord sheath induced by incomplete metamorphosis causes deformities in eel larvae. The reason why deformed larvae have greater thyroid hormone levels is still unknown. It is important to assess how environmental and dietary factors affect the thyroid hormone levels of eel larvae raised in captivity.
... However, the commercialization of lab-produced larvae is 43 still hindered by the low survival rate of pre-leptocephalus larvae (Okamura, et al., 2014). 44 Low survival rate in lab is likely due to nutritional deficiencies in their artificial diets, despite 45 previous efforts to provide diets including shark egg yolk, Antarctic krill, and rotifer paste, additional 46 nutritional improvements are still needed (Liu, et al., 2017;Okamura, et al., 2019;Okamura, et al., 47 Agilent 7890 gas chromatograph (Agilent Technologies, CA, USA) equipped with a flame ionization 102 detector instrument. The 37-FAME Mix (Supelco, Bellefonte, PA, USA) was used to identify the FAMEs, 103 and the fatty acid C19:0 (nonadecanoic acid, Sigma) was used as an internal standard for fatty acid 104 quantification. ...
The starter diet for Japanese eel (Anguilla japonica) has always been a difficult problem for the realization of total artificial reproduction. Therefore, this research analyzed the nutritional composition of artificially fertilized eggs, and transcriptome of samples from early hatchlings of fry to better understand nutrients requirements. The composition of crude lipid and crude protein in fertilized eggs was 7.24% and 10.56%, respectively. Seven kinds of essential amino acids (EAA) were detected but took a comparable lower content (3.19%) than other marine fish eggs. We randomly assembled 265.74 million clean reads and identified 1751 differentially expressed genes (DEGs) (P < 0.01) from pre-leptocephalus larvae. A total of 23 KEGG pathways related to the digestive and metabolic system were detected. Genes related to the secretion pathway of saliva, pancreatic juice and other digestive juices were significantly changed. The genes of carbohydrate metabolism, glycerolipid metabolism and glycerophospholipid metabolism were up-regulated significantly with the growth of the larvae (day 0 to 12). This study will facilitate future studies on the nutrition of eel larvae and other biological traits to reproductive research.
... Flow rates were kept at ~500 mL/min. Temperature was maintained at ~20 • C and light (~500 lux) was only turned on during feeding (Butts et al., 2016;Okamura et al., 2019). ...
The transition from endogenous to exogenous feeding is critical during fish early life, where appropriate feed availability and timing of initiation of feeding influence survival. For European eel (Anguilla anguilla), establishing first feeding culture is at a pioneering state, where successful production of larvae has recently enabled feeding experiments. In the present study, three diets and potential benefits of early feeding during the transition from yolk-sac stage to feeding larvae were explored, including molecular analyses of genes involved in digestive functions and growth. Three consecutive trials were performed using hatchery produced eel offspring. In Feeding regimes 1 and 3, expression of npy and cck (appetite regulation) was higher, while expression of pomca (food intake) was lower in non-prefed larvae, indicating increased fasting and higher starvation risk. In contrast, Feeding regime 2 led to the highest survival ever registered for European eel larvae i.e. 20% at 20 dph, in spite that prefeeding resulted in reduced survival rate during the endogenous feeding stage. This was associated with initial hsp90 (stress/repair) upregulation in larvae receiving prefeeding, however, with subsequent downregulation during exogenous feeding. Notably, the growth related gh expression was higher in prefed larvae, indicating growth benefits of prefeeding. Likewise, prefeeding resulted in pomca as well as try, tgl, and amyl2a (digestion) upregulation, providing evidence of beneficial maturation of gut functionalities. Essentially, Feeding regime 2 demonstrated a continuous upregulation of growth, appetite and digestion related genes, which in combination with the highest survival suggest that dietary requirements were partially met. Moreover, in Feeding regime 2, gh and tgl were expressed at a higher level in prefeeding larvae than in the control, indicating that prefeeding might be advantageous in spite observed mortality, but further research is needed, including timing of feed application.
... Mochioka [27] reported the morphology and growth characteristics of larvae from 10 to 60 mm in total length (TL) in wild. Although there have been many studies related to the morphology of eel larvae under culture conditions, such as the morphology and TL during 0-8 DAH [28], the morphology, TL, and preanal length during 0-15 DAH [29], the morphology, TL, head length, and trunk height during 0-100 DAH [30], the morphology during 100-270 DAH [7], the morphology during 0-268 DAH [31], TL and trunk height at 25 and 55 DAH [32], TL during 0-80 DAH [11], distribution of TL at [33], and the morphology, TL, and preanal length during metamorphosis [34][35][36], there have been no previous studies regarding morphological development and allometric growth patterns during long-term. ...
The freshwater eel Anguilla japonica is rapidly decreasing in number and has not yet been successfully mass produced. This may be at least partially attributable to the unique and long early life history of the eel. Therefore, we investigated its ontogeny of morphometry and growth pattern in larval stages to provide baseline information for understanding the early life history and improving seed rearing technology. This study was conducted for 200 days after hatching (DAH) and analyzed morphometry and allometry for eel larvae. The following cultured eel larval stages were identified: the yolk sac larvae stage (0–6 DAH, 3.23–6.85 mm total length (TL)), the pre-leptocephalus stage (7–30 DAH, 6.85–15.31 mm TL), and the leptocephalus stage (50–200 DAH, 15.31–60.06 mm TL). Cultured and wild eel larvae could be divided into characteristic larval stages at similar sizes. However, compared to wild eels, cultured eels had a slower growth rate and fewer preanal myomeres. Meanwhile, cultured eel larvae rarely had a mixed feeding period as the absorption of endogenous reserves was completed by 7 DAH. The lower jaw of eel larvae was significantly longer than the upper jaw from 50 DAH. In the pre-leptocephalus and leptocephalus stages, eel larvae showed continuous positive allometric growth at trunk height and tail muscle height with change to the willow leaf-like form. These growth characteristics may be the result of adaptation to the migration over long distances and to a diel vertical migration. The inflection point in the body parts growth patterns showed only before 30 DAH, and mass mortality appeared at this period. Therefore, to improve the growth and survival rates of cultured eel seed, it is necessary to focus on improving the feeding and rearing protocol until 30 DAH.
... Ingestion refers to the feeding behavior carried out by the body in order to survive, ensure organ function and obtain energy for various activities. Ingestion is essential for growth and reproduction (Hatef and Unniappan 2019;Okamura et al. 2019). Animal adjusts food intake according to the change of external environmental factors, the change of energy state and appetite factor in the body, which is a complex and precise process (Volkoff 2019). ...
Amylin is a 37-amino acid polypeptide that has been shown to be involved in feeding regulation in a few mammals, birds and goldfish. To study whether amylin regulates the feeding of Siberian sturgeon, this study cloned amylin of Siberian sturgeon and detected the distribution pattern of amylin in 15 tissues, the expression level in periprandial (pre- and post-feeding), and the changes of food intake and related appetite factors after intraperitonal injection experiment. The results showed that the expression of amylin was highest in hypothalamus, followed by duodenum, telencephalon, forebrain, midbrain, heart and liver, and low in cerebellum, pancreas, valvular intestine and other detection tissues. Compared with 1h pre-feeding, the expression level of amylin in hypothalamus was significantly increased at 1h post-feeding (P < 0.05), and the expression level of amylin in duodenum was extremely significantly increased at 1h post-feeding (P < 0.01). Compared with the control group (normal saline), the food intake at 0-1h and 1-3h after intraperitoneal injection of 100 ng/g BW and 200 ng/g BW was significantly decreased (P < 0.05), and the food intake at 3-6h in 100 ng/g BW group was extremely significantly decreased (P < 0.01), and the food intake at 3-6h in 200 ng/g BW group was significantly decreased (P < 0.05). After 1h of injection of 100 ng/g amylin, MC4R in hypothalamus was significantly increased (P < 0.05) and somatostatin was extremely significantly increased (P < 0.01), while amylin and NPY were significantly decreased (P < 0.05). CCK in valvular intestine was extremely significantly increased (P < 0.01), insulin in duodenum was significantly increased (P < 0.05), but ghrelin in duodenum had no significant change (P > 0.05). These results showed that Amylin inhibited feeding in Siberian sturgeon by down-regulating appetite stimulating factor NPY and up-regulating appetite suppressing factors somatostatin , MC4R , CCK and insulin .
... The need for artificial production of glass eels of the Japanese eel Anguilla japonica has increased because natural sources of Japanese eels have been declining in recent decades . Although 2500-3000 glass eels can be produced in laboratory-scale tanks, commercialscale production is not yet feasible (Okamura et al. 2014(Okamura et al. , 2019Tanaka 2015). ...
This study assessed the effects of diets supplemented with different types of chitin hydrolysate on larval growth of the Japanese eel Anguilla japonica . Larvae were reared for 26 days on artificial diets containing 2% of purified monomeric N-acetylglucosamine (NAG1), a mixture of mono-, di- and trimers (NAG3), a mixture of mono-, di-, tri-, tetra-, penta- and hexamers (NAG6), or a control diet. The larvae fed NAG3 and NAG6 grew significantly more rapidly than the control larvae (p < 0.05). However, the survival rate of the NAG6-fed larvae was low (8%) for unknown reasons, whereas the survival rates of the NAG1 and NAG3-fed larvae were similar to that of control larvae, about 46% (p > 0.9). These results indicate that NAG3 is the most effective supplement for eel larvae. A dose–response analysis showed that the optimal level of NAG3 in the diet was around 1%. Although the biological activity of chitin hydrolysates in eel larvae remains unclear, part of these hydrolysates are likely utilized by eel larvae as components of glycosaminoglycan, a major constituent of their bodies. Alternatively, the hydrolysates may act as a bioactive substance, influencing fish health, or in the effective utilization of energy in eel larvae, enhancing their growth.
... To determine survival and growth of post-larval stage (leptocephalus stage) eels, 10 batches were reared using the method deemed most practical (Okamura et al., 2019). Pre-larvae soon after hatching were transferred from the hatching tank to a 180-l polycarbonate tank (Fig. 1) at a mean density of approximately 100 individuals per liter. ...
Because the post-larval (leptocephalus) survival rates of batches of Japanese eels vary widely, a useful indicator is needed to identify good-quality larval batches before the time of first feeding. This study therefore evaluated the effects on post-larval survival of early ontogenetic variables, including rates of fertilization, abnormal cleavage, hatching, survival during the larval stage until 5 days post hatching (dph), and deformities of individual larvae reared in wells of 48-well microplates. In addition, the survival rate of larvae reared in a 180-l mass rearing tank with flowing seawater was measured. Both simple and multiple linear regression analyses showed that none of the variables assessed by the microplate method correlated with post-larval survival, although the survival in the mass rearing tank at 5 dph was significantly associated with post-larval survival. These results suggest that early ontogenetic variables were likely strongly influenced by incubation methods, with the optimal method of identifying good-quality larvae being to measure the survival rate of mass reared rather than individually reared larvae. We suggest that larvae reared by the microplate method were likely subjected to excessively reduced environmental stress, due to the presence of antibiotics and the lack of interference between individuals, resulting in an overestimation of larval quality.
... Anguilla celebesensis that had just started their spawning migration showed remarkably advanced gonadal development compared with migrating eels of other extensively studied species (Hagihara et al., 2012;Arai, 2014), so understanding the reproductive physiology of A. celebesensis may be important not only for basic science but also for applied science. Since succeeded in producing fertilised eggs and larvae by artificial hormone treatment, research on artificial induction of gametogenesis and seed production of anguillid eels has greatly increased (Ohta et al., 1996Kagawa et al., 1995Kagawa et al., , 1997Kagawa et al., , 1998Tanaka et al., 2003;Palstra et al., 2005;Oliveira & Hable 2010;Masuda et al, 2012;Tanaka, 2015;Okamura et al., 2019;Yamada et al., 2019). However, unstable egg quality that leads to low hatching and survival rates remains one of the main obstacles to mass production of glass eels (Kagawa et al., 2005;Ijiri et al., 2011) and several previous studies have pointed out that poor egg quality may be associated with physiological abnormalities in maturing process of artificially matured eels ( Saito et al., 2003;Suetake et al., 2003;Izumi et al., 2015Izumi et al., , 2016). ...
Downstream‐migrating (n = 64) and non‐migrating (n = 21) female Celebes eels Anguilla celebesensis were captured from the Poso Lake–River system on Sulawesi Island, Indonesia, and their reproductive physiological characteristics were examined. A histological observation of the ovaries revealed that most non‐migrating eels were at the perinucleolus (43%) or oil‐droplet (48%) stage, whereas most migrating eels were at the early vitellogenic (36%) or midvitellogenic (61%) stage. Transcript levels of gonadotropin genes (fshb, lhb) in the pituitary gland and concentrations of sex steroids [11‐ketotestosterone (11‐KT), testosterone, 17β‐oestradiol (E2)] in blood plasma of migrating eels were significantly higher than those of non‐migrating eels. The fshb messenger (m)RNA levels were lower in perinucleolus and oil‐droplet stages and then significantly increased in the early vitellogenic stage. The lhb mRNA levels in vitellogenic‐stage eels were significantly higher than those in perinucleolus‐ and oil‐droplet‐stage eels. The 11‐KT levels of eels at the oil‐droplet and vitellogenic stages were significantly higher than those of eels at the perinucleolus stage. The E2 levels at the vitellogenic stage were significantly higher than those at the perinucleolus and oil‐droplet stages. These dynamics of the reproductive hormones represented the physiological background of oogenesis in A. celebesensis that has remarkably well‐developed oocytes just before downstream migration.
The only diet that has been capable of rearing eel larvae to the glass eel stage in captivity is the shark egg (SE)-based diet (SE). This study investigated the potential of alternative dietary components, namely hen egg yolk (HEY), milk proteins, and fish protein hydrolysate (FPH), on the growth and survival of eel larvae. In the first experiment, a diet containing HEY and skimmed milk powder (HS) was compared to SE. There were no significant differences in growth and survival rate between the two diets except in the early part of the experiment period. In the second experiment, HS was modified by the addition of FPH and casein (FC), and larval performance was evaluated for three dietary regimens (SE, HS, and FC). The performance of larvae fed HS and FC was found to be comparable or superior to those fed SE. It was observed that larvae fed the alternative diets to SE were able to progress to glass eels. However, more skeletal abnormalities were observed in HS in experiment 1. The results of this study indicate that a combination of HEY and milk proteins is suitable as a larval eel diet, and that it has the potential to replace SE.
Closing the life cycle of European eel (Anguilla anguilla) in captivity is targeted to provide a sustainable, year-round supply of juveniles for aquaculture. Present focus is on the nutritional requirements during the larval first-feeding period. In this study, three experimental diets were tested on hatchery-produced European eel larvae from the onset of the first-feeding stage commencing 10 days post hatch (dph) until 28 dph. Larval mortality was recorded daily, while sampling was conducted at regular intervals to record larval biometrics and analyze the expression of genes related to digestion, appetite, feed intake and growth. Two periods of high mortality were identified: the first appeared shortly after introduction of feeds (10–12 dph), while the second occurred 20–24 dph, indicating the “point of no return”. This interpretation was supported at the molecular level by the expression of the gene encoding the “hunger hormone” ghrelin (ghrl) that peaked at 22 dph in all dietary trials, suggesting that most larvae were fasting. However, in larvae fed diet 3, ghrl expression was downregulated beyond 22 dph, which indicated that those larvae were no longer starving at this stage, while upregulation of genes encoding the major digestive enzymes (try, tgl, and amyl2a) advocated their healthy development. Moreover, for larvae fed diet 3, the expression of those genes as well as genes for feed intake (pomca) and growth (gh) continued to increase towards 28 dph. These results together with the registered highest survival, largest dry weight increase, and enhanced biometrics (length and body area) pointed to diet 3 as the best-performing. As a whole, this first-feeding study represents a landmark being the first to document European eel larval growth and survival beyond the point of no return, providing novel insights into the molecular development of digestive functions during the first feeding stage.
Metamorphosis of teleosts including Anguilliformes is well known to be induced by thyroid hormone (TH), although the underlying mechanism is not fully understood. In this study, we investigated the experimental conditions needed to induce normal metamorphosis in artificially spawned Japanese eel (Anguilla japonica), including initial larval size, TH concentration, and timing of TH immersion. Around 37 mm TL was found to be the minimum size of larvae that underwent successful metamorphosis induced by l-thyroxine (T4); notably, smaller larvae did not show increased expression of TH receptors in response to T4, suggesting that small leptocephali are not sufficiently responsive to TH. Furthermore, successful completion of metamorphosis depended on sensitivity to TH, which changed with metamorphic stage; for example, prolonged exposure to higher TH concentrations led to morphological defects. Collectively, these results reveal that the induction of metamorphosis by TH is dependent on larval size, and that the concentration of TH must be adjusted in line with metamorphic stage to achieve successful progression of metamorphosis. Our findings will contribute to improving production technology in the aquaculture of Japanese eels by facilitating the earlier induction of metamorphosis in artificial leptocephali.
Amylin is a 37-amino acid polypeptide that has been found to be involved in feeding regulation in some mammals, birds, and goldfish. We cloned amylin of Siberian sturgeon and detected its distribution pattern in 15 tissues. The expression levels in the periprandial period (pre-and post-feeding), the changes in the food intake, and the expression levels of related appetite factors after the intraperitoneal injection of amylin were detected. The expression of amylin was found to be the highest in the hypothalamus. Compared with 1 h pre-feeding, the expression levels of amylin in the hypothalamus and duodenum were increased significantly 1 h post-feeding. Compared with the control group (saline), intraperitoneal injection of 50 ng/g, 100 ng/g, and 200 ng/g of amylin significantly inhibited food intake at 1 h post injection, but not at 3 h and 6 h. The injection of 50 ng/g, 100 ng/g, and 200 ng/g amylin significantly inhibited the cumulative feed. After 1 h of 50 ng/g amylin injection, the levels of MC4R and somatostatin in the hypothalamus increased significantly, while the levels of amylin and NPY decreased significantly. The levels of CCK in the valvular intestine were increased significantly. Insulin in the duodenum was also increased significantly, but there was no significant change in ghrelin in the duodenum. These results show that amylin inhibits feeding in Siberian sturgeon by down-regulating the appetite-stimulating factor NPY and up-regulating the appetite-suppressing factors somatostatin, MC4R, CCK, and insulin. This study provides a theoretical basis for studying the feeding function and action mechanisms of amylin in Siberian sturgeon.
We conducted sensory evaluation and physicochemical analyses of grilled Japanese eel Anguilla japonica farmed by hatchery-based aquaculture (HBA) following controlled captive reproduction. A total of 4939 artificial glass eels were reared in a commercial farm in Shizuoka, Japan for 12 months among which 50 adults (201 g average body weight) were grilled without sauce in a continuous two-sided grilling machine. Eels cultured by capture-based aquaculture (CBA) from seedlings of wild-caught glass eels in the same farm, were used as controls. Sensory evaluation showed that HBA grilled eels had a more fatty taste and harder texture than CBA grilled eels did. Chemical analyses revealed no significant difference in crude fat content between HBA and CBA eels, but the CBA eels contained more n-3 polyunsaturated fatty acids, possibly influencing the evaluation of fatty taste. Texture analysis indicated significantly harder flesh and skin in HBA than CBA eels, corresponding to the sensory evaluation. This information should be useful for product development of HBA eels.
The European eel has recently been included on the Red List of the International Union for Conservation of Nature (IUCN) as a critically endangered species. The rearing of Anguilla larvae is seen as a key bottleneck to the mass production of glass eels since very little ecological information is available regarding their natural nutrition. Studies of digestive physiology and ontogenetic development in eel larvae could provide useful information for solving some of the puzzles regarding larval fish culture. The aim of this study was to characterize the ontogeny of pancreatic enzymes (trypsin, lipase and amylase) and a peptide hormone regulator of pancreatic secretion (cholecystokinin) in terms of gene expression in European eel larvae from day 0 (P0) of hatching to 5, 10, 15 and 20 days post hatching during fasting. The results in the present study showed that all the genes selected were present, with different levels of expression and increasing trends, during larval development. At P0, the increase in the gene expression of lipase and amylase was higher than that of trypsin and cholecystokinin, confirming that enzymatic activity began before mouth opening and that larvae, provided with a complete enzymatic set, might have the capacity of digesting and absorbing various nutrients.
We assessed the effects of starvation, body size, and water temperature on the onset of metamorphosis in leptocephali of Japanese eel (Anguilla japonica Temminck and Schlegel, 1846) as determined by the morphological criteria of proportion, preanal length, and body depth to total length. Leptocephali of mean total length 55.6 mm that had been reared in captivity for 241 days from hatching were divided into unfed (n = 28) and fed (n = 30) groups in triplicate and reared for an additional 2 weeks. The mean percentage of larvae starting metamorphosis within 2 weeks was significantly higher in the unfed than in the fed groups (70% vs. 28.6%), suggesting that food deprivation acted as a cue for metamorphosis. The critical size for metamorphosis was a total length of 50–55 mm; smaller larvae did not start metamorphosis even in the absence of food, whereas larvae reaching that critical size were induced to undergo metamorphosis by starvation. The start of metamorphosis under unfed conditions was independent of diel-varying water temperature (day 23 °C; night 21–29 °C), suggesting a high plasticity in response to a wide range of environmental temperatures. These findings suggest methods for the efficient production of glass eels, as well as new insights into the mechanism of eel metamorphosis.
Eel aquaculture, though thriving nowadays, is totally dependent on the successful capture of wild eel fry and glass eels for its seedlings. The declination of eel resources in recent years has resulted in an urgent need for technology development in artificial seedlings production on an aquaculture basis, in order to protect natural resources and to stabilize the eel supply in the farming industry. Since the life history of the eel holds many mysteries, artificial hatching and rearing of larvae has long been regarded as an extremely difficult task. However, in recent years, the spawning ground of the Japanese eel has finally been located after continuous effort with intensive marine surveys, in which wild parental eels were captured, followed by the collection of fertilized eggs and the harvest of newly hatched preleptocephali. Meanwhile, through the collaborative efforts of many researchers, progress has also been made in improving technologies for artificial maturation of parental eels, which do not mature naturally in captivity, as well as in the technology for artificial hatching. Moreover, a technology for producing feed-rearing eel hatchlings, the most challenging process of all, has advanced rapidly after suitable feed was developed in the 1990s. Then, in 2002, for the first time in the world, larvae were successfully reared up to the glass eel stage, and second generation artificial hatchlings were born in 2010. In this way, eel farming technology that is not reliant on natural resources has been developed. There are strong hopes now for a technology for stable mass production of glass eels to be developed in the near future.
Usual diets for rearing leptocephalus larvae of Japanese eel Anguilla japonica include eggs of the endangered spiny dogfish Squalus acanthias (SE). We investigated the effects of alternative food materials, hen egg yolk (HEY) and exoskeleton-free (skinned) Antarctic krill (SAK), on the growth and survival of eel larvae. We found that feed comprising whole krill including exoskeleton (WAK) containing higher levels of fluoride (37.89 mg kg−1) was acutely toxic to eel larvae exposed to this alone. In contrast, extract from SAK containing lower concentrations of fluoride (4.25 mg kg−1) showed no apparent adverse effects. Growth of larvae fed a mixture of SE and SAK in a feed trial of 58 days [mean body weight (BW), 6.0 mg] was about twofold higher than that of larvae fed a mixture of SE and WAK (3.2 mg) (P < 0.01). A mixture of HEY and SAK also had some dietary benefits for eel larvae, enabling them to survive for up to 58 days and to grow significantly (mean BW, 2.4 mg), compared with their initial weight (mean BW, 0.2 mg) (P < 0.001). Although additional nutritional improvements are needed, the present results suggest that combination diet HEY and SAK may be a good alternative to SE as an effective diet for eel larvae.
Remarkable progress in techniques for maturation of adults and rearing of larvae has been achieved for the Japanese eel Anguilla japonica over the past 50 years, but recent efforts have not yet succeeded in the mass production of glass eels. This article reviews recent advances in techniques for artificial production of A. japonica glass eels. Successful new protocols for obtaining viable eggs and larvae of A. japonica are largely resulting from using artificially feminised eels instead of wild silver eels. The feminisation technique by administration of 17β-estradiol to glass eels not only provides an opportunity for using females as experimental broodstock throughout the year, but also accelerates their oocyte development or even increases the competence of females to respond to salmon pituitary extract. Induced spontaneous spawning with paired eels can improve the success rate for obtaining good-quality eggs compared with the traditional stripping-insemination method. As a result of the accumulation of field data on natural environmental conditions, such as an optimum temperature, where eel eggs and larvae were collected, the survival and growth rate of captive eel leptocephali have been much improved under these conditions. However, 50% diluted sea water (17.5 psu) that is far from the natural condition can result in better growth and survival performance in A. japonica leptocephali, possibly because it reduces the energy necessary for osmoregulation. Starvation can be a cue for triggering the onset of metamorphosis in A. japonica leptocephali, reducing the prolonged duration of the larval stage in captivity. To reach the mass production of glass eels at a commercial level, further improvement in quality of an artificial larval diet not containing eggs of the endangered shark Squalus acanthias is needed. The control of deformities in captive leptocephali and glass eels is also an important task. New techniques reducing the excessive use of exogenous hormones with parent eels should also be developed, to avoid public perception of an unhealthy product. If these techniques are completely established, we can exchange artificially produced glass eels with some part of the wild glass eels that are heavily exploited and provided to the eel aquaculture industry, and this can help to reduce the impact of eel consumption, which will contribute to the conservation of eel species worldwide.
The larvae of anguillid eels are termed leptocephali, among other reasons, because they differ greatly in morphology and physiology from other fish larvae. Leptocephali grow to large sizes but are transparent and fragile and have an unusual feeding strategy, so their behavior or ecology has rarely been Studied. We used leptocephali, metamorphosing larvae and glass eels reared artificially from eggs to study the ontogenetic change in phototaxis in Japanese eel Anguilla japonica. By Studying the early life history of this eel, Our intent was to learn more about behavioral mechanisms associated with their vertical migration in the ocean and their long migration from the offshore spawning area to coastal waters. Horizontal distribution experiments found a clear negative phototaxis in the leptocephalus and metamorphosing larval stages, but no phototaxis was detected after metamorphosis into glass eels. Vertical distribution experiments found that overhead lighting caused downward movement in leptocephali and metamorphosing larvae. When kept in darkness, leptocephali and metamorphosing larvae were distributed near the surface layer at night and at the bottom (during daylight hours, Suggesting an endogenous circadian rhythm that. may control their diel vertical migration. Glass eels exhibited benthic distribution Under any light conditions (0.0039 to 3.9 mu mol quanta m(-2) s(-1)), but showed random vertical distribution in darkness during both daytime and night-time, a behavior which appeared to be controlled only by light. Swimming speeds of leptocephali during horizontal movement and rising behavior were significantly greater than during diving behavior. These swimming behaviors and ontogenetic changes in phototactic behavior may help regulate the vertical distribution and inshore migration of anguillid eel larvae in the ocean.
To improve techniques for artificial seed production of the Japanese eel Anguilla japonica, we evaluated two types of artificial fertilization methods: i) the spontaneous spawning method of paired specimens and ii) the stripping-insemination method, by comparing the quality of eggs and larvae obtained. Females artificially matured by the injection of salmon pituitary extract were kept: at 20 degrees C before final treatment with 17 alpha,20 beta-dihydroxy-4-pregnen-3-one. Afterwards, each female for i) the spontaneous spawning method was kept in an individual tank with three males and water temperature was raised to 22 degrees C. For ii) the stripping-insemination method, ovulated fish kept at 20 degrees C for about 15 hrs were stripped and eggs obtained were inseminated. Eggs and larvae obtained by i) the spontaneous spawning method (n = 12) showed significantly higher fertility (80 +/- 14%), hatching rate (62 +/- 23%), survival rate at 5 days after hatching (54 +/- 22%), and lower deformity at 5 days after hatching (60 23%) than those obtained by ii) the stripping-insemination method (n = 14, 41 +/- 32%, 31 +/- 29%, 27 +/- 25%, 79 +/- 19%, respectively). These results show that the spontaneous spawning method at 22 degrees C is better than the stripping-insemination method at 20 degrees C for the fish quality in artificial eel seed production.
What eel larvae feed on in the surface layer of the ocean has remained mysterious. Gut contents and bulk nitrogen stable isotope studies suggested that these unusual larvae, called leptocephali, feed at a low level in the oceanic food web, whereas other types of evidence have suggested that small zooplankton are eaten. In this study, we determined the nitrogen isotopic composition of amino acids of both natural larvae and laboratory-reared larvae of the Japanese eel to estimate the trophic position (TP) of leptocephali. We observed a mean TP of 2.4 for natural leptocephali, which is consistent with feeding on particulate organic matter (POM) such as marine snow and discarded appendicularian houses containing bacteria, protozoans and other biological materials. The nitrogen isotope enrichment values of the reared larvae confirm that the primary food source of natural larvae is consistent only with POM. This shows that leptocephali feed on readily available particulate material originating from various sources closely linked to ocean primary production and that leptocephali are a previously unrecognized part of oceanic POM cycling.
We investigated the effects of low-salinity water on the growth, survival, and activity of artificially reared Japanese eel larvae (Anguilla japonica), proceeding from the assumption that such water quality saves energy due to lower cost for osmoregulation. We reared 5-day-old larvae in 0%, 10%, 30%, 50%, 70%, and 100% seawater (SW) with or without diet for 13 days. All larvae reared in 0% and 10% SW died within 6 days, while larvae in 70% and 100% SW survived until 9 days. Larvae in 30% and 50% SW further survived until 13 days without diet. Significant growth in body depth was observed in 30% and 50% SW after 7 days rearing with diet (0.65 ± 0.02 and 0.62 ± 0.02 mm, respectively) as compared with the initial size (0.49 ± 0.03 mm), while no significant growth was observed under the other salinity conditions examined. Larvae swam actively in the light (about 2000 lx) in 50%, 70%, and 100% SW, while they were apparently inactive in 0%, 10%, and 30% SW. The long-term rearing trial showed a 2.2-fold higher 2-month cumulative survival rate in 50% SW (18.2%) than in 100% SW (8.2%). The body depth of larvae in 50% SW (1.58 ± 0.47 mm) was also significantly larger than in 100% SW (1.32 ± 0.35 mm). These findings indicate that the intermediate salinity can result in better growth and survival performance in Japanese eel larvae.
European eels (Anguilla anguilla) undertake spawning migrations of more than 5000 km from continental Europe and North Africa to frontal zones in the Sargasso Sea. Subsequently, the larval offspring are advected by large-scale eastward ocean currents towards continental waters. However, the Sargasso Sea is oligotrophic, with generally low plankton biomass, and the feeding biology of eel larvae has so far remained a mystery, hampering understanding of this peculiar life history. DNA barcoding of gut contents of 61 genetically identified A. anguilla larvae caught in the Sargasso Sea showed that even the smallest larvae feed on a striking variety of plankton organisms, and that gelatinous zooplankton is of fundamental dietary importance. Hence, the specific plankton composition seems essential for eel larval feeding and growth, suggesting a linkage between eel survival and regional plankton productivity. These novel insights into the prey of Atlantic eels may furthermore facilitate eel larval rearing in aquaculture, which ultimately may replace the unsustainable use of wild-caught glass eels.
Development of artificial production of glass eels still presents many problems to be solved. What and how to feed larvae is one of the most crucial problems for commercial mass production of glass eels. We evaluated a new liquid-type diet and feeding method against the conventional slurry-type diet by comparing the growth and survival of larvae in triplicate for 27 days after hatching. Larvae were fed by immersion in a liquid-type diet that was a 1.5 times dilution of the conventional slurry-type diet based on dogfish yolk spread over the bottom of the rearing tank. Larvae took more and denser liquid-type food in the intestine than the conventional slurry-type diet. Survival rates at 3 weeks after first feeding were about 90% in most experimental groups (5 groups) except one of three slurry-type diet groups (about 70%). Growth rates were 1.35 times higher in liquid-type diet groups (0.27 mm/day at 27 dph) than in conventional slurry-type diet groups (0.20 mm/day). These results suggest that the immersion feeding method in conjunction with the liquid-type diet has the potential to enable large scale production of glass eels by ensuring high growth and survival.
Being part of the elopomorph group of fishes, Anguillidae species show a leptocephalus larval stage. However, due to largely unknown spawning locations and habitats of their earliest life stages, as well as their transparency, these Anguilla larvae are rarely encountered in nature. Therefore, information regarding the early life history of these larvae, including their exogenous feeding strategy and feeding performance, is rather scarce. To better understand the structural basis and functional performance of larval feeding in captivity, the functional morphology of the cranial musculoskeletal system in pre- and first-feeding engyodontic leptocephali of the European eel (Anguilla anguilla) was studied. A 3D reconstruction of the feeding apparatus (head of the leptocephali < 1 mm) was used to visualize and describe the musculoskeletal changes throughout these stages. To analyze the ontogenetic changes in the functionality of the feeding apparatus towards the active feeding phase, 3D data of joints, levers and muscles derived from the reconstructions were used to estimate bite and joint reaction forces (JRFs). Observing a maximum estimated bite force of about 65 μN (and corresponding JRFs of 260 μN), it can be hypothesized that leptocephalus larvae are functionally constrained to feed only on soft food particles. Additionally, potential prey items are size delimited, based on the theoretically estimated average gape of these larvae of about 100 μm. This hypothesis appears to be in line with recent observations of a diet consisting of small and/or gelatinous prey items (Hydrozoa, Thaliacea, Ctenophora, Polycystenia) found in the guts of euryodontic leptocephalus larvae.
© 2015 Anatomical Society.
The high fecundity of many marine fishes implies that an extremely high rate of mortality must be experienced by each year-class. Most of this mortality occurs during the pelagic larval stage, and for this reason the characteristics of larval mortality are intimately related to basic problems in the population dynamics of fishes, including density-dependent regulatory mechanisms, the relation between stock and recruitment, and the determination of year-class strength (Beverton, 1962; Gulland, 1965; Hempel, 1965; Cushing, 1969). The critical period concept may be defined as the proposal that most larval mortality is concentrated during a relatively short period in early development. This review will first consider several variants of this broad definition and then examine evidence for a critical period (in its most widely accepted sense) in marine fishes.
The gut contents and ultrastructure of midgut mucosal cells were examined in the leptocephali of Conger myriaster. Detrital aggregates less than 20 mum in diameter and fecal pellets of zooplankton, which were collectively called 'particulate organic matter' (POM), were found in the gut of 78 % of leptocephali examined. These particulate materials were also present in over 90 % of the gut of C japonicus and Muraenosox cinereus leptocephali which were collected in the same waters. The POM is thought to be a major food item of leptocephali. Gut pigment contents were low in C. myriaster, indicating no feeding on phytoplankton. Active seawater ingestion was ultrastructurally indicated in midgut mucosal epithelium, which suggested that dissolved organic matter (DOM) was another possible nutritive source. The stable nitrogen isotopic composition of C. myriaster leptocephali, a reliable indicator of trophic position in food webs, was at the lowest level, equal to that of POM. This result strongly supports our suggestion that POM and DOM are food sources for eel leptocephali.
Gut contents of 234 leptocephali comprising eight species of eels were examined from five families (Congridae, Muraenidae, Muraenesocidae, Nettastomatidae, and Ophichthidae). The larvae belonged to developing leptocephalus (215 specimens) and the early metamorphic stage (19 specimens). Visible gut contents were recognized in 111 individuals among the eight species, regardless of developmental stage. Two kinds of visible objects, transparent (0.4 to 1.2 mm) and opaque (20 to 380 m), were found in the gut of leptocephali. From their morphological characteristics, the former and the latter were identified as larvacean houses and zooplankton fecal pellets, respectively. Furthermore, most fecal pellets in the gut were identified as oikopleurid larvacean's fecal pellets. No trace of the many other phytoplankton or zooplankton, which were found with leptocephali in the ambient waters and could be suitable-size food, was found in the gut of any leptocephalus. On the basis of the importance of larvecean houses in the diet of several species of leptocephalus larvae, it is proposed that the peculiar, large, fang-like teeth of leptocephali are used for feeding, and evolved to pierce and grasp the mucous houses of larvaceans.
Repeated injections of salmon pituitary extract (20 mg per fish per week) induced vitellogenesis in feminized, cultivated Japanese eels (Anguilla japonica). Oocytes were attained at the migratory nucleus stage after 11 or 12 injections. Addition of 17,20-dihydroxy-4-pregnen-3-one (DHP) into the incubation medium induced germinal vesicle breakdown (GVBD) in the oocytes at the migratory nucleus stage. An injection of DHP (2 g g-1 BW), given 24h after an injection of salmon pituitary extract (20 mg fish-1), succeeded in inducing maturation and ovulation in females which contained occytes at the migratory nucleus stage. Most fish ovulated 15–18h following the DHP injection. Eggs that were ovulated within 15h after the DHP injection showed high fertility and hatchability, but eggs ovulated 18 or 21h after the DHP injection, showed considerably lower fertility and hatchability. A delay between ovulation and stripping of the eggs rapidly decreased both the fertility and hatchability within 6–9h after ovulation, indicating that artificial fertilization must be carried out immediately after ovulation. Repeated injections of human chorionic gonadotropin (hCG) at a concentration of 1 IU g-1 BW week-1 induced spermatogenesis, spermiation, and the acquisition of potential for sperm motility in cultivated males. Most males spermiated after the fifth or sixth injection of hCG, and the milt weight gradually increased and remained constant (1–2 g) from the 11th to 31th injection. Sperm motility peaked 24h after each weekly injection, and decreased from the 3rd day after the injection. Potassium ions are an essential constituent for the maintenance of motility in the eel spermatozoa. Artificial seminal plasma containing 15.2 mM KCl is applicable as a milt diluent. Using these techniques developed for female and male eels, we have succeeded in obtaining many fertilized eggs from cultivated eels.
Despite intensive research on wild and captive eels, no resource has so far provided access to all life cycle stages of the Japanese eel Anguilla japonica. The transition from the preleptocephalus (newly hatched larva) to the leptocephalus stage (typical leaf-like eel larva) has, therefore, remained the missing link in the eel life cycle. We recently found that a slurry-type diet made from shark egg powder is suitable feed for captive-bred eel larvae. The larvae were successfully reared with this diet in aquaria for 100 days and raised to 22.8 mm in total length (TL). Age, TL, and body proportions of the reared specimens overlapped with those of wild leptocephali. We revealed for the first time the transition from the preleptocephalus to the leptocephalus stage of the eel.
To investigate the unavailability of Brachionus rotifers as an initial food for rearing preleptocephalus larvae of the Japanese eel, the ontogenetic morphological changes during the early developmental stages of eel with special reference to the organs for larval feeding were studied in captivity. Fertilized eggs were obtained from matured adults induced by hormonal treatments. The eggs were incubated at 21°C and 25°C in 33-34 psu sea water, and the newly hatched larvae were reared until the completion of yolk-absorption. Hatching of larvae began 45.0 h after the fertilization at 21°C and 31.5 h at 25°C, and terminated within 3.5 to 4.0 h after the onset of hatching. The absorption of yolk completed on 10 dph at 21°C (211.7 day-degrees) and 8 dph at 25°C (201.5day-degrees), and simultaneously four pairs of characteristic sharp-pointed larval teeth fully developed on both of upper and lower jaws. A series of histological sections of oesophageal part from 6 dph to 10 dph specimen showed that eel larvae had a characteristic thicker tissue layer and well-developed circular muscles surrounding larval gullet without mucous cells. ホルモン処理によって人工催熟した親魚から搾出した卵及び精子を人工授精し, その受精卵から孵化仔魚を得た。孵化仔魚は卵黄吸収が終了するまで, 21°C と 25°C, 塩分 32-34, 0-数 lx の暗条件下で飼育管理し, 日々の連続標本からその外部形態及び摂餌に関わる器官の発達を観察した。孵化仔魚は, 水温 21°C では日齢 10 (211.7°C・日), 25°C では日齢 8 (201.5°C・日) で卵黄吸収を終了し, 口には上下顎それぞれ 4 対の針状幼歯が特徴的となった。この時期, HE 染色された組織切片を見ると, 咽頭部に食道壁の肥大により狭窄した部位が観察され, またその内壁には粘液細胞もほとんど発達しないため, ワムシのような固形分の嚥下の困難が想像された。
A colloid-type diet can be ingested by larvae of the Japanese eel
- Y Masuda
- H Oku
- K Nomura
- K Teruya
- H Tanaka
A colloidtype diet can be ingested by larvae of the Japanese eel Anguilla japonica
- Y Masuda
- H Oku
- K Nomura
- K Teruya
- H Tanaka
Masuda Y, Oku H, Nomura K, Teruya K, Tanaka H (2010) A colloidtype diet can be ingested by larvae of the Japanese eel Anguilla
japonica. J Fish Technol 2:99-104 (in Japanese with English
abstract)
Larval mortality in marine fishes and the critical period concept
- R C May
May RC (1974) Larval mortality in marine fishes and the critical
period concept. In: Blaxter JHS (ed) The early life history of fish.
Springer, Berlin, pp 3-19