Temporal changes in digestive enzyme activities in the gastrointestinal tract of European eel (Anguilla anguilla) (Linneo 1758) following feeding

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Temporal changes in digestive enzymeactivities in the gastrointestinal tractof European eel (Anguilla anguilla)(Linneo 1758) following feedingGabriella Caruso a , Maria G. Denaro b & Lucrezia Genovese aa Instituto per l’Ambiente Marino Costiero (IAMC), NationalResearch Council, Messina, Italyb Department of Life Sciences ‘M. Malpighi’, University ofMessina, Messina, ItalyPublished online: 11 Dec 2008.

To cite this article: Gabriella Caruso , Maria G. Denaro & Lucrezia Genovese (2008): Temporalchanges in digestive enzyme activities in the gastrointestinal tract of European eel (Anguillaanguilla) (Linneo 1758) following feeding, Marine and Freshwater Behaviour and Physiology, 41:4,215-228

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Marine and Freshwater Behaviour and PhysiologyVol. 41, No. 4, December 2008, 215–228

Temporal changes in digestive enzyme activities in the gastrointestinal tract

of European eel (Anguilla anguilla) (Linneo 1758) following feeding

Gabriella Carusoa*, Maria G. Denarob and Lucrezia Genovesea

aInstituto per l’Ambiente Marino Costiero (IAMC), National Research Council, Messina, Italy;bDepartment of Life Sciences ‘M. Malpighi’, University of Messina, Messina, Italy

(Received 19 December 2007; final version received 15 September 2008)

Changes occurring after feeding in the digestive enzyme activities of European eelwere investigated to provide some insights into the digestive physiology of thisfish. Total and specific proteases, amylase and lipase activities were measuredusing standard biochemical assays over a 24 h cycle in fed eels, compared tostarved ones, under the same rearing conditions. In the gastrointestinal tract offed eels quantitative changes started 4 h after feeding and continued later on;conversely, in starved eels enzyme activities remained unchanged over time. In fedeels, total and specific protease activities showed an overall increasing trend in theintestine, while in the stomach they progressively decreased to values 22–50%lower than those measured at the pre-feeding time; this behaviour probablyreflected the progression of digesta along the intestinal tract. The prolongedsecretory response of European eel to food ingestion proved its extended activityin the digestive process.

Keywords: eel; Anguilla anguilla; digestive enzyme; digestive tract; fed; non-fed

Introduction

European eel (Anguilla anguilla, Linneo 1758) represents an important economic resourcefor Italian fish farming because of its high nutritional quality and high adaptability tofreshwater and brackish rearing conditions; due to the increasing market demand, it is themost common cultured fish after trout and carp. The increased fishing pressure inassociation with the decline in its continental and spawning stock (Dekker 2003), has led toexpanding interest towards its basic biology and husbandry as a prerequisite to improve itsculture practices (Brusle 1991; Gallagher and Degani 2000; Van Ginneken and Maes2005). Studies on fish digestive physiology are of primary importance to set up nutritionalprotocols responsive to the metabolic capabilities of feed utilization of reared individuals(Smith 1989). Compared to a great amount of research dealing with the dietary qualitativerequirements of European eels (Degani et al. 1985; Degani and Levanonz 1987; De laHiguera et al. 1989; Heinsbroek 1989, 1991; Heinsbroek et al. 1989; Seymour 1989;Garcıa-Gallego et al. 1993; Hidalgo et al. 1993; Sanz et al. 1993; Degani and Gallagher1995), only a few studies have been addressed to the aspects concerning the utilization offeed components such as carbohydrates (Spannhof 1976; Spannhof and Kuhne 1977;Garcıa-Gallego et al. 1995; Suarez et al. 2002) and proteins (Schmidt et al. 1984), as well asto the effects on metabolism and digestibility of feed composition (Heinsbroek et al. 2007)

*Corresponding author. Email: [email protected]

ISSN 1023–6244 print/ISSN 1029–0362 online

� 2008 Taylor & Francis

DOI: 10.1080/10236240802492931

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and feeding schedule (Heinsbroek et al. 2008). The chemical digestion of food is strictlyrelated to the secretion of specific digestive enzymes along the gastrointestinal tract, whichaffect the ability of fish to transform and use macromolecules such as proteins,carbohydrates and lipids. The distribution and activity of the enzymes present in thedigestive tract of fish reflect the feeding habits of each species and the residence time of themeal along the gastrointestinal tract varies depending on the patterns of availableenzymes. Therefore, the analysis of the metabolic profiles and of the changes occurring inthe secretory response after feeding is of crucial importance to know the actual enzymeactivity along the gastrointestinal tract and to investigate the evolution of the digestiveprocess. To date, temporal changes in enzyme patterns during digestion have beeninvestigated in a few fish species only, among which the carp Cyprinus carpio (Onishi et al.1976), the rainbow trout Oncorhynchus mykiss (Fal’ge and Spannhof 1976), the Japaneseeel A. japonica (Takii et al. 1985), the African catfish Clarias gariepinus (Uys et al. 1987),the Atlantic salmon Salmo salar (Einarsson et al. 1996), and several aspects of digestiveprocess remain still unclear.

The gastrointestinal tract of eels is characterized by the absence of pyloric caeca andincludes three main organs: the stomach, Y-shaped and with an epithelium involved ingastric digestion through the secretion of pepsin, HCl, gastrin, sulphomucins; the smallintestine, that receives the enzymes trypsin, chymotrypsin, amylase and lipase secreted bythe pancreas, and the large intestine (Smith 1989). While the digestive process has beenfully investigated in the Japanese species A. japonica (Morishita et al. 1964; Takahashiet al. 1964; Takii et al. 1985; Chiu and Pan 2002), also at early life stages (Kurokawa et al.1995; Kruse et al. 1996; Ozaki et al. 2006), little is known on the enzyme profiles of theEuropean eel. In a comparative study, Hidalgo et al. (1999) showed that this fish exhibitedthe lowest digestive proteolytic potential among all the carnivorous species studied.

The scarce knowledge of fundamental aspects of physiology and biochemistry ofdietary nutrients in European eel has stimulated an increased interest on the effectof feeding on the digestive enzyme activities of this species. A recent study of the course ofdigestive enzyme activities following food ingestion (Mancuso et al. 2005) showedsignificant increases in proteolytic, lipolytic and amylolytic activities of A. anguillaspecimens 4 h after feeding. As a further development of this preliminary study, a 24 hcycle of observations on the distribution and activity of the main enzymes (total andspecific proteases, amylase and lipase) along the gastrointestinal tract of European eel wasundertaken, focusing on the temporal changes occurring in the enzyme activity profiles atdifferent times after feeding, compared to the previous data obtained in fish examined atthe same development stage.

Materials and methods

Experimental study

Specimens of A. anguilla (average weight 133� 2.3 g and length 26� 1.2 cm, presumed age48 months), obtained from an aquaculture farm in Siracusa, Sicily, were initially acclimatedfor two months at the CNR-IAMC rearing plant before the onset of the experiment. Duringthis adaptation period, they were maintained in a 300 l PVC tank supplied with runningwater directly pumped from the sea (salinity 38ø), exchanged at a rate of three times perday. The rearing conditions were: natural photoperiod and aeration provided to maintainthe oxygen concentration nearly at the saturation level, water temperature maintained atthe optimum level of 20�C; the choice of such thermal conditions derived from the

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observation that food uptake in eel was dependent on water temperature, while feeding

was suspended during cold periods. Oxygen and temperature levels were checked twicedaily. Throughout the experiment, fresh diet was offered ad libitum every morning at 7:00;

the administered diet consisted of wastes of fish markets, such as raw meat of fish

(Trachurus trachurus) cut into cubes, which were preserved by freezing and thawed beforefeeding. This fresh diet, whose proximate composition is reported in Table 1, was also

supplemented with vitamin and mineral salts. Its dietary composition was similar to thatreported by Degani and Gallagher (1995) to meet nutritional requirements of European eel

(crude protein: 30–48%, lipid: 15–20%, carbohydrate: 20–30%).After acclimation, fish were randomly assigned to two groups (‘fed’ and ‘starved’,

assumed as test and control groups, respectively), each of them consisting of 30 specimens

and kept under the same experimental conditions (temperature, oxygen, salinity) for

15 days. Prior to the experiment, eels were starved for 20 h, after which five individualswere taken from each tank at 7:30 am just prior to feeding and designated as T0

(pre-feeding sampling). To study changes occurring at different times after food ingestion,the fresh diet was offered to satiation at 8:00 am to fish of the fed tank only. In most cases,

feed were completely consumed within 30min. After feeding, residual food particles weresiphoned out in order to avoid that fish could find alternative food sources. Unfortunately,

no replicate tanks for each treatment (fed and starved) were used, due to the limited

availability of space in the experimental rearing plant during the study period. Five fishwere randomly taken from each (fed and starved) tank and sacrificed at 4, 8, 12, 16 and

Table 1. Proximate composition (as fed basis) of the fresh dietused for European eel.

(%)

Dry matter 41.46Crude protein 54.92Crude lipid 17.46Crude fiber 2.82N-free extract 23.72Ash 1.08Supplemented with vitamin–mineral premix (administered at2% of the biomass)

Vitamin premix (per kg of diet)Vit. A 6000 IUVit. D 4000 IUVit. E 250 IUVit. K 30mgThiamin 40mgRiboflavin 50mgFolic acid 15mgNiacin 200mgAscorbic acid 200mgInositol 400mg

Mineral premix (per kg of diet)Mn SO4 �H2O 123.1mgCuSO4 � 5H2O 39.4mgZnSO4 � 7H2O 220.3mgMgSO4 � 7H2O 4000mgCoCl2 � 6H2O 40.3mg

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24 h after feeding by anaesthetic treatment with MS 222 (SIGMA-ALDRICH, St. Louis,Missouri, USA, final concentration 0.1 ppm). The sampling times used in this study were

chosen to perform enzyme activity determinations at regular, constant, time intervals.

Measurements of digestive enzyme activities

From each individual, the digestive tract was removed and divided into stomach andintestine, which were treated separately, taking care to avoid reciprocal contamination.From each organ, the mucosa and the content were scraped with a metallic spatula and

homogenized in Tris-HCl buffer 50mMpH 7.0, added in a 1:10 weight/volume ratio.Supernatants obtained after centrifuging at 3000 rpm� 20min were stored at �20�C untilthe analysis of the following enzymes: total proteases, pepsin, trypsin and chymotrypsin,

carboxypeptidases A and B, amylase and lipase.Total protease activities were determined using the casein hydrolysis method of Kunitz

(1947); assays were performed at pH values of 1.5 and 8.5, previously determined as theoptimum pH values for gastric and intestinal proteases, respectively (Mancuso et al. 2005).Absorbance readings were carried out at 280 nm, using L-tyrosine (SIGMA-ALDRICH)

as the standard.Pepsin activity was determined according to Anson (1938), using bovine haemoglobin

(SIGMA-ALDRICH) as the substrate and measuring changes in absorbance occurring at750 nm.

Trypsin and chymotrypsin activities were measured using N-toluen-sulphonyl-L-arginine methyl ester (TAME, SIGMA-ALDRICH) and N-benzoyl-L-tyrosine ethyl ester(BTEE, SIGMA-ALDRICH) as the specific substrates (Hummel 1959), respectively;

absorbance readings were performed at 247 and 256 nm, respectively.Carboxypeptidase A and B activities were determined using L-hippuryl-L-phenylala-

nine and L-hippuryl-L-arginine (SIGMA-ALDRICH) as the substrates (Appel 1974) andmeasuring absorbance changes at 254 nm.

Amylase activity was measured by the hydrolysis method of starch (SIGMA-ALDRICH), using maltose (SIGMA-ALDRICH) as the standard, according toBernfeld (1955); amylase activity was measured at 540 nm and expressed as units of

maltose released from starch.Lipase activity was determined by the titrimetric evaluation of the degradation of

triacylglycerols to free fatty acids, using a kit Lipase (SIGMA-ALDRICH) relying onTietz and Fiereck’s method (1966).

Samples of the feeds used during the experiment were assayed for the above-reportedenzyme activities, in order to exclude the presence of exogenous enzymes.

All the obtained enzymatic values were expressed as specific activities (Umg�1 protein)after normalization to the protein content, estimated according to Lowry et al. (1951).

Statistical analysis

Prior to statistical analysis, all enzymatic data were logarithmically transformed in orderto attain their normal distribution. Data were then subjected to a two-way analysis ofvariance (ANOVA) to assess significant differences produced by the factors ‘time

samplings’ (six levels: 0, 4, 8, 12, 16, 24 h) and ‘feeding treatment’ (two levels: fed andstarved) as sources of variability. In the case of statistically significant differences, the leastsignificant difference (LSD) test was then applied to ANOVA results for post-hoc

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comparisons among means. For each enzyme, Student’s t test was applied to find thespecific differences occurring between starved and fed groups at the same time afterfeeding.

Results

The patterns of the specific activities of digestive enzymes recorded in the stomach andintestine of fed and starved individuals are shown in Figures 1–8. The mean activityvalues� SD are reported.

In the stomach, total protease activities (Figure 1a) ranged from 457.60 to99.99Umg�1 protein and from 450.0 to 76.0Umg�1 protein measured at T0 and at

Figure 1. Time changes of specific activity of total proteases in the stomach (a) and intestine (b) offed and starved European eel. Reported are the mean values� SD, n¼ 5. Within each experimentalgroup, different letters indicate significant differences by LSD test. Asterisks indicate significant(p50.05) differences detected between fed and starved fish per each parameter and sampling time byStudent’s t test.

Figure 2. Time changes of specific pepsin activity in the stomach of fed and starved European eel.See Figure 1 for explanations of other details.

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Figure 4. Time changes of specific carboxypeptidase B activity in the stomach (a) and intestine(b) of fed and starved European eel. See Figure 1 for explanations of other details.

Figure 3. Time changes of specific carboxypeptidase A activity in the stomach (a) and intestine(b) of fed and starved European eel. See Figure 1 for explanations of other details.

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Figure 6. Time changes of specific amylase activity in the stomach (a) and intestine (b) of fed andstarved European eel. See Figure 1 for explanations of other details.

Figure 5. Time changes of specific lipase activity in the stomach (a) and intestine (b) of fed andstarved European eel. See Figure 1 for explanations of other details.

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24 h in fed and starved eels, respectively. In both the groups, enzyme activity followed asignificant (p50.01) decrease, both overall and at individual sampling times (0–4 h and8–12 h after feeding). Student’s t test showed that in fed eels enzyme activityvalues recorded 4 h after feeding and later on were significantly (p50.01) higher than instarved ones.

Similarly to total proteases, pepsin activity (Figure 2) decreased from a maximumvalue of 163.85 and 155.0Umg�1 protein measured at T0 in fed and starved eels,respectively, to significantly (F: 203.26 and 200.62, p50.01, respectively) lower valuesrecorded at 24 h (76.45 and 67.0Umg�1 protein in the same groups, respectively). Instarved eels, pepsin activity underwent a sharp decrease at 4 h, maintaining a ratherconstant level over the successive period; significant (p50.01) differences in enzymeactivity values between starved and fed fish were observed since 4 h after feeding.

Carboxypeptidase A activity (Figure 3a) of fed eels was 11.02Umg�1 protein at T0 anddecreased to 4–4.3Umg�1 protein after feeding; a similar trend was observed in starvedeels, where the enzyme activity values measured at individual sampling times weresignificantly (p50.01) lower than those recorded at the same time in fed eels since 8 h afterfeeding. Carboxypeptidase B activity (Figure 4a) was high at T0 in both fed and starvedeels (9.36 and 9.2Umg�1 protein, respectively), later on it decreased significantly

Figure 7. Time changes of specific trypsin activity in the intestine of fed and starved European eel.See Figure 1 for explanations of other details.

Figure 8. Time changes of specific chymotrypsin activity in the intestine of fed and starvedEuropean eel. See Figure 1 for explanations of other details.

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(p50.01); activity values recorded in starved eels were significantly (p50.01) lower than

those measured in fed eels from 8 to 24 h after feeding.Lipase activity (Figure 5a) ranged from 17.76 to 247.3Umg�1 protein and from 13 to

24Umg�1 protein in fed and starved eels, at T0 and at 8 h, respectively. In fed eels, enzyme

activity levels peaked 8 h after feeding (vs. T0, F¼ 79.39, p50.01), keeping values

significantly (p50.01) higher than those observed in the starved ones till 24 h after feeding.Amylase activity (Figure 6a) in fed eels decreased significantly (F: 16.44, p50.01) from

1.54 to 0.73Umg�1 protein, at T0 and 24 h after feeding, respectively; in starved eels,

enzyme activity ranged from 1.42 to 0.7Umg�1 protein, with an early reduction at 4 h.

Significantly (p50.01) different values of amylase activity were found between fed and

starved eels from 4 to 16 h after feeding.In the intestine, total protease activities (Figure 1b) in fed eels showed a significant

increase from an initial value of 31.20Umg�1 protein to a peak value of about 112Umg�1

protein reached 24 h after feeding (F¼ 104.29, p50.01, vs. T0); in starved eels, no significant

changes were detected over time in the activity values of total intestinal proteases, which

were significantly (p50.01) lower than those detected in fed eels since 4 h after feeding.Trypsin activity increased progressively during digestion in fed eels (Figure 7), reaching

a peak 8 h after feeding (5.7Umg�1 protein, F: 492.79, p50.01 vs. T0); activity values

remained unchanged during the successive digestive phases. In starved eels, the trypsin

activity seemed to be unaffected by time. Significant (p50.01) differences were found in

the enzyme activity values recorded 4, 8, 12, 16, 24 h after feeding in fed eels compared to

those measured at the same times in the starved ones.Similarly to trypsin, chymotrypsin activity (Figure 8) increased progressively in fed eels

(F: 15.16, p50.01, T0 vs. 24 h); in starved eels no significant changes over time were

detected by ANOVA, and enzyme activity values were significantly (p50.01) different

from the values observed in the fed eels, from 4 to 24 h after feeding.Carboxypeptidase A activity (Figure 3b) in fed eels doubled 24 h after feeding

compared to T0 value (1.8Umg�1 protein) (F: 198.58, p50.01); conversely, in starved eels

this enzyme activity remained at a constant value, significantly (p50.01) different from the

ones measured in fed eels from 4 to 24 h after feeding.Carboxypeptidase B activity (Figure 4b) in fed eels showed a maximum value

(3.66Umg�1 protein), twice the value measured at T0, 24 h after feeding (F: 317.01,

p50.01); in starved eels low activity values were always detected, which remained

unchanged over time and were significantly (p50.01) different from the ones measured in

fed eels since 4 h after feeding.Lipase activity (Figure 5b) ranged from 14.25 to 1209.36Umg�1 protein and from 13

to 145Umg�1 protein in fed and starved eels, at T0 and at 8 h, respectively; enzyme

activity values measured in fed eels from 8 to 24 h after feeding were significantly (p50.01)

higher than those recorded in starved ones.Amylase activity (Figure 6b) increased significantly (p50.05) in fed eels, reaching

1.29Umg�1 protein 24 h after feeding; in starved eels enzyme activity values did not

change significantly over time and were significantly (p50.01) lower than those found in

fed eels since 4 h after feeding. No enzyme activities were found in the feed samples.

Discussion

To our knowledge, the present study provides the first description of the quantitative

response of digestive enzyme activities to one single meal in European eel over a 24 h cycle

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of observations. The rationale for the research was that current knowledge of the temporal

patterns of digestive enzyme activities in European eel is still incomplete, being limited to

some enzymes only (Hidalgo et al. 1999), or to short-time periods (0 to 4 h after feeding,

Mancuso et al. 2005). In this investigation, the digestive enzyme activity was reported as

specific activity [i.e. enzyme activity per milligram of protein content], which is different

from the digestive capacity [i.e. per gram of tissue activity� the corresponding tissue

weight], or from the total enzyme activity [i.e. enzyme activity in the homogenate of total

gastro-intestinal tract normalized to fish body weight, according to Kuz’mina (1996);

Kroghdhal and Bakke McKellep (2005)]. Nevertheless, the definition of digestive enzyme

activity profiles and the study of changes in the enzyme expression in response to feeding

may provide a useful background on the time evolution of the digestive process which may

contribute to improve nutritional protocols currently adopted for aquaculture farming of

European eel.The results obtained during the experiment showed that in fed eels digestive enzyme

activities were significantly (p50.01) higher than those measured in starved fish; this

indicated that the digestive enzyme secretion is stimulated by the presence of nutrients in

the gastrointestinal tract. Similar findings were obtained 4 h after feeding (Mancuso et al.

2005), although only amylase and chymotrypsin activity values were in the same range as

the present data, while the activity levels of other enzymes were not consistent probably

due to intra-specific diversity. Moreover, a different time evolution of digestive enzyme

activities was observed between starved and fed eels: in the starved fish, enzyme activities

decreased initially but remained mostly unaffected over time; in contrast, in fed eels,

enzyme activity levels varied significantly following feeding, suggesting that the secretion

of these enzymes was regulated by the arrival of organic substrates through food ingestion.

Variations in the levels of digestive enzyme activity along the gastrointestinal tract

occurred mostly within few hours after food ingestion and did not follow a circadian

rhythm; in the stomach, total proteases and pepsin activities decreased over time, more or

less gradually, reaching 24 h after feeding values 22–50% (on average) lower than the ones

measured at pre-feeding time. The general decreasing trend observed for most of the

gastric enzyme activities was not followed by carboxypeptidase B and lipase activities,

although no clear explanation for this behaviour was found. Conversely, in the intestine

proteolytic enzyme activities (trypsin, chymotrypsin and carboxypeptidases), as well as

amylase activities increased significantly already 4 h after feeding; intestinal lipase activity

peaked 8 h after food ingestion, similar to what was observed in the stomach. The enzyme

activity profiles found in the examined eels reflected the initial gastric attack of peptides,

mediated by pepsin, at the first step of the digestion, similar to what was found by Hidalgo

et al. (1999) as well as by Morishita et al. (1964) and Takahashi et al. (1964). The

importance of the enzymatic breakdown played by proteolytic enzyme activities (trypsin,

chymotrypsin and carboxypeptidases) as well as amylase and lipase activities was more

evident 4 h after feeding and over the following times, suggesting a greater contribution of

these enzymes to the digestive process when digesta from the stomach entered into the

intestine. The lag phase between food ingestion and enzyme secretion in the intestine was

consistent with that observed by Takii et al. (1985) in Japanese eel, where maximum levels

of protease activity were found 5 h after feeding. In other fish species, such as carp (Onishi

et al. 1976), the peak of enzyme activities (proteases and amylase) in the digestive tract was

detected 6–7.5 h after feeding. Conversely, in C. gariepinus, the response of protease

activity to feeding was quicker than in carp and in eels, reaching a peak 2.5 and 4 h after

feeding in the gastric and intestinal contents, respectively (Uys et al. 1987).

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The digestive enzyme activity profiles observed at the pre-feeding sampling suggested

that European eel displayed high ability to hydrolyse proteins, carbohydrates and lipids

even at basal physiological conditions. The particularly high enzyme activity levels

recorded at the initial phase (T0) especially in the stomach could derive from a

magnification or concentration effect due to the fact that only the specific enzyme activities

(i.e. normalized to protein content) were considered. The initial fasting period applied

before feeding, a common practice in physiological studies which normally consists in

suspending food administration for 24 h before fish analysis, could be hypothesized to

cause a significant reduction in the protein content, which resulted in high specific enzyme

activity levels. Krogdhal and Bakke-McKellep (2005) reported that fasting produced a

similar decrease in the total body protein content of Atlantic salmon already within the

first two days from the onset of starvation. Pepsin activity values recorded at T0 indicated

that this enzyme still held high activity levels in the gastric mucosa despite the absence of

food; its further decline was probably due to enzyme denaturation or its dilution or

irreversible bound with feed substrates after their arrival in the stomach. Moreover, the

storage of digestive enzymes in the secretory tissues was also reported by Einarsson et al.

(1996) in Atlantic salmon during starvation; the presence of trypsin in Japanese eel pre-

leptocephalus larvae with empty guts was explained by Pedersen et al. (2003) by

hypothesizing that enzyme secreted in response to a previous meal ingestion could be

retained in the intestinal tissue.In the intestine of European eel, digestive enzyme activities displayed an increasing

trend and levels remained high at a late phase (i.e. 24 h after feeding); this suggested that

the secretion of the enzymes was continuous and prolonged after food intake. A similar

observation was reported in the Japanese eel (Chiu and Pan 2002), where trypsin and

chymotrypsin activities reached a maximum 11 h after feeding. If it is true that the

digestive process depends both on digestive enzyme activity levels and the time along

which nutrients are exposed to the enzyme hydrolysis, the finding that intestinal enzymes

could retain high activity levels for a long time after feeding proved that the European

eel had active digestive processes over extended time intervals; this implied that this fish

is able to process efficiently protein- and carbohydrate-rich diets, as previously observed

(Spannhof 1976; Spannhof and Kuhne 1977; Lecomte-Finiger 1983; Hidalgo et al. 1993;

Degani and Gallagher 1995; Garcıa-Gallego et al. 1995; Suarez et al. 2002). This is in

apparent contrast with Hidalgo et al.’s (1999) findings, who reported for this fish a low

proteolytic activity compared to trout, but based their conclusions on observations made

without taking into account the quantitative changes associated with feeding in all the

enzyme activities; in addition, in that case analyses were performed on fish left unfed for

48 h without food, therefore it is likely that a part of overall enzyme activity was

measured only. The enzyme activity patterns observed in this study in response to

feeding supported the opinion that European eel possesses a metabolism efficient enough

to allow the net uptake of dietary components; therefore it appeared only apparently

inactive, in agreement with Owen et al. (1998). This consideration is also consistent with

previous observations on digestibility by Heinsbroek et al. (2007), which showed that

this fish species can digest very well a range of feeds differing in macronutrient

composition (i.e. crude protein, fat and carbohydrates) and that digestibility is little

affected by diet composition. Improvement of current understanding of the digestive

process of European eel provides an effective approach that could have important

implications for optimizing feeding regimes of this fish, opening new perspectives for

future nutritional studies.

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References

Anson ML. 1938. The estimation of pepsin, trypsin, papain and cathepsin with haemoglobin. J Gen

Physiol. 22:79–89.Appel W. 1974. Carboxypeptidases. In: Bergmeyer HU, editor. Methods of enzymatic analysis.

New York: Academic Press. p. 996–997.

Bernfeld P. 1955. Amylase � and �. In: Colowich SP, editor. Methods in enzymology. New York:

Academic Press. p. 149–150.Brusle J. 1991. Anguila y anguilicultura. In: Barnabe G, editor. Acuicultura, Vol II. Barcelona:

Ediciones Omega. p. 679–707.Chiu ST, Pan BS. 2002. Digestive protease activities of juvenile and adult eel (Anguilla japonica) fed

with floating feed. Aquaculture. 205:141–156.Degani G, Gallagher ML. 1995. Growth and nutrition of eels. Jerusalem: Laser Pages

Publishing Ltd.Degani G, Horowitz A, Levanon D. 1985. Effect of protein diet and of density, ammonia, and O2

levels on growth of juvenile European eels. Aquaculture. 46:193–200.

Degani G, Levanonz D. 1987. Effects of dietary carbohydrates and temperatures on slow growing

juvenile eels Anguilla anguilla. Environ Biol Fishes. 18:149–154.Dekker W. 2003. Did lack of spawners cause the collapse of the European eel, Anguilla anguilla?

Fisheries Manag Ecol. 10:365–376.De la Higuera M, Garcıa-Gallego M, Sanz A, Hidalgo MC, Suarez MD. 1989. Utilization of dietary

protein by the eel (Anguilla anguilla). Optimum dietary protein levels. Aquaculture. 79:53–61.

Einarsson S, Davies PS, Talbot C. 1996. The effect of feeding on the secretion of pepsin, trypsin and

chymotrypsin in the Atlantic salmon, Salmo salar L. Fish Physiol Biochem. 15:439–446.Fal’ge R, Spannhof L. 1976. Amylase, esterase and protease activity in the gut contents of the

rainbow trout Salmo gairdneri after feeding. J Ichthyol. 16:672–677.Gallagher ML, Degani G. 2000. Eel culture. In: Stickney RR, editor. The encyclopedia of

aquaculture. New York: J Wiley and Sons. p. 277–283.

Garcıa-Gallego M, Bazoco J, Suarez MD, Sanz A. 1995. Utilization of dietary carbohydrates by

fish: a comparative study in eel and trout. Animal Sci. 61:427–436.Garcıa-Gallego M, Hidalgo MC, Suarez MD, Sanz A, De La Higuera M. 1993. Feeding of the

European eel Anguilla anguilla. II. Influence of dietary lipid level. Comp Biochem Physiol A.

105:171–175.

Heinsbroek LTN. 1989. Preliminary investigations on husbandry, nutrition and growth of glass eels

and elvers, A. anguilla L. Aquac Fish Manag. 20:119–127.Heinsbroek LTN. 1991. A review of eel culture in Japan and Europe. Aquac Fisher Manag.

22:57–72.Heinsbroek LTN, Goedegebuur BJ, Bloemhof G, Flach RB, de Jong GDC. 2008. Gastrointestinal

and metabolic effects of feeding schedule on voluntary feed intake and growth of European eel,

Anguilla anguilla. Aquacult Int. 16:93–108.

Heinsbroek LTN, Van Hooff PLA, Swinkels W, Tanck MWT, Schrama JW, Verreth JAJ. 2007.

Effects of feed composition on life history developments in feed intake, metabolism, growth and

body composition of European eel, Anguilla anguilla. Aquaculture. 267:175–187.Heinsbroek LTN, van Thoor RMJ, Elizondo LJ. 1989. The effect of feeding level on the

apparent digestibilities of nutrients and energy of a reference diet for the European eel,

Anguilla anguilla L., and the African catfish, Clarias gariepinus (Burchell, 1822). In: Takeda M,

Watanabe T, editors. Proceedings of the 3rd International Symposium on Feeding and nutrition

in fish, Toba, Japan, Abstracts, 039.

Hidalgo MC, Sanz A, Garcıa-Gallego M, Suarez MD, De la Higuera M. 1993. Feeding of the

European eel Anguilla anguilla: I. Influence of dietary carbohydrate level. Comp Biochem

Physiol. A. 105:165–169.Hidalgo MC, Urea E, Sanz A. 1999. Comparative study of digestive enzymes in fish with different

nutritional habits. Proteolytic and amylase activities. Aquaculture. 170:267–283.

226 G. Caruso et al.

Dow

nloa

ded

by [

Con

sigl

io N

azio

nale

del

le R

icer

che]

at 0

7:38

14

June

201

3

Hummel BCW. 1959. A modified spectrophotometric determination of chymotrypsin, trypsin and

thrombin. Can J Biochem Physiol. 37:1393–1399.

Kroghdhal A´, Bakke-McKellep AM. 2005. Fasting and refeeding cause rapid changes in intestinal

tissue mass and digestive enzyme capacities of Atlantic salmon (Salmo salar L.). Comp Biochem

Physiol Part A. 141:450–460.Kruse C, Strehlow B, Schmidt H, Muller PK. 1996. Presence of trypsin in distinctive body segments

of leptocephalus larvae of Anguilliformes. Aquaculture. 142:237–244.Kunitz M. 1947. Crystalline soybean trypsin inhibitor. II. General properties. J Gen Physiol.

20:291–310.Kurokawa T, Kagawa H, Ohta H, Tanaka H, Okuzawa K, Hirose K. 1995. Development of

digestive organs and feeding ability in larvae of Japanese eel (Anguilla japonica). Can J Fish

Aquat Sci. 52:1030–1036.

Kuz’mina VV. 1996. Influence of age on digestive enzyme activity in some freshwater teleosts.

Aquaculture. 148:25–37.Lecomte-Finiger R. 1983. Regime alimentaire des civelles et anguillettes Anguilla anguilla dans trois

etangs saumatres du Roussillon. Bull Ecol. 14:297–306.Lowry OH, Rosebrough NJ, Farr AL, Randall RJ. 1951. Protein measurement with the Folin

phenol reagent. J Biol Chem. 193:265–275.Mancuso M, Caruso G, Denaro MG, Costanzo MT, Maricchiolo G, De Pasquale F. 2005. Patterns

enzimatici del tratto digestivo di Anguilla anguilla a digiuno e dopo alimentazione. Biol Mar

Medit. 12:198–200.

Morishita T, Noda H, Kitamikado M, Takahashi T, Tachino S. 1964. The activity of the digestive

enzymes in fish. J Fac Fish Univ Mie (Japan). 6:239–246.

Onishi T, Murayama S, Takeuchi M. 1976. Changes in digestive enzyme levels in carp after feeding –

III. Response of protease and amylase to twice-a-day feeding. Bull J Soc Sci Fish. 42:921–929.Owen SF, Houlihan DF, Rennie MJ, van Weerd JH. 1998. Bioenergetics and nitrogen balance of the

European eel (Anguilla anguilla) fed at high and low ration levels. Can J Fish Aquat Sci.

55:2365–2375.Ozaki Y, Tanaka H, Kagawa H, Ohta H, Adachi S, Yamauchi K. 2006. Fine structure and

differentiation of the alimentary canal in captive-bred Japanese eel Anguilla japonica

preleptocephali. Fisheries Sci. 72:13–19.Pedersen BH, Ueberschar B, Kurokawa T. 2003. Digestive response and rates of growth in

pre-leptocephalus larvae of the Japanese eel Anguilla japonica reared on artificial diets.

Aquaculture. 215:321–338.

Sanz A, Suarez MD, Hidalgo MC, Garcıa-Gallego M, de la Higuera M. 1993. Feeding of the

European eel Anguilla anguilla: III. Influence of the relative proportions of the energy yielding

nutrients. Comp Biochem Physiol A. 105:177–182.Schmidt O, Greuel E, Pfeffer E. 1984. Digestibility of crude protein and organic matter of potential

sources of dietary protein for eels (Anguilla anguilla L.). Aquaculture. 41:21–30.Seymour EA. 1989. Devising optimum feeding regimes and temperatures for the warm water culture

of eel, Anguilla anguilla L. Aquac Fisher Manag. 20:311–323.Smith LS. 1989. Digestive functions in teleost fishes. In: Halver JE, editor. Fish nutrition. 2nd ed.

New York: Academic Press. p. 405–407.

Spannhof L. 1976. A study of the carbohydrate metabolism of the freshwater eel (Anguilla anguilla)

and the rainbow trout (Salmo gairdneri). J Ichthyol. 16:165–167.

Spannhof L, Kuhne H. 1977. Studies regarding the utilization of different feed mixes by European

eel (Anguilla anguilla). Arch Tieremahr. 27:517–531.Suarez MD, Sanz A, Bazoco J, Garcia-Gallego M. 2002. Metabolic effects of changes in the dietary

protein: carbohydrate ratio in eel (Anguilla anguilla) and trout (Oncorhynchus mykiss). Aquac

Int. 10:143–156.Takahashi T, Morishita T, Tachino S. 1964. On the digestive enzymes in Anguilla japonica: I.

Proteolytic enzyme and its action on raw and heated fish muscle. Rep Fisheries Univ Mie.

5:127–144.

Marine and Freshwater Behaviour and Physiology 227

Dow

nloa

ded

by [

Con

sigl

io N

azio

nale

del

le R

icer

che]

at 0

7:38

14

June

201

3

Takii K, Shimeno S, Takeda M. 1985. Changes in digestive enzyme activities in eel after feeding. BullJ Soc Sci Fish. 51:2027–2031.

Tietz NW, Fiereck EA. 1966. A specific method for serum lipase determination. Clin Chem Acta.13:352–358.

Uys W, Hecht T, Walters M. 1987. Changes in digestive enzyme activities of Clarias gariepinus(Pisces: Clariidae) after feeding. Aquaculture. 63:243–250.

Van Ginneken V, Maes GE. 2005. The European eel (Anguilla anguilla, Linnaeus), its lifecycle,

evolution and reproduction: a literature review. Rev Fish Biol Fish. 15:367–398.

228 G. Caruso et al.

Dow

nloa

ded

by [

Con

sigl

io N

azio

nale

del

le R

icer

che]

at 0

7:38

14

June

201

3