Tropical Zoology 23: 75-89, 2010
Feeding habits and digestive enzymes
in the gut of Mormyrus rume (Valenciennes 1846)
(Osteichthyes Mormyridae)
D.O. Odedeyi 1 and O.A. Fagbenro 2,3
Department of Environmental Biology and Fisheries, Adekunle Ajasin University, Akungba, Nigeria
2
Department of Fisheries and Aquaculture Technology, Federal University of
Technology, Akure, Nigeria
1
Received 6 August 2009, accepted 29 April 2010
The feeding habits of 436 elephant snout fish, Mormyrus rume
(Valenciennes 1846), from River Ose were investigated. Stomach contents were identified and analyzed by occurrence and volumetric methods, and the prominence of individual food items was determined by
the ranking index I. Only 3.2% of the specimens had empty stomachs
while the others had stomachs in varying degrees of fullness. Food items
included plant and animal matter, mainly arthropods. A preponderance of insects, crustaceans and detritus, which accounted for 100%
occurrence and > 16% of total volume in the stomachs of M. rume
specimens, suggests a benthic feeding habit. The high percentage occurrence of detritus and its high I value established M. rume as a detritivore. Assays were conducted to determine the occurrence, distribution
and specific activities of digestive enzymes in different gut regions of
juvenile (SL < 35.0 cm) and adult (SL > 35.0 cm) M. rume. Alphaamylase, maltase, lactase, sucrase, chitinase, pepsin, trypsin and lipase
were present in the different gut regions at varying quantities and with
specific activities. The wide distribution of enzymes in both juvenile
and adult M. rume reflects its ability to digest the carbohydrate, protein
and lipid portions of its foods. There were no differences (P > 0.05)
in specific activities of digestive enzymes between wet and dry seasons.
key words: feeding habits, stomach contents, digestive enzymes, gut,
Mormyrus rume.
Introduction .................................................................................. 76
Materials and methods ................................................................... 77
3
Corresponding author: O.A. (Dapo) Fagbenro, Tel: + 234-(0)8077788688 (E-mail: [email protected]).
76
D.O. Odedeyi and O.A. Fagbenro
Results ........................................................................................... 80
Discussion ..................................................................................... 82
References ...................................................................................... 87
INTRODUCTION
The food and feeding habits of different fishes often differ widely. The
same fish also may show a preference for different types of food as it grows or at
different times of the year (Maar et al. 1983). The stomachs of many tropical
fishes have been studied to ascertain their feeding habits in natural habitats and
the relationship between the fishes and their biotic environments (Ugwumba et
al. 1990). Fagbenro et al. (2000) established Heterotis niloticus (Cuvier 1829)
as a benthic feeder in River Oluwa and a planktonic feeder in Owena Reservoir
and Mahin Lagoon. Kouamelan et al. (1999) reported that M. rume stomach
contents from a man-made lake on River Bia (Côte d’Ivoire) comprised mainly
chironomid larvae and bacillariophyceae in young fish, while Chaoborus Lichtenstein 1800 species formed the main food item in larger specimens. Fawole
(2002) reported that the major food items in stomachs of M. rume in Lekki
Lagoon (Nigeria) were detritus and plants. Omotosho (1993) reported that M.
rume fed on detritus, algae and macrophytes in Oyun mini-dam (Nigeria) while
Ipinjolu et al. (2005) reported that M. rume in River Rima and Goronyo reservoir (Nigeria) fed on items of both plant and animal origin.
Assays of enzymes in the gut of a fish provide information about its nutritional physiology. The quality of a given food item is directly proportional to
its ability to support growth and its nutritional value is determined by the ability
of the animal to digest and absorb it (Akintunde 1985). Tengjaroenkul et al.
(2000) reported that the distribution and specific activity of digestive enzymes
along the gut change with feeding habits. Tramati et al. (2005) noted that the
age and/or stage of development influence the anatomical and physiological
development of the digestive organs, and the digestive processes are correlated
with the size and type of food items in fishes, thus explaining different feeding
habits at various stages of the life cycle (Kuz’mina et al. 2002). Under natural
conditions, adults tend to capture larger prey, which demands a greater digestive effort due to the smaller surface area exposed to enzymatic action. Uys &
Hecht (1987) reported that knowledge of the digestive enzymes enhances the
development of more efficient diets and rearing techniques. Studies have been
conducted on digestive enzymes of various tropical fishes in sub-Saharan Africa
(Olatunde & Ogunbiyi 1977; Uys & Hecht 1987; Fagbenro 1990; Fagbenro et al. 1993, 2000, 2001, 2005; Ugwumba 1993) but no such information is
available for M. rume.
The objectives of this study were to determine the feeding habits and the
occurrence, distribution and specific activities of digestive enzymes in different
regions of the gut of juvenile (SL < 35.0 cm) and adult (SL > 35.0 cm) specimens
of M. rume.
Feeding habits and digestive enzymes in Mormyrus rume
77
MATERIALS AND METHODS
River Ose is a major perennial river in southwestern Nigeria (Fig. 1). Its
source is in the Apata hills and it flows through savannah, rainforest, mangrove forest before discharging into the Atlantic Ocean through a series of creeks and lagoons.
The river lies between longitudes 5°20’E to 6°10’E and latitudes 6°20’N to 8°00’N.
It flows approximately 300 km from its source before breaking into a series of creeks
and lagoon.
M. rume specimens (436) were obtained from artisanal fishermen using gill nets
set daily at 18:00 hr and recovered at dawn (06:00 hr) in River Ose. The specimens
Fig. 1. — Location of River Ose, southwestern Nigeria.
78
D.O. Odedeyi and O.A. Fagbenro
were collected over 24 months. The total and standard lengths (cm) and weight (g)
of each specimen were taken using a fish measuring board and Ohaus Triple Beam
Balance (Model 610), respectively. The specimens were dissected on the ventral side
from the anal opening to the pectoral fin region. The stomach was removed and
the degree of fullness was assessed visually and recorded as empty, ¼ full, ½ full, ¾
full, and full. The stomach of each specimen was opened and the contents emptied
into a Petri dish containing a known volume of water for subsequent identification
and estimation of food items under a binocular dissecting microscope. For the
quantification of microscopic food items, a drop of the food mixture was placed on
a glass slide for identification and counting with a binocular light microscope. The
various food items were analyzed by occurrence and volumetric methods (Arawomo
1976). The prominence of individual food items was determined by the ranking
index I (Oda & Parrish 1981):
I = (% occurrence × % volume) × 10-2 .
Forty live M. rume were obtained from fishermen in River Ose. The total and
standard lengths (cm) and weight (g) of each fish were taken using a fish measuring
board and Ohaus Triple Beam Balance, respectively. The specimens were kept for 48
hr in glass tanks without feeding to bring them to similar physiological conditions
and to ensure the emptiness of their guts. The entire gut was removed, measured on
ice, and then separated into oesophagus, stomach, pyloric caeca, anterior intestine,
posterior intestine and rectum. Tissues from 10 specimens each of juvenile and adult
fish were pooled and preserved separately in a chest freezer. Each of the regions was
weighed after thawing, homogenized in an all-glass homogenizer with 10 times its
weight of ice-cold neutralized 1% KOH to give a 1:10 homogenate. The homogenates were centrifuged at 2500 rpm for 20 min at 4 oC in a refrigerated centrifuge.
The precipitant was discarded and the clear supernatant was used as a crude enzyme
solution without further purification.
Specific activities of proteases
The specific activities of proteases in the digestive tract of M. rume were determined in triplicate samples using the method of Rinderknecht et al. (1968) and
Balogun & Fisher (1970). Trypsin was determined in a reaction mixture consisting of 1 ml/10 mg egg-albumin solution, 2 ml phosphate buffer (pH 8.0) and 0.5
ml enzyme solution. The enzyme in the control was boiled at 37 oC for 20 min in
a water bath. The reaction mixture for the determination of pepsin was like that of
trypsin but the phosphate buffer for the pepsin was pH 2.0. Both the test and control samples were incubated for 1 hr at 100 oC. During incubation, there was regular
shaking of the tubes to ensure uniform reaction. At the end of incubation, 3 ml of
ice-cold phosphate buffer was added to the test tubes. The mixture was filtered immediately through Whatman No. 1 filter paper and the absorbance of the filtrate was
determined at 595 nm with a Gallenkamp color-spec colorimeter (model COJ-580010N). The blank was used to adjust the colorimeter to zero.
Specific activities of glycosidases
Glycosidases were determined in triplicate samples by the dinitrosalicylate
(DNS) methods described by Plummer (1978) and Olatunde et al. (1988). The
presence of free reducing sugars was initially determined and the amounts of DNS
Feeding habits and digestive enzymes in Mormyrus rume
79
reduced in the presence of appropriate substrate and extracts were compared. The
substrates used for the assays were starch, maltose, sucrose, lactose, chitin and cellulose. The reaction mixture consisted of 0.4 ml of 1% substrate, 0.2 ml phosphate
buffer pH 8.0, 1.6 ml DNS and 0.1 ml of enzyme extract. The enzyme extract in
the control was replaced with distilled water. The control and test samples were allowed to stand at room temperature for 3 min, then 1.0 ml (DNS) was added to each
sample and they were placed in boiling water for 5 min. The samples were removed
and left to cool for 30 min, after which 1.8 ml of distilled water was added to each
sample to bring it up to 4.0 ml (for dilution) and it was mixed properly. The amount
of reducing sugar produced in the enzymatic reaction was estimated with a colorimeter at 540 nm.
Specific activities of lipases
Specific activities of lipases were determined at 37 oC in triplicate samples as
described by OgunbiyI & Okon (1976). The reaction mixture consisted of 1.0 ml
of 25% olive oil emulsion pH 7.0 and 0.2 ml of enzyme extract. The test and control
samples were incubated for 1 hr at 37 oC in a water bath, after which 3.0 ml of 95%
ethanol and two drops of phenolphthalein were added to each test tube including the
control. The reaction mixture was titrated against 0.05 M NaOH to a similar pink
colour. An increase in the titre value of the test sample was compared with the control
to confirm the presence of lipase in the test sample.
Preparation of the calibration curve for protein
The stock consisted of 100 µg/ml egg-albumin (10 mg of egg-albumin dissolved
in 100 ml of water). Serial dilutions of stock solution were prepared, ranging from
20-100 µg/ml egg-albumin. Each concentration was prepared in duplicate tubes,
each tube containing 1 ml of the dilution and 3 ml of biuret. The egg-albumin in
the blank tube was replaced with distilled water. Both the test samples and controls
were incubated for ten minutes at 37 oC in a water bath for colour development. The
optimal density of the solution was determined with a colorimeter at 540 nm, and
was used in the preparation of a calibration curve for determination of protein using
egg-albumin. Absorbance was converted to enzyme activity:
Unit of enzyme activity = Amount of amino acid released (µg/ml)
Time
Preparation of the calibration curve for glucose
The stock solution consisted of 0.1 g D-glucose/100 ml. Serial dilutions
ranging from 0 to 0.8 mg/ml were prepared from the stock. Each dilution was
brought to 1 ml by the addition of distilled water. A blank tube that contained 1
ml of distilled water was used to adjust the colorimeter to 0. Each concentration
was prepared in duplicate test tubes; 1.6 ml of alkaline 3.5-DNS reagent was added
to each test tube and the sample was mixed properly. The samples were placed in a
boiling water bath for 10 min. One millilitre of each sample of diluted stock was
further diluted by the addition of 1 ml of distilled water. The absorbance of the
reaction mixture was read at 540 nm against a blank containing buffer on the digital
colorimeter.
Unit of enzyme activity = Amount of glucose released (mg/ml)
Time
80
D.O. Odedeyi and O.A. Fagbenro
Statistical analyses
All data obtained from the analyses were analyzed in terms of means and
standard deviation. The data were also subjected to analysis of variance (ANOVA)
with size and gut region as sources of variation.
RESULTS
The stomach fullness values for the specimens are presented in Table 1.
M. rume in River Ose fed mainly on insects and crustaceans, which were found
in all the stomachs containing food. Plant parts and detritus were also present
in all stomachs containing food. Other food items were rotifers, diatoms, algae,
nematodes, annelids, and protozoans.
Insects, crustaceans, detritus and plant parts were widely consumed by
the small (15.0-20.0 cm), medium (20.1-35.0 cm) and large (35.1-50.0 cm)
M. rume individuals. Insects and crustaceans occurred in all the stomachs. By
volume, insects and crustaceans (arthropods) accounted for 23 and 18%, 21 and
20%, 15 and 22% of the food of large, medium and small M. rume specimens,
respectively (Table 2). According to the ranking index, insects were the prominent food item for large and medium specimens, with values of 23 and 21%,
respectively, while crustaceans were the prominent food items for small fishes,
making up 22% of the stomach contents (Table 2). As the fish size increased, the
preference for insects increased and the preference for protozoans, rotifers and
diatoms decreased. Detritus and plant materials occurred in 100% of the stomachs of large specimens, while the respective values were 98 and 92% in medium
specimens and 93 and 96% in small specimens. According to the ranking index
values (Table 2), detritus was more prominent among the food items consumed
by smaller fish. Rotifers were significant food items consumed by small specimens
and occurred in all the stomachs examined; however, they showed a low occurrence and low ranking index in large specimens.
Table 1.
Number of Mormyrus rume specimens examined and the degree of stomach fullness.
Stomach fullness
Full
15.0-20.0
1
25
9
4
1
Size (cm)
20.1-35.0
10
187
67
26
10
35.1-50.0
3
60
22
8
3
14 (3.2%)
272 (62.4%)
98 (22.5%)
38 (8.7%)
14 (3.2%)
Total
40
300
96
436
Empty
1/4
1/2
3/4
Total
Feeding habits and digestive enzymes in Mormyrus rume
81
Table 2.
Food items in the stomachs of small, medium and large Mormyrus rume specimens.
SL 15.0-20.0 cm
SL 20.1-35.0 cm
SL 35.1-50.0 cm
Food items
%
volume
%
occurrence
RI
% volume
%
occurrence
RI
%
volume
%
occurrence
RI
Protozoa
0.6
7.1
0.0
0.5
4.3
0.0
0.3
5.7
0.0
Crustacea
22.0
100.0
22.0
20.0
100
20.0
18.0
100.0
18.0
Insecta
15.0
100.0
15.0
21.0
100
21.0
23.0
100.0
23.0
Algae
2.0
11.1
0.2
1.5
70.2
1.1
1.3
57.1
0.7
Rotifera
5.0
100.0
5.0
4.0
51.0
2.0
3.0
65.7
2.0
Nematoda
0.3
3.7
0.0
0.2
2.1
0.0
0.3
5.7
0.0
Annelida
0.5
3.7
0.0
0.4
2.1
0.0
0.5
8.6
0.0
Arthropoda
11.0
100.0
11.0
12.0
95.7
11.5
18.0
100.0
18.0
Detritus
22.0
92.6
20.4
18.0
97.9
17.5
16.0
100.0
16.0
Sand grains
0.5
66.7
0.3
0.3
68.1
0.2
0.2
68.6
0.1
Plant parts
16.0
96.3
15.4
18.0
91.5
16.5
14.9
100.0
14.9
Unidentified
mass
5.1
81.5
4.2
4.1
78.7
3.2
4.5
88.6
4.0
Total
100
100
100
The specific activities of digestive enzymes in the gut of M. rume juveniles
and adults are presented in Table 3. The assay of enzymes showed that the strength
of lipase activity was different in the different gut regions of both the juveniles and
adults. Lipase activity was very strong in the anterior intestine, strong in the posterior intestine and pyloric caeca but weak in the oesophagus, stomach and rectum
of both juveniles (SL < 35.0 cm) and adults (SL > 35.0 cm) in both seasons. The
highest lipase activity occurred in the anterior intestine and was significantly different (P < 0.05) from the other regions, while the lowest activity was recorded in
the oesophagus (Table 3). Lipase activity was higher in adults than in juveniles and
particularly significant in the rectum (Table 3).
82
D.O. Odedeyi and O.A. Fagbenro
Pepsin was present in the stomach, pyloric caeca and anterior intestine; it
was strong in the stomachs of both juveniles and adults but weak in the pyloric
caeca and anterior intestine, and absent in the oesophagus, posterior intestine
and rectum (Table 3). Trypsin was present in the anterior and posterior intestine
and was strong in the posterior intestine (Table 3). Proteases were not detected
in the oesophagus and rectum of juveniles and adults (Table 3). Protease activity
was higher in the stomach and posterior intestine than in the other gut regions.
Pepsin activity was very high in the stomach and low in both the pyloric caeca
and anterior intestine, with significant differences (P < 0.05) between juveniles
and adults. Trypsin activity was high in the posterior intestine, low in the anterior
intestine, and there were significant differences (P < 0.05) between juveniles and
adults. Proteolytic activity was not recorded in the oesophagus and rectum of
either juveniles or adults.
Glycosidases (a-amylase, maltase, lactase, sucrase, chitinase, and cellulase)
were present in all the gut sections except the rectum (Table 3). Alpha-amylase
was very strong in both the pyloric caeca and anterior intestine and strong in
the stomach, while the other glycosidases detected were weak in the various gut
regions. Chitinase and cellulase were detected in the stomach, pyloric caeca and
anterior intestine of juveniles and adults. Glycosidase activities in different gut
regions of juveniles and adults varied somewhat between the wet and dry seasons
but the differences were not significant (P > 0.05). The highest activity was in
the pyloric caeca and anterior intestine and the lowest activity in the oesophagus
(Table 3). There were significant differences (P < 0.05) between sizes.
DISCUSSION
The low incidence of full stomachs, coupled with the high incidence of
¼ full stomachs, uggests poor feeding activity in River Ose but may also be attributed to the method of catching, as many of the specimens were caught using gill nets. Arawomo (1976) reported > 67% empty stomachs for Citharinus
Cuvier 1816 species caught with gill nets in Lake Kainji. Ipinjolu et al. (2005)
reported that 48.1% of M. rume specimens caught in River Rima and Goronyo
Dam (Nigeria) had empty stomachs and no sample had 100% fullness. This may
have been due to the food items having been regurgitated or digested as the fish
struggled during the gill net catches.
M. rume in River Ose fed mainly on benthic insects (Notonectidae, Belastomatidae, Hydrometridae, Nepidae, Corixidae, Chironomidae, and Chaoboridae) and crustaceans (Potamonidae, Hydracarina spp., remains of decapods),
which were found in all the stomachs containing food. This observation agrees
with Ugwumba et al. (1990) that the mormyrids of Lekki Lagoon (Nigeria) fed
mainly on insects and crustaceans. It also agrees with Kouamelan et al. (1999)
who reported that M. rume in River Bia consumed invertebrates. There were also
plant parts, sand grains and detritus in all stomachs containing food. Sand grains
and detritus were probably ingested along with food items during feeding at the
river bottom. This observation revealed that M. rume is a bottom dweller, feeding on benthic organisms. PaugY (2002) also reported that M. rume in Baoule
Feeding habits and digestive enzymes in Mormyrus rume
83
River was insectivorous and that it was a benthic dweller. However, Omotosho
(1993) reported that M. rume fed on detritus, algae and macrophytes in Oyun
mini-dam, Ilorin (Nigeria), which agrees with Fawole (2002) who reported
that the major food items of M. rume in Lekki Lagoon were detritus and plant
parts. According to Ugwumba & Ugwumba (2007), these differences can be
attributed to differences in food availability between the different habitats.
All the food items were encountered in the stomachs of specimens irrespective of size and season. This agrees with Kouamelan et al. (1999) who
reported that there were no significant variations in the food composition of M.
rume from River Bia (Cameroon). However, during the dry season, chironomid
larvae dominated the insect fauna while chaoborid larvae became more abundant
in the rainy season. Moreover, during the rainy season, there is a continuous
input of materials of allochthonous origin, notably insects (coleopterans, dipterans, ants, termites), seeds, leaves and pollen from flooded/inundated forests into
River Ose, which settle at the bottom where they are decayed by bacterial and
fungal activities. The high percentage occurrence of detritus coupled with the
high ranking index of detritus suggest that M. rume is a bottom dweller and a
detritivore; indeed, the trunk-like snout coupled with the small terminal mouth
of M. rume encourages detritivory. Crustaceans and phytoplankton dominated
the plankton found in the environment. However, insects were more prominent
than rotifers in the stomachs of M. rume despite the relatively low occurrence
in the open waters and benthos of the river, showing that M. rume has a higher
preference for insects than for rotifers. This is evident since fish tend to optimize
the energy content of prey ingested by maximizing their size in relation to their
mouth gape.
There was high glycosidase activity, an indication that M. rume is capable
of digesting carbohydrates in its food. The presence of cellulase was also reported
in the gut of the catfish, Heterobranchus bidorsalis Geoffroy St. Hilaire 1809 by
FAgbenro et al. (1993) who noted that the occurrence of cellulase is a rather
unusual development because it occurs in few vertebrates and that when it occurs
the source is usually traced to the microflora inhabiting their guts. Tramati et
al. (2005) also reported cellulase activities in the stomach, pyloric caeca, foregut,
midgut and hindgut of Diplodus puntazzo Cetti 1777 juveniles and adults.
The digestion of starch appears to have started in the oesophagus and
continued to the posterior intestine. The high glycosidase activity in the pyloric
caeca and anterior intestine ensured complete digestion of carbohydrates in these
regions. The presence of lactase was also reported by Fagbenro et al. (1993) in
H. bidorsalis and by Lagler et al. (1977) in the pyloric caeca of trout, though
lactase is known to be associated with milk digestion in mammals.
The strong activities of chitinase in the stomach of adult M. rume specimens
and average activities in both the pyloric caeca and anterior intestine (Table 3)
showed that as the fish size increased the preference for insects increased as well;
this agrees with the increasing specificity of feeding habits of the adult fish (SL
> 35.0 cm) which had benthic insects and crustaceans as the prominent food
items (Ugwumba et al. 1990, Ipinjolu et al. 2005). This finding agrees with
Fagbenro et al. (2000) who recorded high chitinase activity in both the stomach
and anterior intestine of the electric catfish, Malapterurus electricus Gmelin
84
D.O. Odedeyi and O.A. Fagbenro
Table 3.
Specific activities of enzymes in the gut of juveniles and adults of Mormyrus rume from River
Ose.
Proteases2
Gut regions
Lipases1
Pepsin
Trypsin
Juveniles (SL < 35.0 cm)
Oesophagus
0.84+0.15a*
0
0
Stomach
2.33+0.15b*
0.15+0.02a*
0
Pyloric caeca
4.62+0.34c*
0.100+01b*
0
Anterior intestine
6.64+0.40d*
0.10+0.01b*
0.08+0.03a*
Posterior intestine
5.04+0.07e*
0
0.16+0.02b*
Rectum
1.30+0.10f*
0
0
Oesophagus
0.90+0.10a**
0
0
Stomach
2.50+0.37b**
0.19+0.01a**
0
Pyloric caeca
5.10+0.46c**
0.11+0.01b**
0
Anterior intestine
7.23+0.11d**
0.11+0.01 c**
0.10+0.01a**
Posterior intestine
5.27+0.14e**
0
0.18+0.01b**
Rectum
2.90+0.20f**
0
0
Adults (SL > 35.0 cm)
(continued)
Feeding habits and digestive enzymes in Mormyrus rume
85
Table 3. (continued)
Glycosidases3
a-Amylase
Maltase
Lactase
Sucrase
Cellulase
Chitinase
0.04+0.00a*
0.01+0.00a*
0
0
0
0
0.14+0.02b*
0.09+01b*
0.07+0.01a*
0.07+0.01a*
0.02+0.01a*
0.03+0.00a*
0.50+0.03c*
0.09+0.00c*
0.08+0.01b*
0.05+0.01b*
0.03+0.00b*
0.03+0.00a*
0.36+0.03d*
0.06+0.01d*
0.05+0.02c*
0.04+0.01c*
0.02+0.00c*
0.02+0.00b*
0.03+0.00e*
0.02+0.00e*
0.02+0.00d*
0.02+0.00d*
0
0
0
0
0
0
0
0
0.11+0.01a**
0.02+0.00a**
0
0
0
0
0.34+0.02b**
0.10+01b**
0.08+0.01a**
0.08+0.02a**
0.02+0.00a**
0.05+0.00a**
0.53+0.03c**
0.13+0.02c**
0.06+0.01b**
0.07+0.01b**
0.05+0.02b**
0.06+0.01a**
0.39+0.02d**
0.07+0.02d**
0.05+0.02c**
0.05+0.01c**
0.04+0.01c**
0.04+0.00b**
0.04+0.01e**
0.03+0.00e**
0.03+0.00d**
0.02+0.00d**
0
0
0
0
0
0
0
0
a-f = for each enzyme, data in the same column with different superscript letters are significantly different (P < 0.05); *,** = for each enzyme, data along the same row with different
asterisks are significantly different (P < 0.05); 1 milliequivalents of fatty acids/mg protein at 37
o
C, 2 change in optical density at 595 nm/hr/mg of L-tyrosine/hr at 37 oC, 3 mg glucose/min/
mg protein at 37 oC.
86
D.O. Odedeyi and O.A. Fagbenro
1789, and associated this with its chitin-eating habit (feeding on crustaceans and
insects). Similarly high chitinase activities were detected in the stomachs and
intestines of Atlantic cod fed on whole crustaceans (Danulat 1986). Lindsay
(1984) suggested that the primary function of gastric chitinase in fishes is to
chemically disrupt the chitin envelope of the prey.
Digestion of protein in M. rume started in the stomach. The absence of
proteases in the oesophagus showed that this region only serves for the passage
of food items to the stomach while the rectum retains undigested food materials
or waste prior to being voided from the body (Olatunde & Ogunbiyi 1977,
Olatunde et al. 1988). The high peptic activities recorded in the stomach of
both juvenile and adult M. rume suggest the consumption of protein-rich food
items by this species. High peptic activity was also reported in the stomach of
the electric catfish by Fagbenro et al. (2001). Smith (1980) reported that peptic
activity in fishes occurs in acid conditions (pH 2.0-4.0) as in higher vertebrates
and is about 150 times greater than the activity of mammalian pepsin in the
affinity for the substrate (Ananichev 1959). The high activity of trypsin, an
alkaline protease, in the posterior intestine of M. rume agrees with Fagbenro
et al. (2001) who also recorded high trypsin activity in the posterior intestine
of the electric catfish, M. electricus. Olatunde & Ogunbiyi (1977) attributed
high trypsin activities in Schilbe mystus Linnaeus 1758 to the large amounts of
fish and insect materials in the food of the species. Pepsin activity was high in
both juveniles and adults of M. rume. The high pepsin and trypsin activities
recorded in this study can be attributed to M. rume consuming protein-rich food
items, in a similar manner to H. bidorsalis (Fagbenro et al. 1993, 2001) and
other tropical catfishes, Parailia (Physailia) pellucida Boulenger 1901, Eutropius
niloticus Rüppell 1829 and S. mystus (Olatunde & Ogunbiyi 1977). Protein
digestion was initiated by pepsin and completed by trypsin; Uys & Hecht
(1987) and Fagbenro et al. (1993) reported that partial hydrolysis of protein is
subsequently completed by the combined action of trypsin and chymotrypsin
when food reaches the intestine.
Lipase activity was detected in all the regions of the M. rume gut,
indicating a uniform distribution in the entire gut system, as reported in Heterotis
niloticus Cuvier 1829 by Fagbenro et al. (2000). Swarup & Goel (1975) also
observed lipases along the entire gut of some teleosts. The lipase activity was
lowest in the oesophagus and more abundant in the pyloric caeca, anterior and
posterior intestine, which are the neutral and alkaline regions of the tract. This
agrees with Tramati et al. (2005) who reported that lipase was more abundant in
the neutral-alkaline gut regions of juvenile and adult D. puntazzo. Fagbenro et
al. (1993) reported that lipase activity was average and restricted to the posterior
regions of the H. bidorsalis gut, while Olatunde & Ogunbiyi (1977) reported
that lipase activities were not detected in the guts of P. pellucida, E. niloticus
and S. mystus and remarked that this was a surprise in view of the fact that the
food items consumed by E. niloticus and S. mystus included fatty fishes such as
clupeids. The higher values (P < 0.05) of lipase activity in the rectum of adult
M. rume than of juvenile M. rume (Table 3) are unusual and can be attributed
to forceful ejection of enzymes while the fish struggle immediately after being
captured in the gill nets.
Feeding habits and digestive enzymes in Mormyrus rume
87
According to Buddington (1985), the presence or absence of certain
digestive enzymes in fishes depends on the food and feeding habits as well as the
functional morphology of the various parts of the gut. In addition, the presence
of some enzymes in the oesophagus might be an artefact due to reflux of the
chyme and/or digestive juices caused by stress suffered when the fish are captured
in gill nets and handled by the fishermen. The occurrence of various glycosidases,
proteases and lipases in the gut of M. rume was correlated with detritivorous
food habits. The digestive enzyme activities in the adults were higher than in the
juveniles, which agrees with Tramati et al. (2005) that the digestive processes
of D. puntazzo are correlated with the size and composition of the food. Lee
et al. (1984) and Le François et al. (2000) also reported that the activities of
glycolytic enzymes increased with fish mass.
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