Fish diseases are the major problems confronting the fish culturists.
In Nigeria, the demands for fish exceed supply and the proportion of annual
protein in the daily diet is generally low.
Gymnarchus niloticus (Cuvier, 1829) is a common fish species in
Nigeria and several West African countries. The fish species lives in
demersal, potamodromous, freshwater environment with a pH range of 6.5-8.0
and a dH range of 10-25 (Riede, 2004). The species occur widely in the
Nile, Volta, Chad, Senegal, Gambia basins and Lake Rudolf. The fish species
is known to lay about one thousand amber-like eggs; larvae hatching after
five days (Budgett, 1901).
Gymnarchus niloticus is a carnivorous fish feeding on crustaceans,
insects and fish (Bennett, 1971). The body is elongated with no anal and
Bigorne (1990) reported a standard length of 167 cm and a weight of 18.5
kg for male of Gymnarchus niloticus. The fish is also reported
to possess an electric organ that extends along almost the entire trunk
to the tip of the tail (Bennett, 1971). It is also equipped with ampullary
receptors and two types of tuberous receptors for electroreception (Szabo,
The fish breeds in well-vegetated, marginal areas of swamps and rivers
where large, floating nest, about 1 m in diameter is constructed. The
eggs laid are later guarded by one of the parents (Greenwood and Wilson,
Heterotis niloticus, family Osteoglossidae is an omnivorous and
microphagous species. The fish is predominantly a bottom feeder. The fish
species also occur widely in the Nile, Senegal, Gambia and Niger Rivers
and also Lake Rudolf and Lake Chad. It has an elongate and robust body
with height of 3.5 to 5 times in standard length (Paugy, 1990). The fish
also has a relatively short head with length of 3.5 to 5 times in standard
length (Paugy 1990; Moreau, 1982). The young feed on zooplankton while
adults feed mostly on dipteran insect larvae and copepods which is supplemented
with molluscs and higher plants. Acclimated specimen may accept pellets.
The sex is difficult to distinguish and breeding has been unsuccessful
in captivity because of the size of the fish. The fish also feeds on algae.
Heterotis niloticus is found in swamps, weedy areas of rivers
and shallow well-vegetated lakes. It is also found in flowing channels
and floodplains and also in brackish water in estuaries. The fish tolerates
very low limits of dissolved oxygen. Since the swimbladder is modified
to form suprabranchial organ, the fish can breathe atmospheric oxygen.
The fish dermal bones of the cranium are reported to be deeply carved
by large sensory pits (Boulenger, 1990; Paugy, 1990). The young specimens
of the fish possess external gills (Dankwa et al., 1999).
In the present research a comparative study of the gut helminthes of
Gymnarchus niloticus and Heterotis niloticus were undertaken.
MATERIALS AND METHODS
Study area: Lekki lagoon supports a major fishery in Nigeria. The lagoon is
located in Lagos State, Nigeria and lies between longitudes 4°00
and 4°15 E and between latitudes 6°25 and 6°37 N. It has
a surface area of about 247 km2 with a maximum depth of 6.4
m, a greater part of the lagoon is shallow and less than 3.0 m deep.
The Lekki lagoon is part of an intricate system of waterways made up
of lagoons and creeks that are found along the coast of South-western
Nigeria from the Dahomey border to the Niger Delta stretching over a distance
of about 200 km. it is fed by the River Oni discharging to the North-eastern
and the Rivers Oshun and Saga discharging into North-western parts of
the lagoon. Lekki lagoon experiences both dry and rainy seasons typical
of the southern part of Nigeria.
The vegetation around the lagoon is characterized by shrub and raphia
palms, Raphia sudanica and oil palms, Elaeis guneensis.
Floating grass occur on the periphery of the lagoon while coconut palms
Cocos nucifera are widespread in the surrounding villages.
The rich fish fauna of the lagoon includes Heterotis niloticus,
Gymnarchus niloticus, Clarias gariepinus, Malapterurus electricus,
Synodontis clarias, Chrysichthys nigrodigitatus, Channa
obscura, Mormyrus rume, Calabaricus calamoichthys,
Tilapia zilli, Tilapia galilae, Hemichromis fasciatus
and Sarotherodon melanotheron (Kusemiju, 1981). Figure
1 shows map of Lekki lagoon, Lagos, Nigeria.
Collections of specimens: The two specimens, Gymnarchus niloticus and Heterotis
niloticus were caught at Lekki lagoon, Lagos, Nigeria. The fishes
were kept in a long tank that suits their elongated and robust body. This
is done to keep them alive for sometime. The collections of the specimens
were done for more than one year.
Determination of fish parameters: The fishes were identified. The standard length of the fishes were
measured from the head to the region where the tail develops. The total
length is the measurement of the entire length of the fish which is from
the head to the end of the tail. The lengths were measured using a metre
rule. The weight of each fish was obtained using a chemical balance. The
sex of the fishes were determined based on the presence or absence of
Examination of specimens for parasites: The abdominal cavity of each fish was cut open and the gastrointestinal
parts was removed and cut into parts.
||Map of Lekki lagoon
The gastrointestinal parts were separated from the other visceral organs and placed in petri
dishes containing physiological saline. The intestines were further carefully
slit open to aid the emergence of the parasitic helminthes. The emergence
of any worm was easily noticed by its wriggling movement in the saline
solution. Some of the worms however remained permanently attached with
their attachment organs to the gut walls. They were carefully removed
and put into the physiological saline.
Processing of recovered parasites: The different kinds of helminth parasites found were immediately
fixed in 70% alcohol. They were counted and recorded against each of the
specimen. Identification of the helminthes to species level was undertaken
at the British Museum (Natural History), Parasitic Worm Division and United
A total of thirty-eight specimens of Gymnarchus niloticus
20 and Heterotis niloticus 18 were subjected to parasitologic investigations.
In Heterotis niloticus 11 specimens were infected with gastrointestinal
helminth parasites with a prevalence of 61.1%. Gymnarchus niloticus
had a prevalence of 10% with two specimens infected with gastrointestinal
helminth parasites. A total of 13 specimens were infected with an overall
prevalence of gastrointestinal helminth infections of 34.2%. All helminthic
infections were restricted to the intestine. A total of 14 parasites were
recovered from Heterotis niloticus and 4 parasites recovered from
In Gymnarchus niloticus, two nematodes Raphidascaroides
species (Heterocheillidae) (Khalil, 1961) (Anisakidae) (recovered from
the stomach) and Nilonema gymnarchi (Khalil, 1960a) (Philometridae)
(recovered from the intestine) were obtained. Females parasites of the
latter were recovered.
Heterotis niloticus were infected with three kinds of gastrointestinal
helminth parasites. A trematode, Brevimulticaecum heterotis (Petter, 1978) were recovered and seen perforating the liver. Tenuisentis
niloticus (Meyer, 1932) (Tenuisentidae), an acanthocephala and Sandonella
sandoni (Lynsdale, 1960) (Proteocephalidae) a cestode were obtained
from the walls of the intestine. There was no nematode infections observed
in Heterotis niloticus while there was only nematode infections
observed in Gymnarchus niloticus. Single and mixed infections with
two and three helminth worms was observed in the two specimens. Table
1 shows sex in relation to gastrointestinal helminth infections in
||Helminth infection in relation to sex of Heterotis
niloticus in Lekki lagoon
|The Chi-square = 3.841
Helminth infection in relation to size of Heterotis niloticus
|Chi-square = 9.488, Chi-square = 3.841
Gastrointestinal helminth infections in relation to sex of
|Chi-square = 3.841
The male specimens of Heterotis niloticus had a prevalence of
38.9% which accounts for 7 out of the 10 male specimens examined to be
infected with gastrointestinal helminth parasite.
The female specimens had a prevalence of 50% which is higher than the
prevalence of infection in that of the male specimens. A total of 8 female
specimens were examined for gastrointestinal helminth infections and 4
were infected with helminth parasites.
Table 2 reveals intestinal helminth infections in relation
to size in Heterotis niloticus. The length groups 66-70 and 71-75
cm had a prevalence of infections of 66.7 and 80%, respectively. The other
length groups 76-80, 81-85 and 86-90 cm had a prevalence of infection
of 50% each.
The total length of Heterotis niloticus examined ranged from 69.00
to 89.50 cm. An overall prevalence of infections was 61.1%.
There was no relationship between size and infection in Heterotis
niloticus. The results of the gastrointestinal helminth infections
in Heterotis niloticus reveals that smaller specimens are more
liable to infections than the bigger ones.
Table 3 reveals sex in relation to gastrointestinal
helminth infection in Gymnarchus niloticus. The male specimens
had zero percentage of infection while the female specimens had a prevalence
The results of the gastrointestinal helminth infections in relation to
sex of Gymnarchus niloticus was found to be significant. This implies
that there is relationship between sex and helminth infections in the
Table 4 also reveals intestinal helminth infection in
relation to size of Gymnarchus niloticus. The length groups 41-60,
81-100 and 101-120 cm recorded zero
Gastrointestinal helminth infections in relation to size in
|Chi-square = 7.815
Overall Prevalence of gastrointestinal helminth infections
in the two specimens
Scattered diagram total length against total weight of Heterotis
niloticus (Combined Sexes)
Diagram of total length against total weight of Gymaorclus
niloticus (Combined Sexes)
prevalence of infections. The length groups 61-80 cm had a prevalence
of gastrointestinal helminth infections of 25%. The two infected female
specimens of Gymnarchus niloticus belong to this length group.
The results of gastrointestinal helminth infections in relation to size
in Gymnarchus niloticus reveals that there is no relationship between
size and infection in the fish specimen. The total length of Gymnarchus
niloticus examined ranged from 48.80 to 110 cm. An overall prevalence
of 10% was recorded in the fish specimen.
Figure 2 and 3 showed scatter diagrams of total length
against weight of combined sexes in Heterotis niloticus and Gymnarchus
The results reveal that a total of 26 male specimens were infected with
a prevalence of 26.9%. The female specimens recorded a prevalence of 50%
with 6 infected specimens out of the 12 examined.
An overall prevalence of 34.2% was observed in the two specimens (Table
The results also reveal that the female specimens are more liable to
infections than the male specimens in the two fish species.
Gymnarchus niloticus (Cuvier, 1829) (Gymnarchidae) and Heterotis
niloticus (Cuvier, 1829) (Osteoglossidae) were randomly selected from
Lekki lagoon and subjected to parasitologic examinations. A total of 20
specimens of Gymnarchus niloticus and 18 specimens of Heterotis
niloticus were examined for gastrointestinal helminth parasites. A
high prevalence of 61.1% were recorded in Heterotis niloticus.
Gymnarchus niloticus had a prevalence of 10%. An overall prevalence
of 34.2% were recorded from the two specimens.
Heterotis niloticus were infected with Brevimulticaecum heterotis
(Petter, 1978) in the liver, Tenuisentis niloticus
(Meyer, 1932) (Tenuisentidae) and Sandonella sandoni (Lynsdale,
1960) (Proteocephalidae) were recovered from the walls of the intestine.
According to the host parasite checklist on African freshwater fishes
of Khalil and Polling (1997), Tenuisentis niloticus (Rhadinorhynchus
niloticus) has been documented in Heterotis niloticus (Meyer,
1932; Khalil, 1969; Van cleave, 1936; Dollfus and Govan, 1956).
Dollfus and Govan (1956) also recovered encysted immature of Tenuisentis
niloticus in Hydrocynus brevis in Mali.
Golvan (1957, 1965) and Khalil (1971) reported that the thorny-headed
worms are present in representatives of diverse African fish families.
Other parasites such as Heterotesia voltae, Nephrocephala bagri incapsulatum
(metacercaria) and Sardonella sandoni have also been documented
in Heterotis niloticus (Khalil and Polling, 1997). Khalil (1969) recorded 5-27 Tenuisentis niloticus in 93% of Heterotis niloticus
in the Sudan White and Blue Nile. Pathogenic effects of the thorny-headed
worms are simply due to their attachment in the digestive tract of the
fish and also to the encapsulation of larval stages in the tissues.
The gastrointestinal helminth infections of the different length categories
were also recorded. The length groups 66-70 and 71-75 cm recorded a high
prevalence of gastrointestinal helminth infections of 66.7 and 80%, respectively.
The other length groups recorded a lower prevalence of infections of 50%
each. This may no doubt be related to the random selection of the specimens
and to the probable high level of immunity builded up in the fish specimens.
An overall prevalence of 61.1% recorded in Heterotis niloticus
is due to the omnivorous and microphagous feeding habits of the fish species.
Gymnarchus niloticus harboured two nematodes; Raphidascaroides species
(anisakidae) recovered from the stomach and Nilonema gymnarchi (Khalil,
1960b) (Philometridae) recovered from the intestine.
The occurrence of Raphidascaroides species in the stomach of Gymnarchus
nioticus in this present study is in conformity to the work done by
Khalil (1961, 1969) in Sudan who also recovered Raphidascaroides bishaii
from the stomach of Gymnarchus niloticus.
Moravec (1974b) reported that larvae in copepods or other invertebrate
intermediate hosts, will develop to fourth stage larvae and further into
adult males and females when ingested by a suitable definitive host. Petter
et al. (1989) however demonstrated in the anisakids (Heterocheillidae)
Dujardinascaris and Raphidascaroides and in species of Anguillicola
that larvae ingested by wrong piscine hosts often survive as waiting stages
in the gut or other tissues for a variable length of time and continue
development into the adult stage if their carrier host is predated by
the suitable host.
The host parasite checklist of Khalil and Polling (1997) also recorded
Acanthostomum gymnarchi, Opisthorchis, Piscicola, Phyllodistomum linguale
and Nesolecithus africanus in Gymnarchus niloticus.
In the present study, Nilonema gymnarchi was isolated from the
intestine of Gymnarchus niloticus. Khalil 1969 however recovered
Nilonema gymnarchi in the lung-like air bladder sacs of the fish
species and Thwaitia bagri), under the skin lateral to the mouth,
in Bagrus bayad. The helminth parasites (Nilonema gymarchi
and Thwaitia bagri are the two representatives genera of the family
Philometridae in Africa. Gravid Ngymnarchi presumably escape from the
lungs into the water to discharge larvae (Khalil, 1969). Khalil (1969,
1971) and Moravec (1974a) also emphasized very strict host specifically
of Nilonema gymnarchi and Thwaitia bagri.
Yakubu et al. (2002) in a comparative study of gut helminthes
of Tilapia zilli and Clarias gariepinus from River Uke Plateau
State, Nigeria also found a prevalence of 61% in T. zilli and 55%
in Clarias gariepinus respectively.
In Gymnarchus niloticus, the length groups 41-60 cm, 81-100 and
101-120 cm recorded zero (0) prevalence of gastrointestinal helminth infections.
This may also nodoubt be attributable to the random selection of the specimens.
The length groups 61-80 cm had a prevalence of gastrointestinal helminth
infections of 25%.
This comparative study on the parasitic helminth fauna of Gymnarchus
niloticus and Heterotis niloticus, revealed that the female
specimens of Heterotis niloticus are more susceptible to infections
than the male specimens.
The comparative study also reveals the female specimens of Gymnarchus
niloticus to be more susceptible to gastrointestinal helminth infections
than the male specimens. The nature of the Lekki lagoon may however influence
the low prevalence of intestinal helminth infections in the male specimens
of the two fishes.
In conclusion, the incidences of helminth infections of fishes of
Lekki lagoon should not be taken with levity. The helminth parasites recovered
from the intestines of Gymnarchus niloticus and Heterotis niloticus
will no doubt have depended on the presence of absorbable food materials
in the lumen of the gut of the fishes. Fish have carbohydrate, lipid,
vitamin and inorganic ion requirements and are adapted to obtain these
requirements from dietary intake. The absorption of the absorbable food
materials from the gut of the fishes reduced fecundity and market value
of the fishes. Further studies are still required to establish the changes
in the environment, whether natural or man-made and to proffer probable
biological control of the parasites in Lekki lagoon, Lagos, Nigeria.