Bioaccumulation of Heavy Metals in Tissues of Callinectes sapidus from the Iko River, Nigeria:
Implications for Human Health Risk Assessment
1
Okpoji, Awajiiroijana U., 2Warder Amaminor B., 3Ekwere, Ifiok. O., 4Ogwu Nkechi G.
1
Department of Pure and Industrial Chemistry, Nnamdi Azikiwe University, Awka, Nigeria.
2
Department of Biology, Ignatius Ajuru University of Education, Rumuolumeni, Port Harcourt, Nigeria
3
Department of Chemistry, Akwa Ibom State University, Ikot Akpaden, Nigeria.
Department of Chemistry Education, Federal College of Education (Technical), Umunze, Nigeria.
Corresponding email: ao.okpoji@stu.unizik.edu.ng
Abstract
This study assessed the bioaccumulation of three heavy metals—nickel (Ni), lead (Pb), and cadmium
(Cd)—in selected tissues (hepatopancreas, gills, and muscles) of male and female Callinectes sapidus
(blue crab) collected from two stations along the Iko River in Nigeria. Samples were digested and
analysed using Atomic Absorption Spectrophotometry (Unicam 969, Thermo Elemental). The
concentrations of Ni, Pb, and Cd in all tissues were found to exceed the permissible limits set by the
United States Environmental Protection Agency (USEPA) and the Food and Agriculture
Organisation/World Health Organisation (FAO/WHO). A distinct accumulation pattern was observed in
the tissues, following the order: hepatopancreas > gills > muscles. Estimated Daily Intake (EDI) and
Target Hazard Quotient (THQ) were calculated to evaluate potential human health risks associated with
crab consumption. THQ values for the three metals in most tissues exceeded 1.0, suggesting a potential
non-carcinogenic risk. Additionally, long-term exposure assessments indicated a possible carcinogenic
risk from the ingestion of Ni, Pb, and Cd. The findings also demonstrated spatial variation in metal
accumulation, with crabs from Station A showing significantly higher concentrations, likely due to
proximity to known industrial discharge and oil-related activities. This research emphasises the potential
health concerns linked to consuming C. sapidus from the Iko River and underscores the urgent need for
regular monitoring, pollution control, and public health awareness in the region.
Keywords: Heavy metals, Callinectes sapidus, bioaccumulation, Iko River, risk assessment, Nigeria.
1.0 Introduction
Heavy metal pollution has become a significant global environmental issue, especially within aquatic
ecosystems. Since the 1960s, when thousands of people suffered mercury poisoning from contaminated
seafood in Minamata, Japan, the ecological and public health effects of heavy metal contamination have
become increasingly clear (Oladimeji, 2021).
In Nigeria, industrialisation, oil exploration, and poor waste management practices have substantially
contributed to the release of heavy metals into the environment. The main sources of heavy metal
contamination in aquatic systems include industrial effluents, domestic sewage, oil and chemical spills,
combustion emissions, metallurgical processes, and mining activities (Odukoya & Ajayi, 1987). These
metals are particularly concerning because of their non-biodegradable nature, long biological half-lives,
and tendency to bioaccumulate within aquatic organisms, ultimately entering the food chain and posing
health risks to humans (Wang et al., 2005).
Recently, invertebrates such as crabs have received growing attention as bioindicators for assessing
metal pollution due to their sedentary behaviour and ability to reflect local environmental conditions.
Although sampling in such studies can be challenging, especially where species diversity is high,
invertebrates are especially valuable since they serve as both ecological indicators and potential
pathways for toxic metals to transfer to higher trophic levels (Peterson et al., 2002).
The Niger Delta region of Nigeria, including Akwa Ibom State, is recognised for its high levels of oil
extraction and related environmental degradation. Illegal oil bunkering, gas flaring, improper waste
disposal, and aquaculture activities contribute to considerable ecological stress in the region. Crude oil
spills are particularly harmful, releasing a complex mixture of hydrocarbons and associated heavy
metals into land and water environments. These contaminants tend to build up in sediments and aquatic
organisms, potentially harming both biodiversity and human populations. The Iko River, situated in the
Eastern Obolo Local Government Area of Akwa Ibom State, is an estuarine system that supports various
aquatic species, including the blue crab (Callinectes sapidus). The crab holds ecological and economic
significance and is widely consumed by local communities. However, its proximity to oil exploration
sites raises concerns about contamination and food safety.
This study was therefore conducted to examine the concentration and distribution of Nickel (Ni), Lead
(Pb), and Cadmium (Cd) in the gills, hepatopancreas, and muscle tissues of male and female C. sapidus
from two locations along the Iko River. Furthermore, the study aimed to evaluate the potential health
risks linked to consuming contaminated crab tissues using Estimated Daily Intake (EDI) and Target
Hazard Quotient (THQ) models. The results are intended to guide policies on environmental health, food
safety, and pollution management in the Niger Delta region.
2.0 Materials and Methods
2.1 Study Area
The study was conducted along the Iko River, situated in the Eastern Obolo Local Government Area of
Akwa Ibom State, Nigeria. The river is bounded to the south-west by Rivers State and discharges into
the Atlantic Ocean. Two sampling stations were selected within the riverine system, located between
latitudes 4°30′ and 4°45′N, and longitudes 7°35′ and 7°40′E. The area is characterised by an estuarine
environment, supporting diverse aquatic species including fish, periwinkles, shrimps, and crabs. It is
also subject to environmental stressors such as artisanal fishing, aquaculture, oil exploration, and
industrial waste discharge.
Figure 1: Map of the Iko River showing the sample stations.
2.2 Sample Collection and Preparation
Adult male and female blue crabs (Callinectes sapidus) were collected during daylight hours using
baited crawl nets at both sampling stations. Immediately after capture, the crabs were placed in an
insulated container to preserve freshness and transported to the laboratory for analysis. In the laboratory,
the crabs were washed thoroughly under running tap water to remove debris and then allowed to thaw at
room temperature. Using sterilised stainless steel dissection tools, three tissue types—gills,
hepatopancreas, and muscle—were carefully excised from each specimen. The samples were replicated
for both sexes and from each station, then transferred to sterile Petri dishes. The dissected tissues were
oven-dried at 60°C for 72 hours using foil-lined trays to obtain constant dry weight. The dried samples
were then ground to a fine powder using a mortar and pestle, and 1 gram (g) of each powdered tissue
sample was weighed into a digestion flask.
2.3 Sample Digestion and Heavy Metal Analysis
Each 1 g tissue sample was digested using 10 mL of concentrated nitric acid (HNO₃, 16 M, 68%) and 2
mL of perchloric acid (HClO₄, 11 M, 70%) under a fume hood. The digestion was carried out on a hot
plate until the Figure
emergence of whitecrab,
2: Female fumes indicated complete digestion.
Male crab After cooling, the digested
samples were filtered and diluted to a final volume of 100 mL using deionised water in a volumetric
flask. Heavy metal concentrations (Ni, Pb, and Cd) were determined using Atomic Absorption
Spectrophotometry (AAS), model Unicam 969 Thermo Elemental. All measurements were conducted in
triplicate, and blank determinations were included to ensure quality control. Calibration curves were
prepared using standard metal solutions of known concentrations.
2.4 Estimated Daily Intake (EDI)
The Estimated Daily Intake (EDI) of each metal was calculated to assess the average daily exposure
through crab consumption using the following formula:
(Cmetal × IR × CF)
EDI=
BW
Where:
Cmetal = Concentration of metal in tissue (mg/kg)
IR = Ingestion rate (60 g/day or 0.06 kg/day)
CF = Conversion factor (20.5 for C. sapidus) (Ricciardi & Bourget, 1998)
BW = Average body weight of adult (60 kg) (USEPA, 2006)
2.5 Target Hazard Quotient (THQ)
Non-carcinogenic risk was assessed using the Target Hazard Quotient (THQ), which was calculated
following the United States Environmental Protection Agency (USEPA) methodology (USEPA, 1989;
Singh et al., 2010; USEPA, 2011). The equation is as follows:
EF x ED x IR x C
The THQ=
RfD x BW x AT
Where:
EF = Exposure frequency (350 days/year)
ED = Exposure duration (54 years, average Nigerian lifespan)
IR = Ingestion rate (0.06 kg/day)
CCC = Metal concentration in tissue (mg/kg)
RfD = Oral reference dose (mg/kg/day)
BW = Average body weight (60 kg)
AT = Averaging time for non-carcinogens (ED × 365 days = 19,710 days). A THQ value ≥ 1 implies
a potential health risk from prolonged exposure, while a value < 1 suggests negligible risk.
3.0 Results
3.1 Heavy Metal Concentration in Tissues of Callinectes sapidus
The concentrations of Nickel (Ni), Lead (Pb), and Cadmium (Cd) in the gills, hepatopancreas, and
muscles of male and female C. sapidus from Stations A and B are presented in Tables 1a and 1b.
Nickel exhibited the highest levels among the three metals analysed. At Station A, Ni concentrations in
the hepatopancreas ranged from 230.97–231.17 mg/kg in males and 311.67–313.53 mg/kg in females.
Gills showed 138.72–139.65 mg/kg (males) and 210.00–213.51 mg/kg (females), while muscles
recorded the lowest concentrations: 103.86–105.02 mg/kg (males) and 168.32–170.01 mg/kg (females).
Similarly, at Station B, the hepatopancreas had the highest Ni levels: 182.97–183.65 mg/kg (males) and
272.61–272.85 mg/kg (females). Gills ranged from 129.93–130.52 mg/kg (males) to 192.06–192.14
mg/kg (females), while muscle concentrations ranged from 93.09–93.33 mg/kg (males) to 165.63–
165.84 mg/kg (females).
Lead concentrations followed a similar trend. At Station A, Pb levels in the hepatopancreas were 29.72–
29.76 mg/kg (males) and 38.99–39.63 mg/kg (females), while gills contained 13.46–14.94 mg/kg
(males) and 21.17–21.35 mg/kg (females). Muscle tissues had the lowest Pb concentrations: 3.51–3.60
mg/kg (males) and 6.29–6.32 mg/kg (females). At Station B, Pb levels in the hepatopancreas ranged
from 22.86–23.15 mg/kg (males) to 37.01–37.08 mg/kg (females), with lower concentrations in the gills
(10.07–10.25 mg/kg in males; 12.74–12.80 mg/kg in females) and muscles (2.73–3.02 mg/kg in males;
4.77–4.82 mg/kg in females).
Cadmium concentrations, though lower than Ni and Pb, showed a significant pattern of
bioaccumulation. At Station A, Cd in the hepatopancreas ranged from 3.98–4.10 mg/kg (males) to 8.91–
9.03 mg/kg (females), while gill concentrations were 0.45–1.50 mg/kg (males) and 4.43–4.95 mg/kg
(females). Muscle tissues ranged from 0.26–0.35 mg/kg in males to 2.81–2.84 mg/kg in females. At
Station B, Cd concentrations in the hepatopancreas ranged from 2.04–2.10 mg/kg (males) and 6.83–7.08
mg/kg (females), with gill values of 0.12–0.18 mg/kg (males) and 3.59–3.66 mg/kg (females), and
muscle concentrations of 0.105–0.15 mg/kg (males) and 1.47–1.56 mg/kg (females). Across both
stations, the hierarchical pattern of metal accumulation in tissues was: hepatopancreas > gills > muscle,
and females consistently showed higher concentrations than males.
3.2 Estimated Daily Intake (EDI) and Target Hazard Quotient (THQ)
Tables 2a and 2b summarise the Estimated Daily Intake (EDI) and Target Hazard Quotients (THQ) of
Ni, Pb, and Cd from consumption of C. sapidus tissues. At Station A, EDI values for Ni ranged from
2.14–4.73 µg/day in males and 3.47–6.41 µg/day in females. Corresponding THQ values exceeded 1.0
in all tissues, suggesting potential non-carcinogenic health risks. Lead EDIs ranged from 0.07–0.61
µg/day (males) and 0.13–0.81 µg/day (females), with THQs also above 1.0. Cd EDI values were lowest
but still yielded THQ values ranging from 0.37 to 8.70, especially elevated in the hepatopancreas and
gills of female crabs. At Station B, a similar trend was observed. EDI and THQ values for all three
metals were generally lower than those from Station A but still exceeded safe thresholds, especially for
females. Ni THQ values ranged from 4.44–14.44, Pb from 0.56–10.15, and Cd from 0.12–6.69. Overall,
THQ values >1.0 across most tissues and metals indicate a significant risk of chronic health effects
upon regular consumption of contaminated crab tissues, particularly from Station A.
Table 1a. Concentration of Heavy Metals (mg/kg) in Tissues of Callinectes sapidus from Station A
Metal Sex Tissue Range Mean ± SD
Ni Male Gills 138.72–139.65 139.25 ± 0.48
Hepatopancreas 230.97–231.17 231.06 ± 0.10
Muscles 103.86–105.02 104.23 ± 0.63
Female Gills 210.00–213.51 211.18 ± 2.02
Hepatopancreas 311.67–313.53 312.59 ± 0.93
Muscles 168.32–170.01 169.40 ± 0.94
Pb Male Gills 13.46–14.94 14.38 ± 0.80
Hepatopancreas 29.72–29.76 29.74 ± 0.02
Muscles 3.51–3.60 3.57 ± 0.05
Female Gills 21.17–21.35 21.24 ± 0.10
Metal Sex Tissue Range Mean ± SD
Hepatopancreas 38.99–39.63 39.32 ± 0.33
Muscles 6.29–6.32 6.30 ± 0.01
Cd Male Gills 0.45–1.50 0.82 ± 0.59
Hepatopancreas 3.98–4.10 4.04 ± 0.06
Muscles 0.26–0.35 0.30 ± 0.05
Female Gills 4.43–4.95 4.78 ± 0.31
Hepatopancreas 8.91–9.03 8.99 ± 0.07
Muscles 2.81–2.84 2.82 ± 0.02
Table 1b. Concentration of Heavy Metals (mg/kg) in Tissues of Callinectes sapidus from Station B
Metal Sex Tissue Range Mean ± SD
Ni Male Gills 129.93–130.52 130.16 ± 0.32
Hepatopancreas 182.97–183.65 183.40 ± 0.38
Muscles 93.09–93.33 93.18 ± 0.13
Female Gills 192.06–192.14 192.12 ± 0.06
Hepatopancreas 272.61–272.85 272.74 ± 0.12
Muscles 165.63–165.84 165.71 ± 0.11
Pb Male Gills 10.07–10.25 10.12 ± 0.12
Hepatopancreas 22.86–23.15 22.99 ± 0.15
Muscles 2.73–3.02 2.91 ± 0.16
Female Gills 12.74–12.80 12.77 ± 0.03
Hepatopancreas 37.01–37.08 37.04 ± 0.04
Muscles 4.77–4.82 4.80 ± 0.02
Cd Male Gills 0.12–0.18 0.16 ± 0.03
Hepatopancreas 2.04–2.10 2.07 ± 0.03
Muscles 0.105–0.15 0.13 ± 0.02
Female Gills 3.59–3.66 3.63 ± 0.04
Hepatopancreas 6.83–7.08 6.97 ± 0.13
Muscles 1.47–1.56 1.52 ± 0.04
Table 2a. Estimated Daily Intake (EDI) and Target Hazard Quotient (THQ) of Heavy Metals at
Station A
Metal Sex Tissue EDI (µg/day) THQ
Ni Male Gills 2.85 6.67
Hepatopancreas 4.73 11.11
Muscles 2.14 5.00
Female Gills 4.33 10.19
Hepatopancreas 6.41 15.00
Metal Sex Tissue EDI (µg/day) THQ
Muscles 3.47 8.15
Pb Male Gills 0.29 3.89
Hepatopancreas 0.61 8.15
Muscles 0.07 0.98
Female Gills 0.44 5.83
Hepatopancreas 0.81 10.74
Muscles 0.13 1.67
Cd Male Gills 0.02 0.74
Hepatopancreas 0.083 3.89
Muscles 0.01 0.37
Female Gills 0.12 4.26
Hepatopancreas 0.184 8.70
Muscles 0.15 3.70
Table 2b. Estimated Daily Intake (EDI) and Target Hazard Quotient (THQ) of Heavy Metals at
Station B
EDI
Metal Sex Tissue THQ
(µg/day)
Ni Male Gills 2.67 6.30
Hepatopancreas 3.76 8.89
Muscles 1.91 4.44
Female Gills 3.94 9.26
Hepatopancreas 5.59 14.44
Muscles 3.40 8.33
Pb Male Gills 0.21 2.78
Hepatopancreas 0.47 6.30
Muscles 0.06 0.56
Female Gills 0.26 3.50
Hepatopancreas 0.76 10.15
Muscles 0.10 1.34
Cd Male Gills 0.0034 0.16
Hepatopancreas 0.04 1.99
Muscles 0.0025 0.12
Female Gills 0.007 3.48
Hepatopancreas 0.18 6.69
Muscles 0.031 1.44
350
300
Concentration in mg/kg
250
200
150 Male
Female
100
50
0
GA GB HA HB MA MB
Level of Nickels
Figure 1: Comparison of Nickel Concentration in the Organs of the Male and Female C. Sapidus
at Stations A and B.
45
40
35
Concention in mg/kg
30
25
20
Male
15 Female
10
5
0
GA GB HA HB MA MB
Levels of Lead
Figure 2: Comparison of Lead Concentration in the Organs of Male and C. Sapidus at Station A and B.
10
9
8
Concentration in mg/kg
7
6
5
4 Male
3 Female
2
1
0
GA GB HA HB MA MB
Level of Cadmium
Figure 3: Comparison of Cadmium Concentration in the Organs of the Male and Female C.
Sapidus at Stations A and B.
4.0 Discussion
The findings of this study revealed substantial concentrations of Nickel (Ni), Lead (Pb), and Cadmium
(Cd) in the tissues of Callinectes sapidus collected from the Iko River, with variation across tissues,
sexes, and sampling stations. These concentrations, in many instances, exceeded the regulatory limits set
by global agencies such as the FAO/WHO and USEPA, underscoring the serious environmental
implications and potential health risks for local consumers.
A consistent bioaccumulation pattern was observed across all heavy metals, in the order hepatopancreas
> gills > muscles. This trend is consistent with the physiological functions of these organs in
crustaceans. The hepatopancreas acts as a major site of metal uptake, storage, and detoxification, largely
due to its role in synthesising low-molecular-weight, metal-binding proteins such as metallothioneins,
which have a high affinity for heavy metals (Chou et al., 2002; Yezereroglu et al., 2010). Previous
research has demonstrated that the hepatopancreas can accumulate heavy metals at concentrations
significantly higher than those found in muscle tissue—sometimes up to 30-fold—owing to its high
metabolic activity and detoxification functions (Kargin et al., 2001).
The gills, being in direct contact with the ambient water, serve as critical interfaces for ion exchange and
gas regulation. This constant exposure makes them particularly susceptible to environmental
contaminants. Metals present in water can be absorbed directly across the gill epithelium, contributing to
the relatively high levels of Ni, Pb, and Cd observed in this study. Their osmoregulatory function also
facilitates the transportation and exchange of ionic substances, including metals, which explains the
intermediate levels recorded in this tissue. The comparatively low concentrations observed in muscle
tissues are consistent with previous studies that attribute it to lower metabolic activity and the limited
presence of metallothioneins (Ubong et al., 2023).
Sex-based differences were also pronounced, with female crabs consistently recording higher metal
concentrations across all tissues. This variation may be attributed to physiological differences linked to
reproductive cycles. During oogenesis, females require substantial nutrients and energy, which may
result in the mobilisation and retention of higher metal loads, particularly in tissues like the
hepatopancreas. Larger body size, lipid content, and hormonal fluctuations may also contribute to higher
bioaccumulation potentials in females compared to males, as similarly reported in other ecotoxicological
studies involving crustaceans and molluscs.
Notably, spatial differences in metal concentration between the two stations were significant. Crabs
collected from Station A exhibited consistently higher levels of all three metals when compared to those
from Station B. This discrepancy strongly suggests site-specific contamination, likely due to localised
anthropogenic inputs. Station A is proximate to the Utapete oil field, an area previously reported to
experience frequent oil spills, gas flaring, and the discharge of industrial effluents (Etesin et al., 2013).
These activities are known to release trace metals directly into aquatic systems, either through liquid
waste or atmospheric deposition, thus increasing bioavailability and uptake by aquatic fauna. This
supports the hypothesis that Station A is more impacted by industrial pollution than Station B.
The health risk assessment component of this study—evaluated through the Estimated Daily Intake
(EDI) and Target Hazard Quotient (THQ)—showed that consumers who regularly ingest crab tissues,
particularly the hepatopancreas and gills, are at potential risk of chronic toxicity. THQ values for Ni, Pb,
and Cd were notably greater than 1.0 across several tissues, especially in female samples and at Station
A, indicating a potential for non-carcinogenic health effects. Long-term exposure to these metals, even
in trace amounts, has been associated with renal dysfunction, cardiovascular disorders, neurological
impairment, and carcinogenesis. Lead and cadmium, in particular, are classified among the most toxic
environmental pollutants with documented risks including kidney damage, reproductive toxicity, and
developmental disorders.
The ecological ramifications of these findings are also profound. Persistent contamination of aquatic
ecosystems with heavy metals can impair the reproductive capacity, growth, and survival of various
aquatic species. Moreover, bioaccumulation in edible aquatic organisms like C. sapidus raises concerns
of biomagnification in higher trophic levels, thereby disrupting aquatic food webs. From a socio-
economic perspective, this pollution threatens local fisheries, reduces biodiversity, and endangers the
livelihoods of fishing communities in Eastern Obolo and surrounding regions.
Conclusion
This study has provided compelling evidence of significant bioaccumulation of heavy metals—Nickel
(Ni), Lead (Pb), and Cadmium (Cd)—in the tissues of the blue crab, Callinectes sapidus, collected from
the Iko River in Eastern Obolo Local Government Area of Akwa Ibom State, Nigeria. The results
revealed tissue-specific accumulation patterns, with the hepatopancreas recording the highest
concentrations, followed by the gills and then the muscles. Notably, female crabs exhibited higher metal
loads than their male counterparts, and crabs from Station A—located nearer to oil exploration zones—
contained higher levels of contamination than those from Station B.
These findings highlight the persistent influence of anthropogenic activities such as oil spills, gas
flaring, and industrial discharge on aquatic ecosystems in the Niger Delta. The elevated concentrations
of Ni, Pb, and Cd across multiple tissues of C. sapidus are not only indicative of environmental
degradation but also pose potential non-carcinogenic health risks to consumers. The Estimated Daily
Intake (EDI) and Target Hazard Quotient (THQ) values for these metals, particularly from the
hepatopancreas and gills, exceeded safe thresholds recommended by international regulatory bodies.
This suggests that regular consumption of these contaminated tissues may lead to chronic health
complications, including renal, neurological, and cardiovascular disorders, and possibly cancer.
The ecological implications are equally concerning. Bioaccumulation of heavy metals in economically
and ecologically important aquatic species can lead to trophic transfer and biomagnification, resulting in
ecosystem-level disruptions and biodiversity loss. The evidence presented in this study underscores the
urgent need for sustained environmental monitoring, stricter enforcement of pollution control
regulations, and the adoption of preventive and remedial strategies in affected regions.
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