Parasitism of Egeria radiata (Lamarck, 1804) in Lower Sanaga delta, Cameroon: Prevalence, Diversity, Intensity and its Impact on This Economic and Nutritional Clam Species

Background: Clams are an important source of economic incomes and animal protein source in Cameroon. In contrast, parasitism is a serious threat to the economy and health risk to Cameroonian populations. The present study aimed at determine diversity and intensity of parasitism along with its impact on E. radiata, a major clam species in the country. Methods: Parasitic screening of freshwater Egeria radiata was carried out December 2017 to July 2018 at lower Sanaga delta in the Littoral region of Cameroon. Bolounga and Maldjedou were local stations of sampling, where clams were collected and in situ measurements of weight, length and width and were transported to the laboratory for parasites survey. Parasites were examined and attempt was made on the identi�cation using appropriate keys. Results: Statistical analysis of results showed that the parasitic prevalence was 74.5% overall and was 56% and 90% at Bolounga and Maldjedou respectively. The prevalence of parasites habour found were Protozoans (10% vs 0%; p=0.01), Trematodes (4% vs 16.7%; p=0.04), Cestodes (6% vs 23%; p=0.01) and Nematodes (12% vs 31.7%; p=0.01) at respectively Bolounga and Maldjedou stations. Parasites diversity of Egeria radiata show Intensity index at Bolounga and Maldjedou was respectively 1.35 and 1.31 with 1.32 overall. Abundance rate was 0.99 overall and was 0.76 and 1.18 at Bolounga and Maldjedou respectively. Signi�cative correlation can be observed with Burrow and parasitic index ; The size of clams in two stations show lengths (5.2mm vs 4.5mm), height (4.1cm vs 3.5cm), total shell weight (38.9g vs 30.0g) and left shell weight (19.6g vs 15.2g). higher values were encountered in Maldjedou with signi�cant differences that were observed. Conclusion: Velocity


Background
Urbanization and demographic growth increasingly observed in developing countries, especially inCameroon,have contributed to an increasing and urgent demand in proteins of animal origin [1].In Cameroon, bivalve mollusks (i.e., clams) are exploited by a restricted local community shing groups in the District of Mouanko (Littoral region) along the Sanaga Delta for their esh and shell, with an estimated annual production of more than800 000 tons of clams [2].Many Cameroonian authors expressed their fears regarding declining production of these palatable mollusk species in actual this context of increased demand of animal proteins.
Clams are greatly appreciated by Cameroonian populations for the quality of their wite esh and secondary services produced by their shells notably as a source of calcium in poultry feed and in lime manufacturing [2; 3].The exploitation of clams is an income-generating activity as it generates enormous pro ts for local communities through their employment along the Cross-River waters in Nigeria, Volta in Ghana, and Sanaga in Cameroon, and positivelyimpacts the economy of these three counties [4;5].However, a set of biotic and abiotic factors including parasitism have contributing to decrease in clam populations, and as a consequence a decline in food production and nancial resources over the course of the shing season [2].Parasitism is important in the process of regulating aquatic ecosystem invertebrate populations [6; 7; 8].Unfortunately, parasitism has received little attentiondespite its impact inclam mortality and economic losses in the exploitation of edible species such as bivalves [8;9].Egeria radiata (Lamarck, 1804) is one of the main bivalves exploited in Cameroon by local population.Previous studies in Ghana and Nigeria outlined that this clam species was highly vulnerable to contamination by diverse pathogens includingcoliform bacteria [5], and parasites (trematodes, cestodes, nematodes and protozoans) [10].
There is a lack of parasitological studies on E. radiata in Cameroon, especially in the Lower Sanaga river.The implementation of such study is a crucial prerequisite in an attempt to successfullydomesticate this clam species in the country.We therefore conducted a study to determinethe diversity and intensity of parasite fauna, and its impact on characteristics of E. radiate populations from the Lower Sanaga river.

Study area
The study took place from December 2017 to July 2018 in two sites of Mouanko area, namely Bolounga and Maldedjou.Mouanko is a district of the Division of Sanaga Maritime, Littoral region, Cameroon, located on the banks of the Sanaga River (Fig. 1) [2].Its average geographical coordinates range from 3-38' north latitude, to 9-47' east longitude.The district of Mouanko has about 17,000 inhabitants.The majority of local populations live mainly from shing and their activities are restricted to the vicinity of streams and alluvial forests.River shing is mainly based on Cichlidae, cat sh and clams.

Study Design
The E. radiataclams were collected at the study sites of interest with help of local shermen.The shermen were informed on the objectives of the study, and only those who gave their approval were included in the study for captures.The measurements (length, height, width and bulge) on each clam individualwere made on spot.Thereafter, the valves of each clam were opened, and different body parts were collected for identi cation of parasite diversity and intensity (Fig. 2).

Collection and measurements ofE. radiatapopulations
The clams were collected with the help of local shermen along the study sites of the Sanaga river, and morphometricparameters (i.e., length, height, and bulge) were made on spot using a sliding foot to the nearest millimeter.The clams were then placed in individual plastic bags, brought to the Laboratory for further identi cation of parasitic organisms (Fig. 3).

Identi cation Of The Parasites
The identi cation of different parasite classes was made using published taxonomic identi cation keys [11; 12; 13; 14].Different body parts (gills, foot, siphons, visceral mass and mantle) and soft body partof each E. radiataindividual were analyzed to identify different development stages of main parasite classes (trematodes, cestodes, nematodes and protozoans).Brie y, metazoans were identi ed by microscopic observation of the crushed sections of the above mentioned body parts.Biopsies of the E. radiata's soft body were made for the microscopic identi cation of endoparasites.

Condition, Diversity And Ecological Indexes
The analysis of parasite composition was made using several condition, parasitological and ecological indexes [15; 16].Parasite communities were studied at the infrapopulation level (i.e., all members of a parasite species found in/on the host), and the different indexes were determined as follows [17; 18; 19].: Elongation index (Ei) is the ratio between height and lengthof the clam.In practice, Ei is low when environmental conditions are favorable while high Ei values re ect unfavorable environmental conditions ; Condition index (K) is calculated using the following formula K = 10 4 × (W/L 3 ) where W and L are weight and length of clam, respectively ; Parasite prevalence (P) was de ned as the ratio between the number of parasitized host individuals and the total number of host individuals; Parasite abundance (AM) was computed by dividing the number of parasite individuals found to the number of clams examined; Mean intensity of the parasitofauna (IM) was de ned as the total number of a given parasite species divided by the total number of clams infected with this parasite species.IM was categorized as very low (< 10), low (10 ≤ IM ≤ 50); moderate (50 < IM ≤ 100), and high (IM > 100) as described.
Parasite speci c richness (S) is the number of species found in each clam examined.

Statistical analysis
The data were keyed into an Excel spreadsheet, and then exported to the statistical package for social sciences v17 (SPSS, IBM, Inc., Chicago, USA) and GraphPad v5.03 (GraphPad PRISM, Inc., San Diego, USA) for statistical analysis.Qualitative variables were presented as percentages while quantitative variables were presentedas mean ± standard deviation (SD).Non-parametric tests of Kruskal-Wallis and Mann-Whitney, and parametric unpaired sample Student tests were used to compare mean values.Pearson's independence chi-square and Fisher's exact tests were used to compare percentage values.Statistical signi cance was set at p-value less than 0.05.

Characteristics of the clams collected in the study sites
A total of 110 clams were collected during the study, 50 in Bolounga and 60 in Maldjedou.The comparative analysis of morphometric characteristics of clams between the study sites revealed that mean values of total shell weight, length and height of clams collected in Madjedou were signi cantly higher than those of their counterparts collected in Bolounga: 30.0 ± 17.1 vs 38.9 ± 22.9, p = 0.02 (total shell weight, g), 4.5 ± 1.2 vs 5.2 ± 1.4, p = 0.005 (length, cm), and 3.5 ± 0.8vs 4.1 ± 0.8, p = 0.001 (height, cm) (Table 1).

Prevalence Of The Different Parasites Classes By Study Sites
The analysis of clams revealed different parasite groups including protozoans, cestodes, trematodes and nematodes.Protozoans were found only in Bolounga (10 %).The prevalence of cestodes, trematodes and nematodes was signi cantly higher in Maldjedou compared to Bolounga (e.g., 12.0 % vs 31.7 %, p = 0.01 for nematodes).It should be noted that nematodes were the main parasite classes found in both the study sites (Fig. 4).

Elongation And Condition Indexes
The elongation and condition indexes were higher in Bolounga compared to Maldjedou, but statistical difference was found only for condition index (p = 0.0067) (Fig. 5).

Parasitological And Ecological Indexes
Page 6/16 The parasite abundance was signi cantly lower in Bolounga as compared to Maldjedou (p = 0.0001) as depicted in Table 2.The same pattern was observed for speci c richness, where a maximum of three parasite species were found in Bolounga.Parasite intensity rates were similar between the two sites (p = 0.841).

Impact Of Parasitism On The Characteristics Of Clams
No statistically signi cant difference was found for morphometric parameters between clams infected and uninfected with protozoans and trematodes.In contrast, the bulge of clams was signi cantly higher in those infected with cestodes and nematodes compared to their uninfected counterparts (Table 3).All morphometric parameters, with the exception of bulge, were signi cantly lower in clams infected with parasites of not speci ed class (Table 3).We compared the morphometric characteristics of the E. radiata populations with respect to the speci c richness (Table 4).Only width was signi cantly lower in clams infected with two parasite species compared to clams infected with one parasite species.

Discussion
Clams are an important source of economic incomes and animal protein source for population of sub-Saharan Africa countries, especially in Cameroon.Unfortunately, parasitism is a serious threat to the economy and health risk to these populations.The present study aimed at determine parasite diversity and intensity as well as the impact of this parasitism on E. radiata populations in the Lower Sanaga river, Cameroon.
All main groups of parasites (i.e., protozoans, trematodes, cestodes and nematodes) were found in this study, and this is consistent with works of Adeyemo et al. [10] who reported the same groups of parasites in the Nun River in Nigeria.The natural capacity of this bivalve species to burrow and lter water expose it to infection with these above mentioned groups of parasites which also live in the aquatic environment of E. radiata clam.In general, clams are victims of a large number of parasites that cause a mass mortality and signi cant economic losses [7; 8; 9].Marine bivalves are often served as a host of metazoan parasites or symbiosis such as trematode, cestode, nematode and copepode.Such metazoans have been identi ed from various marine bivalves including oysters, mussels, cockles and clams [20; 21].Several authors have reported on the occurrence of trematode and cestode in clams in Jeju Korea and Ariake Bay, Japan [22; 23]. .
The parasitofauna is mainly dominated by nematodes while protozoans were found with the lowest prevalence.This pattern is in line with that of Adeyemo et al. [10].In addition, a signi cant difference was found in prevalence of the groups of parasites between the two study sites.This discrepancy is likely the fact of a geographic area-speci cdistribution of the groups of parasites.The environment conditions are well known shape the distribution of numerous living organisms including parasites.High number of trematodes may affect reproductive processes and possibly retarding gonadal development [24].The authors found that mite and trematode abundance were greater in smaller freshwater mussels [25].
Values of parasitological clues at Egeria radiata (Lamarck, 1804) Depending on the area, there was a higher average speci c wealth in Maldjedou (0.75 ± 0.51) than in Bolounga signi cantly (p-0.0033).That parasite indexes were higher for larger clams and could be explained by higher ltration rates.Furthermore could be explained also by longer exposure to infective particles [26].In the present study, although clam shell length was 5.2 ± 1.4 cm on average at Maldjedou compare to 4.5 ± 1.2 cm at Bolounga.However, the maximum value of 3 was obtained for the entire population, as well as in Bolounga.This could be explained by the fact that in Bolounga the parasitic species here would have been more diverse than in Maldjedou and thus being a lterer more bioaccumulated of parasitic species.This result is lower than that of Adeyemo et al. [10] having found a value of 4 in the same host Galatea paradoxa (Born, 1778).The study showed a higher average speci c richness associated with Bulging signi cantly (p-0.0035).The speci c richness in relation to the morphometric parameters depending on the zones, revealed that the Bulging of clams in Bolounga would be the only parameter related to the speci c parasitic richness.This could be explained by the fact that according to authors [10] molluscs are bioaccumulators that concentrate many parasitic species present in the water all around it.Acquisition of parasite infective stages can be increased for larger bivalves with high ltration capacities.

Parasite Intensity and Abundance
The overall prevalence of infection was 74.5 %.This value is higher than the 50% threshold used to identify the presence of dominant species according to authors [17].Similar prevalence values were reported in Nigeria [10].
This high value of infection burden could be due to anthropic activity as riverside residents used to dump wastes of varied nature, and parasitized feces, and thus contribute to the contamination of the aquatic milieu of E. radiata.As above stated, clams are important lterer organisms, and as a consequence are exposed to infection when ltering parasitized water.Parasite indexes were below 10 in the study sites, thereby implying that majority of parasite species have a very low mean intensity of infection [19].A similar result was reported in Nigeria, where the authors found abundance values of 1 and 1.09 in their study sites [10].Also prevalence and infection intensities of parasites at Bolounga were comparatively lower than the prevalence and intensities reported from Maldjedou, suggesting that overall health condition of Bolounga E. radiata clams was graded to be good.
Existing relationship between some biotic parameters of bivalve and Egeria radiata parasitolofauna.
Regarding the in uence of parasitofauna on metric parameters, only the Bulge was in uenced for clams parasitized at Cestodes (p-0.028) and parasitized to nematodes (p-0.017).This can be explained by the fact that helminths in general and especially these two classes contain endoparasites and mesoparasites that would have a direct in uence on the weight of the soft body of the bivalve.Therefore, an in uence of pathology on the metric parameters of the clam [27] to the example of the Egeria radiata Bulge.Parasite prevalence was higher in adults than in juveniles and could be explained by higher ltration rates and also by longer exposure to infective particles [26].Condition index at Maldjedou show that despite low infection intensity an overall infection, Bolounga have better condition with E. radiata bivalves.The low infection intensity at two stations could be associated with a low density of clam and this index is positively correlated with the density of host organisms and environmental factors such substrate of area as bed [9;28].
However, including the group of parasites of unidenti ed species, signi cant associations were observed with metric parameters such as the weight of the left shell (p-0.036), the length of the shell (p-0.007), and the height of the shell (p-0.018).This can only be explained after extensive study on the identity of unidenti ed parasites.
statistical analyses.KOJOM FOKO Loïck Pradel from University of Douala performed the data analysis.Minette TOMEDI EYANGO from the University of Doula designed freely the study.Authors' contributionsAD designed the study, conducted the experiments, and wrote the manuscript.GA conducted the experiments, did statistical analyses.KFLP helped write-up, statistical analysis and critically revised the manuscript for intellectual content.MTE designed the study.All authors read and approved the nal manuscript.

Figures
Figures

Figure 1 Map
Figure 1

Figure 2 Flow
Figure 2

Table 2
Comparative analysis of indexes between the study sites *Statistically signi cant at p < 0.05

Table 3
Variation of morphometric characteristics with respect to infection by different parasite groups

Table 4
Impact of parasite species richness on E. radiata morphometric characteristics