Parasitic infestation on coral associated fishes of saint Martin`s Island, Bay of Bengal: Molecular, histopathological and heavy metal analysis

Abstract

The current study was conducted in order to observe the parasite communities and their characteristics, assemblage, prevalence, intensity, and dominance, their effects on fish, contamination from heavy metals in sediment, water, fish, and parasites from Saint Martin’s Island, in the Bay of Bengal, and evaluate bioaccumulation and the hazards to human health. A sum of 259 reef fishes under 4 families were autopsied during October to March in the years 2019 to 2022, of which 65 fishes were infected with 539 (153 metazoan ectoparasites and 386 endoparasites) parasites. In case of metazoan ectoparasites, prevalence (17.14%) was highest in Terapontidae but intensity was highest in Lutjanidae (6.8±8.15), whereas, for endoparasites, both prevalence (25%) and intensity (19.64±35.07) were highest in Sciaenidae. The analysis of species richness revealed that the ectoparasites demonstrated higher species richness than endoparasites within each family. Among the four fish families, female Terapontidae fish showed higher ectoparasite prevalence than male fish. However, in all four fish families the prevalence of endoparasites was higher in female fish compared to male fish. In endoparasites, only fishes of the Serranidae family showed a significant difference (χ 2 =10.434, p<0.05). Gill and body cavity became the most favourable sites for crustacean parasites [prevalence (12.74%), intensity (3.88±6.55)] and helminth parasites [prevalence (7.69%), intensity (15.7±33.44)] respectively. A strong positive association between host length and prevalence was found in Lutjinidae (rs =0.90, P = 0.04 at p<0.05) and Serranidae (rs =1, P = 0.002 at p<0.01) fish families according to Spearman rank correlation analysis. On the other hand the relationship between host weight and prevalence was inverse but not statistically significant (rs= -0.56, P = 0.32; rs= -0.35, P = 0.55) in Lutjanidae and Serranidae fish families, respectively. A total of 15 metazoan ectoparasites species and 8 endoparasites species were identified, belonging to isopoda, copepod, monogenea and nematoda. Copepod Lernanthropus otolithi showed comparatively highest prevalence (3.86%) among other ectoparasites, whereas isopod Argathona macronema showed the highest intensity (18±0.00) among others. Gonad infecting nematode Philometra cephalopholidis showed the comparative highest prevalence (3.47%) among other nematodes and Hysterothylacium thalassini showed the highest intensity (29.25±50.88). Hysterothylacium thalassini was the highest dominant species. Five classes of dominance were distinguished based on the dominance coefficient value and the most common class of dominance was subrecedents (14 species). Length-weight relationship between uninfected and infected fish demonstrates that both groups maintained the same general body shape and showed no significant differences due to infection status and b-values were less than 3 in all cases except for infected Terpontidae (b=3.07). The condition factor results indicated that fish from all four families were in good condition (K>1.5), with the Serranidae family performing the best (K>2). We identified four parasites through DNA barcode, of which three are up to species and one up to genus. Thymine (T/U) exhibited the highest proportion, ranging from 39.87% to 41.64%, while cytosine (C) showed the lowest content, varying between 14.29% and 15.84% across the four species. The intra-species genetic distances were low, ranging from 0.00351 to 0.02177. The low K2P distance between H. thalassini and P. decipiens implies a close evolutionary connection. The species Hysterothylacium thalassini is clustered together in a strongly supported monophyletic clade, with multiple sequences from different sources (DUZM and NCBI). Pseudoterranova decipiens forms a separate clade with two closely related sequences. Philometra sp. is positioned as a distinct branch, separating it from the other taxa. Lernanthropus otolitii forms the most divergent group in the tree, positioned as like as an outgroup relative to the other species. We employed several significant pollution indicators to evaluate ecological quality. The following heavy metals were measured in samples taken from five locations throughout the Island using atomic absorption spectrometry (AAS): Fe, Co, Pb, Cu, Mn, Ni, Zn, Cr and Cd. The results showed that Fe, Pb, Cu, Mn, Co, and Ni were the heavy metals with the lowest mean amounts in the water. The water’s mean lead content was higher than recommended levels, although the heavy metal pollution index indicated low to medium contamination levels. Cu and Fe (r = 0.910, p<0.05) revealed a significant positive correlation, according to Pearson's correlation study. Fe > Zn > Cr > Pb > Cd was the directive of concentrations in sediment, with Pb concentrations exceeding acceptable standards and a high pollution level for Cr according to geo-accumulation index. Cd and Zn (r = 0.935), Pb and Fe (r = 0.938), and Cd and Cr (r = 0.935) all showed significant correlations at p<0.05, with Zn and Cd showing a perfect connection (r = 1.000, p<0.01). According to the ecological risk measure and contamination variables, there was low to moderate sediment pollution. Fish and parasite muscle heavy metal concentrations were ranked as Fe>Pb>Zn>Cr>Cd and Fe>Zn>Pb>Cr>Cd, respectively, with Cr and Pb levels in fish muscle exceeding regulatory limits. The highest bioaccumulation was observed in fish, Terapon jerbua, and the parasite, Philometra cephalopholidis, with Fe and Cd have a higher propensity to bioaccumulate in parasites than fish. The target hazard quotient indicated significant possible health concerns from consuming Cr, Fe, and Pb-exposed fish, while the target cancer risk analysis revealed significant hazards of carcinogenesis linked to Cd and Cr. Overall, the study concluded that the nematode parasite is a suitable indicator of pollution from heavy metals. We also detected the histopathological alterations in fish tissues and found that some major changes occurred due to parasitic infestation in the stomach, intestine, liver, and gonadal tissue. Both the infected fish stomach and intestine showed haemorrhage, vacuolation, damage to microvilli, sub-mucosal breakage, breakage of microvilli, shrinkage of tissue and parasite attachment. Necrosis in the hepatocytes, haemosiderotic nodules, vacuolation, extensive vacuolar degeneration, hemorrhage, hepatocytes degeneration, blood vessels congestion, edema and focal necrosis were discovered in the infected liver. Infected ovary showed ovarian follicle degeneration, yellowish-brown pigment accumulate encircled by cells that resemble fibroblasts, breakdown of vitelline envelope, oocyte necrosis and ovarian follicle rupture.