Marine fish parasites of Vietnam: a comprehensive review and updated list of species, hosts, and zoogeographical distribution

With a long coastline stretching from tropical to subtropical climate zones, and an immense exclusive economic zone with over 4000 islands, the Vietnamese marine waters support a rich and biodiverse parasite fauna. Although the first parasitological record was in 1898, systematic studies of the parasite fauna have increased during the last 50 years. This comprehensive review covers the current state of knowledge of marine fish parasites in Vietnam and lists 498 species found in 225 fish species, and their geographical distribution. In addition, 251 marine parasite species have newly been added to the already known fauna of 247 species since 2006 (more than two-fold increase). The most speciose group was the Digenea, which accounted for 43% of the total parasite species biodiversity, followed by Monogenea (23.5%), Crustacea (11.6%), Nematoda, and Acanthocephala (8.0% each). The shallow and muddy Gulf of Tonkin showed a rich parasite fauna, accounting for 66.3% of the whole marine parasite fauna of Vietnam, with Digenea accounting for 51% of the regional total parasite richness, followed by Monogenea (27%), Acanthocephala (8.8%), and Nematoda (5.8%). Only a few species belonged to Hirudinea, Myxozoa, and Cestoda, suggesting that these taxa may be understudied. Despite significant progress in studies of marine fish parasites in Vietnam since 2006, only about 12% and 13% of the total fish species have been examined for parasites in the whole country and the Gulf of Tonkin, respectively.


Introduction
Vietnam has a long coastline of more than 3260 km and an immense exclusive economic zone (one million km 2 ), comprising the coastlines of more than 4000 islands and thousands of square kilometers of coral reefs [122,131]. The Vietnamese Sea stretches from tropical to subtropical climate zones (from the Namzu Islands, 9°40 0 N, 104°22 0 E, in the Gulf of Thailand to Daochao Island, 20°50 0 N, 107°20 0 E, in the Gulf of Tonkin), opening to the Pacific Ocean and being known for its biodiverse marine ecosystems supporting a rich fauna and flora, including fish parasites. Although Billet [26] recorded the first parasite, Distomum hypselobagri (Trematoda), systematic studies of the parasite fauna did not begin until the 1960s (e.g., [69,71,82,[97][98][99], etc.), and the first review was published only 16 years ago [19]. Recently, studies using both molecular [11-13, 16, 20, 24, 25, 36] and morphological methods [12,13,20,24,92] have been conducted in order to elucidate the systematics of marine parasites in Vietnam.
According to Palm and Bray [103], marine fish parasites are important in ecosystems. In open Hawaiian waters (Pacific Ocean), the authors discovered an average of 2.2 parasite species per marine fish species. In contrast, Klimpel et al. [66] discovered 1.5 metazoan parasite species per deep-sea fish species. According to Palm [102], each fish species in the world harbors up to 3-4 metazoan parasite species on average. In this sense, Arthur and Te [19] reported that the estimated number of parasite species per marine fish species in Vietnamese waters was as high as 3.0. There are currently 1876 species of marine fish reported in Vietnam's marine waters [45]. Despite this, only 247 parasite species infecting 82 marine fish species were documented by 2006, accounting for about 4.4% of the total fish community [19]. Such data demonstrate that the number of parasites known to science is insufficient to provide credible estimates of the local parasite community.
Because of the importance of marine parasites in fisheries, aquaculture, and human health, more research has been conducted recently, focusing on commercially important fish (e.g., [88,90,132,135]). Furthermore, because parasites can be used as a tool for monitoring climate change and environmental health [86,102,129], studies regarding these organisms are essential. However, no systematic update pertaining to parasites in marine fish from Vietnam has been published since 2006, and since that date, much has changed in the knowledge of such organisms. However, Poulin [123] asserts that, due to the absence of an adequate method for determining parasite diversity, species richness remains the most straightforward and thus most relevant indicator of diversity. Therefore, the present study is aimed at: (1) shedding more light on the marine parasite richness in Vietnam by listing, correcting, and arranging the latest information, including the host and geographical distribution of parasites of marine fish; and (2) providing a current state of knowledge on parasite research in the Vietnamese marine waters, with particular emphasis on the Gulf of Tonkin (GOT).

Study sites and data collection
Vietnam is a Southeast Asian country divided into three distinct geographical and climatic regions: North, Central, and South Vietnam. The aquaculture industry is vital to the Vietnamese economy, especially brackish and marine fish farming along the country's coastline, particularly in the north [67].
A total of 137 references published between 1901 and 2022 were retrieved to gather knowledge on marine fish parasites and their occurrence in cultured and wild marine fish in Vietnam, focusing on the latest studies. This list was also based on information from the checklists of parasites of fishes in Vietnam [19], and it was expanded to provide a parasite-host list organized on a taxonomic basis, including information about each parasite species, sites of infection, the known geographical distribution in Vietnam, and references. More recently, reviews of individual species of parasites, including morphology, host-parasite distribution, zoogeography, and occasionally molecular analysis have become available; for instance, Amin et al. [2,8,14,15] on Rhadinorhynchus trachuri Harada, 1935, Neoechinorhynchus johnii Yamaguti, 1939, Sclerocollum neorubrimaris Amin, Heckmann, Ha, 2018, respectively and Serrasentis sagittifer (Linton, 1889) Linton, 1932. The animal science databases (https://www.cabi.org/animalscience/disease-health/), MED-LINE, PubMed, Scopus, Web of Science, and Google Scholar were searched using the keywords: parasite, fish, marine, and Vietnam.
Vietnam's 3260 km of coastline is home to 25 coastal provinces and three cities by the sea. The following abbreviations are used to denote administrative and oceanic divisions where the parasites have been reported (Table 1).

Data analysis
The retrieved literature data, such as the reference, study period, study location, site of infection, fish host, taxa, and parasite species studied, were entered into an excel spreadsheet. This analysis only included parasites identified at the species level. In the list, the parasite taxon levels, i.e., subfamilies, genera, and species, were organized alphabetically. For the entire country of Vietnam and the Gulf of Tonkin, parasite species were classified into eight main taxa: Myxozoa (My), Ciliophora (Ci), Monogenea (Mo), Digenea (D), Cestoda (C), Nematoda (N), Acanthocephala (A), and Crustacea (Cr).
In order to have an insight into parasite richness, the total number of parasite species for Vietnam and the GOT was used to calculate the parasite species ratio per fish host. It is determined by dividing the total number of parasite species parasitizing fishes by the number of infected hosts. Microsoft Excel software was used to perform calculations and descriptive analysis of the collected data.

Results and discussion
Parasites in marine fish of Vietnam

Parasite richness
A total of 498 parasite species have been recorded from 225 marine fish in Vietnam belonging to the following taxa: Myxozoa and Ciliophora (8 each), Monogenea (117), Digenea (214), Cestoda (17), Nematoda (37), Acanthocephala (37), Hirudinea (2), and Crustacea (58), demonstrating the high diversity of the marine fish parasites inhabiting Vietnamese waters. The current average number of parasite species per fish species was 2.2, which was lower than the previously estimated species richness in Vietnamese (3.0) and German coastal waters (3.1) [19,104]. This lower species richness appears to be due to a higher proportion of the fish community being investigated compared to 16 years ago (225 species vs. 82 fish species reported in 2006) so that a better prediction of the species richness can be made. It could also be explained by a decline in host species richness and population density, the major universal determinants of variations in parasite species richness [123]. However, the new figure of 2.2 is comparable to that of New Caledonia (1.9), the Indo-West Pacific (1.7), and Hawaii's open Pacific waters (2.2) ( [58,103,126], respectively). These regions' similar latitudinal patterns could account for their parasite richness [28].
Our findings showed that only 225 of the total 1876 species of marine fish recorded in Vietnam [21] had been investigated for parasite species, accounting for only 12% of the entire fish community; this indicates that the parasite fauna of marine fishes in Vietnam is still poorly known.   Mo Epinephelus sexfasciatus Gi QN [34] (Continued on next page)     (Continued on next page) (Continued on next page)    (Continued on next page)         (Continued on next page)   Kabata, 1979 Chauvanium chauvani Kazatchenko, Kovaleva, Nguyen and Ngo, 2017 Cr Alepes melanoptera Gi GOT [61] (Continued on next page) Parasite composition and proportion in marine fish of Vietnam The present results showed that the proportion of marine fish parasites observed in Vietnam is similar to that previously described in Hawaiian waters ( [103] ; Fig. 2b). As mentioned, the proximity of the latitudinal range between Vietnam and Hawaii might be the reason for such similarities in the proportion of the parasite taxa.
Digenea and Monogenea were the most common taxa, accounting for 43% and 23.5% of Vietnam's total marine fish parasite fauna, respectively. The overall taxon proportions are similar to those found in Hawaii, except for Nematoda and Acanthocephala, which were higher in Vietnam than in Hawaii (7.4% and 7.4% in Vietnam vs. 3.1% and 1.4% in Hawaii, respectively). Many factors can influence the richness of these two parasite taxa, e.g., the zooplankton (e.g., Copepoda, Amphipoda), which may act as intermediate hosts for Nematoda and Acanthocephala and were reported to be more diverse and abundant in Vietnam than in Hawaii [56,87]. Furthermore, since acanthocephalans use amphipods or ostracods more frequently as intermediate hosts, they are abundant in shallow waters in Ha Long Bay but not on the Hawaiian coast [56,87]; these parasites are typically found in benthic and benthopelagic fish final hosts [54]. As a result, when compared to the 2006 list, 29  To date, it is known that the Cestoda, particularly the orders Trypanorhyncha and Tetraphyllidea, are found in a wide range of marine fish species worldwide [101,103]. They are tapeworms from elasmobranchs that have been considered excellent indicators of host ecology [105]. In Vietnam, cestodes accounted for only 3.4% of the total parasite fauna in marine fish (17 out of 498 species; Fig. 2a), a much lower proportion than in Hawaii (9.2% represented by 60 species; [103]; Fig. 2b); this could be because of a lack of definitive hosts (elasmobranchs) due to shallow waters or, more likely, a lack of research interest in these parasites in Vietnam.
The hirudinean parasites were in very low proportion compared to other parasite taxa. These annelids have one of the largest body sizes among the groups of parasites in marine fish and are well known to be pathogenic in farmed finfish [40]. Moreover, hirudineans occur mainly in freshwater [134] which may explain their low proportion of occurrence in marine fish observed here (0.4%; Fig. 2a). This low proportion was also observed by Palm and Bray [103] in Hawaiian waters (0.5%; Fig. 2b).
In comparison to data from Hawaii [103], Crustacea were found in similar proportions in Vietnamese marine fish (11.6% vs. 10.3%; Fig. 2a), but Myxozoa proportions were higher (1.6% vs. 0.9%), and no Ciliophora were found in Hawaii, while there was 1.6% in Vietnam. However, myxozoans have recently been reported in a low proportion in Vietnamese marine waters (0.09% equal to 2 species) when compared to other groups. However, Mackenzie and Kalavati [79] reported 223 myxosporean parasites from North Pacific marine fish, identifying them as one of the most common parasite groups in marine fish [79]. Thus, the true diversity of myxozoans in Vietnam is expected to be much higher than the current findings. The lack of information and data for this group could be attributed to rudimentary sampling methods and a lack of well-trained personnel to investigate this parasite group. Similarly, the Ciliophora, a worldwide distributed parasite of aquatic teleosts, was only found in small numbers. Depending on their host species, this taxon can range from harmless ectocommensals to dangerous parasites in fish aquaculture [79]. Bui [34] and Phan [121] reported Ciliophora as parasites causing disease in Vietnam's finfish mariculture. These parasites that infected marine fish in Vietnam were most likely reported as multi-species groups (e.g., Trichodina spp.), making identification difficult. This means that the actual number of ciliates in Vietnamese marine waters may be much higher than the eight species found so far.
Parasitic crustacean species accounted for 11.6% of total marine fish parasites reported in Vietnam thus far, slightly higher than the figure found in Hawaiian marine waters (10.3%; Fig. 2). This parasite group has been identified as the most diverse and widespread subphylum of arthropods in marine fish [29,126]. They have also been identified as critical pathogenic agents of cultured marine fish in the Asian-Pacific region [77]. Given the critical role that disease control plays in ensuring the success of aquaculture, further research on parasites is both necessary and warranted.

Parasite composition and proportion in the Gulf of Tonkin
The current study presents 330 marine fish parasite species in the GOT. They are classified into eight taxa, Myxozoa (3), Ciliophora (3), Monogenea (89), Digenea (168), Cestoda (11), Nematoda (19), Acanthocephala (29), and Crustacea (8), demonstrating a high number and rich species composition in the GOT. The parasite richness in the GOT may be influenced V.T. Truong et al.: Parasite 2022, 29, 36 by several factors, including geographical latitude, the environment, and the availability of intermediate hosts [55,85]. The GOT is in South-East Asia, a region with a diverse range of parasites and hosts [125,126], including the first intermediate host, mollusks [91], which may be the primary factor affecting endoparasite diversity, such as the digeneans (168 species) we discovered here. However, the GOT's shallow water depth and muddy sediment limit the distribution of open-water elasmobranchs (e.g., sharks and rays), the expected final hosts of marine Cestoda, particularly Trypanorhyncha (Fig. 3). As a result, only seven Trypanorhyncha species have been identified in the GOT so far.
Most of the recorded marine parasite species in Vietnam (66.3%) have been found in the GOT, indicating that the GOT is a hotspot for interested collectors and institutions supporting research due to its high parasite biodiversity. However, apart from its natural biodiversity, most parasitological research may be concentrated in the GOT due to the urgency of fish disease research serving the high-density aquaculture area in the GOT or because conducting research here is more convenient than in other regions. The composition related to the parasite fauna in the GOT was generally similar to that of all Vietnamese marine environments (Figs. 2a and 3a). Digenea were proportionally the taxon in the GOT with the highest species richness (51%; Fig. 3a). This proportion in the GOT was higher than that observed for the whole of Vietnam (this study; Fig. 2a) and similar to that reported in Hawaii ( [103], Fig. 2b). A possible explanation for the previous results relates to the environmental features of the GOT, i.e., muddy and shallow waters with rocky and limestone substrates [89,120] which support an abundant fauna of mollusks that act as intermediate hosts for digenetic trematodes. This is also consistent with the findings of Sujatha and Madhavi [128] and Bray et al. [31], which indicated that the Digenea were typically shallow-water parasites.
The parasite-host ratio of 2.7 was calculated based on a total of 330 parasite species that have been recorded in 122 marine fish species in the GOT. This ratio is higher than the average for Vietnamese marine water (2.2), Hawaii (2.2), New Caledonia (1.9), and the Indo-West Pacific (1.7) [58,103,126]. Since the GOT is thought to have around 928 marine fish species [144], only a tiny portion of the fish species has been  examined for parasites (13.1%), indicating that more attention needs to be paid to exploring the region's high diversity of marine fish parasites.

Conclusions
This is a comprehensive review to understand the diversity and richness of Vietnam's marine parasite fauna. In 225 fish species, 498 marine fish parasite species have been identified, compared to 247 parasites in 82 fish species in 2006. Digenea (43%) and Monogenea (23.5%) had the highest levels of species richness. According to the data currently available, most parasites in Vietnamese fish come from the GOT (330 species, equivalent to 66.3%).
In Vietnam, the estimated marine parasite richness decreased from 3.0 in 2006 to 2.2 in 2022. Only 12% of the marine fish species have been studied for parasites, so a large part of the fish community still needs to be studied to evaluate and predict better parasite richness and diversity in this tropicalsubtropical country. This compilation of parasite records shows significant progress in studying marine fish parasites in Vietnam over the last 16 years. However, it is still in its infancy, leaving a sizable task for the future, as species classification is the first critical step in characterizing any ecosystem. It is a challenging but fascinating task to learn about evolutionary biology and the history of nature, while discovering the true diversity of these marine ecosystems. Understanding pathogenic threats is critical for Vietnam's growing finfish mariculture industry. Recent research shows that parasites can be used to study climate change and the environment. Thus, studying marine fish parasites in Vietnam is urgently needed in the future, especially using molecular data to characterize and classify the fauna.
The findings of this study will help create a database for marine fish parasites in Vietnam. Additionally, it promotes aquaculture success in Vietnam, reduces the risk of fish disease transmission between countries, and reduces the risk of consuming parasite-infected fishery and aquaculture products in Vietnam and elsewhere.