Helminth parasites of grey mullets (Teleostei: Mugilidae) in the Mediterranean region: a review

Helminths are a diverse and rich group of fish parasites, some of which are dangerous pathogens of epidemic potential. The present review is focused on helminth parasites from grey mullets (Mugilidae) in the Mediterranean and Azov-Black Seas. These fish are of great economic importance, for supplying food and recreation services. This study covers helminth parasites of six species: Mugil cephalus (L.), Chelon labrosus (Risso), C. auratus (Risso), C. ramado (Risso), C. saliens (Risso) and Planiliza haematocheila (Temminck, Schlegel). This paper gives a brief overview of the history of taxonomic and faunistic studies of helminth parasites from grey mullets in the Mediterranean. The history of the helminth parasites from grey mullets goes back to over 200 years ago to the first species described by Rudolphi in 1819. One hundred and four helminth species have been recorded in about 130 published papers. Fewer than 10 species have been recorded prior to 1900. Since 1960s, taxonomic efforts have rapidly increased, reaching the highest level in the first decade of the 21st century. Only one new species has been recorded in


Introduction
Helminths represent one of the most diverse group of fish parasites.Many species are dangerous pathogens of epidemic potential, that may cause serious damage in both wild and farmed animals 1,2 .Taxonomic and faunistic studies are the first step in ecological and epidemiological investigations and are important for discovering and documenting species diversity, their geographic distribution, host specificity and pathogenicity.
The present study focuses on a variety and number of helminth parasites from grey mullets (Mugilidae) in the Mediterranean and Azov-Black Seas.Mullets are of great economic importance 3 , supplying food and recreation services.The demand for mullet roe has grown considerably in recent decades elevated the status of grey mullets to be being called «grey gold» 4,5 .This review covers helminth parasites of six species from three genera of the Mugilidae family: flathead mullet Mugil cephalus, thicklip mullet Chelon labrosus, golden grey mullet Chelon auratus, thinlip mullet Chelon ramado, leaping mullet Chelon saliens and so-iuy mullet, also known as haarder or red-lip mullet, Planiliza haematocheila 6 .Oedalechilus labeo (Cuvier, 1829) also occurs in the Mediterranean 6 , but only one reference reports two species of flatworms from this host 7 .P. haematocheila was deliberately introduced in the Black and the Azov Seas.The main goal of the introduction was seeking ways to increase the fishing capacity of natural water reservoirs and to use abundant and underutilized amount of detritus 8 .The fish translocation with subsequent induced breeding and fry release in the 1970s-1980s resulted in the establishment of a self-reproducing population of the so-iuy mullet in the new distribution range 9 .The soiuy mullet has been officially subject to commercial fishing in Ukraine since 1993 10 .This fish is one of the most important and common commercial fish, which is dominant in catch composition, and effectively replacing the depleted stocks of local mullets 9,11 .
The history of the helminth parasites from grey mullets goes back to over 200 years ago to the first species described 12 .The first review of the parasitic species recorded from worldwide mullets was provided by Paperna and Overstreet in 1981 2 .More recently, Radujković and co-authors 7,[13][14][15][16] , and Dmitrieva and Gaevskaya 17 reviewed the parasite species list of grey mullets from the Adriatic and the Azov-Black Seas, respectively.A review of microparasites, also including viral, prokaryotic, and fungal pathogens, of mullets worldwide was performed by Ovcharenko 18 .Thus, a review of helminth parasites of grey mullets at the large geographic scale of the Mediterranean region is needed to analyse the literature available, compile a species list, examine the structure of the helminth fauna and evaluate further perspectives.The present work is specifically designed to cater for these objectives.

History of taxonomic and faunistic studies of helminth parasites from grey mullet fish in the Mediterranean and Azov-Black Seas
It is likely that the first record of parasites from a grey mullet host was published by Rudolphi 12 in 1819.Rudolphi's monograph includes a description of Neoechinorhynchus agilis (as Echinorhynchus agilis) from the Mediterranean flathead grey mullet.Since this first description, this acanthocephalan species has been recorded in numerous marine and freshwater locations in the Northern Hemisphere and from a broad range of host species 15,17,[19][20][21][22][23][24] [26][27][28] .The scarce information gained in 19 th century indicate that parasites of grey mullets were studied sporadically at that time (fig.1).
In the early 20 th century, Looss published two papers with description of eight new intestinal digenean species from grey mullets off the coast of Trieste 29 and Egypt 30 .Looss 29 erected four new genera (Haploporus, Dicrogaster, Saccocoelium and Lecithobothris) to allocate digeneans from Mediterranean mullets, which were later assigned to Haploporidae Nicoll, 1914 31 .The extent of intra-and interspecific variation in species from these four genera is virtually unknown, since most species within the genus are known only from their original descriptions.Although the Mediterranean forms of Haploporus, Dicrogaster, Saccocoelium and Lecithobothris, are the most widely reported species, there are few documented reports providing data on their morphology [32][33][34] .Therefore, a taxonomic review of Haploporidae was needed and this huge work has been recently done by Blasco-Costa and co-authors [32][33][34][35] 43 studied parasite fauna of the Black Sea fish, including grey mullets, extending the list of parasite fauna to eight species.The subsequent publication of Reshetnikova 44 merits special attention.This work provided an analysis of faunal, as well as regional, age and seasonal dynamics of infection parameters of parasites from three grey mullet hosts across different localities in the Azov-Black Seas.Thirteen, 15 and 11 species were reported from M. cephalus, C. auratus and C. saliens, respectively.Based on the patterns of seasonal and age dynamics practical recommendations were given for the optimal age for introduction of grey mullets in estuaries for aquaculture exploitation 44 .Thus, studies of the Black Sea grey mullet parasites in the first half of the 20 th century were mostly carried out at the Caradag Biological Station, Kurortne, Crimea, Ukraine, but with Department of Ecological Parasitology, O. O. Kovalevsky Institute of Biology of the Southern Seas, Sevastopol, Ukraine in the second half of the 20 th century and up to the present time.The team of the Department of Ecological Parasitology together with the Odessa and Karadag branches published over 25 papers devoted to the parasite fauna 17,[45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63] , taxonomy 53,56,60,[64][65][66][67][68][69][70] , ecology 70,71  Polyclithrum ponticum Gerasev, Dmitrieva and Gaevskaya, 2002 53,66 .Recently, a number of monographs published by Gaevskaya [73][74][75] were devoted to studies of fish parasites and pathogens from the Azov-Black Seas, including parasites of grey mullets.The Black Sea mullets have also been studied along the Bulgarian 76,77 and Turkish 78-80 coastal waters.These studies report new faunistic records of parasites.
Coming back to the Mediterranean, some studies of Euzet and co-authors devoted to monogenean parasites of grey mullets were published in the second half of the 20 th century [81][82][83] 92 .
Paperna and co-authors published a set of papers on helminth parasites from the Eastern Mediterranean grey mullets (Israel and Egypt) 2,86,103,104 .Abu Samak [105][106][107] reported three species of Ligophorus from C. ramada in the Mediterranean coastal waters of Egypt.There are also a few papers with helminths records from the Marmara Sea and Turkish Mediterranean [108][109][110] .
First reports of parasites from the introduced population of P. haematocheila in the Azov Sea appeared in papers of Sabodash and Semenenko 111,112 .These authors recorded five parasite species in so-iuy mullets from the Molochny Estuary.Maltsev and co-authors studied parasites of the migrated population of P. haematocheila through the Kerch Chanel [113][114][115] .Their taxonomic works were predominantly focused on monogeneans from the so-iuy mullet.Maltsev and Miroshnichenko 114,115 redescribed Gyrodactylus mugili Zhukov, 1970, G. zhukovi Ling 1962 and G. anguillae Ergens, 1960 and description of a new species Ligophorus gussevi Maltsev and Miroshnichenko, 2004 across different localities in the Azov Sea.However, based on comparative analysis of shape and measurements of sclerotized characters of worms from the so-iuy mullet L. gussevi was considered as the junior synonym of Ligophorus pilengas Sarabeev and Balbuena, 2004 116 .
Sarabeev and co-authors studied the parasite fauna of P. haematocheila and local fish species across localities in the north-western Azov Sea.Sixty-three species of parasites were recorded in the fish studied 88,[117][118][119][120][121][122] .Two new species were described from the so-iuy mullet in the Azov Sea, digenean Bunocotyle constrictus Domnich and Sarabeev, 1999 and monogenean L. pilengas.The former species was later considered as the junior synonym of S. papernai 90 123,124 .The taxonomic status of N. agilis from grey mullets (Mugilidae) across localities in the North-east Atlantic and the North-west Pacific areas was reviewed by Tkach et al. 125 .This review based on morphological features of acanthocephalans showed that there are three different species of Neoechinorhynchus occurring in grey mullet, two of those were recorded in the Atlantic (Neoechinorhynchus (Neoechinorhynchus) agilis and Neoechinorhynchus (Hebesoma) personatus Tkach, Sarabeev et Shvetsova, 2014) and one in the Pacific waters (Neoechinorhynchus (Hebesoma) yamagutii Tkach, Sarabeev et Shvetsova, 2014).Molecular analysis performed on 18S rRNA partial gene sequences has confirmed the species status of N. personatus and N. yamagutii 126 .
Míguez-Lozano et al. 127 provided a detailed description of the helminth communities of C. auratus in the Spanish Mediterranean and analysed the role of spatial, temporal, and host variables in shaping the infracommunities.The spatial structure of helminth infracommunities has been found to be determined by a combination of differences in local environmental conditions and the transmission ability of each helminth species along spatial and temporal scales 127 .A quantitative complex analysis of helminth species richness of invasive so-iuy mullet across different native and introduced populations was performed by Sarabeev 128 .This study showed that the helminth diversity was apparently higher in the introduced population of P. haematocheila than in those of their native habitat, but this trend was not confirmed when the sampling efforts were controlled for.
The cumulative number of helminth species described in grey mullets is presented in Figure 1.
Fewer than 10 species, predominantly large ones, were recorded prior to 1900.After 1900, the three waves of species description are remarkable in the history of taxonomic studies of helminth parasites.Those are related with papers of Looss 30,129 , Euzet and Suriano 82 and Blasco-Costa et al. 32- 35,91,92,130-132 , Sarabeev and co-authors [122][123][124] .An increase in the number of authors suggests that taxonomic effort has increased since the 1960s reaching the highest number of species described (13) in the first decade of the 21st century, w hile only one new species was found in the last decade.Since the number of species is limited for any community 133 , we suggest that the decrease in the number of described species observed for the last decade is the consequence of previous intensive taxonomic efforts probably resulted in actual estimation of species richness for the studied area.
Although taxonomic and faunistic studies of helminth parasites from grey mullets have a long history originating from Rudolphi's work in 1819 12 and were frequent enough (counting up to 130 references), they could not be considered as completed.There are two reasons for this.First, regardless of whether the cause is anthropogenic or natural, the environment is changing mostly due to climate change, pollution and introduction of alien species 134,135 .Environmental change affects species diversity of aquatic and terrestrial ecosystems resulting in the extinction of species or loss of populations from a particular region, and expansion of invasive or introduced species to new areas 134,135 .Since every fish species usually harbors a number of species, it is no surprising that co-introduced parasites commonly make the trip with their invasive host.The Pacific so-iuy mullet introduced in the Azov-Black Seas provides one example of this.Six helminth species of this invasive host have also invaded the new areas 128 (Appendix A).In this context, the Lessepsian migration of grey mullets increases the probability of alien helminth species introduced from the Red Sea to the Mediterranean 136 .For instance, Forticulcita glabra and Saccocoelium gohari reported in Eastern Mediterranean localities obviously have a Red Sea origin.Second, application of modern tools and techniques, such as transmission and scanning electron microscopy, molecular and population approaches provides new possibilities for the discovery of new species.The description of two species of mixosporidians of the genus Kudoa from Mediterranean grey mullets 137,138 is a good example of such research achievement.

Structure of helminth communities from grey mullets in the Mediterranean
Altogether 104 species of helminth parasites from 53 genera and 29 families have so far been reported in Mediterranean grey mullets; of these, 17 and 3 have been identified to generic and family levels only (Appendix A).
The digestive tract harboured the highest helminth species richness (38 species, 36%).There is almost equal number of species occurring on gills, the external surface of the body (29 species, 28%) and in the body cavity, internal organs and tissues (31 species, 30%).
The number of parasite species reported is unequal in the four Mediterranean geographic areas considered (table 1).The highest number of species was found in the Azov-Black Seas, followed by the Western, Eastern and Central Mediterranean areas.We tend to associate the richest fauna of helminth parasites revealed from the Azov-Black Seas region with the intensity of research efforts.Since the first report of flatworms by Wlassenko 36 in 1931, the studies of grey mullet parasites has been quite intensive amounting to over 60 papers.One interesting finding of the present study is that each geographic region is characterized by a specific set of euryxenic parasites.In contrast to oioxenic and stenoxenic helminths, some of which have been reported in all four areas, euryxenic parasites predominantly known for one or maximum two regions (Appendix A).The exception from this pattern is Ascocotyle longa metacercaria, which were registered in the Western, Eastern Mediterranean and Azov-Black Seas.This finding opens new possibilities to use helminth parasites as bioindicators of fish catches and the stock location.

Fig. 1 .
Fig. 1.Cumulative number of helminth species described from grey mullets per decade since 1819.

Fig. 2 .
Fig. 2. Structure of the helminth fauna of grey mullets in the Mediterranean region: Ahigher-level taxonomic groups and stages, Bhost specificity, Cinfection site.

Table 1 -
Number of helminth species reported from different parts of the Mediterranean region: WM -Western Mediterranean; CM -Central Mediterranean; EM -Eastern Mediterranean; AZ-BL -Azov-Black Seas ISSN