An Updated Greek National Checklist of Chondrichthyans

: Accurate checklists of species are essential for evaluating their conservation status and for understanding more about their distribution, biology and ecology and, therefore, the ﬁrst step in order to effectively protect them. According to the existing literature, the Greek seas are rich in chondrichthyan biodiversity and herein, we update the most recent chondrichthyan checklist for the country regarding the species that are present in the Greek waters, correct unvalidated miscellaneous sightings and observations and provide guidelines about future research to improve their conservation. In total, 330 sources were collected from which 276 were used for further analysis, resulting in 1485 records of 67 species, among which 61 are conﬁrmed by experts, including 34 sharks, 26 batoids and one chimaera. We are further listing six species as “Questionable/Not Conﬁrmed”.


Introduction
Chondrichthyans are a taxonomic group that contains about 1296 species worldwide [1]. The chondrichthyans belong to two subclasses, Holocephali (Chimaeras) and Elasmobranchii (sharks and batoids). These organisms have been living on earth for about 400 million years. In their majority, they are meso-to upper-level predators in marine ecosystems and may play an important ecosystem role. The Mediterranean Sea, despite its small acreage, is an important area for chondrichthyans, as it is characterized by moderate biodiversity [1], with 88 species already recorded at present [2]. Unfortunately, at the same time, it is probably the most impacted area for chondrichthyans, as they are the most threatened fish group. Particularly, from the 73 species populations in the Mediterranean Sea, which are assessed by the Red List of Threatened Species of the International Union for Conservation of Nature (IUCN), a percentage of 53.4% has been classified as Threatened and more than one-third as Data Deficient or Not Evaluated due to a lack of relevant data [3].
According to the existing literature, the Greek seas are rich in chondrichthyan biodiversity, with 36 species of sharks, 30 species of batoids and 1 species of chimaera being reported [4]. The Greek Red Book, published in 2009, includes 63 species of chondrichthyans, of which 50 are listed as "Not Assessed", while all the rest belong to one of the threatened categories [5]. In recent years, considerable efforts have been made to study the ecology and biology of these species, both in Greece but also in the Mediterranean [2]. However, knowledge of chondrichthyans in the Greek territorial waters is still limited, especially when it comes to their interaction with fisheries.
Accurate checklists of species are essential for evaluating their conservation status and for understanding more about their distribution and their biology and ecology [6][7][8]. This can help us understand more about the human impact on their populations and, consequently, design proper management action, supporting their conservation.
For this reason, a comprehensive review of the existing knowledge regarding chondrichthyan species' presence in the Greek territorial waters was conducted through a systematic and in-depth review of the current situation. The primary goal was to update the most recent chondrichthyan checklist for the country regarding the species that are present in Greek waters, correct possible miscellaneous sightings and observations and provide guidelines for future research in an attempt to increase their conservation.
Further chondrichthyan records were searched through: • recognition tool and identify all original pictures to the lowest possible taxonomic level. Whenever possible, specimens are collected and experts record data such as maturity, gestation and sex. Finally, there is also a two-way dialogue between citizen participants and scientific experts to retrieve historical records based on old pictures and social media posts. An expert opinion was also used from researchers invited to participate in the preparation of the list. All researchers were asked to provide additional publications that contained original data regarding the presence of chondrichthyans in Greece that were not included in the database after the search in the abovementioned sources.
All sources (publications, reports, biodiversity databases) were organized in a single database including information regarding the species, the year of the sighting, the location (organized by Geographical subareas (GSAs) of General Fisheries Commission for the Mediterranean; Resolution GFCM/33/2009/2), the year of publication and a link to the publication. When a source included reports of observations from one GSA only one record was inserted in the database, while for sources that included reports of observations from different GSAs, we included as many records as the GSAs. Species names and families followed Eschmeyer's Catalog of Fishes [1] and the Red List of Threatened Species of the International Union for Conservation of Nature (IUCN).

Exclusion and Inclusion Criteria
Studies were considered vague when:

1.
They did not provide sufficient information about the location of the observation and pictures of the individual(s) observed; 2.
(For review papers) the original sources could not be tracked or the original sources do not provide sufficient information (location of the observation and pictures of the individual(s); 3.
They were performed in fish and auction markets; 4.
They were referring to ancient specimen remaining; 5.
They were referring to museum collection specimens, of which the capture location was not provided.
Presence index in the Greek waters was based on the following criteria:  Studies from Turkey referring to the GSAs that Greece and Turkey share were included due to the migratory nature of most studied species and the proximity between the two countries that share sea borders. Species records that included exclusively vague sources were considered as "Questionable/Not Confirmed" for Greek waters.
All studies before 1990 were excluded from further analysis, given that the aim of this work is to present an updated list of chondrichthyan species that still exist in the Greek waters.

Results
Source collection resulted in a total of 330 sources, from which 276 were used for further analysis while the rest were excluded (see Supplementary Materials). The analysis of the sources after 1990 resulted in 1485 records of 67 species, among which 61 are confirmed by expert, including 34 sharks, 26 batoids and one chimaera and belonging to 10 orders. Overall, 30 species of batoids, 1 chimaera species and 36 species of sharks were found through the review (Table 1). Based on the qualitative analysis of the sources and the expert's opinion, the Greek waters host 61 chondrichthyan species, represented by 26 batoids, 34 sharks and 1 chimaera ( Figure 1). The six non-retained species are considered misidentification or questionable species.    [10], after the examination of Scyliorhinus canicula (Linnaeus, 1758) specimens from the Mediterranean and elsewhere the species was separated into two distinct species S. canicula and Scyliorhinus duhamelii with the examined specimens of the latter distributed along Adriatic and Mediterranean Seas, along the continental shelves of Croatia, Greece, Tunisia and Argelia. Therefore, it is possible some of the sources presented here refer to S. duhamelii; however, further research is required to confirm the presence of the species in Greece. * Dipturus cf. batis could refer either to Dipturus batis, or Dipturus intermedius or a mix of the two species. More research is required on these species. Four batoids were listed as "Questionable/Not Confirmed" (Dasyatis tortonesei, Dipturus cf. batis, Leucoraja fullonica and Rhinoptera marginata), as well as two species of sharks (Centroscymnus coelolepis and Carcharhinus obscurus), while one shark species was considered as "Not valid" (Sphyrna tudes). For Dipturus cf. batis, the rationale based on [2] was followed. Although [11] stated that D. nidarosiensis might be involved, as it has been found in the Mediterranean, ref. [2] considers the species questionable for the Mediterranean and suspects that Dipturus cf. batis records refer to a species complex, including potentially D. nidarosiensis and D. intemedius. Regarding C. coelolepis, C. obscurus and R. marginata, the available sources did not provide sufficient evidence about the presence of the species in Greek waters, because either identification was vague through the photographic evidence presented or they were only mentioned in review papers, with original publications not being able to be tracked. C. coelolepis appeared in five records (Table 1) [4,[12][13][14][15]. However, all publications, apart from [12,13], do not present original data but were referring primarily to the record of [12]. In [13], the authors claim that it "was believed to be either Centrophorus granulosus or C. coelolepis" and, thus, no definite conclusion can be made. In [12], C. coelolepis is listed among the species observed; however, no further information is provided nor a picture and, therefore, we considered the status of the species "Questionable/Not Confirmed" and further research is required. Based on the recent study of [10], after the examination of Scyliorhinus canicula (Linnaeus, 1758) specimens from the Mediterranean and elsewhere the species was separated into two distinct species S. canicula and Scyliorhinus duhamelii with the examined specimens of the latter distributed along Adriatic and Mediterranean Seas, along the continental shelves of Croatia, Greece, Tunisia and Argelia. Therefore, it is possible some of the sources presented here refer to S. duhamelii; however, further research is required to confirm the presence of the species in Greece. * Dipturus cf. batis could refer either to Dipturus batis, or Dipturus intermedius or a mix of the two species. More research is required on these species.

Figure 1. (A). Percentage of entries per group of chondrichthyans in Greek waters (B). Percentage of entries examined per species for batoids (C).
Percentage of records per species for sharks. Species with less than 2% of records are not shown in the graph but can be found in Table 1.
Four batoids were listed as "Questionable/Not Confirmed" (Dasyatis tortonesei, Dipturus cf. batis, Leucoraja fullonica and Rhinoptera marginata), as well as two species of sharks (Centroscymnus coelolepis and Carcharhinus obscurus), while one shark species was considered as "Not valid" (Sphyrna tudes). For Dipturus cf. batis, the rationale based on [2] was followed. Although [11] stated that D. nidarosiensis might be involved, as it has been found in the Mediterranean, [2] considers the species questionable for the Mediterranean and suspects that Dipturus cf. batis records refer to a species complex, including potentially D. nidarosiensis and D. intemedius. Regarding C. coelolepis, C. obscurus and R. marginata, the available sources did not provide sufficient evidence about the presence of the species in  Table 1. In total, 1012 records were found for the Aegean Sea (GSA 22), 336 for the Ionian Sea (GSA 20) and 137 for Crete (GSA 23) (Figure 2). The majority of the records referred to sharks (60.31%), followed to a lesser extent by batoids (38.03%) and to a minor extent by chimaeras (1.66%), with the percentages slightly differing among the three GSAs ( Figure 2). Greek waters, because either identification was vague through the photographic evidence presented or they were only mentioned in review papers, with original publications not being able to be tracked. C. coelolepis appeared in five records (Table 1), [4,[12][13][14][15]. However, all publications, apart from [12,13], do not present original data but were referring primarily to the record of [12]. In [13], the authors claim that it "was believed to be either Centrophorus granulosus or C. coelolepis" and, thus, no definite conclusion can be made. In [12], C. coelolepis is listed among the species observed; however, no further information is provided nor a picture and, therefore, we considered the status of the species "Questionable/Not Confirmed" and further research is required. In total, 1012 records were found for the Aegean Sea (GSA 22), 336 for the Ionian Sea (GSA 20) and 137 for Crete (GSA 23) (Figure 2). The majority of the records referred to sharks (60.31%), followed to a lesser extent by batoids (38.03%) and to a minor extent by chimaeras (1.66%), with the percentages slightly differing among the three GSAs ( Figure  2). Two records of Carcharhinus obscurus appeared in our review ( Table 1). The first was reported in the citizen-science platform iNaturalist and the second was reported by [16]. In both cases, we considered the photographic evidence not robust enough to confirm the presence of the species in Greek waters, while the records presented in [16] are dated back to 1942. In this work, we consider the current presence of the species in Greek waters as  Two records of Carcharhinus obscurus appeared in our review ( Table 1). The first was reported in the citizen-science platform iNaturalist and the second was reported by [16]. In both cases, we considered the photographic evidence not robust enough to confirm the presence of the species in Greek waters, while the records presented in [16] are dated back to 1942. In this work, we consider the current presence of the species in Greek waters as "Questionable/Not Confirmed" and we suspect that it might be a vagrant species.
Rhinoptera marginata appeared in three records (Table 1) [17,18], with all of them being review sources and not referring to any original observation of the species but to older reviews, such as [19][20][21]. Therefore, we considered the species presence in Greek waters as "Questionable/Not Confirmed". Regarding D. tortonesei, D. cf. batis and L. fullonica, we are following the suggestions by [2] that, for all these species, further investigation is required to confirm their presence in the Mediterranean and, thus, in Greece.
The vast majority of the batoid species (27 species out of 30) present in the Greek waters have not been assessed in the Greek Red Book [5] (Figure 3); however, from those, the Mediterranean population of 11 was assessed as threatened (Critically Endangered; Endangered; Vulnerable) in the IUCN Red List for Threatened Species. The same is true for sharks, with 25 of the 36 species not being assessed in the Greek Red Book (Figure 3), while the Mediterranean population of 12 (50%) was assessed as threatened (Critically Endangered; Endangered; Vulnerable) in the IUCN Red List for Threatened Species (Figure 3).  Regarding the research effort on elasmobranchs within Greek waters, it appears to be increasing rapidly in the last 10 years, having doubled compared to the period 2000-2020 and more than tripled from the period 1978-2000 (Figure 4). The increase in the research effort coincides with programs to evaluate fish stocks in European seas. In particular, the MEDITS program of the EU made it possible to produce numerous scientific papers, not only relating to the stock assessment but also concerning taxonomic items. This allowed one to update the faunal lists of many marine areas, including Greek seas [22][23][24][25][26][27][28][29][30][31][32].  Regarding the research effort on elasmobranchs within Greek waters, it appears to be increasing rapidly in the last 10 years, having doubled compared to the period 2000-2020 and more than tripled from the period 1978-2000 ( Figure 4). The increase in the research effort coincides with programs to evaluate fish stocks in European seas. In particular, the MEDITS program of the EU made it possible to produce numerous scientific papers, not only relating to the stock assessment but also concerning taxonomic items. This allowed one to update the faunal lists of many marine areas, including Greek seas [22][23][24][25][26][27][28][29][30][31][32].

Discussion
Our work presents an exhaustive review of the available bibliography regarding occurrences of elasmobranchs in Greek waters, over almost two centuries (1858-2021). Given the analysis and the validation of the sources, we confirm the presence of 26 batoid species, 34 shark species and 1 chimaera, while we consider D. tortonesei, D. cf. batis, L. fullonica, R. marginata, C. obscurus and C. coelolepis as "Questionable/Not Confirmed" and further research is required for their confirmation. Potentially, some of these species are transient in Greek waters, while for others, further research is required regarding their taxonomical status in the Mediterranean Sea [2]. For the species listed as "Questionable/Not Confirmed", a dedicated campaign on social media, local mass media and peerto-peer with fishers is required to confirm their presence in Greek waters. The latter might be beneficial for some other species, currently listed as "Rare", such as Echinorhinus brucus and Rhinobatos rhinobatos, for which the last observation was reported decades ago. Further research is also needed for some others (such as S. canicula) in order to validate the most recent findings of S. duhamelii revalidation from Greece.
The difference in the sources and the records among GSA 20, 22 and 23 cannot be attributed to differences in the abundance of the elasmobranch species (the study did not assess by the abundance of elasmobranchs in Greek waters), but we believe that it is more related to the scientific effort that seems to be very limited around Crete. Moreover, a number of records from the Ionian Sea are, to a small extent, duplicate records, resulting from publications of a project that conducted field work and utilized multidimensional research on the same species. For example, in the context of the project CoralFISH (https://imbriw.hcmr.gr/coralfish/, accessed on 23 December 2021), several publications were published, including on the diet and feeding strategy of blackmouth catshark Galeus melastomus and on the reproductive biology and length-weight relationships of G. melastomus in the eastern Ionian Sea, with both studies using the same specimens and, therefore, occurrences [30,31]. The same is true for some studies in the Aegean Sea; however, the large number of publications from this GSA undermines the issue. Following the aforementioned research, we suggest that future research in Greece could focus more on Crete and then in the Ionian Sea. Apart from the fact that possibly more species might be present

Discussion
Our work presents an exhaustive review of the available bibliography regarding occurrences of elasmobranchs in Greek waters, over almost two centuries (1858-2021). Given the analysis and the validation of the sources, we confirm the presence of 26 batoid species, 34 shark species and 1 chimaera, while we consider D. tortonesei, D. cf. batis, L. fullonica, R. marginata, C. obscurus and C. coelolepis as "Questionable/Not Confirmed" and further research is required for their confirmation. Potentially, some of these species are transient in Greek waters, while for others, further research is required regarding their taxonomical status in the Mediterranean Sea [2]. For the species listed as "Questionable/Not Confirmed", a dedicated campaign on social media, local mass media and peer-to-peer with fishers is required to confirm their presence in Greek waters. The latter might be beneficial for some other species, currently listed as "Rare", such as Echinorhinus brucus and Rhinobatos rhinobatos, for which the last observation was reported decades ago. Further research is also needed for some others (such as S. canicula) in order to validate the most recent findings of S. duhamelii revalidation from Greece.
The difference in the sources and the records among GSA 20, 22 and 23 cannot be attributed to differences in the abundance of the elasmobranch species (the study did not assess by the abundance of elasmobranchs in Greek waters), but we believe that it is more related to the scientific effort that seems to be very limited around Crete. Moreover, a number of records from the Ionian Sea are, to a small extent, duplicate records, resulting from publications of a project that conducted field work and utilized multidimensional research on the same species. For example, in the context of the project CoralFISH (https://imbriw.hcmr.gr/coralfish/, accessed on 23 December 2021), several publications were published, including on the diet and feeding strategy of blackmouth catshark Galeus melastomus and on the reproductive biology and length-weight relationships of G. melastomus in the eastern Ionian Sea, with both studies using the same specimens and, therefore, occurrences [30,31]. The same is true for some studies in the Aegean Sea; however, the large number of publications from this GSA undermines the issue. Following the aforementioned research, we suggest that future research in Greece could focus more on Crete and then in the Ionian Sea. Apart from the fact that possibly more species might be present in these two areas, such limited research might result in insufficient understanding about the role of elasmobranch species in these areas and the threats they face.
It is also important to highlight the knowledge gap in the evaluations of elasmobranch species in the Greek Red Book, which was published in 2009 [5]. From the 61 species definitely present in the Greek waters, 50 (≈82%) were not evaluated due to a lack of data. Since then (2009) [5] a few more publications focusing on Greek waters have been published. Hence, a Red Book update is on its way in Greece as an initiative of the Ministry of Environment and Energy. For this reason, it is of particular importance to fulfill the following: (i) Dedicated research to take place on measuring the impact of fisheries and other human activities on the populations of elasmobranchs in Greek waters; (ii) Research centers participating in the national Data Collection Framework to utilize all the collected information, particularly from the MEDITS survey for providing abundance estimates for species in different locations around Greece; (iii) Funders to support initiatives in the country that aim to estimate the impact of fisheries on the population of elasmobranchs or that provide population abundance estimates, especially in Crete and the Ionian Sea.

Conclusions
Our study update the most recent chondrichthyan checklist for Greek waters, correct unvalidated miscellaneous sightings and observations, and provide several avenues for further research in an effort to improve chondrichthyan conservation. Although the Greek seas are rich in chondrichthyan biodiversity, only during the last decade research effort on this class have been largely increased mostly attributed to MEDITS program. However, in several locations, there are still significant gaps in knowledge (e.g., Corinth Gulf; Crete, etc.) and a lack of understanding about the pressure that several métiers appear to have on them (e.g., in the North Aegean Sea; [32]). Before the Red Book is updated, it is critical to fill these gaps in order to produce a comprehensive assessment of the elasmobranch species found in Greek waters. Funding: This research was funded by the Green Fund of the Hellenic Ministry of Environment and Energy in the context of the project "Updating of the Greek National Chondrichthyans Checklist". Additionally, the Shark Conservation Fund, the Save Our Seas Foundation, OceanCare and Shark Foundation/Hai Stiftung have been supporting projects of iSea that contributed significant data for this work.

Institutional Review Board Statement: Not applicable.
Data Availability Statement: The dataset analyzed in the current study is available from the corresponding author upon reasonable request.