![Numbers (and percentages) of Mediterranean fish species with published information on eight biological characteristics, namely, length-weight relationships (LWR), growth parameters (G), maximum age (t max ), mortality rate (M), spawning period (Spawn), size at maturity (L m ), fecundity (Fec) and feeding preferences (Diet). Top row: blue colour: all Mediterranean fish species (top left panel), red colour: Mediterranean fish species listed in the IUCN Red List of Threatened Species under the categories near threatened (NT), vulnerable (VU), endangered (EN) and critically endangered (CR) (top middle panel), green colour: highly commercial Mediterranean fish species under the categories high (H) and very high (VH) commercial value in FishBase (top right panel). Bottom row: orange colour: Western Mediterranean fish species (bottom left panel), yellow colour: central Mediterranean fish species (bottom middle panel) and brown colour: eastern Mediterranean fish species (bottom right panel). Light coloured data are from Dimarchopoulou et al., (2017) [1], while darker coloured data are from the present study and data coloured in light gray represent the number of fish species lacking published information on the respective biological characteristic. https://doi.org/10.1371/journal.pone.0277383.g001](https://www.researchgate.net/publication/365308306/figure/fig1/AS:11431281096627881@1668193528090/Numbers-and-percentages-of-Mediterranean-fish-species-with-published-information-on_Q320.jpg)
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RESEARCH ARTICLE
Scientific progress made towards bridging
the knowledge gap in the biology of
Mediterranean marine fishes
Eva DaskalakiID
1
*, Evangelos KoufalisID
1
, Donna DimarchopoulouID
2,3
, Athanassios
C. Tsikliras
1
1Laboratory of Ichthyology, Department of Zoology, School of Biology, Aristotle University of Thessaloniki,
Thessaloniki, Greece, 2Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada,
3Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, United States
of America
*daskalakieva2@gmail.com,evangeld@bio.auth.gr
Abstract
The Mediterranean Sea is a renowned biodiversity hotspot influenced by multiple interacting
ecological and human forces. A gap analysis on the biology of Mediterranean marine fishes
was conducted in 2017, revealing the most studied species and biological characteristics,
as well as identifying knowledge gaps and areas of potential future research. Here, we
updated this gap analysis five years later by reviewing the literature containing information
on the same eight biological characteristics, namely length-weight relationships, growth,
maximum age, mortality, spawning, maturity, fecundity and diet, for the 722 fish species of
the Mediterranean Sea. The results revealed a considerable knowledge gap as 37% of the
species had no information for any of the studied characteristics, while 13% had information
on only one characteristic. Out of all the biological characteristics, the smallest knowledge
gap was found in the length-weight relationships (studied for 51% of the species, mainly in
the eastern Mediterranean), while the least studied characteristic was mortality (studied for
10% of the species). The western and eastern Mediterranean Sea were leading forces in
data collection exhibiting the narrowest gaps between current and desired knowledge. The
most studied species across the entire region were the highly commercial European hake
(Merluccius merluccius), red mullet (Mullus barbatus), European anchovy (Engraulis encra-
sicolus), European pilchard (Sardina pilchardus), common pandora (Pagellus erythrinus),
and annular seabream (Diplodus annularis). The knowledge gap has shrunk by 6% during
the last five years, with 40 new species having at least one study on their biology. Moreover,
research has slightly shifted towards species that have been traditionally neglected, e.g.,
sharks, rays and chimaeras (chondrichthyans). It is recommended that research becomes
less focused on commercial species and more targeted towards the identified gaps, vulnera-
ble species (e.g., deep-sea species and chondrichthyans) and species that could potentially
pose a threat (e.g., non-indigenous species) to the ecosystems of the everchanging Medi-
terranean Sea.
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OPEN ACCESS
Citation: Daskalaki E, Koufalis E, Dimarchopoulou
D, Tsikliras AC (2022) Scientific progress made
towards bridging the knowledge gap in the biology
of Mediterranean marine fishes. PLoS ONE 17(11):
e0277383. https://doi.org/10.1371/journal.
pone.0277383
Editor: Roberta Cimmaruta, Universita degli Studi
della Tuscia, ITALY
Received: August 3, 2022
Accepted: October 25, 2022
Published: November 10, 2022
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0277383
Copyright: ©2022 Daskalaki et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
available from review articles cited within the paper
and at www.fishbase.org.
Introduction
The Mediterranean Sea has been identified as an area with a large information gap on the bio-
logical characteristics of fish species [1]. The lack of biological information impedes the efforts
of scientists working in different fields, as this information is needed for stock assessments and
ecosystem models. Fish stock assessment models alone require an array of biological informa-
tion such as length-weight relationships, maximum age, size at maturity, and growth parame-
ters [2], leading scientists to develop new and less data-demanding methods for estimating the
desired parameters [3]. In addition to assessment models, life history traits such as maximum
age, growth rate and age at maturation, affect species resilience against overexploitation [4,5]
and climate change [6], therefore playing an important role in modelling species resilience.
Moreover, the more holistic approach of the ecosystem-based fisheries management (EBFM)
requires biological data on all ecosystem components, both commercial and non-commercial,
along with the characteristics of individual species, to be taken into consideration for the deci-
sion-making process [7,8].
As a recognized global biodiversity hotspot, the Mediterranean Sea needs to be protected
[9,10]. Despite the fact that there are established marine protected areas (MPAs) around the
Mediterranean basin covering 8.33% of its surface, the cumulative surface of no-go, no-take or
no-fishing area represents only 0.04% of it [11]. Therefore, only 0.23% of the Mediterranean
basin is actually and effectively managed and protected [12], alongside the species living in
these areas. Given the importance of data in systematic management, the reported lack of data
regarding Mediterranean biodiversity [1,9,13] is a handicap to any conservation effort.
In the gap analysis conducted by Dimarchopoulou et al. (2017) [1] on the biological charac-
teristics of Mediterranean marine fishes, sharks, rays and chimaeras (chondrichthyans) were
identified as one of the least studied groups in the Mediterranean region, with scarce informa-
tion available for only a handful of species. Many of these species are top predators in the envi-
ronments they inhabit and as such, they play a key role in the food web through direct and
indirect cascading effects [14,15]. Additionally, chondrichthyans are characterized by low
fecundity, late maturity and slow growth and are exposed to considerable fishing pressure all
over the world, which makes them extremely vulnerable [16]. Although chondrichthyans are
not explicitly targeted in the Mediterranean region, they are often caught as bycatch [17] and
research has shown that Mediterranean elasmobranchs (sharks and rays) are either of poor
conservation status compared to the rest of the world or simply neglected [17,18]. Further-
more, historically declining trends in the abundance of chondrichthyan species have been
reported in the Mediterranean region. For some species like hammerhead (Sphyrna spp.), blue
shark (Prionace glauca), Shortfin mako (Isurus oxyrinchus), Porbeagle (Lamna nasus), and
thresher sharks (Alopias vulpinus) the decline in abundance has been estimated between 96
and 99.99% in the western Mediterranean region [19]. At the same time, the fishing effort is
evidently becoming more intense, pushing these vulnerable species to their limits [15,20,21].
Other Mediterranean fish species adding to the Mediterranean’s unique biodiversity are
deep-sea species. This diverse group of species (Actinopterygii and Elasmobranchii) inhabits
deep-sea ecosystems covering 79% of the entire basin [22] and is found beneath approximately
200 m depth [23]. Deep-sea species are particularly vulnerable due to ‘‘K-selected” life history
characteristics, as a consequence of the low productivity environment they inhabit [24].
Research has revealed that Mediterranean deep-sea biodiversity is richer than once expected,
and yet still largely undiscovered and understudied [23] due to intricate and laborious logistics
required for its exploration [25]. Nevertheless, fishing [26] and various maritime exploitative
activities have been reportedly pushed towards larger depths on a global scale [27] and espe-
cially in the Mediterranean region [22], as the fish stocks of the continental self are being
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Funding: This research was partly funded by the
European Union’s Horizon 2020 Research and
Innovation Program (H2020-BG-10-2020-2), grant
number No. 101000302 - EcoScope (Ecocentric
management for sustainable fisheries and healthy
marine ecosystems). The funders had no role in
study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
depleted [28,29]. The lack of data regarding basic biological characteristics of many deep-sea
species makes implementation of effective management regulation for these vulnerable and
important ecosystems a challenging task [30].
Moreover, the Mediterranean region is facing rapid changes including acidification [31,32]
climate-driven warming [33] and the influx of non-indigenous species [34]. Non-indigenous
species that establish reproductive populations become invasive species [35], posing a threat to
human health [36], local populations [37] and the economy [34,38], additionally to the effects
on the ecosystem, as a whole [39]. The rate at which the invasion is taking place is alarming
[40], thus gathering data on the biological characteristics of non-indigenous species in the
Mediterranean region, is of major importance in order to identify, monitor and predict the
behavior and distribution of these species and successfully integrate non-indigenous species
into an ecosystem-based management approach over the entire basin [41].
Considering that five years have passed since the initial gap analysis by Dimarchopoulou
et al. (2017) [1] and given 1) the numerous stressors co-occuring in the Mediterranean region,
2) the imperative need for monitoring and efficient management, activities that require all-
encompassing ecosystem modelling and data availability and 3) the reported knowledge gap in
fundamental biological characteristics of marine fish, we revisited the topic to keep track of the
existing gap and identify any progress made. Finally, we aim to provide some updated recom-
mendations pointing at the existing gaps as priority targets for future scientific projects and
put a spotlight on groups of species of great value to the Mediterranean biodiversity and there-
fore, of conservation interest (deep-sea species and chondrichthyans) as well as species
described in the literature as potential stressors for the ecosystems (invasive non-indigenous
species).
Materials and methods
Following the methodology of Dimarchopoulou et al. (2017) [1] we updated the gap analysis
on the biology of Mediterranean marine fish species, taking into consideration the Mediterra-
nean Sea as a whole, but also analyzing the data on a subregional basis (W: western; C: central;
E: eastern, Mediterranean). The gap between the current and desired knowledge was investi-
gated for the same eight biological characteristics, i.e., length-weight relationships, growth,
maximum age, mortality, spawning, maturity, fecundity and diet. The desired knowledge level
has been defined as having information on most biological characteristics for at least half of the
Mediterranean marine fishes [1].
The updated gap analysis performed here covers the years 2015–2021 and investigates the
relevant studies on the biology of Mediterranean marine fishes that have been published after
the original gap analysis. We took into consideration the two-year literature overlap (2015–
2016) with the initial gap analysis by excluding the papers published in 2015 and 2016 that had
already been included in the Dimarchopoulou et al. (2017) paper [1]. For our analysis, we col-
lected information on all fish species that have been recorded in the Mediterranean Sea large
marine ecosystem as they are listed in FishBase [42]. Specifically, we recorded the existence (or
not) of papers containing information on eight biological characteristics for every species in
the list (S1 Appendix). Altogether, 758 Mediterranean marine fish species were listed in Fish-
Base [42], of which 36 species were excluded as misidentified and/or questionable records. The
current list of Mediterranean fish species is not identical to the one used in Dimarchopoulou
et al. (2017) [1]. Our updated list consists of 722 species, while the first gap analysis used a list
of 714 species. Apparently, there have been a few changes in the FishBase Mediterranean spe-
cies list [42], during the past five years. Nevertheless, the changes regarding the fish species are
minor: 709 species are the same in both lists, 5 species existed only in the Dimarchopoulou
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et al. (2017) [1] list but have been removed from the Mediterranean fish species list in FishBase
most probably to identified synonyms [42], while 13 new species were added in that list since
the first gap analysis.
For each of the 722 analyzed species, the available information on length-weight relation-
ships, growth parameters, maximum age, mortality rate, spawning period, size at maturity,
fecundity and diet composition was extracted from FishBase [42] and published literature that
was searched through SCOPUS. For the length-weight relationships (LWR) we recorded spe-
cies with information on both the slope (b) and intercept (a) of the equation; for somatic
growth (G) we considered records with the asymptotic length (L
1
) and the rate at which L
1
is
approached (K), leaving out all growth records of “questionable” status in FishBase [42] and
for the lifespan we included records with the maximum age (t
max
). Concerning the reproduc-
tion parameters, we considered the onset and duration of spawning (Spawn) and length at
maturity (L
m
) to identify spawning and maturity related information, respectively. Addition-
ally, we regarded absolute (the actual number of eggs) and relative (the number of eggs per
unit of weight) number of oocytes for fecundity (Fec) [43]. Lastly, we considered as records for
diet papers that included information on prey items, stomach content and feeding preferences
as records for diet, while the natural mortality rate was used as natural mortality (M), regard-
less of the estimation method.
Altogether, 444 papers published between 2015 and 2021 were screened, i.e., 35 and 409
papers extracted from FishBase [42] and SCOPUS, respectively (S2 Appendix). We did a
search for each Mediterranean fish species in FishBase [42] and extracted information coming
from the Mediterranean Sea on the eight studied characteristics which are presented in the
“More information” section of each species page. Regarding the SCOPUS literature review, we
conducted a search using the scientific name of each fish species in our list alongside the name
of the region, e.g., “Merluccius merluccius” AND “Mediterranean”. We then went over the title
and abstract of each search result to identify the ones that met the aforementioned criteria and
could be included in this review.
To accommodate our research, we divided fish species into categories: commercial, non-
commercial, deep-sea (species that live in depths of 200m and below), non-indigenous (species
that have been introduced into the Mediterranean ecoregion), chondrichthyans (sharks, rays
and chimaeras), protected, and species with atypical life history strategies (e.g., those exhibiting
very slow growth or providing parental care to their offspring). Regarding protection, we
recorded the protection status (IUCN Red List of Threatened Species) of each species based on
the IUCN categories (LC: least concern; EN: endangered; DD: data deficient; NE: not evalu-
ated; NT: near threatened; VU: vulnerable; CR: critically endangered), alongside their com-
mercial value (Val), which was shown as “price category” in FishBase [42] (VH: very high; H:
high; M: medium; L: low) and their trophic level, as it was calculated in FishBase [42]. Further-
more, we used the depth range of each species provided by FishBase [42], to discern which fish
species inhabit the deep-sea ecosystems of the Mediterranean Sea. The deep-sea zone is consid-
ered to begin from 200m underneath the surface [23], so we included all the fish species with
depth ranges starting from around 200 meters and deeper. We also extracted the Mediterra-
nean Sea catches averaged for the years 2015–2019 from the FAO-GFCM database [44]. We
chose to keep only the species-specific catch data and not the catches assigned to higher taxo-
nomic groups (e.g., genera, families).
At last, we cross-correlated the number of records per species with the number of total
catches, the maximum reported somatic length (L
max
) and the trophic level for species with at
least one record and available data (total catches: n = 125, L
max
: n = 451 and trophic level:
n = 452), to detect any bias of the scientific effort towards commercial species, larger species or
species of higher trophic level, respectively. We started by checking the normality of our data,
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using the Shapiro–Wilk test of normality and then we proceeded to search for linear correla-
tion between each one of the three variables and total catches using the Pearson’s correlation
coefficient (r) for the parametric data and Spearman’s correlation coefficient (ρ) for the non-
parametric data. Our data were logarithmically transformed to improve the visualization pro-
cess. Finally, following the Dimarchopoulou et al. (2017) [1] methodology, we calculated the
number of records per metric ton of catch (records/t) to allow comparisons between highly
commercial and non-commercial species and per centimeter of somatic length (records/cm)
to allow comparisons among sizes.
Results
Based on the present analysis, there is no information on any biological characteristic for 270
out of the 722 Mediterranean fish species (37%), while for 95 (13%) of them there is informa-
tion for only one characteristic. Regarding the biological characteristics separately, the gap is
narrower for length-weight relationships, as they have been studied for 366 (51%) species, fol-
lowed by spawning (312 species; 43%), diet (284 species; 39%), growth (211 species; 29%),
maturity (192 species; 27%), maximum age (184 species; 25%) and fecundity (142 species;
20%) (Fig 1,top row). Most new studies focused on length-weight relationships, diet and
spawning, with 708 new length-weight relationship records for 56 species, 467 new diet rec-
ords for 76 species and 190 new spawning records for 34 species (Table 1) in the last five years.
Fig 1. Numbers (and percentages) of Mediterranean fish species with published information on eight biological characteristics, namely, length-weight relationships
(LWR), growth parameters (G), maximum age (t
max
), mortality rate (M), spawning period (Spawn), size at maturity (L
m
), fecundity (Fec) and feeding preferences
(Diet). Top row: blue colour: all Mediterranean fish species (top left panel), red colour: Mediterranean fish species listed in the IUCN Red List of Threatened Species under
the categories near threatened (NT), vulnerable (VU), endangered (EN) and critically endangered (CR) (top middle panel), green colour: highly commercial
Mediterranean fish species under the categories high (H) and very high (VH) commercial value in FishBase (top right panel). Bottom row: orange colour: Western
Mediterranean fish species (bottom left panel), yellow colour: central Mediterranean fish species (bottom middle panel) and brown colour: eastern Mediterranean fish
species (bottom right panel). Light coloured data are from Dimarchopoulou et al., (2017) [1], while darker coloured data are from the present study and data coloured in
light gray represent the number of fish species lacking published information on the respective biological characteristic.
https://doi.org/10.1371/journal.pone.0277383.g001
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The largest gap was found in natural mortality for which information was sparse (70 species;
10%) (Table 1). Concerning the studied species that are listed under the categories near threat-
ened (NT), vulnerable (VU), endangered (EN) and critically endangered (CR) of the IUCN
Red List (n = 91), the percentages for all biological characteristics except mortality and growth
were higher compared to the total of the species (Fig 1,top row). In terms of commercial value,
the species listed under the categories high (H) and very high (VH) (n = 167), had higher per-
centages in every biological characteristic, compared to all species listed (Fig 1,top row).
With respect to the studied characteristics, the record distribution pattern was consistent
among the western, central and eastern Mediterranean, with the exception of some spatial var-
iations within each biological characteristic (Fig 1,bottom row). Spawning, maturity, fecundity
and diet were most extensively studied in the western subregion, while length-weight relation-
ships and maximum age were studied mostly in the eastern subregion. Growth and mortality
records were rather evenly distributed among the three subregions (Fig 1,bottom row).
In terms of number of records, the most studied species were European hake Merluccius
merluccius (227 records in total with 46 new records in the last five years), red mullet Mullus
barbatus (163 records in total with 34 new), European anchovy Engraulis encrasicolus (142 rec-
ords in total with 55 new), European pilchard Sardina pilchardus (125 records in total with 43
new), common pandora Pagellus erythrinus (124 records in total with 26 new), annular seab-
ream Diplodus annularis (115 records in total with 17 new), surmullet Mullus surmuletus (115
records in total with 15 new) and bogue Boops boops (100 records in total with 24 new)
(Table 2). Furthermore, we identified many new records for chondrichthyans, resulting in
some chondrichthyan species with more than 30 records, although there still are 13 species
(out of 82 total chondrichthyan species) without a single record on their biological characteris-
tics (Table 3).
The least studied species were those belonging to the Blennidae, Gobiesocidae, Gobiidae,
Moridae, Myctophidae, and Syngnathidae families (Table 4), as well as to two groups of fish
species: non-indigenous species and deep-sea species (Figs 2and 3). There are 126 non-indige-
nous species in FishBase [42] under the “introduced” status, from the list of 722 Mediterranean
fish species (17.5%). For 60% of these non-indigenous fish species there is no available record
on the specific biological characteristics studied in this work. Additionally, for 15% of them,
information exists on one out of eight biological characteristics (Fig 2). Only eight non-indige-
nous fish species can be considered well studied (Goldband goatfish Upeneus moluccensis,
Por’s goatfish Upeneus pori, Redcoat Sargocentron rubrum, Shrimp scad Alepes djedaba, Silver-
cheeked toadfish Lagocephalus sceleratus, Devil firefish Pterois miles, Dusky spinefoot Siganus
Table 1. The number of records from Dimarchopoulou et al. (2017) [1] and from the papers published between2015–2021, as well as the total number of records
from the present study for each one of the studied characteristics.
Characteristic Number of records from Dimarchopoulou
et al. (2017) [1]
Number of records published from
2015 to 2021
Total number of records_Present study
Diet 563 467 1030
Fecundity 184 62 246
Maturity 393 143 536
Spawning 670 190 860
Mortality 85 37 122
T
max
283 187 470
Growth 691 132 823
LWR 1738 708 2446
T
max
: maximum age; LWR: length-weight relationships
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luridus and Marbled spinefoot Siganus rivulatus), as there is available information on at least
seven biological characteristics for these eight species. With respect to the Mediterranean fish
species dwelling in deep-sea habitats, 45% of them have zero available information on their
biological characteristics, while only 24% have recorded data on at least half of the studied
characteristics (Fig 3).
The available information on the previously least studied fish species in the Mediterranean
Sea (Table 2 in [1]), has increased, as many of these species have had new records on their bio-
logical characteristics published within the last five years (Table 5).
The highest ratio of records per metric ton of production was observed for the velvet belly,
Etmopterus spinax (5.1 records/t), longnose spurdog, Squalus blainville (4.6 records/t), big-
scale sand smelt, Atherina boyeri (4.4 records/t) and Mediterranean sand smelt, Atherina hep-
setus (2.2 records/t). The highest number of records to somatic length ratio was observed for
the European pilchard, Sardina pilchardus (4.6 records/cm), red mullet Mullus barbatus barba-
tus (4.3 records/cm), common two-banded seabream, Diplodus vulgaris (3.1 records/cm),
European anchovy, Engraulis encrasicolus (2.4 records/cm), picarel, Spicara smaris (2.2 rec-
ords/cm) and common pandora, Pagellus erythrinus (2.1 records/cm).
Concerning the Mediterranean fish species with at least one studied biological character-
istic and with available data on catches, maximum length and trophic level, the number of
records had a moderately positive correlation to total catches (n = 125, Pearson r = 0.51,
P<0.001), and a very weak positive correlation to maximum reported length (n = 451,
Pearson ρ= 0.17, P <0.001). In contrast to total catches and maximum somatic length, the
trophic level did not correlate with the number of records (n = 452, Spearman ρ= 0.09,
P = 0.068) (Fig 4).
Table 2. List of the most studied fish species in the Mediterranean Sea based on the number of records (Number of Rec.), the number of studied characteristics
(Number of Char.), and the number of records per characteristic.
Species Common Name Family
IUCN
Commercial Value
Number of Rec.
Number of Char.
Number of records per characteristic_Present study
Merluccius merluccius European hake Merlucciidae LC H 227 8/8 59 LWR, 58 G, 29 Diet, 24Sp, 23 Mat, 15 A, 10 Fec, 9 M
Mullus barbatus Red mullet Mullidae LC M 163 8/8 49 LWR, 47 G, 19 Diet, 13 A, 12 Sp, 12 Mat, 7 Fec, 4 M
Engraulis encrasicolus European anchovy Engraulidae LC M 142 8/8 39 LWR, 23 Sp, 23 G, 23 Diet, 13 A, 10 Mat, 7 Fec, 4 M
Sardina pilchardus European pilchard Clupeidae LC L 125 8/8 29 G, 24 LWR, 22 Diet, 19 Sp, 11 Mat, 10 A, 5 Fec, 5 M
Pagellus erythrinus Common pandora Sparidae LC M 124 8/8 49 LWR, 17 G,15 A, 14 Diet, 13 Sp, 12 Mat, 3 Fec, 1 M
Mullus surmuletus Surmullet Mullidae LC VH 115 7/8 48 LWR, 18 Sp, 17 G, 14 Diet, 8 Mat, 7 A, 3 M, 0 Fec
Diplodus annularis Annular seabream Sparidae LC L 115 8/8 57 LWR, 17 Sp, 13 G, 8 A, 7 Mat, 7 Diet, 4 Fec, 2 M
Boops boops Bogue Sparidae LC H 100 8/8 36 LWR, 19 G, 15 Sp, 10 A, 9 Mat, 5 Diet, 3 M, 3 Fec
Diplodus vulgaris Common two-banded seabream Sparidae LC L 80 8/8 30 LWR, 14 Diet, 10 Sp, 8 G, 7 Mat, 7 A, 3 M, 1 Fec
Serranus cabrilla Comber Serranidae LC M 74 7/8 35 LWR, 9 Sp, 8 G, 8 Diet, 7 A, 4 Mat, 3 M, 0 Fec
Thunnus thynnus Atlantic bluefin tuna Scombridae EN VH 73 8/8 16 Diet, 14LWR, 14 G, 10 Sp, 8 A, 5 Mat, 3 M, 3 Fec
Note: The commercial value (Val) is shown as price category (VH: very high; H: high; M: medium; L: low) and the protection status (IUCN) as IUCN Red List status
category (LC: least concern; EN: endangered; DD: data deficient; NE: not evaluated; NT: near threatened; VU: vulnerable; CR: critically endangered). We included all
the species with 8/8 and 7/8 characteristics studied. For the complete table, the reader may refer to the (S1 Table).
LWR: length-weight relationships; G: growth parameters; A: lifespan; Mat: length at maturity; Sp: onset and duration of spawning; Fec: fecundity; M: mortality; Diet:
feeding preferences.
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Discussion
Since the publication of the first gap analysis in 2017 [1], the research effort regarding the knowl-
edge gaps in biological characteristics of Mediterranean marine fish species has intensified. The
results showed that during the last five years (2016–2021) the knowledge gap on the biology of
Mediterranean marine fishes was slightly narrowed as the percentage of fish species without any
biological characteristics has dropped from 43% in 2016 [1] to 37% in 2021 (Fig 1 and Table 5).
With respect to the number of records per species, the most studied species remain the same as
in the previous gap analysis (Table 2). The general rule appears to be that species of high com-
mercial value are also more likely to have been studied across their biological characteristics
(Tables 2and 5), reflected in the missing knowledge regarding the Gobiidae, Blennidae, Mycto-
phidae, Gobiesocidae and Syngnathidae families (Table 4). The commercial status of the species
belonging in the above families is described, mostly, as “NA” in FishBase [42].
Prompted by the original gap analysis, several journal articles were published aiming to
reduce the identified knowledge gap. Two of these articles provided the first published diet
Table 3. List of all the Mediterranean chondrichthyan species with studies on their biological characteristics.
Species Common Name Family
IUCN
Commercial Value
Number of Rec.
Number of Char.
Number of records per characteristic_ Present study
Raja clavata Thornback ray Rajidae NT M 55 7 18 LWR, 12 Diet, 8 G, 6 A, 6 Mat, 3 Fec, 2 Sp
Galeus melastomus Blackmouth catshark Pentanchidae LC H 54 6 18 Diet, 15 Mat, 10 LWR, 8 Sp, 2 Fec, 1 G
Scyliorhinus canicula Lesser spotted dogfish Scyliorhinidae LC M 45 6 16 Diet, 12 LWR, 8 Mat, 6 Fec, 2 Sp, 1 G
Squalus blainville Longnose spurdog Squalidae DD M 37 7 9 LWR, 9 Diet, 7 Mat, 5 G, 3 Sp, 2 A, 2 Fec
Torpedo marmorata Marbled electric ray Torpedinidae DD NA 34 7 14 LWR, 6 Diet, 5 Mat, 4 Fec, 3 Sp, 1 G, 1 A
Raja radula Rough ray Rajidae EN M 15 7 7 LWR, 3 Diet, 1 G, 1 A, 1 Sp, 1 Mat, 1 Fec
Tetronarce nobiliana Electric ray Torpedinidae DD NA 15 6 8 LWR, 2 A, 2 Diet, 1 G, 1 Sp, 1 Fec
Raja polystigma Speckled ray Rajidae LC NA 13 7 4 Diet, 3 LWR, 2 Fec, 1 G, 1 A, 1 Sp, 1 Mat
Mustelus punctulatus Blackspotted smooth hound Triakidae DD M 12 5 5 Diet, 2 LWR, 2 Sp, 2 Mat, 1 Fec
Dalatias licha Kitefin shark Dalatiidae VU M 11 4 6 Diet, 3 LWR, 1 Sp, 1 Mat
Dasyatis marmorata Marbled stingray Dasyatidae DD NA 10 6 4 LWR, 2 Mat, 1 G, 1 A, 1 Fec, 1 Diet
Leucoraja melitensis Maltese ray Rajidae CR NA 5 3 3 LWR, 1 Sp, 1 Fec
Alopias vulpinus Thresher Alopiidae VU H 2 2 1 LWR, 1 Diet
Cetorhinus maximus Basking shark Cetorhinidae EN L 1 1 1 LWR
Glaucostegus halavi Halavi ray Glaucostegidae CR NA 1 1 1 LWR
Carcharhinus longimanus Oceanic whitetip shark Carcharhinidae CR M 0 0
Pristis pectinata Smalltooth sawfish Pristidae CR M 0 0
Pristis pristis Common sawfish Pristidae CR M 0 0
Dipturus batis Blue skate Rajidae CR M 0 0
Sphyrna lewini Scalloped hammerhead Sphyrnidae CR M 0 0
Sphyrna mokarran Great hammerhead Sphyrnidae CR H 0 0
Note: The commercial value (Val) is shown as price category (VH: very high; H: high; M: medium; L: low) and the protection status (IUCN) as IUCN Red List status
category (LC: least concern; EN: endangered; DD: data deficient; NE: not evaluated; NT: near threatened; VU: vulnerable; CR: critically endangered). For the complete
table, the reader may refer to the (S2 Table).
LWR: length-weight relationships; G: growth parameters; A: lifespan; Mat: length at maturity; Sp: onset and duration of spawning; Fec: fecundity; M: mortality; Diet:
feeding preferences).
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records for the species phaeton dragonet Synchiropus phaeton [45] and yellowspotted puffer
Torquigener flavimaculosus [46]. It is explicitly stated, in these publications, that the aim of the
study was to reduce the knowledge gap, a notion that is also reflected, even if not explicitly
stated, among other papers gathered for this study. Nevertheless, a considerable knowledge
gap still exists as 37% of the Mediterranean marine fish species have no available information
on their biological characteristics and 13% of them have very little information available
regarding mostly the length-weight relationship trait.
Table 4. List of selected least studied families of non-commercial fish species in the Mediterranean Sea based on the number of studied characteristics and the num-
ber of records per characteristic.
Species Common Name Family IUCN
Lipophrys pholis Shanny Blenniidae LC
Omobranchus punctatus Muzzled blenny Blenniidae LC
Apletodon incognitus (clingfish) Gobiesocidae LC
Diplecogaster bimaculata Two-spotted clingfish Gobiesocidae LC
Benthophilus stellatus Stellate tadpole-goby Gobiidae LC
Buenia lombartei (goby) Gobiidae NA
Chromogobius quadrivittatus Chestnut goby Gobiidae LC
Coryogalops ocheticus (goby) Gobiidae EN
Eretmophorus kleinenbergi (morid cod) Moridae LC
Guttigadus latifrons (morid cod) Moridae NA
Diogenichthys atlanticus Longfin lanternfish Myctophidae LC
Gonichthys cocco Cocco’s lanternfish Myctophidae LC
Entelurus aequoreus Snake pipefish Syngnathidae NA
Hippocampus fuscus Sea pony Syngnathidae NA
Note: The commercial value (Val) is shown as price category (VH: very high; H: high; M: medium; L: low) and the protection status (IUCN) as IUCN Red List status
category (LC: least concern; EN: endangered; DD: data deficient; NE: not evaluated; NT: near threatened; VU: vulnerable; CR: critically endangered). For all the species
listed in this table, commercial value is not known (“NA”) and both the number of records and characteristics are equal to 0. For the complete table, the reader may refer
to the (S3 Table).
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Fig 2. Percentages of Mediterranean fish species with studied biological characteristics (0–8). Blue colour: all Mediterranean species; purple colour:
introduced/non-indigenous species; green colour: chondrichthyan species).
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Length-weight relationships (LWR) remain the most studied biological characteristic [1], as
expected from the low cost and little effort and time required for its study [47,48]. Contrary to
the recommendations provided in Dimarchopoulou et al. (2017) [1] towards filling the gaps in
less popular characteristics, papers published in the past five years, that focus on narrowing the
knowledge gap, have been massively producing LWR records in a single publication [49,50].
At the same time, information on other biological characteristics that can be cumbersome to
obtain are harder to come across [51]. To this day, fecundity of marine Mediterranean fish spe-
cies, remains one of the least studied characteristics.
On the other hand, maximum age, which was reported as “oddly missing” in the previous
analysis, seems to have gained quite a few new records (n = 187; Table 1) in the last five years,
for instance, for the species Sciaena umbra [52], Trachinotus ovatus [53] and Alepes djedaba
[54]. Conversely, mortality records are still very few and exist for only a small percentage of
the Mediterranean fish species (Fig 1, top row and Table 1). As stated in the initial gap analysis
“the maximum age and natural mortality of a population should be reported in all articles on
growth” [1], as maximum age is measured regardless, while natural mortality may be empiri-
cally estimated from growth parameters and sea temperature [55,56].
In the first gap analysis it was also proposed that maximum economy of sampling should be
applied, and the biological material should be thoroughly studied across their biological char-
acteristics, specifically in cases of protected or vulnerable species [1]. Nevertheless, there is an
issue emerging from this, as working with a single specimen produces sparse data with “very
little potential for analysis” and a “limited insight into its biology” [51]. However, a recently
developed approach [57] can be used to estimate LWR for data-poor species, based on a Bayes-
ian hierarchical method. This method categorizes species in body shape groups and uses the
prior knowledge and existing LWR studies of well-studied species to derive species-specific
LWR parameters for data-deficient species that belong to the same group. The above-men-
tioned method was recently used to estimate the parameters of LWR of uncommon Mediterra-
nean sharks and rays, utilizing even single records, thus making the examination of single
records valuable [58].
The knowledge gap in the chondrichthyan life-history traits has been acknowledged [17]
and efforts have been made for it to be narrowed (Tables 3and 5). Reviews have been
Fig 3. Percentages of deep-sea and non deep-sea Mediterranean fish species with studied biological characteristics (0–8). Dark blue colour: deep-sea species;
light blue colour: non-deep-sea species).
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published in the last five years regarding the large and overlooked group of chondrichthyans.
For example, a significant gap in the length-weight relationships of 46 uncommon sharks and
rays has been filled in using the valuable information from single records or few individuals
and producing values reported for the first time at a global scale [58]. A number of new proj-
ects and studies showcase the rigorous effort to fill in the knowledge gap in the chondrichthyan
fish species of the Mediterranean region. Boldrocchi et al. (2017) [59] have gathered bibliogra-
phy on the feeding ecology of the elusive great white shark (Carcharodon carcharias). Geraci
et al. (2021) [60] have published an extensive review on life history traits of the batoids (rays)
in the Strait of Sicily. In the paper by Karachle et al. (2020) [51] there is a literature review of
Table 5. Comparison between current and previously published records on the biology of selected Mediterranean fish species from Dimarchopoulou et al. (2017)
[1], based on the number of studied characteristics and the number of records per characteristic.
Species Common
Name
Family
IUCN
Commercial Value
Number of Rec. Present
study Present study
Number of Rec. Dimarchopoulou
etal. 2017 [1]
Number of Char. Present
study Present study
Number of Char. Dimarchopoulou
etal. 2017 [1]
Number of records per
characteristic_ Present study
Number of records per characteristic_
Dimarchopoulou et al. (2017) [1]
Species with commercial value
Epinephelus
costae
Goldblotch
grouper
Serranidae DD VH 10 7 3/8 3/8 6 LWR, 2 G, 2 Diet 4 LWR, 2 G, 1 Diet
Argyrosomus
regius
Meagre Sciaenidae LC M 956/8 3/8 3 Sp, 2 LWR, 1 G, 1 A, 1 Mat,
1 Diet
3 Spawn, 1 G, 1 Mat
Lepidotrigla
dieuzeidei
Spiny gurnard Triglidae LC VH 10 16/8 1/8 3 LWR,2 G,2 A,1 Sp,1 Mat,
1 Fec
1 LWR
Non-commercial species
Blennius ocellaris Butterfly
blenny
Blenniidae LC NA 10 83/8 2/8 7 LWR, 2 Sp, 1 Diet 6 LWR, 2 Sp
Callionymus lyra Dragonet Callionymidae LC NA 2 2 2/8 2/8 1 LWR, 1 Sp 1 LWR, 1 Sp
Species with atypical life strategies
Squatina
aculeata
Sawback
angelshark
Squatinidae CR M 16 35/8 3/8 6 LWR,3 Sp,3 Mat,3 Fec,1
Diet
1 Sp, 1 Mat, 1 Fec
Aetomylaeus
bovinus
Bull ray Myliobatidae CR M 11 16/8 1/8 4 LWR,2 Sp,2 Diet,1 G,1 A,
1 Fec
1 Fec
Pterois miles Devil firefish Scorpaenidae LC NA 12 07/8 0/8 3 Diet,2 LWR,2 Sp,2 Mat,1
G,1 A,1 Fec
Protected species
Mobula mobular Devil fish Mobulidae EN NA 401/8 0/8 4 LWR
Carcharodon
carcharias
Great white
shark
Lamnidae VU L 502/8 0/8 3 Diet,2 LWR
Note: The commercial value (Val) is shown as price category (VH: very high; H: high; M: medium; L: low) and the protection status (IUCN) as IUCN Red List status
category (LC: least concern; EN: endangered; DD: data deficient; NE: not evaluated; NT: near threatened; VU: vulnerable; CR: critically endangered). The differences
between the results of the present study and the Dimarchopoulou et al. (2017) [1] study are highlighted in bold. For the complete table, the reader may refer to the (S4
Table).
LWR: length-weight relationships; G: growth parameters; A: lifespan; Mat: length at maturity; Sp: onset and duration of spawning; Fec: fecundity; M: mortality; Diet:
feeding preferences
https://doi.org/10.1371/journal.pone.0277383.t005
_
_
_
_
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Fig 4. The relationship between the number of records and total catches (in metric tons), maximum reported
somatic length (cm) and trophic level, for the Mediterranean fish species with at least one studied biological
characteristic and with available data on total catches (n = 125), length measurements (n = 451) and calculated
trophic level (n = 452). All variables were logarithmically transformed to improve visibility of the figure.
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the near threatened sharpnose sevengill shark (Heptranchias perlo). In addition to these
reviews, there is also a new data collection project, the Mediterranean Large Elasmobranchs
Monitoring Program (MEDLEM), aiming to contribute to biodiversity management and con-
servation of the Mediterranean basin [61]. Chondrichthyans of the Mediterranean Sea have
been described as “Endangered”, “Vulnerable” and “Data deficient” by the IUCN Shark Spe-
cialist Group [17]. Data Deficient species are excluded from regional priority species lists,
which impedes a much-needed status improvement for these species [18]. Although there
have been attempts to predict the conservation status of Data Deficient chondrichthyan species
through models [18], data collection for this group of species should still be a top priority for
future research effort.
Another group of vulnerable Mediterranean fish species lacking data, is the deep-sea species
group (Fig 3). Deep-sea fish show remarkable adaptations to this low productivity, low temper-
ature and high-pressure environment [62]. These environmental conditions are tied to fish life
history characteristics such as slow growth, delayed maturity and high maximum age [24],
which affects negatively their resilience against fishing pressure [5,63]. Our results confirm
the findings of other studies [23] stressing out the knowledge gap regarding this particular
group of fish in the Mediterranean region. As our view on these species remains obscure,
exploitation of deep-sea fish stocks [64] and other deep-sea services intensifies [27], causing
habitat disturbance, biodiversity loss [26,65] and exponential decline in deep-sea functioning
[66]. The Marine Strategy Framework Directive (MSFD 2008/56/EC) was EU’s Integrated
Maritime Policy plan to reach a good environmental status in the EU’s marine ecosystems and
to protect marine socio-economic resources. Although at first, the implementation of this
directive was focusing on coastal areas, currently, it is migrating towards the deep Mediterra-
nean Sea, to establish an ecosystem-based approach for its management. The approach is
based on 11 qualitative descriptors, with the first one being “biodiversity” and requiring knowl-
edge over “the population demographic characteristics of the species”, meaning maximum age
and fecundity, among other parameters [67]. Bridging the knowledge gap in Mediterranean
deep-sea species will help in the establishment of a management strategy for the protection of
the deep basin, and the eco-friendly blue growth of the Mediterranean countries [22,68,69].
Biological invasions from the Red Sea and the Indo-Pacific Ocean are a well-documented
phenomenon [70], with the influx of species from the Suez Canal growing each year [34] and
threatening Mediterranean biodiversity. This threat is amplified by climate change, as the ther-
mophilic species arriving from the Red Sea spread faster due to the rising water temperature
[71,72]. The impacts that non-indigenous species have on local ecosystems vary from preying
on native species and competing against them over the natural resources [39], to population
losses and extinctions on a local scale [73]. Non-indigenous species impacts also affect societies
on a socioeconomic level, as they introduce toxins and pathogens to the food webs, thus threat-
ening aquaculture and fisheries and consequently human health [36,39]. The current study
showed that non-indigenous fish species are significantly less studied than any other group in
this paper (chondrichthyans, deep-sea species or the whole of Mediterranean marine fish spe-
cies; Figs 2and 3). Although there have been several papers and reports that detail the sightings
of non-indigenous fish species in the Mediterranean Basin [74], the effort to document their
biological characteristics doesn’t appear to be as intense, probably due to the laborious nature
of collecting data on biological characteristics. Nevertheless, it would be a good first step if, at
least LWR was documented for the non-indigenous fish species of the Mediterranean, as this
is the least demanding biological characteristic to obtain [47,48]. The incomplete, inaccurate
and uncertain data on these species’ biological characteristics, abundances, distributions etc.,
hinder any management attempt [34,41].
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It seems that an ongoing, simultaneous effort is being made regarding species of the Medi-
terranean marine biodiversity once neglected. The Mediterranean international trawl survey
(MEDITS) programme [75], the EU Biodiversity Strategy for 2030 [76], the Mediterranean
Biodiversity Records [70], frequent stock assessments [29,77] and even citizen science projects
[78,79] are indicative of the newfound interest and intensive effort of scientists, stakeholders
and the public towards discovering, monitoring and managing marine biodiversity in the
Mediterranean region. Moreover, during record collection we also came across the data paper
by Albouy et al. (2015) [80] which contains extensive information on biological characteristics,
functional traits and the phylogeny of Mediterranean marine fishes, as well as their current
and projected distributions. Although this is undoubtedly a valuable source of data, such
papers and datasets containing raw data do not lie within the scope of this work and were
therefore not included in our analysis. There are indeed multiple sources of information that
could help fill in the gap of knowledge, but in the initial and current gap analysis, we focused
on information that was derived from published papers found in FishBase [42], the largest
database for fish, and the global scientific search engine SCOPUS.
To prioritize the actions needed to be taken, is key to any successful management strategy
[81], especially when resources for management are limited [82]. In character with effectively
targeting the identified gaps, the geographical and temporal aspect should also be taken into
consideration. There have been efforts to identify existing data gaps of all Mediterranean
marine databases, in an attempt to use them as a guide in the prioritization process [83]. The
biological characteristics of fish species can be useful in this process, for example, data regard-
ing the spawning period of species could provide temporal insight. Additionally, the subre-
gional patterns detected would give spatial insight, as they revealed that the western and
eastern Mediterranean are leading forces in biological data collection (Fig 1.bottom row),
while the central part of the Mediterranean is sparse on information.
Conclusions
The demand for an informed and unanimously shared management strategy of the Medi-
terranean Sea is urgent, as the understudied aspects of the its biodiversity need to be revis-
ited within the framework of climate change, overfishing and the constant influx of new
invasive species from the Red Sea [84]. Basic biological data are essential for future models,
projections and scenario testing, priority species lists, and vulnerability assessments, all of
which facilitate the understanding of the functioning of Mediterranean ecosystems and
their response to change and can, therefore, inform ecosystem-based planning and manage-
ment for the basin as a whole. Hence, it is encouraging that the gap in the biological knowl-
edge of Mediterranean marine fish species has been reduced during the past few years, with
new studies producing first records of species characteristics in the Mediterranean area and
even globally.
In this pursue of data and knowledge, we recommend that the scientific society prioritizes
identified gaps on key species over the well-studied commercial species, namely chondrichth-
yans and deep-sea fish species, as well as the infamous non-indigenous fish species. Addition-
ally, the expansion of citizen science projects is commendable both for raising public
awareness on ecological issues and for producing truly useful data [85,86]. Finally, employing
new methods of producing data through non-traditional sampling [3,57,58] is a practice
aligned with the spirit of economy and efficacy, which is a notion embraced by both the initial
and the present gap analysis.
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Supporting information
S1 Table. List of the most studied fish species in the Mediterranean Sea based on the num-
ber of records (Number of Rec.), the number of studied characteristics (Number of Char.)
and the number of records per characteristic (LWR: Length-weight relationships; G:
Growth parameters; A: Lifespan; Mat: Length at maturity; Sp: Onset and duration of
spawning; Fec: Fecundity; M: Mortality; Diet: Feeding preferences). The commercial value
(Val) is shown as price category (VH: very high; H: high; M: medium; L: low) and the protec-
tion status (IUCN) as IUCN Red List status category (LC: least concern; EN: endangered; DD:
data deficient; NE: not evaluated; NT: near threatened; VU: vulnerable; CR: critically endan-
gered). We included all the species with 8/8 and 7/8 studied characteristics and only the species
with 6/8 studied characteristics and more than 40 records.
(DOCX)
S2 Table. List of all the Mediterranean chondrichthyan species with or without studies on
their biological characteristics. (LWR: length-weight relationships; G: growth parameters; A:
lifespan; Mat: length at maturity; Sp: onset and duration of spawning; Fec: fecundity; M: mor-
tality; Diet: feeding preferences). The commercial value (Val) is shown as price category (VH:
very high; H: high; M: medium; L: low) and the protection status (IUCN) as IUCN Red List
status category (LC: least concern; EN: endangered; DD: data deficient; NE: not evaluated; NT:
near threatened; VU: vulnerable; CR: critically endangered).
(DOCX)
S3 Table. List of some of the least studied families of non-commercial fish species in the
Mediterranean Sea based on the number of studied characteristics and the number of rec-
ords per characteristic. The commercial value (Val) is shown as price category (VH: very high;
H: high; M: medium; L: low) and the protection status (IUCN) as IUCN Red List status category
(LC: least concern; EN: endangered; DD: data deficient; NE: not evaluated; NT: near threatened;
VU: vulnerable; CR: critically endangered). For all the species of this table, commercial value is
not known (“NA”) and both the number of records and characteristics are equal to 0.
(DOCX)
S4 Table. Comparison between current and previously published records on the biology of
selected Mediterranean fish species (Dimarchopoulou et al., 2017) [1], based on the num-
ber of studied characteristics and the number of records per characteristic (LWR: Length-
weight relationships; G: Growth parameters; A: Lifespan; Mat: Length at maturity; Sp:
Onset and duration of spawning; Fec: Fecundity; M: Mortality; Diet: Feeding preferences).
The commercial value (Val) is shown as price category (VH: very high; H: high; M: medium;
L: low) and the protection status (IUCN) as IUCN Red List status category (LC: least concern;
EN: endangered; DD: data deficient; NE: not evaluated; NT: near threatened; VU: vulnerable;
CR: critically endangered). The differences between the results of the present study and the
Dimarchopoulou et al. (2017) [1] study are highlighted in bold.
(DOCX)
S1 Appendix. The number of records per characteristic per species gathered from litera-
ture published from 2015 to 2021 and the total number of records per characteristic per
species up until 2021.
(XLSX)
S2 Appendix. List of literature sources screened in the present study.
(XLSX)
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Acknowledgments
We would like to thank the two reviewers for their comments which have greatly improved
the manuscript.
Author Contributions
Conceptualization: Eva Daskalaki, Donna Dimarchopoulou, Athanassios C. Tsikliras.
Data curation: Eva Daskalaki, Evangelos Koufalis.
Formal analysis: Eva Daskalaki, Evangelos Koufalis.
Methodology: Eva Daskalaki, Donna Dimarchopoulou.
Resources: Athanassios C. Tsikliras.
Supervision: Athanassios C. Tsikliras.
Writing – original draft: Eva Daskalaki.
Writing – review & editing: Evangelos Koufalis, Donna Dimarchopoulou, Athanassios C.
Tsikliras.
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