Reconstructing the Historical Density, Size, and Age Structure of the Noble Pen Shell (Pinna nobilis) Population: Insights from Malo Jezero Lagoon, Mljet National Park (Adriatic Sea)

Abstract

The noble pen shell (Pinna nobilis) is a key bivalve species found in the Mediterranean that has suffered dramatic declines due to mass mortality events (MMEs) caused by pathogens like Haplosporidium pinnae. This study looks at the historical population structure of P. nobilis in Malo jezero, a coastal lagoon in Mljet National Park, Croatia, using data collected before the MME. During a field survey in 2018, data on the population density, size, and age of 3800 individuals, using a grid-based transect method, were collected. The population density ranged from 7.50 to 55.83 individuals per 100 m², with an average of 25.42 individuals per 100 m², over 11 520 m², reflecting a high abundance compared to other populations. All individuals were mature, with no juveniles or signs of recent recruitment. The estimated ages ranged from 8.00 to 44.34 years, with 20 individuals exceeding the expected maximum size. The population was comprised of older individuals, making it vulnerable to sudden events, due to a lack of younger individuals. The isolation of Malo jezero may limit larval exchange with other populations, contributing to recruitment challenges. This study provides important information for understanding P. nobilis populations and supports the potential of Malo jezero for future conservation and reintroduction efforts.

Full text

Academic Editor: Michele Mistri
Received: 20 January 2025
Revised: 15 February 2025
Accepted: 22 February 2025
Published: 25 February 2025
Citation: ˇ
Cižmek, H.; ˇ
Coli´c, B.;
Zubak ˇ
Cižmek, I. Reconstructing the
Historical Density, Size, and Age
Structure of the Noble Pen Shell (Pinna
nobilis) Population: Insights from Malo
Jezero Lagoon, Mljet National Park
(Adriatic Sea). Water 2025,17, 663.
https://doi.org/10.3390/w17050663
Copyright: © 2025 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license
(https://creativecommons.org/
licenses/by/4.0/).
Article
Reconstructing the Historical Density, Size, and Age Structure
of the Noble Pen Shell (Pinna nobilis) Population: Insights
from Malo Jezero Lagoon, Mljet National Park (Adriatic Sea)
Hrvoje ˇ
Cižmek 1, * , Barbara ˇ
Coli´c 1and Ivana Zubak ˇ
Cižmek 1,2
1Marine Explorers Society 20000 Leagues, 23000 Zadar, Croatia; bcolic@drustvo20000milja.hr (B. ˇ
C.);
izubak@unizd.hr (I.Z. ˇ
C.)
2Department of Ecology, Agronomy, and Aquaculture, University of Zadar, 23000 Zadar, Croatia
*Correspondence: hrvoje@drustvo20000milja.hr
Abstract: The noble pen shell (Pinna nobilis) is a key bivalve species found in the Mediter-
ranean that has suffered dramatic declines due to mass mortality events (MMEs) caused by
pathogens like Haplosporidium pinnae. This study looks at the historical population structure
of P. nobilis in Malo jezero, a coastal lagoon in Mljet National Park, Croatia, using data
collected before the MME. During a field survey in 2018, data on the population density,
size, and age of 3800 individuals, using a grid-based transect method, were collected. The
population density ranged from 7.50 to 55.83 individuals per 100 m
2
, with an average of
25.42 individuals per 100 m
2
, over 11 520 m
2
, reflecting a high abundance compared to other
populations. All individuals were mature, with no juveniles or signs of recent recruitment.
The estimated ages ranged from 8.00 to 44.34 years, with 20 individuals exceeding the
expected maximum size. The population was comprised of older individuals, making it
vulnerable to sudden events, due to a lack of younger individuals. The isolation of Malo
jezero may limit larval exchange with other populations, contributing to recruitment chal-
lenges. This study provides important information for understanding P. nobilis populations
and supports the potential of Malo jezero for future conservation and reintroduction efforts.
Keywords: age; density; noble pen shell; population structure; Pinna nobilis; recruitment; size
1. Introduction
The noble pen shell (Pinna nobilis L.) is a large bivalve species endemic to the Mediter-
ranean Sea, found on sandy and muddy sediments and within seagrass ecosystems, partic-
ularly Posidonia oceanica and Cymodocea nodosa meadows, where it plays a critical ecological
role that involves filtering water, stabilizing sediments, and providing a habitat for other
organisms, which enhances biodiversity and habitat complexity [
1
–
3
]. This species is an
integral part of these ecosystems and serves as a sentinel for environmental health, due to
its sensitivity to habitat changes [4].
The population density of P. nobilis varies significantly across the Mediterranean,
especially within lagoon environments, showing a wide range of densities and patchy
distribution [
5
–
7
], and references therein]. In some regions, these densities are exceptionally
high, indicating the capacity of some habitats to sustain dense populations. Some of the
highest population densities were reported in Tunisia, 56 ind/100m
2
[
5
], and France,
70 ind/100m2[7].
Along the Mediterranean and Adriatic coast, P. nobilis has been documented to reach
heights of up to 120 cm, making it one of the largest Mediterranean bivalves [
8
]. Individuals
Water 2025,17, 663 https://doi.org/10.3390/w17050663

Water 2025,17, 663 2 of 12
can live for over 20 years and exhibit rapid early growth, with subsequent slowing as they
age [
2
,
9
]. Based on long-term monitoring data on their estimated total height, individuals
aged 45–50 years have been reported [6].
Size (and age) population structure differ across the Mediterranean basin. Most popu-
lations show unimodal [
2
] or multimodal [
10
] size distributions, but some were reported to
have bimodal distributions [
3
], indicating recruitment issues or environmental pressures,
suggested by the absence of certain size and/or age categories. For example, surveys
conducted in the Mar Menor lagoon between 2014 and 2019 revealed an absence of indi-
viduals smaller than 10 cm, suggesting recruitment failure or high predation during early
life stages [
11
]. Populations in Sardinia were also found to be dominated by medium-sized
individuals, with smaller size classes absent, pointing again to recruitment limitations [
12
].
The health status of P. nobilis populations depends on favorable environmental factors,
such as water quality, substrate type, and recruitment rates; therefore, the average size
distribution of this population typically includes a wide range of size classes, reflecting
successful recruitment over multiple years [
1
]. Unfortunately, that is not always the case.
P. nobilis populations have experienced sharp declines due to habitat loss, pollution, and
illegal harvesting, over the last few decades [
1
,
13
]. Since 2016, mass mortality event (MMEs),
most likely caused by Haplosporidium pinnae and Mycobacterium sp. infections, have further
devastated populations, with mortality rates reaching up to 100% in some areas [
14
–
16
].
This has brought P. nobilis to the brink of extinction and resulted in its classification as
Critically Endangered by the IUCN in 2019 [17].
While much research has focused on documenting these declines, less attention has
been paid to the historical population structures of P. nobilis, particularly in enclosed and
semi-enclosed environments. Such habitats are characterized by restricted water exchange
with the open sea, resulting in unique environmental conditions that shape population
dynamics [
18
]. Understanding the historical population structure of a species on the verge
of extinction might be the key to tracking ecological trends and evaluating the resilience of
P. nobilis populations to environmental change.
This study examines the historical population structure of P. nobilis in Malo jezero, a
coastal lagoon within Mljet National Park, Croatia. Using data collected before the MME,
we analyze the population’s density, size distribution, and age structure. The goal is to
provide a baseline demographic profile for this distinctive ecosystem and to understand
how the population was structured before its collapse. The results might help to assess the
suitability of the lagoon as a conservation sanctuary, identify potential threats, and inform
management authorities.
2. Materials and Methods
2.1. Study Location
The research was conducted during July and August 2018 in Malo jezero lagoon in
Mljet National Park (Figure 1). Malo jezero is the smaller of two lagoons, formed in a
karstic depression during the post-glacial period [
19
]. The bigger lagoon, Veliko jezero, is
connected to the open sea through a 2.5 m deep and 10 m wide strait and to Malo jezero
through a 0.5 m deep and 30 m long channel. The surface of Malo jezero is 0.24 km
2
, with a
maximum depth of 29.4 m. During warmer periods, the lake exhibits strong stratification,
with a pronounced thermocline, leading to oxygen depletion in deeper layers [
19
]. The
average monthly sea temperature measured at a depth of 80 cm during the survey period
was 25.8
±
0.2
◦
C in July and 27.6
±
0.2
◦
C in August 2018. Veliko jezero and Malo jezero
are no-take zones within the boundaries of Mljet National Park.

Water 2025,17, 663 3 of 12
Figure 1. Geographic location of the study site in Malo jezero, Mljet island, Adriatic Sea.
2.2. Sampling Design and Density Measurement
Due to the high density of P. nobilis individuals spread over the large area within the
lagoon, a grid-based transect system was used. Eight identical grids covering a total area
of 11,520 m
2
were used. Each grid covered an area of 48 m by 30 m (1440 m
2
) and consisted
of six parallel strip transects. Each transect was 30 m long and 4 m wide on either side
(240 m2). A rope of four meters tied to the central line was used to delimit the boundaries
of the strip, following the protocol established in [
20
]. Using the grid, the avoidance of any
individuals was reduced to a minimum. During the research, the grid was repositioned by
a team of four SCUBA divers, so that at least two points of the square remained fixed at
every reposition. The square points were georeferenced with the GPS Garmin GPSMAP
64x, Software version 4.20.
Surveys were conducted by trained SCUBA divers, operating in pairs, with each pair
assigned to a transect. Divers recorded all visible P. nobilis individuals along the transect
lines, measured the pinnid biometrics to the nearest centimeter (1 cm) using calipers, and
recorded the position on the transect for spatial analysis. The P. nobilis population density
was calculated for each transect by dividing the total number of individuals observed
by the transect area (240 m
2
) and standardizing the results according to the number of
individuals per 100 m2.
The total abundance of Pinna nobilis in Malo jezero was estimated and modeled by
multiplying the average population density (individuals per 100 m
2
) by the surface area
of the lagoon within the depth range of 2–15 m. This depth range reflects the habitat
suitability for P. nobilis, based on previous observations wherein the species had been
recorded at depths of up to 15 m [
2
], and the recorded depths from this study (2–8 m).
This expanded depth range was used to ensure comparability with earlier findings and to
provide a comprehensive estimate of population abundance.
2.3. Size and Age Estimation
According to the established protocol [
20
], the height above the sediment (H
s
), maxi-
mum width (W), and width (w) at the sediment level were measured. For the estimated
total height (Ht) of individuals in Malo jezero lagoon, a formula from [21] was used:
Ht= 1.18W + 40.39.

Water 2025,17, 663 4 of 12
Compared to other models, this simple linear regression provides an accurate predic-
tion of the overall height of the individuals.
A growth curve for P. nobilis was generated using the von Bertalanffy growth equation:
Ht= Ht∞(1 −e−kt);
where H
t∞
is the value of the maximum theoretical total height for this population (72.31 cm),
and k is the slope of the line (0.16), calculated for Adriatic populations of P. nobilis [
2
], and
used here to ensure regional specificity.
Using this model and the size–age relationship established in 2003 [
2
], we estimated the
ages of P. nobilis individuals based on their shell height. The equation defines a theoretical
maximum age of 44.34 years, beyond which age estimates are unreliable. Individuals
exceeding this range were excluded from the statistical analysis of age, but they were
included in the histograms to provide a complete overview of the population structure.
2.4. Statistical Analysis
The data were organized and processed using MS Excel for Mac (Version 16.93.1).
The statistical analyses were conducted using the R programming language (ver. 4.3.3,
Angel Food Cake) [
22
] and R Studio software (ver. 2023.12.1, Ocean Storm) [
23
]. Statistical
significance was assessed at an alpha level of 0.05.
The average depth data did not follow the normality assumptions (Shapiro–Wilk W = 0.92,
p= 0.003), so Spearman’s rank correlation was used to assess the relationship between the
average depth and the P. nobilis density for each transect.
Geospatial analyses, including creating heatmaps, geographical maps, and sur-
face models, were performed using QGIS (ver. 3.22.5, Białowie ˙
za) [
24
]. All the
data and protocols associated with this study are available upon request from the
corresponding author.
This study complied with all the ethical guidelines and regulations for field research, and
permissions were granted by the Ministry of the Sea, Transport, and Infrastructure in 2018.
3. Results
A total of 3800 Pinna nobilis individuals were recorded across 48 transects, each cover-
ing an area of 240 m
2
. Among these, 483 individuals were dead, indicating a mortality rate
of 13%. All further calculations were based solely on living individuals.
3.1. Density
The density values were standardized to the number of individuals per 100 m
2
. Across
the transects, the density ranged from 7.50 to 55.83 individuals per 100 m
2
, with a mean of
25.42 individuals per 100 m2(SD = 11.83) and a median of 25.00 individuals per 100 m2.
Higher densities were observed in transects located at greater depths (Figure 2), partic-
ularly in the central sections of Malo jezero (e.g., transect 7E, 55.83 individuals per 100 m
2
).
Spearman correlation analysis confirmed a significant positive relationship between depth
and density (rs = 0.70, p< 0.001, n = 48) (Figure 3). As shown in Table 1, transects exceeding
5 m in depth consistently exhibited greater population densities than shallower areas.
Figure 2provides a heatmap visualization of the density distribution, highlighting the
spatial variation observed across the study site.
Given the mean density of 25.42 individuals per 100 m
2
and the total surface area of
Malo jezero within the modeled depth range of 2 to 15 m (0.123 km
2
), the total abundance
of P. nobilis in this coastal lagoon at the time of the study was estimated to be approximately
31,266 individuals.

Water 2025,17, 663 5 of 12
Figure 2. Heatmap of Pinna nobilis density across transects in Malo jezero, expressed as the number
of individuals per 100 m2.
Figure 3. Spearman correlation analysis between the depth (m) and Pinna nobilis density (ind./100 m
2
)
in Malo jezero. The trendline represents the fitted linear regression with a significant positive
correlation (rs = 0.70, p< 0.001, n = 48).
Table 1. Transect-level data showing Pinna nobilis counts, densities (ind/100 m
2
), and average depths
recorded in Malo jezero, Mljet National Park. Transect areas are standardized to 240 m2.
Transect Code Count (per 240 m2) Density (ind/100 m2)Average Depth (m)
1A 42 17.5 2.4
1B 33 13.75 2.6
1C 38 15.83 2.5
1D 24 10 2.9
1E 55 22.92 3.1
1F 39 16.25 3.2
2A 56 23.33 2.8

Water 2025,17, 663 6 of 12
Table 1. Cont.
Transect Code Count (per 240 m2) Density (ind/100 m2)Average Depth (m)
2B 65 27.08 3.0
2C 77 32.08 3.0
2D 87 36.25 3.0
2E 75 31.25 3.4
2F 61 25.42 4.0
3A 22 9.17 2.0
3B 46 19.17 2.0
3C 37 15.42 3.0
3D 40 16.67 3.5
3E 61 25.42 4.0
3F 45 18.75 4.0
4A 19 7.92 2.5
4B 18 7.5 2.5
4C 23 9.58 3.3
4D 28 11.67 3.7
4E 39 16.25 3.5
4F 37 15.42 5.5
5A 115 47.92 4.4
5B 122 50.83 4.2
5C 88 36.67 3.5
5D 97 40.42 3.2
5E 92 38.33 3.7
5F 74 30.83 2.5
6A 87 36.25 5.0
6B 88 36.67 5.4
6C 79 32.92 5.0
6D 66 27.5 5.0
6E 97 40.42 5.0
6F 91 37.92 4.5
7A 69 28.75 6.0
7B 104 43.33 6.2
7C 78 32.5 6.3
7D 93 38.75 7.0
7E 134 55.83 7.0
7F 91 37.92 7.2
8A 120 50 6.0
8B 87 36.25 5.2
8C 110 45.83 6.0
8D 105 43.75 7.0
8E 110 45.83 7.2
8F 104 43.33 7.0

Water 2025,17, 663 7 of 12
3.2. Size and Age Distribution
The measured shell widths ranged from 10.0 to 33.0 cm, with a mean of 20.41 cm
(SD = 2.20) and a median of 20.0 cm (Table S1).
The estimated total shell height, calculated using the width-to-height equation [
21
],
ranged from 52.19 to 79.33 cm, with a mean height of 64.48 cm (SD = 2.60) and a median of
63.99 cm (Table S1; Figure 4).
Figure 4. Histogram of Pinna nobilis shell heights recorded in Malo jezero.
The estimated ages of P. nobilis individuals in Malo jezero were calculated using
the size–age equation [
2
]. The resulting growth curve (Figure 5) illustrates the modeled
relationship between shell height and age. It reveals rapid growth during the first decade
of life, with growth rates slowing as individuals approach the maximum theoretical height.
The population’s age ranged from 8.00 to 44.34 years. The mean estimated age was
14.29 years (SD = 2.98), and the median age was 13.51 years. Based on the size–age equation,
20 individuals exceeded the theoretical maximum shell height of 72.31 cm predicted by
this growth model, with measured heights ranging from 73.43 cm to 79.33 cm. These
exceptionally large individuals could not be aged accurately using the model and were
likely to be older than 44.34 years.
Figure 5. Growth curve for Pinna nobilis in Malo jezero based on the adapted von Bertalanffy growth
equation [2].

Water 2025,17, 663 8 of 12
Figure 6provides a histogram of the age distribution for individuals included in the
analysis, with outliers visually represented to highlight their distinctiveness within the
population structure.
Figure 6. Histogram of estimated ages of Pinna nobilis individuals in Malo jezero.
4. Discussion
This study provides a retrospective look at the historical population structure, size,
and age distribution of the noble pen shell, Pinna nobilis, in Malo jezero, Mljet MPA. It
offers a baseline for understanding its ecological characteristics and informing future
conservation strategies.
4.1. Density
The mean density (25 ind/100 m
2
over a large area of 11,520 m
2
) recorded during
this research is higher than previously published for Malo jezero [
2
,
25
] or the rest of the
Adriatic Sea, according to which previous reports indicate a mean density of approximately
10 ind/100 m
2
[
8
]. In a systematic review from 2015 [
26
], 24 papers on population density
were identified, and the average population density of 9.78 ind/100 m
2
in the Mediter-
ranean was calculated. The authors noted high and statistically significant variability
among different ecoregions. Moreover, they detected high variability in the methodology,
which is important to consider given the species’ patchy distribution. The high mean
abundance over a large area (and not only isolated patches), in the present study, indeed
indicates an unusually high abundance of P. nobilis in Malo jezero, compared to other
Mediterranean regions.
The observed positive correlation between depth and P. nobilis density (Figure 3)
within the studied depth range (2–8 m) aligns with findings in previous works on
Malo jezero. In 2002, the highest densities were found between a depth of 5 and
10 m [
25
], and high densities were reported in 2003, in Cymodocea nodosa meadows, at
depths of 7–8 m [
2
]. It also corroborates the findings from Tunisia in 2010, where the density
increased with depth, within the studied depth range (0–6 m) [
5
]. These patterns highlight

Water 2025,17, 663 9 of 12
the importance of habitat structure and depth as key drivers of P. nobilis distribution across
the Mediterranean.
4.2. Size and Age Distribution
Given the mean total height of 64.48 cm, the results reveal the presence of many mature
individuals, with no evidence of juveniles or recent recruitment (Figure 4). Moreover, as
previously recorded in Malo jezero for the same population, an individual whose total
height was as much as 84 cm was recorded [
21
]. This specimen was dead and, therefore,
was not included in the current analysis.
The age range from 8.00 to 44.34 years indicates the absence of recent recruitment and
reflects a population dominated by mature individuals. Similar patterns were observed in
other Mediterranean locations, such as Port-Cros National Park, where an extreme case
of a lack of recruitment for more than 40 years was observed [
6
]. Other authors have also
reported the lack of juveniles in populations of P. nobilis [
11
,
12
]. Interestingly, in 1980,
no P. nobilis individuals were recorded in Malo jezero [
27
], yet a dense population was
documented in 1996, with juveniles observed in 1998 [
28
]. Later, juveniles were recorded
in 1998 and 2000, and recruitment and growth patterns for P. nobilis in Malo jezero were
documented [
2
]. However, between 2000 and 2018, there was no recruitment, and no
juveniles were observed in the present study. The reasons for this lack of recruitment are
unclear, but one possible explanation might be that the hydrodynamic isolation of the
lagoon is a barrier to larval exchange with open populations [
18
]. Limited connectivity with
the open sea restricts both horizontal and vertical water circulation, further reducing larval
dispersal and exchange with external populations [
29
]. The prolonged larval retention
time in Malo jezero, estimated to be 160–185 days, suggests that while some larvae may
have settled within the lake, the restricted water exchange likely limited both genetic
diversity and the influx of new recruits from external populations, further exacerbating the
observed recruitment failure [
18
]. Another factor contributing to the existence of the aging
population could be the threshold “refuge size”, where individuals above a certain size
are less vulnerable to predation and environmental stressors and remain in the population
when recruitment is absent [30].
The maximum estimated age of 44.34 years falls within the upper limit of the docu-
mented age for P. nobilis [
6
]. The presence of 20 exceptionally large and old individuals
(>72.31 cm), for which the age could not be estimated, together with 13 individuals with
a total height larger than 72 cm (older than 44 years), but within the model boundaries,
accounts for approximately 1% of the total population that has reached beyond the theo-
retical maximal height. This is in contrast with research from 2020, which discovered that
local hydrodynamism is usually lower in enclosed environments and supports populations
in which the theoretical maximal height is higher than the observed maximal height [
31
].
The individuals recorded in our study could represent a subset of the population, with
growth characteristics not accounted for in previous models (Table S1; Figure 6). This result
underscores the uniqueness of our population and highlights the potential for variability in
growth in specific environmental conditions.
Following the MME that began in 2016, no living P. nobilis individuals have been
observed in Malo jezero. Surveys conducted after 2019 have confirmed 100% mortality,
leaving only empty shells, mirroring the widespread collapse of the species across the
Mediterranean [
14
,
15
,
17
]. The enclosed nature of Malo jezero, which historically may have
contributed to high population densities, likely became a disadvantage following the MME,
as limited water exchange restricts larval dispersal and natural recolonization. Similar
challenges have been documented in other enclosed environments, such as the Mar Menor
lagoon [
11
], reinforcing the need for targeted conservation strategies. The pre-MME data

Water 2025,17, 663 10 of 12
serve as a crucial reference for potential reintroduction and restoration efforts, which can
aid in understanding population dynamics and support efforts to develop strategies for
reintroduction and restocking [32].
5. Conclusions
This study provides a rare insight into a now-extinct population, highlighting both
the past ecological success and the fragility of P. nobilis in the face of widespread environ-
mental threats. Many individuals, a high population density, and the lack of juveniles or
smaller individuals observed in Malo jezero before the MME highlight the vulnerability of
P. nobilis populations when there is no recruitment. Populations consisting mostly of old
individuals are especially at risk from sudden events, as they lack the younger generations
needed to recover and sustain the population. The enclosed nature of the lake, which
may have historically supported high population densities, ultimately became a barrier to
recolonization following the collapse of the population.
Our study offers valuable insights into the historical population structure of P. nobilis.
Based on the legal protection of the site (a no-take zone within the Mljet National Park)
and the sizes and ages reached, Malo jezero could be a favorable location for potential
reintroduction efforts in the future.
The lessons learned from Malo jezero can inform conservation efforts aimed at
safeguarding any remaining populations and, potentially, restoring P. nobilis to its
former habitats.
Supplementary Materials: The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/w17050663/s1, Table S1: Individual measurements, estimated
lengths, and ages of Pinna nobilis recorded in Malo jezero.
Author Contributions: Conceptualization, H. ˇ
C.; methodology, H. ˇ
C. and B. ˇ
C.; validation, H. ˇ
C., B. ˇ
C.
and I.Z. ˇ
C.; formal analysis, H. ˇ
C., B. ˇ
C. and I.Z. ˇ
C.; investigation, H. ˇ
C. and B. ˇ
C.; resources, H. ˇ
C.; data
curation, H. ˇ
C. and I.Z. ˇ
C.; writing—original draft preparation, H. ˇ
C. and I.Z. ˇ
C.; writing—review and
editing, H. ˇ
C., B. ˇ
C. and I.Z. ˇ
C.; visualization, H. ˇ
C. and I.Z. ˇ
C.; supervision, B. ˇ
C.; project administration,
H. ˇ
C.; funding acquisition, H. ˇ
C. All authors have read and agreed to the published version of the
manuscript.
Funding: This research received no external funding.
Data Availability Statement: The data are contained within the article.
Acknowledgments: This work was carried out with the operational support of the Operation Wallacea
organization and the Croatian Science Foundation as part of the ISLAND project (IP-2020-02-9524).
We thank all the students and staff involved in the Operation Wallacea mission at the Croatia marine
site and the staff at Mljet National Park.
Conflicts of Interest: The authors declare that there are no conflicts of interest. The research was
conducted in the absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
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