INTRODUCTION

Comparison of the anatomical characteristics

of organisms has been a key point in biological

research. Studies focused on taxonomic classifi-

cation of organisms have mainly been on the

characterization of body size and shape of indi-

viduals (Rohlf 1990; Adams et al. 2004). In the

case of gastropods, shell morphology has been

one of the most important features to identify

species and to understand phenotypic variation

within species (e.g. Trussell 2000; Hollander et

al. 2006; Conde-Padín et al. 2007, 2009). Several

methods have been used to analyze intra- and

inter-specific shell variation in morphology, but

traditional and geometric morphometrics have

MARINE AND FISHERY SCIENCES 33 (1): 53-68 (2020). https://doi.org/10.47193/mafis.3312020061803

MORPHOLOGICAL SHELL VARIATION OF Zidona dufresnei (CAENOGASTROPODA:

VOLUTIDAE) FROM THE SOUTHWESTERN ATLANTIC OCEAN

ALONSO I. MEDINA1, 2, MARÍA ALEJANDRA ROMERO1, 2, 3, AUGUSTO CRESPI-ABRIL3, 4

and MAITE A. NARVARTE1, 2, 3

1Escuela Superior de Ciencias Marinas, Universidad Nacional del Comahue (UNCo),

San Martín 247, San Antonio Oeste, Argentina

e-mail: alonsoim@gmail.com

2Centro de Investigación Aplicada y Transferencia Tecnológica en Recursos Marinos Almirante

Storni (CIMAS), Güemes 1030, San Antonio Oeste, Argentina

3Laboratorio de Oceanografía Biológica (LOBio), Centro para el Estudio de Sistemas Marinos

(CESIMAR), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET),

Blvd. Almirante Brown 2915, Puerto Madryn, Argentina

4Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia San Juan Bosco (UNPSJB),

Blvd. Almirante Brown 3051, Puerto Madryn, Argentina

ABSTRACT. The volutid gastropod Zidona dufresnei is an important fishery resource from the Southwestern

Atlantic Ocean. This species exhibits strong interpopulation differences in life history features, which lead to postulate

the existence of two morphotype (‘normal’ and ‘dwarf’). In this study, we combine and compare traditional and geo-

metric morphometrics to capture shell shape variation of Z. dufresnei among three populations from Mar del Plata

(37° S) to San Matías Gulf (42° S) to test the hypothesis that the phenotypic variation already described in the life cycle

and size is also expressed in the shell shape. Significant differences in the shell morphology among these three popu-

lations were detected, mainly associated to the maximum size of individuals and shell shape. The Bahía San Antonio

morphotype had shells with higher general roundness and weight compared to San Matías Gulf and Mar del Plata mor-

photypes, which were not differentiated. Our results support the hypothesis of Lahille (1895) who distinguished the

morphotype of Bahía San Antonio (‘dwarf’ morphotype) as Voluta angulata affinis. The functional significance of the

variability found is discussed in terms of the ecological and genetic effects on shape and size.

Key words: Marine gasteropod, shell variation, geometric morphometry, South Atlantic.

been the most frequent since the shell is rigid and

characterized by noticeable anatomical points

(Carvajal-Rodríguez et al. 2005; Marko 2005;

Guerra-Varela et al. 2009; Avaca 2010; Valladares

et al. 2010; Teso et al. 2011).

The volutid gastropod Zidona dufresnei (Dono-

van, 1823), known locally as ‘caracol fino’ (fine

snail) or ‘caracol atigrado’ (tabby snail), is distrib-

uted on the western coast of the South Atlantic

Ocean from Río de Janeiro, Brazil (22° S-42° W)

to Patagonian waters of San Matías Gulf, Argenti-

na (42° S-64° W) (Kaiser 1977; Rosenberg 2009).

This species lives on sandy or muddy bottoms

between the low intertidal zone and 200 m water

depth and exhibits a patchy distribution pattern

(Scarabino 1977; Pereyra et al. 2009; Medina et

al. 2015, 2016).

Z. dufresnei is one of the most important gas-

tropods which have been subject to fishing pres-

sure in Argentina and Uruguay with annual land-

ings ranging from 500 to 3,000 t (Fabiano et al.

2000; Giménez et al. 2005; Roche et al. 2013).

Similar to other volutid gastropods, their life his-

tory parameters (large body size and somatic pro-

duction, slow growth rate, late reproductive

maturity and direct development) make this

species extremely vulnerable to overexploitation

(Giménez and Penchaszadeh 2002; Giménez et

al. 2004; Medina et al. 2015, 2016). Further, the

occurrence of direct (intracapsular) development

and absence of a pelagic larval stage is usually

recognized as a factor preventing gene flow and

leading to genetic differentiation of allopatric

populations (e.g. Scarabino 1977; Darragh et al.

1998; Pereyra et al. 2009). Several studies report-

ed differences in the maximum size and weight of

individuals of Z. dufresnei along the geographical

distribution of the species, possibly due to differ-

ent environmental conditions (Pereyra et al.

2009; Medina et al. 2015). Particularly, two dif-

ferent populations were described in San Matias

Gulf based on the maximum size and weight of

mature individuals (Medina et al. 2015, 2016).

One of these populations, whose individuals

reach 230 mm long and 831 g in weight, inhabits

deep waters (between 35 and 130 m) inside the

gulf. The other population, with individuals

reaching 120 mm long and 113 g in weight is

located in shallow waters of the gulf (less than 2

m depth) (Medina et al. 2015) (Figure 1). These

differences led to postulate the existence of two

morphotypes: a ‘normal’ (from relatively deep

waters) and a ‘dwarf’ morphotype (from shallow

waters) (Lahille 1895). Even Lahille (1895)

referred to a small volutid identified as Voluta

angulata affinis, which would be a specimen

from the San Antonio Bay. Later, Clench and

Turner (1964) based on morphological characters

unified the variety V. angulata affinis with Z.

dufresnei leading to potential taxonomic incon-

sistencies related to the issue of whether these

morphotypes are subspecies or even separate

(cryptic) species.

Considering the high degree of morphological

variation reported for the species (Roche et al.

2013; Medina et al. 2015, 2016), Z. dufresnei

offers the opportunity to investigate morphologi-

cal pattern in heterogeneous environment.

Despite that, no studies have been conducted to

determine differences in shell shape since differ-

ences in shell size have already been investigated.

In this context, the aim of this study was to ana-

lyze the differences in shape between morpho-

types of Z. dufresnei using traditional and geo-

metric morphometrics approaches among and

within three populations distributed along the

Argentine Sea. The results obtained by both

methodologies were also compared. We tested the

hypothesis that the isolation among populations

favors a phenotypic variation expressed at the

shell shape level. Overall, these results are

expected to contribute to a better understanding

of the taxonomic status of Z. dufresnei, and thus

provide basic knowledge to achieve a sustainable

management of this fishing resource by designing

strategies that account for the variability between

local taxonomic units.

54 MARINE AND FISHERY SCIENCES 33 (1): 53-68 (2020)

MATERIALS AND METHODS

Study sites and samples collection

Individuals of Z. dufresnei were collected in

three locations along the Argentine Sea: Mar del

Plata (MDQ), San Antonio Bay (BSA) and San

Matías Gulf (GSM) (Figure 2), from 2007 to

2011. These locations were selected since they are

the only places where stable populations of this

species in Argentine waters were properly

described. At the same time, BSA population is

the only ‘dwarf’ morphotype population of Z.

dufresnei known so far. MDQ site was character-

ized by sandy bottom, mean salinity of 35, sea sur-

face temperature (SST) range of 9-17 °C, and

depth between 40 to 60 m (Guerrero et al. 1997).

GSM is a semi-enclosed gulf with a surface of

19.700 km2, characterized by a high rate of water

retention due to its topography. Its maximum

depth is 200 m in the center of the gulf and

decreases up to 45 m in the mouth (Mazio and

Vara 1983). The SST and salinity in the gulf vary

between 11.3 and 13.5 °C and between 33.5 and

34.1, respectively (Williams et al. 2010). The

seabed of the fishing zone consists mainly of a

mixture of sand and mud. BSA is a shallow

macrotidal system located in the northwestern

region of GSM with tidal amplitudes of up to 9 m

and strong tidal currents within their main chan-

nels. The dominant bottom type is sand, with vari-

able content of interspersed pebble and cobble.

Due to its narrow mouth (5 km long), the bay

presents a low rate of water exchange with GSM.

The water temperature in BSA oscillates between

6 and 28 °C throughout the year and salinity varies

between 31.8 and 39.0 (Piola and Scasso 1988;

Saad 2018 pers. comm.) but it could decrease to

MEDINA ET AL.: SHELL MORPHOLOGY OF ZIDONA DUFRESNEI

Figure 1. Representative specimens of mean total length of each studied population of Zidona dufresnei. BSA: San Antonio Bay,

GSM: San Matías Gulf, MDQ: Mar del Plata. Scale bar =10 mm.

x BSA 95.6 mm=

x GSM 191.8 mm=

x MDQ 152.2 mm=

29.0 with extreme rainfall (Salas 2019 pers.

comm.). Contrary to MDQ and GSM sites, BSA

area is a wave exposed intertidal environment.

In MDQ, individuals were obtained from the

bottom trawl fishery that targets this species at

40-60 m depth. In this fishery, vessels are

equipped with bottom nets of 42 mm mesh size.

In GSM, individuals were collected from the

bycatch of the bottom trawling fleet that targets

the Argentine hake (Merluccius hubbsi). The

depth at which the specimens of Z. dufresnei

were obtained averaged 100 m. Mesh size used

in this fishery ranges between 110 and 120 mm.

In BSA, individuals were hand-collected by arti-

sanal fishermen from the intertidal region (0-1 m

depth), using an iron gaff. All specimens were

sexually mature adults. Maximum size for each

population was recorded (Lahille 1895; Clench

and Turner 1964; Kaiser 1977; Scarabino 1977;

Roche et al. 2013; Medina et al. 2015). Adult

size was established separately for each popula-

tion according to size at maturity described in the

literature (Roche et al. 2015, Giménez and Pen-

chaszadeh 2003). Although smallest individuals

were not sampled in any of the populations we

were able to compare among adults and maxi-

mum sizes.

Morphometric analysis

Both traditional and geometric morphometric

methodologies were used to study shell shape

variation as complementary analysis. The tradi-

tional morphometric analysis was conducted

using 253 individuals (MDQ: 99, GSM: 78 and

BSA: 76). These sample sizes were in concor-

dance to the sample sizes estimated by power

analysis method using G*power software (free-

56 MARINE AND FISHERY SCIENCES 33 (1): 53-68 (2020)

Figure 2. Collection site of Zidona dufresnei. MDQ: Mar del Plata, BSA: San Antonio Bay, GSM: San Matías Gulf.

MDQ

Buenos Aires Province

South Atlantic Ocean

San Matías Gulf

Argentina

Argentine Sea

0 100 200 400 km

BSA

GSM

62° W 60° W 58° W

55° S

50° S

45° S

40° S

35° S

40° S

38° S

36° S

41° S

65° W 64° W

65° W 60° W 55° W

ware, Faul et al. 2009). Power of the study was

95%. Only individuals in good enough condition

to take the measurements were used for the tradi-

tional morphometric analysis (e.g. apically erod-

ed specimens were discarded from the analysis).

Animals were sexed based on the presence of the

pedal gland in females and the presence of a penis

in males. The following measures (mm) were

taken for each individual shell using a digital

caliper: total length (TL), total width (TW), aper-

ture length (AL) and aperture width (AW) (Figure

3 A). Additionally, total weight (TW) and shell

weight (SW) in grams were recorded. To analyze

morphometric variations six indexes were used:

general roundness (GR =TW/TL), relative length

of the aperture (RLA =AL/TL), relative width of

the aperture (RWA =AW/LT), relative shape of

the aperture (RSA =AW/AL), relative expansion

of the aperture (REA =AW/TW) and relative

weight of the shell (RWS =SW/TW). Differences

between populations were analyzed by Principal

Component Analysis (PCA) and nonparametric

tests using all indexes. Also, the following linear

regressions were estimated and differences

between sites and sexes were analyzed using

ANCOVA for the relationships TL versus AL, TL

versus AW, and SW versus TW. Before the analy-

ses, data were tested for normality with the

Shapiro-Wilk test and for homogeneity of vari-

ance with the Levene test.

MEDINA ET AL.: SHELL MORPHOLOGY OF ZIDONA DUFRESNEI

Figure 3. Diagram of the shell of Zidona dufresnei. A) Measurements used in the traditional morphometric analysis: total length

(TL), total width (TW), aperture length (AL), aperture width (AW). B) The eight landmarks used in the geometric analy-

sis. Landmark (L) 1: apex, L2: right border of the suture of the last anfract, L3: left border of the suture of the last anfract,

L4: outer-end of the suture of the last right anfract, L5: posterior border of the outer lip, L6: right border of the siphonal

channel, L7: left border of the siphonal channel, L8: end of the suture of the last left anfract.

ATW B

Geometric morphometrics approach was con-

ducted using a subset of 68 adult snails shells

(MDQ: 19, GSM: 19, BSA: 30). We selected

individuals with unbroken shells, which were dif-

ficult to obtain due to fishing procedures. In this

way, the sample size was limited by the availabil-

ity of samples in good condition. Shell photo-

graphs were taken with a digital camera (Nikon

Coolpix P5100, 12.1 megapixels) mounted on a

table top to ensure parallelism between the focal

plane of the camera and frontal plane of individ-

uals. All photographs were taken at the same res-

olution including a graded scale in each one as a

reference. In order to reduce experimenter bias

the photographic method was carried out by

A.I.M. and repeatability was tested. Repeatability

between sessions was high (t-test, p>0.90 for all

comparisons). Shells were placed with the aper-

ture facing the plane of the camera and distance

between shells and camera was large enough

(respect to shell size) to minimize the error

caused by the optical distortion of the lens

(Zelditch et al. 2004). Eight landmarks to analyze

shape variation were selected following the crite-

ria of Conde-Padín et al. (2007) with slight mod-

ifications (Figure 3 B). Three landmarks (1, 6 and

7) were of type I (points where at least two dis-

tinct structures meet; i.e. the posterior tip of the

body) and the remaining five landmarks of type II

(points that are supported by geometric criteria;

i.e. border of the suture of the last anfract) (Book-

stein 1991). These landmarks are typically chosen

to study shell variation in snails (Chiu et al. 2002;

Cruz et al. 2012; Avaca et al. 2013; Vergara et al.

2016, Vaux et al. 2017; Amini-Yekta et al. 2019).

Landmark coordinates were obtained by using

TPSDig v.2 software (Rohlf 2001).

Translation, rotation and scale effects were

removed by Generalized Procrustes Analysis

(GPA) (Zelditch et al. 1998; Adams et al. 2004).

In this method, landmark configurations are

superimposed by least squares optimization and

the process is iterated to compute the mean shape

(Atchley and Hall 1991; Zelditch et al. 2004).

After GPA, shape differences were analyzed by

Procrustes distance differences. Centroid size

(CS), which is calculated as the square root of

the sum of the squared deviations of landmarks

from a centroid (Bookstein 1991; Zelditch et al.

2004) for each specimen was used as a size

proxy. The centroid size is a measure of size

uncorrelated with all pure shape changes (Book-

stein 1991). One-way ANOVA was used to com-

pare the means of the centroid size between the

three populations. Tukey test was used for post-

hoc analyses.

The presence of allometry (changes in shape

related to changes in size) was examined by a

multivariate regression analysis between shape

scores as a dependent variable (Procrustes coordi-

nates) and centroid size (CS) as an independent

variable. A canonical variation analysis (CVA)

was performed, including the study site as a cate-

gorical variable, in order to obtain the Procrustes

distances matrix. Subsequently, the main tenden-

cies in shape variation between specimens within

samples were summarized through PCA of the

variance-covariance matrix of the Procrustes

coordinates. All shape analyses were performed

by using MorphoJ v1.05d (Klingenberg 2011).

More details of the framework of geometric mor-

phometrics using landmarks can be found in

Zelditch et al. (2004).

RESULTS

The analysis of the six morphometric indexes

based on Z. dufresnei shell morphology showed

significant differences between populations

(Kruskal-Wallis, p<0.01). General roundness

(GR) and relative shape of the aperture (RSA)

indexes were significantly higher in the individu-

als from San Antonio Bay (BSA) compared to the

other populations (Table 1). Regarding relative

length of the aperture (RLA) and relative expan-

sion of the aperture (REA), individuals from Mar

58 MARINE AND FISHERY SCIENCES 33 (1): 53-68 (2020)

del Plata (MDQ) presented higher values than

individuals from BSA and San Matias Gulf

(GSM) (Table 1). Relative weight index of the

shell (RWS) presented higher values in individu-

als from BSA.

Regressions between AL and TL were signifi-

cant for the three populations studied (BSA F1, 75

=114.52, IC β: 0.58 −0.84; GSM F1, 77 =289.80,

IC β: 0.67 −0.85; MDQ F1, 98 =361.16, IC β:

0.68 −0.84) (Table 2). Comparison of regression

model between sexes was not significant for BSA

and GSM (ANCOVA, p>0.05), but was signifi-

cant for MDQ. Regression between AW and TL

was significant for the three populations (BSA F1,

74 =49.52, IC β: 0.18 −0.32; GSM F1, 60 =36.89,

IC β: 0.14 −0.28; MDQ F1, 81 =148.60, IC β:

0.24 −0.33) (Table 2). There were not significant

differences between sexes for the populations.

Regressions between log(SW) and log(TW) were

significant (BSA F1, 52 =216.04, IC β: 0.90 −

1.18; GSM F1, 53 =18.17, IC β: 0.24 −0.67; MDQ

F1, 57 =89.51, IC β: 0.54 −0.83) (Table 2). Com-

parison between sexes revealed no significant dif-

ferences for the populations BSA and MDQ,

MEDINA ET AL.: SHELL MORPHOLOGY OF ZIDONA DUFRESNEI

Table 1. Morphometric indexes for Zidona dufresnei. GR: general roundness, RLA: relative length of the aperture, RWA: relative

width of the aperture, REA: relative expansion of the aperture, RSA: relative shape of the aperture, RWS: relative weight

of the shell, BSA: San Antonio Bay, GSM: San Matías Gulf, MDQ: Mar del Plata.

GR RLA RWA REA RSA RWS

BSA Mean ±SD 0.45 ±0.05 0.73 ±0.05 0.23 ±0.02 0.52 ±0.06 0.32 ±0.04 0.61 ±0.06

Min-max 0.38-0.73 0.44-0.82 0.20-0.36 0.28-0.77 0.27-0.50 0.51-0.74

GSM Mean ±SD 0.33 ±0.03 0.74 ±0.03 0.22 ±0.02 0.65 ±0.05 0.30 ±0.03 0.16 ±0.03

Min-max 0.21-0.40 0.61-0.81 0.18-0.26 0.48-0.80 0.24-0.35 0.10-0.22

MDQ Mean ±SD 0.34±0.04 0.76 ±0.05 0.23 ±0.02 0.66 ±0.06 0.30 ±0.03 0.25 ±0.06

Min-max 0.24-0.60 0.62-0.88 0.18-0.28 0.54-0.78 0.23-0.37 0.17-0.47

SD: standard deviation, min: minimum, max: maximum.

Table 2. Parameters of the relationships (linear regression analyses) obtained for Zidona dufresnei. TL: total length, AL: aperture

length, AW: aperture width, SW: shell weight, TW: total weight, BSA: San Antonio Bay, GSM: San Matías Gulf, MDQ:

Mar del Plata. R2 values are expressed in parenthesis. All regressions were significant ( p<0.01).

Females Males Total

BSA AL =0.7176TL +2.2186 (0.68) AL =0.6918TL +3.5171 (0.55) AL =0.7114TL +2.2889 (0.60)

AW =0.2363TL – 0.9183 (0.39) AW =0.2782TL – 3,8232 (0.50) AW =0,2489TL – 1,6614 (0.46)

SW =0.42TW1.09 (0.84) SW =0.62TW0,996 (0.77) SW =0.5085TW1,0423 (0.80)

GSM AL =0.7701TL – 4.6479 (0.82) AL =0.7407TL – 0.1231 (0.72) AL =0.7634TL – 3.8373 (0.79)

AW =0.2467TL – 4.1621 (0.46) AW =0.1739TL +8.9763 (0.29) AW =0.2142TL +1.6694 (0.38)

SW =4.86TW0.47 (0.32) SW =7.28TW0.44 (0.19) SW =5.096TW0.4563 (0.26)

MDQ AL =0.7317TL +6.1449 (0.81) AL =0.8004TL – 7.4892 (0.79) AL =0.7707TL – 1.3581 (0.78)

AA =0.289LT – 9.407 (0.69) AA =0.2759LT – 7.2562 (0.57) AA =0.2843LT – 8.6399 (0.65)

SW =1.16TW0.76 (0.72) SW =1.77TW0.62 (0.53) SW =1.293TW0.6846 (0.61)

while for GSM females presented heavier shell

than males (ANCOVA, p<0.01). When compar-

ing regression models between BSA and MDQ

and between BSA and GSM, significant differ-

ences in the slope were observed (β BSA ≠β

GSM, p<0.05; β BSA ≠β MDQ, p<0.01), while

comparing GSM and MDQ no significant differ-

ences were observed.

PCA conducted with morphometrical indexes

explained 75.9% of total variation of data when

the first two components were used (Figure 4). In

this analysis, individuals from GSM and MDQ

populations presented some degree of overlap-

ping. A posteriori comparisons revealed signifi-

cant differences between GSM and BSA and

between MDQ and BSA, while no differences

were detected between GSM and MDQ.

Geometric morphometric analyses showed a

significant difference in centroid size (CS) among

populations. Comparison of the centroid size (CS)

between populations showed that individuals from

GSM (CS: 2.99) were significantly larger than

individuals of MDQ (CS: 2.82) and BSA (CS:

2.31) (ANOVA: F2, 65 =843, p<0.01). Multivari-

ate regression of shape on CS was significant (per-

mutation test with 10,000 random permutation, p

<0.01). Thus, subsequent analyses were per-

formed with the residuals of the regression which

are free of allometric effects. PCA explained

87.8% of total shape variation when the first four

components were considered (PC1 61.1%, PC2

12.4%, PC3 9.8% and PC4 4.4%). Individuals

from BSA were represented by positive values of

PC1 which means a more rounded-shape shell

60 MARINE AND FISHERY SCIENCES 33 (1): 53-68 (2020)

Figure 4. Principal Component Analysis (PCA) for shell shape variation of Zidona dufresnei with percentage of explained vari-

ance. GSM: San Matías Gulf, BSA: San Antonio Bay, MDQ: Mar del Plata.

BSA

GSM

MDQ

PC 1 (42.38%)

-4.00

-4.00 -2.00 -0.00 2.00 4.00

-2.00

0.00

2.00

4.00

PC 2 (33.54%)

than individuals from GSM and MDQ (Figure 5).

Analysis of canonical components revealed a

smaller distance between individuals from MDQ

and GSM, and higher distance between individu-

als from BSA and MDQ (Procrustes distance:

BSA-GSM: 0.0878; BSA-MDQ: 0.0954; GSM-

MDQ: 0.0381, p<0.01) (Figure 6).

DISCUSSION

Studying the adaptation of a population to a

changing environment, whether modeled by

selection, plasticity or the interaction of both, is

an ongoing challenge in evolutionary studies

(Reed et al. 2011; Grenier et al. 2016). These

studies contributed to elucidate different local

adaptive strategies to avoid predation or reduce

intraspecific competition, among others (Trussell

1996; Marchinko 2003; Andrade and Solferini

2006; Hollander et al. 2006; Avaca et al. 2013).

Morphometric techniques, both traditional and

geometric, have been widely used in ecological

and evolutionary studies (Carvajal-Rodríguez et

al. 2005; Fedosov et al.2011; Epherra et al.

2015). Shape variation of body structures, such as

shells in gastropods, has a genetic basis but is also

influenced by environmental and epigenetic

processes (Atchley and Hall 1991; Valentin et al.

2002; Rufino et al. 2006; Amini-Yekta et al.

2019). Therefore, to fully understand factors that

determine shape it is necessary to consider the

ontogenetic development and also adaptations to

MEDINA ET AL.: SHELL MORPHOLOGY OF ZIDONA DUFRESNEI

Figure 5. Principal Component Analysis (PCA) of Procrustes coordinates for Zidona dufresnei that explains 87.8% of total vari-

ation of data. GSM: San Matías Gulf, BSA: San Antonio Bay, MDQ: Mar del Plata.

0.1 BSA

GSM

MDQ

0.05

-0.05

-0.1

-0.1 -0.05 00.05 0.1

PC 1

PC 2

environment besides genetics (Hanken and Wake

1991; Lombard 1991; Müller 1991). In this paper,

we combined traditional and geometric morpho-

metric tools to analyze at the first time the shell

morphology variation of Z. dufresnei in different

locations along the geographical distribution of

the species. According to these results, the three

populations presented significant differences in

size, but also in shell shape, showing allometric

effects between populations. The analysis of mor-

phological indexes showed that the shell of the

individuals from BSA presented a higher value of

general roundness compared to individuals from

GSM and MDQ. In the case of relative length and

width of the aperture, individuals from MDQ pre-

sented the highest values. The relative weight of

the shell was higher in individuals from BSA. In

general terms, individuals from BSA presented a

shell characterized by a higher general roundness

and relative weight, and lower relative aperture

compared to individuals from GSM and MDQ.

This was reflected in the multivariate analysis

where individuals from BSA were notably differ-

ent from individuals of MDQ and GSM.

Mean values of shell length and width were sig-

nificantly different between individuals of the

three populations studied. Comparisons between

regression models showed that main differences

between BSA and GSM were related to the size of

individuals since the relationships between length

and width with total length were represented by

the same model. When MDQ and GSM popula-

tions were compared, individuals from GSM pre-

sented larger shells than individuals from MDQ.

The differences found in morphological variables

in the present study may be related to differences

in individual growth of each population (Giménez

et al. 2004), and to particular environmental con-

ditions at each location. The observed differences

in maximum size between MDQ and GSM indi-

viduals could be also related to a long-term

anthropogenic selection pressure by fishing which

decreases the relative frequency of individuals

with large body sizes. MDQ population has been

directly exploited by the Argentinean and

Uruguayan fleets for the last 40 years and seems

to be in the over-exploitation phase (Giménez et

al. 2005) while GSM population is not under

62 MARINE AND FISHERY SCIENCES 33 (1): 53-68 (2020)

Figure 6. Canonical analysis of Procrustes coordinates of the shell of Zidona dufresnei. GSM: San Matías Gulf, BSA: San

Antonio Bay, MDQ: Mar del Plata, CC: canonic components.

BSA

GSM

MDQ

-10 -5 0 5 10

-6

-4

-2

CC 1

CC 2

direct fishing pressure and only sporadically

caught as bycatch of demersal trawling fleet.

Previous studies pointed out that the aperture

of the shell is a highly variable area where sexual

dimorphism is expressed. For example, Family

Bursidae is characterized by differences in the

aperture borders between sexes (Beu 1998). For

genera Buccinum and Buccinanops, differences in

size of the aperture were reported between sexes

with higher apertures in males than in females

(Hallers-Tjabbes 1979; Avaca 2010; Avaca et al.

2013). However, our results did not reveal differ-

ences in aperture length and width between sexes,

suggesting that such responses may vary accord-

ing to the family under analysis.

Geometric morphometrics analysis allowed us

to separate the individuals from the three popula-

tions, being GSM and MDQ the most similar.

Main variations were observed in the size and

volume of individuals. This result is in agreement

with those obtained by traditional morphometric

analysis. In general, size was the variable that

explained the highest variation (70% of the total

variation). When the effects of size and allometry

were removed and only shape variation was con-

sidered for comparisons, a separation of popula-

tions through the principal axis of shape variation

was clearly evident. GSM and MDQ showed sim-

ilar shell shape morphology compared to BSA.

Comparisons between individuals of the same

species from different sites or under different envi-

ronmental conditions, using the combined

approach of traditional and geometric morphomet-

rics have been conducted in previous studies.

Bigatti and Carranza (2007), studying the effect of

the occurrence of imposex in Odontocymbiola

magellanica from Patagonian waters detected

some differences in shell shape and body using

both univariate and multivariate approaches. Addi-

tionally, shape variations were determined for

Buccinanops deformis in three populations of

Patagonia (Argentina) using both techniques

(Avaca 2010). Differences in shell shape were

detected using geometric morphometrics that

remained undetected by traditional morphometrics

in two sympatric ecotypes of Littorina saxatilis

(Carvajal-Rodríguez et al. 2005). This species also

showed a larger aperture on exposed shores and a

smaller aperture on sheltered shores in response to

predation (Conde-Padín et al. 2009). In the case of

Z. dufresnei, traditional and geometric morpho-

metrics were useful both to describe and to quan-

tify the shell shape variation observed between

populations. These methods were reliable for dis-

tinguishing individuals from different locations

based solely on their shell shape. Although the two

morphotypes were much better separated by geo-

metric morphometrics approach, traditional mor-

phometrics were useful as a complementary tech-

nique since it allowed working with a larger num-

ber of samples. The number of samples available

for geometric morphometrics was limited because

it was difficult to access to individuals in good

shape condition since samples from MDQ and

GSM belonged to fisheries catches.

Our results support the hypothesis of Lahille

(1895) who classified the individuals from BSA as

a ‘dwarf’ morphotype based on shell morphology,

highlighting the need to revise the taxonomic sta-

tus of Zidona. Unfortunately, there are not pub-

lished genetic data to validate the two species

hypothesis from a molecular approach. The

marked shell variations detected among popula-

tions of Z. dufresnei may be driven by several eco-

logical factors other than growth pattern, such as

changes in prey availability, presence of predators,

and temperature (e.g. Dalziel and Boulding 2005;

Doyle et al. 2010). BSA corresponds to an inter-

tidal zone where snails are exposed to highly vari-

able environmental conditions with clines of food

availability, wave exposure, desiccation and pres-

ence of predators, contrasted with GSM and MDQ

(Roche et al. 2011). These environmental pres-

sures (Raffaelli and Hawkins 1999; Chapman

2000) may favor smaller size (i.e. the occurrence

of a ‘dwarf’ morphotype population), higher gen-

eral roundness and relative weight, and also small-

er relative aperture in the individuals from BSA.

MEDINA ET AL.: SHELL MORPHOLOGY OF ZIDONA DUFRESNEI

At the same time, certain characteristics of life

history of Z. dufresnei, such as direct intracapsular

development (Penchaszadeh and De Mahieu

1976; Giménez and Penchaszadeh 2002) and

restricted range of spatial dispersion (Pen-

chaszadeh et al. 1999; Pereyra et al. 2009; Roche

et al. 2011, 2013) may have resulted in a reduction

of gene flow among populations leading to such

adaptations to local conditions. Considering that

GSM and MDQ individuals were similar in size

and shell shape morphology but showed the

longest distance between them, ecotypes adapted

to different conditions should be maintained as the

most probable explanation for the variation

between dwarf and normal morphotypes unless

new data contradict this. In summary, issues

affecting size and shell shape variation in Z.

dufresnei are multiple and not mutually exclusive.

Additional experimental studies are needed to sort

out the role of the physical and ecological factors

on the shell shape and to test whether this varia-

tion has an adaptive value. On the other hand, fur-

ther investigation is needed to better understand if

the phenotypic variation observed in shell mor-

phology is also expressed at genetic level. This is

also highlighted in the case of Z. dufresnei which

is under an increasing fishing pressure.

The authors declare that they have no conflict

of interest. No animal testing was performed dur-

ing this study. All necessary permits for sampling

and observational field studies have been

obtained by the authors from the competent

authorities. All applicable international, national,

and/or institutional guidelines for the care and use

of animals were followed.

The study material is cataloged in the collec-

tion deposited at the Laboratory of Benthic

Resources. Center for Applied Research and

Technology Transfer in Marine Resources Almi-

rante Storni (CIMAS). The data sets generated

during and/or analyzed during the current study

are available from the corresponding author at

reasonable request.

ACKNOWLEDGMENTS

We are thankful to Andrés Milessi (INIDEP),

Alejandra Goya and Horacio Sancho (SENASA)

for their help in animal sampling. Alonso Medina

thanks Consejo Nacional de Investigaciones

Científicas y Técnicas (CONICET) for the doc-

toral and posdoctoral fellowships. Thanks very

much to Dr Thomas A. Darragh and Dr Pablo A.

Martinez for the valued comments and sugges-

tions. Dedicated to my friend C. J. Bidau.

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Received: 23 March 2020

Accepted: 20 April 2020

68 MARINE AND FISHERY SCIENCES 33 (1): 53-68 (2020)