Morphometric Analysis of the Critically Endangered Fan Mussel (Pinna nobilis L.) in Maliakos Gulf (Central Aegean)

Abstract

In this study, we present the relationships between total length, unburied length, and shell width and between total length and net weight for the critically endangered Pinna nobilis. This is the first transplantation study in which live specimens of P. nobilis have been used for estimating the length–weight relationship by deploying the unburied length. Length–length relationships were all linear for all cases (r² > 0.900), whereas the length–weight relationship was negative allometric with the values of the exponent b ranging from 2.159 to 2.828. These relationships are important because they offer a restorative monitoring tool without damaging or sacrificing this endangered species, as total length can be computed using unburied length. By examining the relationships between different size dimensions in this re-allocated population, the present study also provided valuable insights for comparative growth studies, stock assessment models, and conservation purposes.

1. Introduction

Pinna nobilis (Linnaeus, 1758) is the largest endemic Pteriomorphian bivalve in the Mediterranean Sea, reaching lengths of up to 120 cm. Over the past three decades, populations of P. nobilis have significantly declined due to factors such as consumption for food or ornaments, as well as the negative impact of trawling and boat anchoring [1]. To protect this species, it is covered by the Mediterranean Specially Protected Areas protocol (95/96 SPA ANNEX II) and European legislation (92/43 EEC). P. nobilis has recently been classified as critically endangered on the IUCN Red List of threatened species. One of the main challenges faced by P. nobilis populations is the impact of a protozoan parasite called Haplosporidium pinnae [2]. This parasite has caused mass mortality events and increased the risk of extinction in many of the habitats where this species is found. The devastating effects of this parasite have been observed in various countries including France, Italy, Tunisia, Turkey, Greece, and even in one of the last remaining safe shelters for the fan mussel in Croatia [3].

The present study focused on the morphometric analysis of P. nobilis population reallocated for conservation purposes in the Maliakos Gulf (Central Aegean, Greece). By examining the relationships between different size dimensions in this particular population, we would provide valuable insights for comparative growth studies, stock assessment models, and conservation purposes [4,5]. To our knowledge, no information is currently available in the SealifeBase (www.sealifebase.or) (accessed on 5 July 2023) ([6] on the above-mentioned relationships, and only a few relationships have so far been estimated for the species in the Mediterranean (Corse: [7]; Greek Lake: [8]; Turkish waters: [9]; Spain: [7,10,11]. This is the first study in which live specimens of P. nobilis have been used for estimating the length–weight relationship by deploying the unburied length. These relationships are important because they offer a restoration monitoring tool for the environment and the organism without damaging or sacrificing this critically endangered species, as total length can be computed using unburied length.

2. Materials and Methods

2.1. Study Area

Maliakos Gulf is located on the eastern side of mainland central Greece and part of the Aegean Sea (Figure 1; more characteristics of the area can be found in [5,12]). The fan mussel was widely distributed in the coastal zone until 2020, having the anterior part of the shell partially buried in soft or hard substrata, and it is attached by threads of its byssus [12]. For a pilot study regarding the transplantation of P. nobilis, local divers removed 100 lived specimens covering all visible mollusk sizes. After the careful removal of mollusks from their natural habitat, paying attention not to lose the byssus threads, certain morphometric and weight parameters were measured were measured on board: total length (TL, in cm), unburied length (UL, in cm), shell width (SW, in cm), and net weight (NW, in g) (Figure 2). All length measurements were taken with an accuracy of 0.1 cm and weight with accuracy of 100 g. The accuracy of the weight measurements was controlled by the fact that these measurements were taken on board as part of a transplant experiment, for which we had no intention of harming the specimens.

2.2. Data Analysis

Length–length relationships were estimated using the linear regression Y = a + bX, and length–weight relationship was estimated through the relationship W = a × L^b [13], where W is the wet weight, L the length, and coefficients a and b are the intercept of the curve on the weight axis and the slope of the line in the linear slope of the equation, respectively. The findings of the present study were also compared with those from the literature to investigate the area effect on these relationships.

3. Results and Discussion

Descriptive statistics of the morphometric measurements are shown in

Table 1. Descriptive statistics for the morphometric measurements for all specimens of Pinna nobilis sampled in Maliakos Gulf (Central Aegean, Greece) in 2019.

Parameters	n	Mean	SD	cv	Minimum	Maximum
Total Length (TL, cm)	100	46.76	8.37	17.90%	27	67
Unburied Length (UL, cm)	100	27.46	5.47	19.93%	16	42
Shell width (SW, cm)	100	18.58	2.19	11.81%	13	25

.

The length class of the sampled specimens peaked at 23 and 26 cm (Figure 3). Considering the fact that the total length of 20 cm represents the threshold value under which individuals should be considered juveniles [14,15], we could assume that all sampled individuals were mature.

All length–length relationships were significant (all r² > 0.425, p < 0.05), with r² values ranging from 0.472, for the relationship between UL and SW, to 0.804, for the relationship between TL and UL (

Table 2. Morphometric relationships between total length (TL, cm) and shell width (SW, cm), unburied length (UL, cm) and shell width (SW, cm) and total length (TL, cm) and unburied length (UL, cm), for all specimens of Pinna nobilis sampled in Maliakos Gulf (Central Aegean, Greece) in 2019. n is the sample size; a and b are the parameters of the linear regression.

Equation	Parameters of the Relationship
	n	a	b	SE (b)	r2
SW = a + b × TL	100	9.551	0.193	0.018	0.543
SW = a + b × UL	100	11.019	0.275	0.029	0.472
UL = a + b × TL	100	0.048	0.586	0.029	0.804

n is the sample size; a and b are the parameters of the relationship; SE (b) is the standard error of the slope b; and r² is the coefficient of determination.

). TL and UL relationships with NW were highly significant (p < 0.001) with r² values being 0.836 and 0.759, respectively (

Table 3. Length–weight relationship (W = a × TL^b) between total length (TL, cm) and -net weight (NW, g) and unburied length (UL, cm) and net weight, for all specimens of Pinna nobilis sampled in Maliakos Gulf (Central Aegean, Greece) in 2019. n is the sample size, a and b are the parameters of the length-weight relationship and r² is the coefficient of determination.

Equation	Parameters of the Relationship W = a × Lb
	n	a	b	SE (b)	r2
NW = a × TLb	100	0.067	2.572	0.012	0.836
NW = a × ULb	100	0.815	2.233	0.021	0.759

). These relationships were also negative allometric, with the values of the exponent b being 2.233 and 2.572, respectively (Table 3). Contrarily with our results, ref. [8] reveals that the TL-W relationships followed a positive allometric growth. Apart from the exponential regression, the authors of [1], to determine the relationship between TL and SW, apply a second-degree polynomial function.

Six equations on the morphometric characteristics of P. nobilis have been established so far, but none of them estimated the relationships with UL (

Table 4. Estimated parameters of the relationships on the morphometric parameters for Pinna nobilis estimated in the Mediterranean. TL is the total length in cm; SW is the shell width in cm; W is the width at depth level in cm; NW is the net weight in g; and UL is the unburied length in cm.

Area	Equation	Reference
Diana Lagoon, Corsica, France	TL = 1.651 + 2.186 × SW	[7]
Moraira, Alicante, Spain	TL = 0.500 + 1.790 × W + UL	[9]
Lake Vouliagmeni, Greece	TL ¼ = 1.460 + 0.0837 × SW − 0.001 × SW2	[1]
Moraira, Alicante, Spain	TL = 1.290 × SW1.240	[10]
Miramare, Triest, Italy	TL = 10.259e0.0809SW	[16]
Maliakos Gulf, Greece	UL = 0.048 + 0.586 × TL	Present study
	SW = 9.551 + 0.193 × TL
Izmir Bay, Turkey	NW = 0.003 × TL3.645	[8]
Maliakos Gulf, Greece	NW = 0.067 × TL2.572	Present study
	W = 0.815 × UL2.233

). In the present study, the relationships incorporated the UL might be useful for estimating the TL and the weight, because the measurement of the UL is a non-destructive sampling method. Differences in morphometric estimation could be attributed to the combination of one or more of the following factors: (a) differences in the number of specimens examined, (b) area/season effect, (c) differences in the observed length ranges of the specimen measured, and (d) abiotic factors such as temperature, salinity, wave regime, surrounding vegetation, and substrate type.