Spatial distribution, abundance and habitat use of the endemic Mediterranean fan mussel Pinna nobilis in Gera Gulf, Lesvos (Greece): comparison of design-based and model-based approaches

Abstract

An important population of the endemic Mediterranean fan mussel Pinna nobilis thrives in the marine protected area of Gera Gulf (Lesvos island, north-eastern Aegean Sea, Greece), and was assessed for the first time. To estimate the abundance, spatial distribution and habitat use of fan mussels in Gera Gulf, a distance sampling underwater survey was conducted. Detectability was modelled to secure unbiased estimates of population density. Two approaches were applied to analyze survey data, a design-based and a model-based approach using generalized additive models. The first approach was based on stratified random sampling on two strata, an assumed 'preferable' zone close to the coastline and an assumed unsuitable habitat, with predominantly muddy sediments, in which low sampling effort was applied. For the needs of the model-based approach, a dedicated cruise was conducted to collect bathymetric data with a single-beam echo-sounder and map the bathymetry of the study area. A very high-resolution image from the Worldview-3 satellite was processed, based on an object-based image analysis, for mapping all main habitat types in the study area. The estimated abundance using the design-based approach was low-biased as the stratum of pre-assumed unsuitable habitat proved to include patches of suitable habitats with high population densities that were missed by sampling. The model-based approach provided an abundance estimate of 213300 individuals (95% confidence interval between 97600-466000 individuals), which renders the fan mussel population of Gera Gulf the largest recorded population in Greece. Population density peaked between 1.5-8 m depth and became practically zero at depths >15 m. A bathymetric segregation of fan mussel size classes was noted, with the density of small individuals peaking in shallow waters, while that of large individuals peaked deeper. The highest population densities were observed in Posidonia oceanica meadows, followed by mixed bottoms (with reefs, rocks and sandy patches), while densities were very low on sandy and zero on muddy sediments. The current assessment provides a baseline for future monitoring of the fan mussel population in Gera Gulf. In view of the current (2017-2018) ongoing mass mortality of the species in the western Mediterranean, continuous monitoring of the main fan mussel populations, such as the one in Gera Gulf, is of utmost importance.

Population status, distribution and trophic implications of Pinna nobilis along the South-eastern Italian coast

“…This is particularly evident along the northern Adriatic coast of the region, where extended seagrass meadows are absent and, no trace of P. nobilis was encountered, except in the Tremiti archipelago where both P. oceanica meadows and pen shells were found. By contrast, present data reporting P. nobilis as associated with various seagrass species, such as P. oceanica, C. nodosa, and Zostera sp., are consistent with the macroscale and mesoscale association between P. nobilis and seagrass meadows sensu lato and most literature reporting ubiquitous distribution of P. nobilis both in lagoon-estuarine 21,22,[24][25][26]31 and in marine ecosystems 4,7,9,14,16,24 .…”

After the Pinna nobilis l. mass mortality event in Bosnia and Herzegovina – A proposal for remediation of biofiltration services in marine ecosystem

“…We did not notice the presence of a habitat type with the flowering plant Posidonia oceanica meadows. As factors potentially affecting detectability, namely, the size of fan mussel individuals, a habitat type and water visibility were all absent and given the good visibility, absence of juveniles and a habitat type inhabited by Posidonia oceanica meadows, we had no visual obstacles during our underwater surveys (as shown in Figure 7), and we did not specifically address the detection function (Tsatiris et al, 2018).…”

Trophic role of marbled crayfish in a lentic freshwater ecosystem

“…The sites were selected on the basis of existing information on the presence of P. nobilis populations. Three of these sites were located in Gera Gulf at locations where previous research (in the summer of 2016) reported very high population densities (Tsatiris et al 2018), and two in Kalloni Gulf, where P. nobilis used to have very high densities as well (unpublished data). Two sites (sites 4 and 6 in Table 1) were surveyed again three months after the initial sampling (i.e.…”

“…It occurs at depths < 60 m, with the apex of the shell anchored in the soft substrate by byssus threads. It typically thrives in seagrass meadows, such as Posidonia oceanica, Cymodocea nodosa, Zostera marina, and Z. noltii (Prado et al 2014) but also in macroalgal beds (Katsanevakis and Thessalou-Legaki 2009), unvegetated sandy habitats (Katsanevakis 2006), estuarine areas (Addis et al 2009), and even in small sandy patches within predominantly rocky habitats (Tsatiris et al 2018). It plays a key ecological role by filtering water and retaining large amounts of organic matter from suspended detritus, thus reducing turbidity (Trigos et al 2014), providing hard substrate colonized by numerous epibionts in soft-bottom habitats (Addis et al 2009;Rabaoui et al 2009), and, at high densities, acting as an ecosystem engineer able to create biogenic reefs (Katsanevakis 2016).…”