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

## TL;DR

This study assesses the population collapse of Pinna nobilis in southern Italy and explores its ecological connections with seagrass meadows.

## Contribution

This is the first large-scale survey of Pinna nobilis in Italy, revealing 100% mortality and new insights into its trophic relationship with seagrass meadows.

## Key findings

- Pinna nobilis populations in Apulia have completely collapsed with 100% mortality.
- The species' distribution overlaps with seagrass meadows at meso- and macro-geographical scales.
- P. nobilis may rely on seagrass-derived detritus for trophic support beyond habitat boundaries.

## Abstract

The dramatic Mass Mortality Event, MME, of Pinna nobilis populations initially detected in the western Mediterranean basin, has also spread rapidly to the central and eastern basin. Unfortunately, there is still a significant lack of information on the status and health of P. nobilis, since only a fragmentary picture of the mortality rate affecting these populations is available. Regarding the Italian coast, several surveys have given only localized or point-like views on the distribution of species and the effect of the MME. Therefore, for the first time, this study investigated P. nobilis density of individuals, distribution and mortality throughout 161 surveys along 800 km of coastline in the Apulia region (South-east of Italy). The geographical scale of this investigation made it the largest ever conducted in Italy, and this was achieved through a rapid and standardized protocol. During this monitoring campaign, 90 km of linear underwater transects were surveyed, along which no live individuals were observed. This result allowed to estimate that the P. nobilis populations had totally collapsed, with a mortality rate of 100% in Apulia. The distributional pattern of the species showed a strong overlap with seagrass meadows on meso- and macro-geographical scale, however this was not the case on a micro-scale. This result evidenced that relationships between P. nobilis and seagrass meadows are not limited to the habitat patch, but cross the boundaries of seagrass leading us to suggest that the distribution of P. nobilis hold a trophic link through the cross-boundary subsidy occurring from seagrass meadows to the nearby habitat, by means of the refractory detrital pathway.