Gustavo Enrique Chiaramonte, Ana Julia Alarcos, Diego Martin Vazquez, Nelson Dario Bovcon, Sergio Matias Delpiani, Leandro Nahuel Chavez, Ruben Francisco Dellacasa, Ezequiel Mabragaña

Recurrent juvenile stranding of the porbeagle shark Lamna nasus along the South West Atlantic coast

Introduction

Recurrent juvenile stranding of the porbeagle shark lamna nasus along the south west atlantic coast. Recurrent porbeagle shark strandings in SWA Argentina. New biological and genetic data (COI gene) reveal high diversity, three distinct clusters, and Southern/Northern Hemisphere partitions.

Abstract

Strandings of juvenile porbeagles are recurrent in the South West Atlantic (SWA). We provide newbiological information and explore to which haplogroups the juveniles may belong. Fourteen stranding events occurred between May 2009 and May 2022 on porbeagle in the beaches of the SWA Argentine coast were identified. Based on a 2019 beached specimen, total length, 83 morphometric measures, and the parasites diversity were provided, as well as the COI gene sequences, derived from DNA extraction from muscle tissue, followed by PCR. The sequence matched Lamna nasus (98.76% – 100.00%; n=100), using the Barcode of Life Data System identification. Molecular data show a great genetic diversity in COI sequences, especially between northern and southern hemisphere samples, in accordance with previous studies. In addition, juveniles recorded in northern Argentinecoastal waters differ in COI nucleotide sequences and are located in different clusters in the maximum likelihood (ML) analysis. The haplotype network built showed a star-shaped topology and, as the ML tree, revealed the existence of three genetically distinct clusters. A partition of the genetic variation into Southern Hemisphere and Northern Hemisphere groups was observed.

Review

This manuscript presents a timely and important investigation into the recurrent stranding events of juvenile porbeagle sharks (*Lamna nasus*) along the South West Atlantic (SWA) Argentine coast. Documenting 14 stranding occurrences over a 13-year period (2009-2022), the study provides new biological information by integrating detailed morphological and parasitological data from a representative specimen with robust genetic analysis. This comprehensive approach offers valuable insights into a phenomenon that is crucial for understanding the ecology and conservation needs of this vulnerable species. A significant strength of the study lies in its genetic characterization of the stranded juveniles. Utilizing COI gene sequencing, the authors confirmed species identification and revealed substantial genetic diversity. In agreement with previous research, a clear genetic partitioning between Southern and Northern Hemisphere populations was observed. Crucially, the analysis identified three distinct genetic clusters among the SWA juveniles, a finding supported by both a star-shaped haplotype network topology and maximum likelihood tree analysis. The observation that juveniles from northern Argentine coastal waters exhibit different COI nucleotide sequences and cluster separately further highlights potential sub-structuring within the regional population. These findings have profound implications for the population structure and conservation management of porbeagle sharks in the SWA. The presence of multiple genetically distinct clusters among the stranded juveniles suggests that these coastal areas may serve as critical habitats for different porbeagle populations or subpopulations, possibly functioning as diverse nursery grounds or migratory pathways. This genetic insight is vital for informing targeted conservation strategies that acknowledge and protect distinct genetic units. The work provides a fundamental baseline for future research aiming to uncover the underlying causes of these recurrent strandings and to more precisely delineate stock boundaries in this ecologically important region.

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