In oceanic ecosystems, the nature of barriers to gene flow and the processes by which populations may become isolated are different from the terrestrial environment, and less well understood. In this study we investigate a highly mobile species (the sperm whale, Physeter macrocephalus) that is genetically differentiated between an open North Atlantic population and the populations in the Mediterranean Sea. We apply high-resolution single nucleotide polymorphism (SNP) analysis to study the nature of barriers to gene flow in this system, assessing the putative boundary into the Mediterranean (Strait of Gibraltar and Alboran Sea region), and including novel analyses on structuring among sperm whale populations within the Mediterranean basin. Our data support a recent founding of the Mediterranean population, around the time of the last glacial maximum, and show concerted historical demographic profiles in both the Atlantic and the Mediterranean. In each region there is evidence for a population decline around the time of the founder event. The largest decline was seen within the Mediterranean Sea where effective population size is substantially lower (especially in the eastern basin). While differentiation is strongest at the Atlantic/Mediterranean boundary, there is also weaker but significant differentiation between the eastern and western basins of the Mediterranean Sea. We propose, however, that the mechanisms are different. While post-founding gene flow was reduced between the Mediterranean and Atlantic populations, within the Mediterranean an important factor differentiating the basins is probably a greater degree of admixture between the western basin and the North Atlantic and some level of isolation between the western and eastern Mediterranean basins. Subdivision within the Mediterranean Sea exacerbates conservation concerns and will require consideration of what distinct impacts may affect populations in the two basins.

The exploitation of sharks and the degradation of their habitats elevate the urgency to understand the factors that influence offspring survival and ultimately shark reproductive success. We monitored and sampled blacktip reef sharks (Carcharhinus melanopterus) in nursery habitats of Moorea Island (French Polynesia), to improve knowledge on shark reproductive behavior and biology. We sampled fin clips and morphometrics from 230 young-of-the-year sharks and used microsatellite DNA markers to process parentage analysis to study the reproductive philopatric behavior in female sharks and the matrotrophy within litters. These traits are driving the success of the local replenishment influencing selection through birth site and maternal reserves transmitted to pups. Parentage analysis revealed that some female sharks changed their parturition areas (inter-seasonally) while other female sharks came back to the same site for parturition, providing evidence for a plastic philopatric behavior. Morphometrics showed that there was no significant relationship between body condition indices and nursery locations. However, similarities and differences in body condition were observed between individuals sharing the same mother, indicating that resource allocation within some shark litters might be unbalanced. Our findings further our understanding of the reproductive biology and behavior that shape shark populations with the aim to introduce these parameters into future conservation strategies.

Human‐caused injuries can have severe consequences for cetaceans, both for the well‐being of individuals and the effects at the population level. Here, we report the first case of a live fin whale in the Mediterranean Sea with completely amputated flukes, a female who suffered two separated incidents: the first one before 1996, when she lost her left lobe, and the second one, in 2019, leading to the severing of the whole tail and finally to presumed death. “Codamozza‐Fluker” was regularly sighted during summer in the North‐Western Mediterranean for 24 years. Entanglement and collision were both hypothesized as potential causes of her injuries. Without flukes, she traveled at least 7000 km across the Mediterranean, despite severe emaciation. Sighting records allowed tracking her from September 2019 to July 2020, providing rare evidence of large‐scale movements within the Mediterranean over almost a year. Her journey, which extended as far as Syria, provides the first evidence of a fin whale from the Ligurian Sea being resighted in the Eastern Basin. This introduces a new perspective on this endangered subpopulation's movement patterns, which are crucial for effective conservation measures. The case gained widespread media attention, representing a dramatic example of usually unnoticed injuries or deaths, and contributed to raising public awareness, essential for driving focused conservation efforts. Her latest accident most likely occurred within the Pelagos Sanctuary for marine mammals, highlighting the need for more effective local protection measures, and underscoring the importance of complementing area‐based conservation efforts with an ecosystem‐based approach.