Field Observations of the Behaviour of Blackfin Reef Sharks (<em>Carcharhinus melanopterus</em>)

Abstract

The chondrichthyan lineage diverged from the osteichthyan line 440 million years ago, resulting in a vast evolutionary gulf between modern elasmobranchs and other vertebrates. Though this has supported the assumption that sharks are ancient, dangerous, and binary-minded, the few ethological studies done have noted intelligent actions including social exchanges. Yet their behaviour remains little known. On seeing that Carcharhinus melanopterus displayed complex actions during incidental meetings, a long-term ethological study of the species was carried out using artificial aggregations, at several sites in the fringe lagoon of Mo&amp;rsquo;orea Island, French Polynesia. Short and long-term behaviour was recorded in 473 individuals, including an ethogram, roaming patterns, social interactions, and cognition. C. melanopterus is considered sedentary, yet the home range could also be viewed as a place to pause between travels, for most individuals left for long periods. The study community and its visitors travelled in correlation with the lunar phase, in groups of up to six individuals, socializing with conspecifics encountered along the way, and displaying fluid social dynamics. C. melanopterus was highly alert to danger yet prone to investigate novel objects, a combination that generated a variety of tactics to remain hidden while investigating the environment. Basic to this was the use of the visual limit for escape or to screen their presence, indicating an awareness of being present and observable. Using their other senses, they could focus their attention on events beyond visual range and made swift decisions to act as circumstances unfolded. In their non-territorial, non-hierarchical society, any shark could lead, but it was usually the same ones that did so. Therefore, unusual individuals had a significant effect on events through social learning, suggesting the potential for culture. Actions in a variety of situations suggested complex cognition, and individuals displayed both positive and negative subjective states including playfulness.

Genetic evidence for plastic reproductive philopatry and matrotrophy in blacktip reef sharks (Carcharhinus melanopterus) of the Moorea Island (French Polynesia)

“…There were no significant differences among sites in the K index neither for the sampling season 2017–2018 nor for the 2018–2019 season. As the body conditions of juvenile sharks, combined, seem to be independent from habitat (i.e., mangroves, sand flats, coral covered areas) and birth location, maternal effects—such as matrotrophy, or the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation 35 may be critical.…”

“…Around the island of Moorea, (French Polynesia, Fig. 1 ), 10 sites (Apaura, Haapiti, Maharepa, Paorea, Papetoai, Pihaena, Tiki, Vaiane, Vaiare, and Valorie), have been annually monitored and sampled during parturition seasons (from October to February each year) as part of long-term population dynamic surveys carried out by the Centre de Recherches Insulaires et Observatoire de l’Environnement (CRIOBE), as these 10 sites have been identified as inshore reef shark nursery areas 10 , 35 (Fig. 1 ).…”