The megamouth shark (Megachasma pelagios) was described as a new species in 1983. Since then, only ca. 100 individuals have been observed or caught. Its horizontal migration, dispersal, and connectivity patterns are still unknown due to its rarity. Two genetic markers were used in this study to reveal its genetic diversity and connectivity pattern. This approach provides a proxy to indirectly measure gene flow between populations. Tissues from 27 megamouth sharks caught by drift nets off the Hualien coast (eastern Taiwan) were collected from 2013 to 2015. With two additional tissue samples from megamouths caught in Baja California, Mexico, and sequences obtained from GenBank, we were able to perform the first population genetic analyses of the megamouth shark. The mtDNA cox1 gene and a microsatellite (Loc 6) were sequenced and analyzed. Our results showed that there is no genetic structure in the megamouth shark, suggesting a possible panmictic population. Based on occurrence data, we also suggest that the Kuroshio region, including the Philippines, Taiwan, and Japan, may act as a passageway for megamouth sharks to reach their feeding grounds from April to August. Our results provide insights into the dispersal and connectivity of megamouth sharks. Future studies should focus on collecting more samples and conducting satellite tagging to better understand the global migration and connectivity pattern of the megamouth shark.
The megamouth shark (Megachasma pelagios) is one of the rarest shark species in the three oceans, and its biological and fishery information is still very limited. A total of 261 landing/stranding records were examined, including 132 females, 87 males, and 42 sex unknown individuals, to provide the most detailed information on global megamouth shark records, and the spatial–temporal distribution of M. pelagios was inferenced from these records. The vertical distribution of M. pelagios ranged 0–1203 m in depth, and immature individuals were mostly found in the waters shallower than 200 m. Mature individuals are not only able to dive deeper, but also move to higher latitude waters. The majority of M. pelagios are found in the western North Pacific Ocean (>5° N). The Indian and Atlantic Oceans are the potential nursery areas for this species, immature individuals are mainly found in Indonesia and Philippine waters. Large individuals tend to move towards higher latitude waters (>15° N) for foraging and growth from April to August. Sexual segregation of M. pelagios is found, females tend to move to higher latitude waters (>30° N) in the western North Pacific Ocean, but males may move across the North Pacific Ocean.
In the Pacific Ocean, the rare Bigeye Sand Tiger shark Odontaspis noronhai has only been recorded twice, once from its central and once from its eastern part. Here we report the first record of this species from the northwestern Pacific. This specimen measuring 312 cm total length (TL) was captured off northeastern Taiwan (25°25’N, 124°10’E) from a depth of 100 m (in waters over 2100 m deep) in mid-December 2019, and was retrieved on 27 December 2019 when landed in port. Photo of the fresh specimen along with morphometric and meristic data and DNA information are provided herein. Dentition of the specimen is different from all other specimens by having two cusplets on at least one side of cusps on most of the teeth (vs. only one cusplet per side), and lower total tooth count (29/29 vs. 34–38/37–46). This record documents an extended distribution (about 7864 km westward from the central Pacific Ocean), and provides strong evidence for the wide distribution and mesopelagic characteristic of this poorly known species.
Background The whale shark, Rhincodon typus, was utilized extensively in Taiwan before 1996. A quota management measure was put in place in 2002, and the whale shark fishery was completely banned in 2008. Biological studies including age and growth, and migration/ movement have been conducted. However, the feeding ecology of whale sharks in different life history stages and sexes in Taiwan waters (Northwest Pacific Ocean) is still unknown. Approach The stable isotope technique was used to analyze the tissue of whale sharks to understanding their feeding ecology. The ´13C value can be used to indicate the foraging habitat of fish, and the ´15N value can be used to estimate the relative position of the consumer in the ecosystem. The specimens (tissue) were taken from individuals entangled in set nets during the period 2008 – 2013 in Taiwan. Results In total, 66 tissue samples from 42 males and 24 females, ranging from 2.84 to 11.90 m TL (total length) were used in stable isotope analysis. Among these specimens 50 and 16 were from the eastern and western waters off Taiwan, respectively. The value of ´13C was from –13.68 to –18.42%, and the value of ´15N was from 5.17 to 13.01%. There was a positive relationship between ´13C and ´15N, and both ´13C and ´15N increased with body size. No gender or geographic difference was found in this study, but the range of stable isotope values of whale shark tissue was wider in eastern Taiwan waters. Conclusions In this study, ontogenetic changes in the diet of whale sharks were found. More specimens are needed to examine the differences in stable isotope values among different genders, seasons, and regions. The results derived from this study can provide useful information on the husbandry of whale sharks, which can help ecotourism operators become more knowledgeable about the ecology of whale sharks. In addition, the results can also be used as an important reference for ecosystem-based management in the future. Future work should focus on discussion on the habitat partition, utilization, and adaption in various marine environments for whale sharks.
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