Identification and Salinity Tolerance of the Western Hemisphere Mussel Mytella charruana (D'Orbigny, 1842) in the Philippines

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Beginning in 2014, mussels have been found by shellfishers in parts of the Philippines that are morphologically distinct from native mytilids. These mussels, with a thick black periostracum, were first found in Manila Bay near an international shipping port, and later in 2014 appeared in western Tambac Bay (approximately 16.28° N, 119.9° E). The next year (July 2015), they were found near the village of Tucok (Dagupan City; 16.0272° N, 120.3147° E), and more recently (early 2016) they have been observed in Longos, San Fabian, Pangasinan (16.1887, 120.4043). About 50 mussels from Tucok were preserved in 95% ethanol and sent to the University of Maine for genetic evaluation. Sequencing of mitochondrial cytochrome oxidase I polymerase chain reaction products identified the nonnative mussels as Mytella charruana, the charru mussel, native to the tropical Caribbean and western Pacific coasts of South America. Further analysis indicates that populations on the Caribbean Coast of South America are the likely source of the Philippine charru mussels. Two salinity tolerance experiments were also conducted; in the first experiment, Philippine charru mussels were conditioned at salinity 5 (similar to the salinity of the collection site) for 7 days, then subjected to a salinity shock by directly transferring them to different salinity levels (10, 15, 20, 25, 30, and 35) for 12 days. In the second experiment, replicate groups of mussels were conditioned in 30 salinity, then for each replicate, salinity was gradually increased by five increments every 5 days until all mussels had died. In both the experiments, all mussels survived at salinities below 35. In the rapid salinity change trial, byssus formation was absent in mussels subjected to salinity 35 shock, and all mussels in this treatment died before the end of the experiment. In the acclimation trial, some mussels survived to salinity 60, but were inactive, and all died when salinity reached 65. These salinity shock and acclimation trials suggest that charru mussels may be best suited to Philippine waters during and after the monsoonal rainy seasons when salinities are routinely below 35, and may be spread among different estuaries via larval transport during the monsoonal rainy season. On the basis of these criteria, charru mussels may be a potential species for aquaculture complementing the culture of the native Perna viridis, a species with a higher preferred salinity that is traditionally cultured in the dry season.

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Journal of Shellfish Research