1. Marine protected areas (MPAs) are set up to conserve biodiversity, but their design is not always based on strictly scientific considerations. Ideally, an MPA should protect all key habitats necessary for a marine species to complete its life cycle. The identification of these key habitats is complex, especially during the early life of marine fishes.2. A widely distributed tropical and important low trophic-level fish species, Spratelloides delicatulus (Clupeidae), was used to evaluate the significance of various coastal habitats for its larvae and juveniles in the Con Dao Archipelago MPA in Vietnam. Early stages (larvae and juveniles) were sampled monthly over one year (June 2016 to May 2017) using light traps in three main habitats (seagrass beds, coral reefs and harbour). The species was identified using morphometry and DNA barcoding. Age and growth variables were estimated using otolith daily growth increments.3. A total of 3,581 fish were caught. The species was not found in captures between January and February, directly linked to the decrease in seawater temperature and was most abundant from April to June. For a subsample of 248 fish (7-38 mm standard length), ages ranged from 7 to 108 days.4. Captures and back-calculated birthdates using otolith daily increments showed that S. delicatulus spawns during the period of high seawater temperature, from March to October. The species colonizes all three habitats during the early stages (0-26 days old), with growth rate lowest on the seagrass beds. Nevertheless, the species occupies seagrass beds exclusively during the older stages.5. The conservation of seagrass beds in the Con Dao archipelago is essential for protection of juvenile stages of this species but this habitat is presently not included in the MPA patches. Establishment of a continuum of protected areas linking habitats, rather than the existing patches is needed to conserve the complete life cycle of this species in the Con Dao MPA.

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The Epinephelidae is a marine fish family that has been the focus of worldwide taxonomical attention due to its economic value. Recent molecular-based phylogenies have improved understanding of species diversity within this family. Nonetheless, species misidentification or hybridization has led to a barcode index number (BIN) being associated with several species, which hampers a clear species identification within the BOLD system. In this study, grouper diversity in Vietnam was investigated to check the species inventory, provide a DNA barcode library for Vietnam, and reevaluate BINs associated with some species that are present in Vietnam. To this end, 157 specimens were barcoded corresponding to 30 species and 31 BINs. Nine species were new records, bringing to 49 the number of species inventoried for Vietnam. In BOLD, these species are associated with 75 BINs, 31 species being represented by more than one BIN. A careful review of these BINs, considering the present results from Vietnam, the species composition and frequency of each BIN in BOLD, and reference species sequences from phylogenetic studies, revealed those misidentified or hybrid specimens that artificially increased the number of BINs per species. After appropriate revision, 15 species still remained associated with more than one BIN. A phylogeographic analysis of these species demonstrated that most of the BINs derived from evolutionary lineages with geographic distributions that match well known biogeographic units in the Indo-Pacific. Beyond the species identification, these multiple BINs of each species can be used to track the biogeographic origin of each of the associated specimens.