**1. Introduction**

Parrotfish (family *Scaridae*), belonging to Actinopterygii: *Perciformes*, are herbivorous fish that live in tropical and subtropical coral reefs and are relatively abundant in biomass [1]. They play a crucial role in coral reef ecosystems [2,3], and as consumers of benthic algae, directly affect the structure and composition of benthic communities, and positively affect coral survival and growth [4]. Parrotfish are also involved in calcium carbonate cycling in coral reefs [5,6], and decompose coral reef skeletons into sand-sized sediments [7,8]. They maintain a coral-dominated community structure by feeding on fast-growing algae and can also influence reef development and complexity by decomposing reef carbonates [2,9]. Thus, sediments produced through parrotfish activities are an important source of island construction and maintenance in atoll coral reef environments, particularly in the setting of current rising sea levels and changes in island morphology [10]. The Healthy Reef Initiative (http://www.healthyreefs.org/cms/; accessed on 1 June 2022) uses parrotfish biomass as one of the key indicators in their coral reef health reports [11]. They have beautiful body shapes and bright colors, which greatly enhance the aesthetic quality of coral reefs and improve their economic value through ecotourism [3]. They are used as important food resources, and their biomass has recently decreased because of increased fishing [12,13].

**Citation:** Liu, B.; Yan, Y.; Zhang, N.; Guo, H.; Liu, B.; Yang, J.; Zhu, K.; Zhang, D. DNA Barcoding Is a Useful Tool for the Identification of the Family *Scaridae* in Hainan. *J. Mar. Sci. Eng.* **2022**, *10*, 1915. https:// doi.org/10.3390/jmse10121915

Academic Editor: Francesco Tiralongo

Received: 11 November 2022 Accepted: 18 November 2022 Published: 6 December 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Parrotfish, which are widely distributed in tropical, subtropical, and temperate areas [14], include approximately 100 species, divided among 10 genera [15]. There are 33 species of *Scaridae* in Taiwan and 31 species in mainland China [16]. The fish are known to change sex (from female to male) and color (inpectoral fin color from brown to blue) during growth [12,17]. Additionally, parrotfish have a behavioral mode called "grouping crypsis" (http://fishdb.sinica.edu.tw/; accessed on 1 June 2022). Juvenile parrotfish are discolored because of their social behavior. When they swim together, regardless of whether they are of the same species, their body color responds to the majority, becoming consistently grayish-brown, or exhibiting a longitudinal gradient in these colors. Once alone, juvenile fish instantly display beautiful and bright colors that blend into the surrounding environment, and these color changes occur very quickly. These extremely variable characteristics lead to difficulties in the morphological identification of *Scaridae*.

In the last decade, research on parrotfish has largely focused on their role and the mechanisms of their effects on coral reef ecosystems [9,18]. These fish can be excavators, scrapers, or grazers [19]. With increasing concern about parrotfish populations, the focus has been on coral reef health [20]. Furthermore, some researchers have explored the feeding ecology, habitat, diet, and habitat shifts of parrotfish by examining their fatty acid concentrations, composition, and levels [17]. Therefore, based on the economic, development and conservation of parrotfish, it is necessary to accumulate basic information about parrotfish and develop appropriate conservation and management measures [21]. A study using otoliths to identify parrotfish found that small otoliths were most similar to large *Scarus oviceps*, and least similar to large *Hipposcarus longiceps* [22]. However, there are several obvious shortcomings in the identification of otoliths: (i) there is no complete database of otolith morphology; (ii) because of the wide variety of parrotfish, it is difficult to find enough variability in otolith shape to identify species; and (iii) polishing otoliths is time-consuming, and the loss is large. Much experience is required to accurately identify otoliths.

In the past two decades, molecular techniques have become a popular and critical method for identifying species and resolving taxonomic ambiguities [23]. Molecular methods are useful in elucidating phylogenetic relationships and evolutionary patterns for biological ecology where classical morphological methods are not applicable [24]. DNA barcoding methods have been used to complement or refine morphological species identifications [25,26]. Studies have shown that the identification and discrimination of DNA barcoding are accurate and rapid [27–29]. DNA barcoding has been used to identify species using cytochrome c oxidase subunit I (*CO* I) sequences [30]. Therefore, they have been widely used for species identification [31–34].

In this study, more parrotfish samples were investigated to further evaluate the effectiveness of DNA barcoding for distinguishing parrotfish. The objectives of this study were to examine the reliability of *CO* I as a DNA barcode in parrotfish gene composition, and to determine intra- and interspecific genetic distances, codon characteristics, and molecular phylogenetic trees. The DNA barcoding data generated can be used as an effective molecular tool to achieve better monitoring and conservation outcomes for the family *Scaridae*.
