Clithon retropictus (Martens 1878) belongs to the Gastropoda (Neritidae) family and is distributed in the brackish water zone of Korea and Japan (Neritopsine Gastropods 2010). In Korea, it is found in some small streams basins in the southern coast and Jeju Island (Fig. 1). Its distribution is restricted to locations with a constant flow of water, large and small stones, and gravels (Kwon et al. 2001; Noseworthy 2012). Because of the ecological characteristics determining a limited distribution, the habitat is likely to be easily destroyed by the development of rivers and coasts (Malmqvist and Rundle 2002).

C. retropictus is a small snail belonging to the Neritidae about 10-15 mm in width and height. The shell is made up of four layers, and shell color is relatively diverse. Generally, it has triangular-shaped yellow and black spots on a brown background, and two rows of white bands appear on the layers of the shell (Fig. 2). Most large individuals have a broken protoconch, and the mouth of the shell is half-moon-shaped with small denticules on the inner lips of the mouth (Biodiversity on the Korea 2018). Among the confirmed species in Neritinidae, C. retropictus is one of the longest-living gas- tropods living in seawater or freshwater, with a lifetime of 12 years (Ohara and Tomiyama 2000; Miyajima and Wada 2014). C. retropictus lay and attach eggs on large stones or pebbles from April to August. As shown in the Fig. 3 each egg has several embryos, which grow safely in white milky colored solid eggs. After growing through the process of cell division, the embryos growing to be a spat, break and come out of the eggs. C. retropictus is one of the major dominant species in the estuarine ecosystem and occupies an ecologically important position. Nonetheless, less well-known species of endangered species, including C. retropictus have been underestimated in ecological value. The species that have been underestimated in ecological value, such as C. retropictus, are now classified as ecosystem major species, which play an important role in the nutrient cycling of ecosystems.

In Korea, morphological and habitual studies of C. retropictus have been carried out (Noseworthy et al. 2012, 2013), as well as genetic studies through transcriptomic analysis (Park et al. 2016). However, these ecological studies were carried out only in the Jeju Island region, and there is no research on the southern coastal area which is one of their main habitats. In Korea, small stream estuaries are a known habitat of C. retropictus (Biodiversity on the Korea 2018), but additional habitat was found in upstream in a small watershed that is almost unaffected by seawater. Kobayashi and Iwasaki (2002) reported that C. retropictus was distributed only in the downstream region of rivers where fresh water was connected to sea water in Japan. Hirata et al. (1999) reported that C. retropictus seemed to prefer water that was affected by sodium. From these results, it can be inferred that C. retropictus can adapt to a wide range of salinity due to its biological success in the brackish water zone.

However, the densely primary habitats are located in the lower stream area where water depth is low, and gravel has developed. C. retropictus is reported to inhabit silt-rich areas with gravel and rocks covered with algae, and occasionally the stems of aquatic plants (Neritopsine Gastropods 2010). In the southern coast of Korea, it is reported to inhabit artificial concrete structures or shellfish shells. Also, as in the results of Noseworthy et al. (2013), this study found that C. retropictus inhabits the lower portions of rocks in the mud and rocks that are not exposed.

C. retropictus has been protected by the Ministry of Environment as an endangered species since 1998 and has been listed on the state red list of endangered wildlife category II. It is categorized as vulnerable (VU) in the national red list of endangered wildlife. The vulnerable (VU) category means indicates a state in which most valid evidence in the national red list of the Ministry of Environment is consistent with one of the criteria for vulnerability. In this case, it is facing a high threat of extinction in the wild. However, the red list of threatened species managed by the International Union for Conservation of Nature (IUCN) does not include C. retropictus.

Although C. retropictus is currently designated as endangered wildlife category II by the Ministry of Environment and is being protected and managed, habitat degradation is increasing because of road construction and small stream improvement projects. If these habitats are continuously influenced by external factors such as development and environmental pollution, the threat of extinction will increase locally area. In addition, there is no worry about being overfished, but it is distributed intensively in the very limited water streams. Because of this, small environmental changes such as the inflow of non-point pollutants can cause great damage to C. retropictus. Restricting seawater inflow into the reservoir facility near the habitat can also be a major threat, C. retropictus inhabits a limited area at a high density per unit area. It is more important to prevent the habitat from being damaged or destroyed than to protect individuals of C. retropictus.

In addition, as the habitats of C. retropictus are located where rivers meet the sea, it is very difficult to prevent damage to or destruction of the habitat due to road construction, small stream development, irrigation for securing agricultural water, and flood prevention. If development is inevitable, it is necessary to protect endangered species by preparing alternative habitats and transplanting individuals, and to develop rivers while maintaining their natural bottom condition (Fig. 4). Once the construction is completed in this habitat, it is necessary to prepare plans to relocate the transplanted individuals by examining the environmental characteristics of the habitat. In order to systematically protect and manage endangered wildlife category II, the systematic improvement plan as shown in Table 1 is needed. The first and foremost step that the Ministry of Environment should take is to improve institutional arrangements through the establishment of measures for habitat conservation and construction methods by supplying information on the distribution of endangered species in each local government.

Generally, the damage to habitats by development can be recognized by strategic environmental evaluation. The departments responsible for the licensing and development of small river basins in local governments are the construction and development departments that do not have sufficient information to recognize the damage. For this reason, the departments need to recognize endangered species during the construction of roads and development of small watersheds, which causes various social and legal problems. Therefore, it is urgent to establish plans for monitoring and managing the population of C. retropictus and to protect its habitat in small streams along the southern coast of Korea.