Distribution of the invasive freshwater shrimp Palaemon sinensis (Sollaud, 1911) in rivers of Hiroshima Prefecture, western Japan

The freshwater palaemonid shrimp, Palaemon sinensis was imported into Japan from China as live fishing bait, and its introduction has become a concern in Japanese freshwater areas. In the present study, field research was conducted to confirm the distribution of this invasive shrimp in eight rivers in Hiroshima Prefecture, western Japan. We found caridean shrimp, such as P. sinensis, Palaemon paucidens, Palaemon serrifer, Palaemon macrodactylus, Palaemon orientis, Macrobrachium nipponense, Caridina leucosticta, and Neocaridina spp.. P. sinensis was found in four rivers. A longitudinal distribution analysis showed that P. sinensis occurred in the lower reaches, whereas P. paucidens showed a wider distribution from the upper to the lower reaches. Palaemon sinensis occurred in a narrow range of stream velocity (0–2 cm/s) compared with that of P. paucidens (0–18 cm/s). P. sinensis density was negatively correlated with river velocity, indicating that this shrimp prefers a lentic water environment.


Introduction
Human-mediated introductions of aquatic organisms beyond their native range occurred with increasing frequency during the latter half of the 20th century (Carlton 1996;Rodríguez and Suárez 2001;Iwasaki 2006;Ashelby et al. 2013).Although the shipping industry has received considerable attention as a dispersal mechanism for aquatic nuisance species, many invasions have been linked to other transfer mechanisms, such as the bait industry (Weigle et al. 2005).Release of unused bait by anglers is an important vector for invasive species (Haska et al. 2012;Kilian et al. 2012).Live fishing bait has been transported from Asian countries, mainly China, to European countries, the United States, and Japan (Olive 1994;Gambi et al. 1994;Costa et al. 2006;Cohen 2012).More than 20 species of live fishing bait, such as polychaetes, bivalves, crustaceans, and fish, have been imported into Japan at a rate of approxi-mately 1,000 tons/year since 1969 (Hayashi 2001;Saito et al. 2011Saito et al. , 2014)).
Although P. sinensis was not reported in Japan before 1990 (Liu et al. 1990), this species was discovered recently in freshwater ponds in central (Shizuoka Prefecture) and western (Kagawa Prefecture) Japan (Oonuki et al. 2010;Imai and Oonuki 2014).Release of the bait by anglers is thought to be why P. sinensis is found in freshwater areas.Therefore, P. sinensis may also be established in other parts of Japan.However, introduction of shrimp remains poorly investigated in Japanese rivers.In the present study, field research was conducted to confirm the distribution of this species in rivers in Hiroshima Prefecture, western Japan and to determine the relationships between distribution and environmental characteristics.

Materials and methods
We surveyed the Oze, Yahata, Ohota, Seno, Kurose, Kamo, Nuta, and Ashida Rivers in Hiroshima Prefecture (Figure 1).Estuarine and freshwater shrimp were sampled at five locations in each river during March 2015.The shrimp was sampled using a D-shaped hand net (opening, 35 × 30 cm; mesh size, 2.5 mm; handle length, 165 cm).Submerged vegetation and root masses in an approximate 1 m 2 area at each station along the riverbanks were scooped with a net during the day.The shrimp was preserved immediately in 10% formalin and identified with a stereoscopic microscope (SMZ745T; Nikon, Tokyo, Japan).
We examined and identified both species based on the difference in carapace colour pattern (Imai and Oonuki 2014).Other estuarine and freshwater shrimp species were identified according to Hayashi (1990Hayashi ( , 1999Hayashi ( , 2000a, b, c) , b, c) and Toyota and Seki (2014).The atyid shrimp Neocaridina denticulate (De Haan, 1844) is distributed in western Japan (Hayashi 1990;Toyota and Seki 2014).However, non-native Neocaridina spp.are imported as live fishing bait from China and South Korea, and have been dispersed in various parts of Japan (Niwa 2010;Niwa et al. 2014).Because the taxonomy of some Neocaridina spp. is controversial (Shih and Cai 2007;Niwa et al. 2010;Yoshigo 2011;Klotz et al. 2013), we denote Neocaridina shrimp as Neocaridina spp.
Distance from the mouth of the river and altitude were calculated for each sampling point using Google Maps API v3.Salinity, dissolved oxygen (DO), electrical conductivity, and water temperature were measured with a YSI model 85 water quality meter (YSI Inc., Ohio, USA).
Velocity at the water surface was measured using the float method.A stepwise multiple regression was performed using Ekuseru-Toukei ver.2015 software (Social Survey Research Information Co., Ltd., Tokyo, Japan) to evaluate the effects of the environmental variables (distance from the mouth of river, altitude, salinity, DO, electrical conductivity and velocity) on P. sinensis density.
P. sinensis appeared mainly in lower reaches, except the tidally influenced area (Figure 3).P. paucidens was widely distributed from the upper to the lower reaches, including the tidally influenced areas.The distributions of P. serrifer, P. macrodactylus, P. orientis, and M. nipponense were restricted to the tidally influenced areas.C. leucosticta also occurred in tidally influenced areas and the adjacent freshwater area.Neocaridina spp. was widely distributed from the upper to the lower reaches, except the tidally influenced area.
The stepwise multiple regression between P. sinensis density and the environmental variables (F in = 2, F out = 2) showed that of the predictor variables only velocity (F = 4.848, p = 0.041) was selected as influential (Table 2).

Discussion
Although P. sinensis has been reported to inhabit freshwater ponds in central and western parts of Japan (Oonuki et al. 2010;Imai and Oonuki 2014), occurrence of this species was also confirmed in freshwater areas of four of the eight surveyed rivers in western Japan; the Ohota, Seno, Kamo, and Nuta Rivers.In the last three rivers, it was found in lower reaches at 3-9 m of altitude and 2.0-4.8km from the river mouth.In the Ohota River, it occurred in the mid-stream at 11 m of altitude and 17.0 km from the river mouth, ca.1.5 km upstream from the Takase dam that intercepts the flowing tide.Our results show that P. sinensis has a narrow range of stream velocity (0-2 cm/s) compared with that of P. paucidens (0-18 cm/s).Stepwise multiple regression revealed that the density of P. sinensis was negatively correlated with water velocity.P. sinensis reportedly occurs in lentic downstream environments of Far East Russia (Kawai and Nakata 2011).In Japan, this introduced shrimp seems to be restricted to lentic environments similar to the native habitats found in continental regions of China and Russia.
The longitudinal distribution of shrimp in a river can be affected by river topography and species life history (Shokita 1979;Suzuki et al. 1993;Saito et al. 2012).In the present study, P. serrifer, P. macrodactylus, and P. orientis occurred in the tidally influenced areas.These species are marine and estuarine shrimp with planktonic larval stages (Chen and Hirano 1989;Kim 2010;Lejeusne et al. 2014).The other caridean shrimp, such as C. leucosticta, M. nipponense, P. paucidens, Neocaridina spp., and P. sinensis had different distributional patterns.Caridean shrimp species that inhabit lotic environments are classified into two types based on their methods of adaptation, i.e., amphidromous shrimp species that metamorphose in a marine environment and lay smaller eggs, and landlocked shrimp that lead their entire life history in a river and lay fewer large eggs (Shokita 1979).C. leucosticta and M. nipponense were collected in the tidally influenced areas and/or adjacent freshwater area.These shrimp are considered to be diadromous and migrate between the river and sea to spawn (Armada et al. 1993;Kawai and Nakata 2011;Saito et al. 2012;Toyota and Seki 2014).In fresh water areas, landlocked shrimp, such as P. sinensis, P. paucidens and Neocaridina spp.were distributed and lay fewer large eggs (Shokita 1979;Shih and Cai 2007;Oonuki et al. 2010;Toyota and Seki 2014).These shrimp are important components of aquatic ecosystems, playing a key role at intermediate tropic levels, as consumers of algae, detritus and small insects, and as prey of many fishes (Kawai and Nakata 2011;Puspitasari 2013).In the six sampling stations where P. sinensis was found, Neocaridina spp co-occurred at all stations, but P. paucidens appeared only one station.Although P. paucidens can live in lentic downstream environments, there is a possibility that its distribution was narrowed by P. sinensis.Further researches are needed to clarify the invasion effects of P. sinensis on the native shrimp (e.g., competition with them or ecosystem change via the food chain).
From the latter half of the 20th century, caridean shrimp species, such as P. macrodactylus, M. nipponense, Macrobrachium dayanum (Henderson, 1893), Palaemon modestus (Heller, 1862) and Neocaridina davidi (Bouvier, 1904) have been reported to be introduced intentionally or unintentionally from East Asian countries worldwide (Rodríguez and Suárez 2001;De Grave and Ghane 2006;Gorgin and Sudagar 2008;De Grave and Mann 2012;Ashelby et al. 2013;Klotz et al. 2013;Lejeusne et al. 2014).In Japan, several non-native Neocaridina spp.have been imported from China and South Korea for use as live fishing bait together with the ectosymbiont annelid, Holtodrilus truncatus (Liang, 1963), and later dispersed and settled after being discarded into Japanese freshwater environments by anglers (Niwa 2010;Niwa et al. 2014).P. paucidens and P. sinensis are supplied under the trade name "Shirasa ebi" in western Japan as live fishing bait (Niwa 2010).P. paucidens has been evaluated as a target aquaculture species in various parts of Japan for food and fishing bait (Ogawa and Kakuda 1988).Since traders and fishermen are unaware of the difference between P. paucidens and P. sinensis (Saito et al. 2011;Imai and Oonuki 2014), P. sinensis will be mixed in shrimp seed for P. paucidens aquaculture.Therefore, it is possible that P. sinensis will become established via not only release by anglers but also escape from shrimp farms, so further investigations are necessary to confirm the occurrence of this species, particularly in areas where "Shirasa ebi" has been cultured.

Figure 1 .
Figure 1.Sampling locations in Hiroshima Prefecture, western Japan.For details see Supplementary material TableS1.

Figure 2 .
Figure 2. Occurrence of estuarine and freshwater caridean shrimp species in Hiroshima Prefecture.

Figure 3 .
Figure 3. Longitudinal distributions of estuarine and freshwater caridean shrimp species The graph for each species was prepared based on shrimp collected from all rivers sampled.

Table 1 .
Physical environments at the sampling stations.

Table 2 .
Resuts of stepwise multiple regressions of P. sinensis density with environmental variables.