Plankton Benthos Res 12(1): 53–60, 2017 Plankton & Benthos Research © The Japanese Association of Benthology Abundance and body size of the moonsnail Laguncula pulchella in the Misuji River estuary, Seto Inland Sea, Japan: comparison with a population in northern Japan 1 2 2 1, KAZUKI YOSHIDA , TATSUMA SATO , KAORU NARITA & TAKESHI TOMIYAMA * 1 Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739–8528, Japan 2 Soma Branch, Fukushima Prefectural Fisheries Experimental Station, Soma, Fukushima 976–0022, Japan Received 6 May 2016; Accepted 30 November 2016 Responsible Editor: Shigeaki Kojima Abstract: The moonsnail Laguncula pulchella (Naticidae, formerly known as Euspira fortunei) is an invasive species that has had a negative impact on the Manila clam (Ruditapes philippinarum) population in northern Japan. In southern Japan, few records of L. pulchella exist; furthermore, the knowledge concerning the ecology of this organism is cur- rently limited. Unexpectedly, we found many L. pulchella individuals in the Misuji River estuary, located in Hiroshima in southern Japan, during May 2015. We conducted a field survey on the density and body size of L. pulchella at a tidal flat in the Misuji River estuary (Site H) and compared those to L. pulchella in Matsukawaura Lagoon, located in Fukushima in northern Japan (Site F). The adult population of L. pulchella in a clam fishing ground (40,500 m2) at Site H was estimated to be 2,048 individuals in 2015. Densities of both surfaced individuals and egg collars of L. pulchella were lower at Site H than at Site F. Sizes of L. pulchella individuals and their egg collars as well as the somatic weight of the individuals at Site H were significantly smaller than those at Site F. These results suggest that the biological char- acteristics of L. pulchella largely differ between the two sites. Key words: invasive species, naticids, tidal flat management 2007). Thus, it is important to elucidate the influence of L. Introduction pulchella on the mollusks at each locality to minimize its The moonsnail, Laguncula pulchella Benson (Naticid- impact. ae, formerly known as Euspira fortunei; Torigoe & Inaba In May 2015, we unexpectedly identified multiple L. 2011), is a predator of mollusks. This species has a native pulchella individuals in the Misuji River estuary in Hiro- population in southern Japan, and has been assigned as an shima Bay, southern Japan. It was suggested that this spe- endangered species (Fukuda & Kimura 2012). However, cies was introduced to this site because R. philippinarum since the late 1980s, L. pulchella has been unintention- juveniles from other localities have been released there. ally introduced to other areas of north and south Japan Actually, L. pulchella individuals collected from the Misu- through the deliberate import of Manila clam Ruditapes ji River estuary are genetically similar to non-indigenous philippinarum (Adams & Reeve) for cultivation and rec- individuals collected from Miyagi Prefecture in northern reational clamming (Okoshi 2004, Okoshi & Sato-Okoshi Japan (Ohtsuki et al. unpublished data). 2011). In northern Japan, the abundance of L. pulchella has This study aimed to determine the abundance, body noticeably increased, where they have decimated R. philip- size, and population size of L. pulchella in the Misuji River pinarum fisheries (Sakai 2000, Okoshi 2007). The biologi- estuary. Field surveys on the density of L. pulchella crawl- cal characteristics of L. pulchella in northern Japan have ing on the tidal flat were conducted. We also investigated been previously defined; however, there is little informa- the egg laying season, the size of egg collars, and the den- tion concerning the biology of this species in southern Ja- sity of egg collars in the field. These surveys were also car- pan (Torigoe 1988, 1989, Masuda & Fukuda 2006, Masuda ried out at a tidal flat in Matsukawaura Lagoon, located in northern Japan, and compared to those found in the Misuji * Corresponding author: Takeshi Tomiyama; E-mail, tomiyama@hiro- River estuary. We estimated the size of the adult popula- shima-u.ac.jp tion of L. pulchella at a clam fishing ground (40,500 m2) in 54 K. YOSHIDA et al. the Misuji River estuary using the density of egg collars as 140°59′E). At Site H, a tidal flat of ca. 60,000 m2 appears a proxy. at low tide (Fig. 1c). The tidal range is <4 m (Japan Meteo- rological Agency website, http://www.data.jma.go.jp/gmd/ kaiyou/db/tide/suisan/index.php [in Japanese]). Between Materials and Methods 2007 and 2014, Ruditapes philippinarum juveniles were released annually (90–330 kg each year) in the 40,500- Study site m2 clam fishing ground at Site H between April and June Two tidal flats were chosen as study sites: one at the (Fisheries Division of Hiroshima City, unpublished data); Misuji River estuary (Site H) located in Hiroshima in the yet, the clam fishery was nearly terminated because of the Seto Inland Sea in southern Japan (34°21′N, 132°21′E; low abundance of clams. It is not known when Laguncula Fig. 1) and another in Matsukawaura Lagoon in Fuku- pulchella was first introduced to Site H, even though local shima along the northern Pacific coast of Japan (37°49′N, fishers have been aware of the presence of this species at Fig. 1. Map of the study sites and the location of the sampling stations. The plots in (c) and (f) indicate the sampling stations at Site H and Site F. Body size of Laguncula pulchella 55 this site for at least 10 years. To determine sediment characteristics, substratum samples Matsukawaura Lagoon covers an area of 646,000 m2 were collected from the sediment surface to a depth of (Ministry of the Environment website, https://www.env. 10 cm at each station. Individuals of L. pulchella and bi- go.jp/water/heisa/heisa_net/waters/matukawaura.html valves, in addition to substratum samples, were transferred [in Japanese]). The maximum tidal range is approximate- to the laboratory immediately after collection. Relative el- ly 1.5 m (Japan Meteorological Agency website). Almost evation was determined by measuring the water depth at half of the lagoon was intertidal before the Great East Ja- each station when the tidal flat was covered with seawater, pan Earthquake (Fig. 1e). Fisheries of R. philippinarum and the difference in elevation between stations, includ- were previously operational in the lagoon, with juveniles ing maximum and minimum elevation, was defined as the transported to this site from other domestic and foreign elevation index. locations until 2010. Laguncula pulchella was first found Egg collars of L. pulchella are usually produced in Sep- at this site in 2002, which has been confirmed by fisher- tember or early October in Matsukawaura Lagoon (Tomi- men since 2003 (Tomiyama et al. 2011). From 2004, fisher- yama 2013). On the other hand, the egg-laying season of men attempted to remove L. pulchella and their egg col- this species is not known in southern Japan, although the lars from the fishing grounds annually, except for in 2011 egg collars were observed in November in Hyogo Prefec- when debris from the tsunami caused by the Great East Ja- ture, located in the eastern Seto Inland Sea (Okoshi & pan Earthquake in March prevented access to the site. The Sato-Okoshi 2011). To investigate the timing of egg collar population of L. pulchella was reduced by the tsunami, but occurrence of L. pulchella at Site H, daytime field obser- again increased between 2012 and 2014 (Okoshi and Su- vations at low tide were conducted every two weeks from zuki 2014, Sato et al. 2016). September to October. Water temperature and salinity In the preliminary survey at Site H in 2015, L. pulchella were measured around the tidal flat. For Site F, fishermen was rarely observed outside the clam fishing ground. Three recorded the date the first L. pulchella egg collars (Sep- transect lines perpendicular to the shoreline were estab- tember 22, 2015). We monitored water temperature and lished at 120 m and 150 m intervals in the fishing ground salinity near Site F with a conductivity-temperature logger at Site H, with five sampling stations (St.) being set on each (Infinity CTW, JFE Advantech Co., Ltd). Egg collars of L. transect line at 30 m intervals from the uppermost zone in pulchella were collected by hand from Site F on September the tidal flat (St. H1–H15, Fig. 1c). In the Matsukawaura 25, 2015, before removal by fishermen, and from Site H Lagoon, we selected a tidal flat where L. pulchella was on October 28, 2015. We assumed that the density of egg abundant (Site F). Site F was a small tidal flat (approxi- collars did not change during the month after first laying, mately 100 m×100 m) on which just one transect line with because individual females produce one or two egg col- five sampling stations set at 20-m intervals was established lars only once during the egg-laying season (Tomiyama (St. F1–F5, Fig. 1f). 2013), and 30 to 40 days from egg-laying to hatching are required (Sakai & Suto 2005). To investigate the density Field survey of adult L. pulchella, egg collars on the tidal flats during Crawling Laguncula pulchella individuals were col- ebb tide were counted three to five times within an area of lected from Sites H and F in August 2015. Laguncula pul- 18 m×3 m at each station (St. H1–H15) and in two areas at chella come to the sand surface and leave a trail on the St. F1–F2 and St. F4–F5. Collected egg collars were trans- tidal flat during the ebb tide (Okoshi & Sato-Okoshi 2011). ferred to the laboratory.