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. Although L. pulchella bury themselves in the sand at low tide, their trails allow buried individuals that crawled on Measurements the surface of the sand to be found, making it easy to col- The shell height (SH, mm) of Laguncula pulchella lect them. To investigate the density of L. pulchella that was measured with a caliper. Subsequently, the shell was emerged from the sand (termed surfaced here), the number crushed and the soft body wet weight (SBWW, g) was of individuals was counted within a 18 m×3 m area at St. measured after removing the operculum. The shell length H1–H10 and St. F1–F5. This procedure was repeated 4 to (SL, mm) of prey bivalves was also measured. The egg col- 17 times over the course of 30 min periods at each station. lar was measured (basal diameter in mm and wet mass in To investigate the abundance of L. pulchella prey, bivalves g), following Tomiyama (2013). Substratum samples were were collected from Sites H and F at the same time as L. dried at 80°C for 48 h after removing organic matter with pulchella were collected. The collection of bivalves was a 30% hydrogen peroxide solution. Then the samples were repeated three times at each station at St. H1–H5 (n=15). sieved through 2, 1, 0.5, 0.25, 0.125, and 0.063 mm meshes, Collection was carried out once at St. F1–F5 (n=5), with and were weighed to determine the median grain diameter additional collections at random (n=9) being conducted at and silt-clay content (%). Site F. Sediment samples, obtained from a depth of 10 cm using a shovel, were collected and sieved with a 5.6 mm Estimation of the population size of L. pulchella at Site mesh. The quadrat sizes were 25×25 cm at Sites H and H F, in addition to five quadrats of 20×20 cm size at Site F. To estimate the size of the Laguncula pulchella popula- 56 K. Yoshida et al. tion at Site H first, the clam fishing ground at this location Results was divided into 15 subareas based on 15 stations (Fig. 2). The L. pulchella egg-collar density at each station was Abundance used to estimate the number of adult individuals at each subarea. The density of surfaced Laguncula pulchella was sig- The adult population of L. pulchella in each subarea nificantly lower at Site H (1.9±2.8 inds 10−2 m−2, n=53; was estimated from the density of egg collars at St. H1– mean±SD) compared to Site F (3.7±3.8 inds 10−2 m−2, H15. The number of egg collars is equivalent to 1.25 times n=63; U=2224.5, P<0.01, Fig. 3). The largest number of the number of adult females, because one mature female surfaced L. pulchella within the 54-m2 area (18 m×3 m) produces either one egg collar (75% of females) or two was 7 and 10 at Site H and Site F, respectively. The density egg collars (25% of females) during the egg laying season, of egg collars was significantly lower at Site H (3.5±4.7 with egg collars only produced for a short period (around 10−2 m−2, n=52) compared to Site F (17.4±10.0 10−2 m−2, September to October, Tomiyama 2013). The number of n=8; U=394.5, P<0.001, Fig. 3). The largest number of egg collars produced by L. pulchella in the subareas was egg collars per 54 m2 was 8 and 18 at Site H and Site F, summed to estimate the total number of egg collars in respectively. The density of egg collars was significantly the fishing ground at Site H. The number of egg collars greater than that of surfaced individuals (GLM, coeffi- was divided by 1.25 and thereafter doubled to convert to cient=0.54, P<0.05). The density of egg collars and sur- the number of adult individuals based on the assumption that all adult individuals had already finished egg collar production before the survey date (two weeks after the first observation of egg collars), and that the sex ratio of adult individuals was approximately 1 : 1 (Okoshi & Sato- Okoshi 2011). Statistical analysis A Mann–Whitney U test was performed to compare the densities of surfaced Laguncula pulchella, egg collars, and Ruditapes philippinarum individuals at the two sites. The body size (SH) of surfaced L. pulchella, basal diameter of L. pulchella egg collars, and SL of R. philippinarum were also compared between the two sites. The densities of L. pulchella were compared between surfaced individuals and egg collars at Site H, using a generalized linear model (GLM) with negative binomial family and log-link func- tion. The number of individuals collected per 18 m×3 m area was used as the response variable, while stations (H1– H10) and targeted samples (surfaced individuals or egg collars) were used as explanatory variables. All data used for figures are available as a supplemen- tary materials (https://www.jstage.jst.go.jp/browse/pbr).

Fig. 3. Densities of (a) surfaced Laguncula pulchella on the tidal flat (Site H, n=53; Site F, n=63), (b) egg collar of L. pulchella Fig. 2. Subareas in the clam fishing ground (40,500 m2) at Site (Site H, n=52; Site F, n=8), and (c) surfaced L. pulchella and egg H used to estimate the population size of Laguncula pulchella. The collars at stations (surfaced: n=4–6 at each; egg collar: n=4–5 at solid square indicates the location of the clam fishing ground at Site each except H10). Vertical bars indicate SD. Shaded and gray bars H. Each subarea is identified by dotted lines. Solid circles indicate show densities of surfaced individuals and egg collars, respec- sampling stations. tively. Body size of Laguncula pulchella 57

Table 1. Environmental conditions at Sites H and F. Densities of Ruditapes philippinarum (Site H, n=15; Site F, n=14) are shown as mean±SD. Salinity was measured on the sea surface at low tide during the daytime on September 22 and October 13, 2015. Median grain diameter is shown as mean±SD at Stations H1–H10 and Sta- tions F1–F5. Silt-clay content is shown as mean±SD at Stations H1–H10 and Stations F1–F5. Elevation index was calculated as the difference between maximum elevation and minimum elevation at Stations H1–H15 and Stations F1–F5.

Site H Site F

Density of R. philippinarum (inds m−2) 61.9±73.0 480.0±213.8 Salinity at low tide 26.6–30.0 25.1–32.7 Median grain diameter (mm) 0.174±0.027 0.163±0.023 Silt-clay content (%) 0.64±0.39 1.58±0.84 Elevation index (cm) 99 26 faced individuals was significantly different between sta- tions (GLM, P<0.001), with that at St. H1 higher than the other stations, except St. H6 (Fig. 3c). Ruditapes philippinarum accounted for 69% of all bi- valves collected from Site H. Other bivalve species that were found included: Moerella rutila (Dunker), Macoma incongrua (Martens), Arcuatula senhousia (Benson), and Laternula gracilis (Reeve). Furthermore, Meretrix lusoria (Röding) and Solen strictus Gould were also identified in another survey at this site (Yoshida K, unpublished data). In the quadrat survey, R. phillipinarum and S. strictus had shells with drill holes (from predators) at frequencies of 27.7 m−2 and 1.1 m−2, respectively. Two L. pulchella indi- viduals were found in the quadrat survey covering 0.94 m2. Bivalves from Site F included R. philippinarum (97%), S. strictus, Nuttallia japonica (Reeve), and M. lusoria. Five individuals of L. pulchella were collected in the quadrat survey covering 0.76 m2. Salinity, as well as median grain diameter, and silt- clay content were similar at both sites (Table 1). The el- evation index was higher at Site H compared to Site F. Fig. 4. (a, b) Body size composition of surfaced Laguncula pul- The density of R. philippinarum was significantly lower chella on the tidal flat and (c, d) the size of L. pulchella egg collars. at Site H (61.9±73.0 inds m−2, n=5) compared to Site F (480.6±213.8 inds m−2, n=14; U=70, P<0.01, Fig. 3). from Site F. The largest individual collected from Site H The SL of R. philippinarum was not significantly dif- had an SH of 42.7 mm and that collected from Site F had ferent between Site H (23.0±8.5 mm, n=58) and Site F an SH of 43.9 mm. The SH of L. pulchella was significant- (24.2±7.4 mm, n=70; U=2141, P=0.60). No significant re- ly smaller at Site H (24.3±6.5 mm, n=107) than that at Site lationship was observed between the relative elevation and F (29.8±8.0 mm, n=164; U=13105.5, P<0.001). Individu- the density of L. pulchella at Site H (Spearman rank cor- als with an SH≤10 mm were not found at Site H, whereas relation, n=10, rs=0.45, P=0.194) or Site F (n=5, rs=–0.41, the smallest individual at Site F had an SH of 8.5 mm. Egg P=0.49). However, both surfaced individuals and egg col- collars with a basal diameter of 75–94 mm accounted for lars were abundant in the upper intertidal zones (St. H1 70% of all egg collar specimens at Site H (Fig. 4), whereas and H6, Fig. 3c). those with a basal diameter of 85–104 mm accounted for 80% of all egg collar specimens collected at Site F. The Size parameters basal diameter of egg collars was significantly smaller for Laguncula pulchella with an SH of 15.1–30 mm ac- the specimens from Site H (86.3±9.1 mm, n=120) than for counted for 74% of the L. pulchella specimens collected the specimens from Site F (93.6±7.5 mm, n=71; U=6290, from Site H (Fig. 4), whereas L. pulchella with an SH of P<0.001). 30.1–35 mm accounted for 66% of the specimens collected Individuals with extremely low soft body weight were 58 K. Yoshida et al.

the distribution of L. pulchella around Hiroshima Bay, western Hiroshima, had not been reported previously, L. pulchella had been previously observed at the Ebi tidal flat (34°39′N, 133°27′E), Onomichi, eastern Hiroshima (Oko- shi & Sato-Okoshi 2011). At the Ebi tidal flat, the highest density of L. pulchella egg collars recorded on October 15, 2015 was 2.4 per 100 m2. The egg collars had a basal diameter of 78.5±12.7 mm (range: 57.3 to 99.5 mm; n=24) (Yoshida K, unpublished data). Thus, the density and size of egg collars at the Ebi tidal flat were smaller than those recorded at Site H. However, large individuals (≥30 mm SH) were abundant in northern Japan before the Great East Japan Earthquake in 2011 (Mangoku-ura Lagoon: Sakai 2000; Matsushima Bay: Okoshi & Sato-Okoshi 2011; Mat- Fig. 5. Relationship between soft body wet weight (SBWW) sukawaura Lagoon: Tomiyama et al. 2011). Thus, L. pul- and shell height (SH) at Site H (open circles) and Site F (cross- chella adult individuals and egg collars may tend to occur es). These relationships are expressed as follows: Site H: ln SBWW=0.30×ln SH>−5.02 (dotted line; n=61, r=0.98), Site F: at lower densities and smaller sizes in southern Japan com- ln SBWW=0.27×ln SH−3.40 (solid line; n=130, r=0.99). pared to northern Japan, including Site F. In the present study, we also found that the soft body weight of L. pulchella was lower at Site H than at Site often observed at Site H, especially for sizes >20 mm SH F. This observation suggests that somatic weight indicates (Fig. 5). Using SH as a covariate, the SBWW of L. pul- the habitat quality of L. pulchella. Prey availability gener- chella was significantly lower at Site H than that at Site ally governs the somatic weight of predators (Tamburi &

F (one-way ANCOVA, F1,188=30.96, P<0.001). No signifi- Martín 2011). The density of Ruditapes philippinarum at cant interaction (SH×site) was observed. Site F was eight times greater than that at Site H. Thus, Site H had lower prey availability, which would affect the Egg laying season growth and somatic weight of L. pulchella. In addition to Fishermen at Site F first observed the egg collars of La- prey availability, water temperature is known to regulate guncula pulchella on September 22, 2015 (Fukushima Pre- growth rates (Ansell 1982), and it may also affect the size fectural Fisheries Experimental Station, personal commu- of L. pulchella. nication). The water temperature at low tide in the daytime The growth rate of L. pulchella may differ between lo- was 22.9°C on that day. At Site H, the egg collars of L. calities. Individuals of around 10 mm SH are considered pulchella and adults (n=2) producing egg collars were first to be 1 year old in September-October (Okoshi & Sato- found on October 13, 2015. The water temperature was Okoshi 2011). Such individuals were rarely found at Site 23.3°C around the tidal flat at low tide on that day, whereas H, but were abundant at Site F. This observation suggests the sea surface temperature in Hiroshima Bay outside of differences in growth rates of L. pulchella between the Site H was 22.0°C (Hiroshima City Fisheries Promotion two sites, or the absence of 1-year-old individuals at Site H. Center, personal communication). Egg collars were not Factors affecting growth, aside from prey availability and found at Site H until September 29, 2015. water temperature, have not been elucidated. The elevation index differed between the two sites, but is not considered Estimation of the size of the L. pulchella population at to affect the growth of L. pulchella because individuals Site H only emerge on the tidal flats for approximately 2–3 hours The number of Laguncula pulchella egg collars in the around the spring ebb tide at both sites, with L. pulchella clam fishing ground at Site H was estimated to be 1,280. inhabiting areas with depths of up to 15 m (Okutani 2000). Using this data, the size of the adult L. pulchella popula- Median grain diameter, silt-clay content, and salinity were tion in the clam fishing ground was estimated to be 2,048 similar at both sites, and are unlikely to affect the growth individuals. of L. pulchella. Mitochondrial DNA analyses have shown that L. pul- chella individuals collected from Site H (Ohtsuki et al. Discussion unpublished data) and Site F (Ohtsuki et al. 2016) belong To our knowledge, the present study is the first to pro- to a genetically foreign population. Pink-colored individu- vide detailed information on the abundance, body size, and als (Okoshi and Sato-Okoshi 2011) were detected at Site egg-laying season of Laguncula pulchella in an estuary in H, with this coloration only being found in foreign popula- southern Japan. To understand how this species affects na- tions (Fukuda & Kimura 2012). These pink individuals oc- tive clams in southern Japan, it is essential to investigate curred at a frequency of six individuals per 158 individuals its distribution and abundance in other tidal flats. Although (3.8%). However, it is not possible to conclude whether all Body size of Laguncula pulchella 59

calculated based on the mean mass (16.3 g) of single egg collar. Misuji River estuary is considerably smaller than Matsukawaura Lagoon. Thus, the removal of egg collars from Site H could considerably inhibit the increase of L. pulchella. It is important to understand how the L. pulchella popu- lation affects clam abundance at Site H. Assuming no mor- tality of L. pulchella and that L. pulchella consumes one clam every 3–4 days (Okoshi & Sato-Okoshi 2011), 2,048 adult individuals would consume 655 kg of R. philippina- rum (mean individual weight of 3.2 g) per year. In addition, L. pulchella individuals prefer clams of particular sizes (Chiba & Sato 2012, Tanabe 2012). Such size-dependent Fig. 6. Temporal changes in water temperature from August to predation would affect the size composition of prey (Com- December 2015. Squares and circles show the sea surface tempera- mito 1982). Moreover, L. pulchella feeds on other mollusks ture at Site H and at Site F, respectively. Solid and dotted arrows when the size of the R. philippinarum population decreases show the dates on which the egg collars of L. pulchella were first (Sato et al. 2012, Chiba & Sato 2013). Dead shells with observed at Site H (October 13) and Site F (September 22), respec- drill holes observed in some bivalve species at Site H in- tively. dicate the predation mortality caused by L. pulchella. In conclusion, further monitoring is necessary to evaluate the L. pulchella at Site H originate from an invasive or native predation impact of L. pulchella on the mollusk commu- population, because a native L. pulchella population inhab- nity in southern Japan. its this region (Okoshi & Sato-Okoshi 2011). The egg collars of L. pulchella were observed after late Acknowledgements September when the water temperature was below 22°C at Sites H and F (Fig. 6). Water temperature affects the tim- We thank the fishermen from both the Misuji River es- ing of egg collar production (Tomiyama 2013); this finding tuary and Matsukawaura Lagoon for cooperating with the suggests that egg collar production starts later at Site H is field surveys. We also thank Dr. G. Kanaya for helping because this site has a higher water temperature than Site F with the identification of clam species, and anonymous two year-round. reviewers for their critical comments on the manuscript. The density of egg collars was greater than that of sur- faced individuals that were counted. Thus, the density of References surfaced individuals cannot be used to estimate the popu- lation size of L. pulchella, because many burrowing indi- Ansell AD (1982) Experimental studies of a benthic predator- viduals remain underground and because the abundance prey relationship. I. Feeding, growth, and egg-collar produc- of surfaced individuals may vary across survey dates. To tion in long-term cultures of the gastropod drill Polinices al- improve the accuracy of this estimate, it would be neces- deri (Forbes) feeding on the bivalve Tellina tenuis (da Costa). J sary to dig a large area of the substratum. This is a time- Exp Mar Biol Ecol 56: 235–255. consuming and laborious approach and was therefore not Chiba T, Sato S (2012) Size-selective predation and drillhole-site feasible to employ in the present study. In contrast, it was selectivity in Euspira fortunei (Gastropoda: Naticidae): impli- feasible to use the density of egg collars to estimate the cations for ecological and paleoecological studies. J Mollusc number of adult L. pulchella. This technique is useful to Stud 78: 205–212. effectively quantify the number of adult L. pulchella. How- Chiba T, Sato S (2013) Invasion of Laguncula pulchella (Gas- ever, this method cannot be used to estimate the number tropoda: Naticidae) and predator–prey interactions with bi- of immature individuals in a population or to estimate the valves on the Tona coast, Miyagi prefecture, northern Japan. Biol Inv 15: 587–598. population size of naticids, which lay several egg collars Commito JA (1982) Effects of Lunatia heros predation on the per individual during an egg-laying season (Ansell 1982). population dynamics of Mya arenaria and Macoma balthica in In addition, if some adult females, such as individuals with Maine. Mar Biol, 69: 187–193. low somatic weight, did not lay egg collars, the adult popu- Fukuda H, Kimura S (2012) Laguncula pulchella Benson, 1842. lation might be underestimated. An alternative method for In: Threatened Animals of Japanese Tidal Flats: Red Data estimating the population size of naticids is the capture- Book of Seashore Benthos (eds The Japanese Association of recapture method (Hunter & Grant 1966). We believe that Benthology). Tokai University Press, Hadano, p. 58. (in Japa- these methods need to be improved to enable accurate esti- nese) mation of the population size of L. pulchella. Hunter WR, Grant DC (1966) Estimates of population density The total mass of the 1,280 egg collars of L. pulchella in and dispersal in the naticid gastropod, Polinices duplicatus, the clam fishing ground at Site H was approximately 21 kg, 60 K. Yoshida et al.

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