VENUS 74 (3–4): 71–78, 2016 DOI: http://doi.org/10.18941/venus.74.3-4_71Shell Growth of giganteum ©The Malacological Society of Japan71

Shell Growth of Umbonium (Suchium) giganteum (: ) in Sagami Bay Based on Oxygen Isotope Profiles

Kentaro Nakayama1*, Yasuo Kondo2 and Takehiro Sato3 1Graduate School of Integrated Arts and Sciences, Kochi University, 2-5-1 Akebono-cho, Kochi City, Kochi 780-8520, Japan 2Sciences Unit, Natural Sciences Cluster, Kochi University, 2-5-1 Akebono-cho, Kochi City, Kochi 780-8520, Japan 3Kanagawa Prefectural Museum of Natural History, 499 Iryuda, Odawara, Kanagawa 250-0031, Japan

Abstract: Investigations of growth rates of marine mollusks are important for understanding paleoenvironmental conditions that influenced their evolution. Here we obtained oxygen isotope (δ18O) profiles for two Umbonium (Suchium) giganteum individuals collected from Sagami Bay on November 25, 1999, to verify the findings of a previous study that used population dynamics to determine growth rates. The oxygen isotope profiles for both individuals exhibited similar cyclic patterns of gradual increases and rapid decreases, with three maximum and three minimum values. Because this inhabits areas that are not influenced by fresh water, these values represent seawater temperatures from spring 1997 to fall 1999. Profiles of both individuals were smooth in both summer and winter, suggesting that this species grows almost continuously throughout the year, with no major seasonal cessation of growth. The findings suggested that the prominent growth lines on the surfaces of the shells coincided with sudden decreases in the seawater temperature. Because this species mainly spawns in fall and winter when the seawater temperature decreases, these prominent growth lines can thus be considered to represent spawning events. This was supported by observations of other individuals that were collected at the same time, which suggested that spawning occurs once or twice each fall. However, spring or early summer spawning was inferred from the oxygen isotope analysis of microsamples from the older parts of one of the individuals, which would likely not have been recorded as prominent growth lines because of the greater growth rate during this season. Our results broadly confirmed those of a previous study on the rate and pattern of shell growth of U. (S.) giganteum. However, we found slightly higher growth rates than those reported by the previous study, particularly in juveniles.

Keywords: Umbonium giganteum, Trochidae, oxygen isotope, growth analysis

Introduction

Umbonium (Suchium) giganteum (Lesson, 1833) is a trochid gastropod that is distributed from the Oga Peninsula to southern Kyushu (Sasaki, 2000), in shallow-sea sandy substrates at depths of 1–5 m (Sato et al., 2009). Fossils of extinct species of Umbonium (Suchium) are abundant in the Neogene and Quaternary formations in Japan and have received special attention because they show successive morphological changes within the stratigraphic successions and are regarded as typical examples of an evolutionary lineage in mollusks (Makiyama, 1925; Shuto, 1956; Ozawa &

* Corresponding author: [email protected] 72 K. Nakayama et al.

Okamoto, 1993). However, the ecological characteristics of extinct and extant species, such as their growth rate, must be studied to better understand the paleoenvironmental conditions that infl uenced morphological and evolutionary patterns. Sato et al. (2009) analyzed the population dynamics of U. (S.) giganteum by conducting continual sampling in Sagami Bay off the coast of Hiratsuka, Japan and concluded that this species has a higher growth rate than the other three extant species [U. (S.) suturale, U. (S.) moniliferum, and U. (S.) costatum]. Oxygen isotope analysis of shells can provide independent information regarding the growth history of shelled by recognizing seasonal changes in the water temperature and can be equally applied to the shells of fossilized and living individuals. Therefore, in this study we conducted oxygen isotope analyses on two of the individuals of U. (S.) giganteum that were collected and studied by Sato et al. (2009), and compared the results of these two studies.

Materials and Methods

Specimens and habitat The specimens analyzed in this study were collected from Sagami Bay (35°48´N, 139°27´E to 35°19´N, 139°27´E; dredged at depths less than 15 m) on November 25, 1999. We collected two specimens (KPM-NGL000117, hereafter the “orange” specimen; and KPM-NGL000118, hereafter the “black” specimen), which were among the specimens used by Sato et al. (2009) (Fig. 1). The “orange” specimen was slightly smaller than the “black” specimen (shell diameter 32.8 vs. 33.7 mm, shell height 21.5 vs. 21.6 mm, respectively). In addition, a further 17 individuals were collected from the same site to determine growth rates. All the voucher specimens are deposited in the molluscan collection in the Kanagawa Prefectural Museum of Natural History (KPM-NGL).

A A’ P2 = KPM-NGL000117 (orange specimen) P1 B B’

P1 P2.1 = P2.2 KPM-NGL000118 (black specimen) P3 10 mm

Fig. 1. The shells of the two Umbonium (Suchium) giganteum specimens analyzed in this study. A. KPM- NGL000117 (“orange” specimen). B. KPM-NGL000118 (“black” specimen). A’ and B’ show the same shells coated with magnesium oxide. P1, P2, and P3 indicate the prominent growth lines. Shell Growth of Umbonium giganteum 73

In Sagami Bay, U. (S.) giganteum inhabits sandy bottoms in shallow water at depths of 1–5 m, alongside Glycymeris albolineata (Lischke, 1872), G. vestita (Dunker, 1877), Dosinorbis bilunulatus (Gray, 1838), Meretrix lamarcki (Deshayes, 1853), Scapharca satowi (Dunker, 1882) and others (Matsushima, 1984; Sato et al., 2009). This depth zone is the second shallowest of the five depth zones recognized along the Sagami Bay coast by Sato et al. (2009), which included the Philyra syndactylus–Chion semigranosus (Dunker, 1887) zone (<1 m depth), the Umbonium giganteum–Mactra chinensis Philippi, 1846 zone (1–5 m), the Diogenes spinifrons–Scaphechinus mirabilis zone (5–8 m), the Astropecten latespinosus– (Valenciennes, 1838) zone (>8 m), and the fragmented shell accumulation zone (approximately 10 m). Sato et al. (2009) stated that the distributions of U. (S.) giganteum and Mactra chinensis vary in contrasting ways according to the extent of freshwater inflow from the Sagami River, with U. (S.) giganteum being dominant when the freshwater influence decreases, and M. chinensis dominating when the freshwater influence increases. Therefore, the oxygen isotope composition of U. (S.) giganteum is inferred to be only slightly affected by fresh water, and so can be considered to reflect the sea water temperature.

Growth analyses (Oxygen isotope and annual ring analyses) For oxygen isotope analyses, powdered microsamples were collected from the outer surface of each shell along the growth line from the umbo to the using a hand router. In total, we analyzed 35 microsamples from the “orange” specimen and 19 microsamples from the “black” specimen. For the “orange” specimen, the microsamples were taken from the parts that corresponded with the juvenile to adult stages. The position of the first sample was tentatively defined as the starting point (0 mm). Oxygen isotope analyses were performed with a Finnigan MAT253 Isotope ratio mass spectrometer at the Center for Advanced Marine Core Research, Kochi University, Japan. The NBS-19 carbonate standard was used for calibration to the Pee Dee Belemnite (PDB) standard. As a result of the oxygen isotope analyses, the prominent growth rings of U. (S.) giganteum are found to be annual rings as described later. On the basis of this association, the shell growth was analyzed for 19 individuals, including the “orange” and “black” individuals. The growth rates were evaluated using the Walford plot, which is often used for analyses of the growth pattern of marine animals, such as bivalves (Koike, 1980). The data are plotted on the diagram, and no additional statistical treatment was made because the life span of the measured specimens was too short to be analyzed statistically. The shell diameter of earlier growth stages was calculated based on the proportional relationship between the shell diameter and the distance from apex to .

Results and Discussion

Seasonal and annual growth The results of the oxygen isotope analyses and their relationships with the seasons and years are shown in Fig. 2. An almost complete oxygen isotope profile was obtained for the “orange” specimen, with the exception of the region correlating to the slightly broken aperture. This profile shows two and a half cycles of seasonal changes in temperature. The final lighter value in this sample does not correspond to the collection time, however, due to the breakage near the aperture. Therefore, the first, second, and third lighter peaks can be correlated with the summers of 1997, 1998, and 1999, respectively; and the first, second, and third heavier values represent the winters of 1996/97, 1997/98, and 1998/99, respectively. δ18O of the first cycle began with isotopically heavier (cooler) values from approximately –0.3 to –0.5‰ in the winter of 1996/97, which gradually decreased to attain isotopically lighter 74 K. Nakayama et al.

Length along suture（mm） 0 10 20 30 40 50 60 70 80 90 100 -3 15 A 10 -2 20

5 30 ） -1 ‰ 30 25

（ 15

O P2

18 0 1 25 P1 δ P1 P2 35 1 20 35 KPM-NGL000117 2 (orange specimen) Direction of growth

Month Jul. Oct. Sep. Jul. Dec. Feb. Jun. Dec. Feb. Jun. Nov. Mar. May May Apr. Apr. Aug.

Age 1997 1998 1999 ~ May

Month ~Jul. Nov. Oct. Jan. Jul. Dec. Feb. Jun. Feb. Nov. Sep. Apr. Aug. -3 B 1 15 -2 20 Direction of growth P1

） -1 P2.1 ‰ 5 （

O 10

18 0 δ P2.2 P3 1 1 10 15 P1 5 KPM-NGL000118 P2.1 2 20 (black specimen) P3 P2.2

0 10 20 30 40 50 60 70 80 90 Fig. 2. The relationship between the oxygen isotope profi les and prominent growth lines (P1, P2, and P3) in two Umbonium (Suchium) giganteum specimens (A, orange specimen; B, black specimen). Sample numbers in the graph correspond to the numbers on the shell diagram. Note that the y-axis for δ18O is reversed so that it corresponds to temperature (°C). The direction of growth is left to right, with open dots indicating the prominent growth lines. The dotted vertical lines in the fi gure show the inferred positions of time between months, judged from the calculated water temperatures and the observed water temperature in Sagami Bay. The vertical gray patterns exhibit the approximate period of relatively low seawater temperature, mainly in winter.

(warmer) values of around –2‰ in the summer of 1997. However, no distinct light peaks were found, suggesting almost continuous shell formation during this summer. The following stage was also characterized by a gradual increase in δ18O, with a fl at pattern during the isotopically heavy stage, suggesting continuous shell formation in February 1998. In contrast to the detailed, full Shell Growth of Umbonium giganteum 75 record of the first cycle, the second cycle was shorter in length along the suture and exhibited a smaller range of δ18O. The isotope oxygen profile of the “black” specimen exhibited two and a half cycles from near the end of the first summer in 1997 to the third winter in 1999. The final record is interpreted as being very close to the aperture during collection on November 25, 1999. The seasons and years are inferred in a similar way as for the “orange” specimen above. The isotope record for winter 1997/98 showed a flat pattern that was similar to that of the “orange” individual during the same winter. However, δ18O values for summer 1999 were lighter than for the “orange” specimen in the same summer. Shell growth and the range of recorded δ18O values were lowest during the last half cycle (1999). Comparison of the two profiles (Fig. 2A and 2B) shows that both specimens had similar minimum δ18O values during the first cycle (summer 1997) and maximum δ18O values during the following winter (1997/98). In contrast, the values during the second and third cycles were more variable, with the δ18O values being lighter in summer 1998 for the “black” individual and summer 1999 for the “orange” individual.

Formation of prominent growth lines The prominent growth lines of the “orange” specimen occurred at microsample nos 25 (–0.56‰; 16.4°C), and 33 (–0.63‰; 16.7°C), whereas those of the “black” specimen occurred at nos 4 (–0.95‰; 18.2°C), 17 (–1.63‰; 21.3°C), 18 (0.02‰; 13.7°C), and 21 (–0.22‰; 14.8°C). The corresponding calculated temperatures are largely consistent with the observed water temperatures at depths of 3–7 m off the coast of Hiratsuka. The only exception is microsample no 21 of the “black” specimen. This microsample is located so close to the aperture that it must have been formed immediately before the time of collection (November 25, 1999). The calculated temperature of microsample no 21 was lower than the observed lowest temperature in November (18.9°C) by 4°C. This is probably because the data of observed seawater temperatures are so scattered that the calculated temperature (no 21; Fig. 2B) was not recorded. Microgrowth lines and prominent growth lines could be seen on the shell surface of both specimens, as well as many other individuals that were collected at the same time. Both specimens tended to form prominent growth lines between fall and winter. Furthermore, growth lines of this species are expected to form regularly, as in other marine molluscs, and the growth increments between the growth lines tended to decrease toward the prominent growth lines, suggesting that the growth of U. (S.) giganteum in Sagami Bay decreased at this time.

Spawning time and its relationship with prominent growth lines Several microsamples were obtained from the older parts of the shell of the “orange” specimen near the protoconch. The calculated water temperatures for these samples of 15–16°C corresponded to those in mid- to late April 1997. Because U. (S.) giganteum is known to start its benthic life only a few days (approximately 50 h) after spawning (Ohata et al., 2002), these calculated water temperatures represent those that were experienced just after spawning and hatching, implying a spawning time of mid- to late April 1997. There were no isotope data to infer the spawning time of the “black” specimen. However, smaller shell size in September 1997 compared with the “orange” specimen suggests that the spawning time was probably much later, possibly late spring or early summer 1997. Therefore, it is concluded that the spawning season for these two individuals was between spring and early summer 1997. Shibata (1993) previously found that gonad ripeness coincided with a sharp decrease in water temperature in a population off the Kujukuri coast, approximately 100 km northeast of Sagami Bay, from which it was inferred that the spawning season of U. (S.) giganteum was from late November 76 K. Nakayama et al. to January and from April to June; and Ohata et al. (2002) also inferred the same time period for spawning. Therefore, it seems likely that spawning in this species occurs in two seasons: fall to early winter and spring to early summer. Sato et al. (2009) observed gonad ripeness from September to March in U. (S.) giganteum in Sagami Bay, with a peak in November, and considered that this population mainly spawns in November. However, these observations were made in 2000 and 2001, rather than in 1999 or earlier, which probably explains the difference between their fi ndings and those of our study, i.e., gonad ripeness may have occurred mainly in spring or early summer in 1997 but in the fall in other years. Alternatively, it is possible that both of the specimens investigated in the present study were rare individuals that hatched from parents that exhibited gonad ripeness and spawned in March or later in 1997, despite most individuals having spawned in the previous November. If two reproductive seasons exist, prominent growth lines would be expected to form in both spring to early summer and fall to early winter. The individuals examined in this study only developed prominent growth lines in fall or early winter. However, this can be explained by the fact that prominent growth lines are easily formed in fall and early winter due to the generally low rate of shell growth in this season, whereas the much greater shell growth rate in spring to early summer prevents the formation of such prominent growth lines.

Shell growth of U. (S.) giganteum Figure 3 shows a Walford plot for U. (S.) giganteum individuals from Sagami Bay collected on

40

35

30

Shell growth rates = 0 25

20

15 Shell diameter t + 1 (mm)

10 “Orange” specimen U. (S.) giganteum “Black” specimen (This study) 5 other specimens n = 19 U. (S.) giganteum (Sato et al.,2009) 0 0 5 10 15 20 25 30 35 40 Shell diameter t（mm） Fig. 3. A Walford plot for the growth of Umbonium (Suchium) giganteum in Sagami Bay, off the coast of Hiratsuka. The x-axis shows the shell diameter at “t” years, whereas the y-axis shows the shell diameter at “t + 1” years. The diagonal black line that passes through the origin represents a growth increment of 0. The data of Sato et al. (2009) are based on the growth formula obtained in the study. If two prominent growth lines were formed consecutively with small distances on the shell, such as P2.1 and P2.2 of the “black” individual in Fig. 2, the average value of shell diameter was plotted. Data of the same individuals are connected by straight lines. Shell Growth of Umbonium giganteum 77

November 25, 1999, including the “orange” and “black” specimens that were analyzed in detail, based on the identification of annual growth rings from the prominent growth lines. This shows that small individuals of up to 15 mm shell diameter grew 15–20 mm in the following year, but the growth rate suddenly decreased after this, with individuals of approximately 30 mm shell diameter exhibiting almost no shell growth. The results from this study based on the oxygen isotope analyses (open dots in Fig. 3) broadly correspond to those of Sato et al. (2009) who inferred the growth history by continually sampling the population in Sagami Bay (black dots in Fig. 3). However, the estimated growth rate by Sato et al. (2009) was lower than the average value inferred in this study. This difference may be explained by the fact that the samples by Sato et al. (2009) did not contain juveniles of shell diameter <20 mm. This may have been due to juveniles passing through the coarse mesh of the sampler, or some other instrumental reasons. However, a more important reason is likely the habitat shift that occurs with growth, meaning that the sampling site (offshore sea bottom) was some distance from the probable main habitat of juveniles. One of the authors (TS) obtained information about the probable habitat shift of U. (S.) giganteum with growth observed by a local fisherman. A similar habitat shift is known to occur in other species of Umbonium; for example, juveniles of U. (S.) moniliferum are known to live in and around the shoreline of Amakusa, whereas the adults live further offshore (Kosuge et al., 1994). Thus, Sato et al. (2009) may have underestimated the higher growth rate of juveniles, which inhabit more nutrient-rich foreshore environments with their much higher phytoplankton productivities (Brown & McLachlan, 1990). The findings that growth rings can be used to estimate past seawater temperatures and spawning seasons in Umbonium species is important for future studies on the fossils of extinct species.

Acknowledgments

We are grateful to Kentaro Shimizu of the Kanagawa Prefectural Fisheries Technology Center and Mitsuhiro Kato of the Kanagawa Prefectural Fisheries Technology Center, Sagami Bay Experiment Station, who provided the data of seawater temperature in Sagami Bay. We also thank Yuta Yamaoka, Kei Yamano, Munehiro Shimooka and Toshiki Yokoyama of Kochi University for assistance in the oxygen isotope analysis. The authors would like to thank Enago (www.enago.jp) for the English language review. This work was supported by JSPS KAKENHI Grant Number 25400499 (Grant-in-Aid for Scientific Research (C) and the Research Center for Global Environmental Change by Earth Drilling Sciences (GEEDS) at Kochi University.

References

Brown, A. C. & McLachlan, A. 1990. Ecology of Sandy Shores. 328 pp. Elsevier, Amsterdam. Koike, H. 1980. Seasonal dating by growth–line counting of the clam, Meretrix lusoria. University Museum, University of Tokyo, Bulletin 18: 1–104. Kosuge, T., Nishihama, S. & Takayama, J. 1994. Macrobenthos at Mogine Sand Flat, Amakusa, Kyushu, with special reference to the occurrence of the two molluscs, Umbonium moniliferum (Lamarck) and Meretrix lusoria (Röding). Nankiseibutu 36: 115–119. (in Japanese with English abstract) Makiyama, J. 1925. The evolution of Umbonium. Japanese Journal of Geology and Geography 3: 119–130, pl. 10. Matsushima, Y. 1984. Shallow marine molluscan assemblages of postglacial period in the Japanese islands: it’s historical and geographical changes induced by the environmental change. Bulletin of the Kanagawa Prefectural Museum, Natural Science 15: 37–109. (in Japanese with English abstract) Ohata, S., Shibata, T., Simizu, T., Tanabe, S. & Ishida, O. 2002. On the spawning and early development of the sand snail Umbonium (Suchium) giganteum. Bulletin of the Chiba Prefectural Fisheries Research Center 1: 45–47. (in Japanese) Ozawa, T. & Okamoto, K. 1993. “Recent development in phyletic evolution by integration of palaeontological and molecular phylogenic approach: examples of the gastropods, Umbonium”. Chikyu monthly 15: 589–595. (in Japanese; title translated by the present authors) Sasaki, T. 2000. Trochidae. In: Okutani, T. (ed.), Marine Mollusks in Japan, pp. 77–83. Tokai University Press, 78 K. Nakayama et al.

Tokyo. Sato, T., Tonami, Y. & Yamamoto, S. 2009. Bathymetrical distribution and growth of the giant button top Umbonium giganteum (Lesson) at off Fujisawa, Sagami Bay, central Japan. Bulletin of the Kanagawa Prefectural Museum, Natural Science 38: 95–106. (in Japanese with English abstract) Shibata, T. 1993. Reproductive cycle of the sand snail Umbonium (Suchium) giganteum in Kujukuri coast, central Japan. Nippon Suisan Gakkaishi 59: 1309–1312. (in Japanese with English abstract) Shuto, T. 1956. Umboniinae from the Miyazaki Group (Palaeontological study of the Tertiary Miyazaki Group-1). Japanese Journal of Geology and Geography 27: 47–66, pl. 4.

(Received March 2, 2016 / Accepted July 7, 2016)

相模湾に生息するニシキウズガイ科腹足類ダンベイキサゴの殻に記録される 酸素同位体比プロファイルからみた殻成長

中山健太朗・近藤康生・佐藤武宏

要 約

本研究では佐藤・他（2009）が 1999 年 11 月 25 日に採集し，成長分析をおこなった相模湾産の 2 標本 について，酸素同位体比分析の手法を用いて，ダンベイキサゴの殻成長を推定した。酸素同位体比分析を 18 行った結果，両個体は類似した酸素同位体比プロファイルの変動パタンを記録しており，δ O の変動は 3 回の極小値と 3 回の極大値を含む，全体として緩やかな増加と比較的急な減少を示した。相模湾のダンベ 18 イキサゴは生息場所から淡水の影響は限定的であると推定される。つまり，殻に記録されている δ O の 値は海水温の変動を示すことから，両分析個体は相模湾の 1997 年春季から 1999 年秋季の海水温を記録し ていることになる。殻に記録される酸素同位体比値と殻表面に見られる明瞭な成長輪の関係から、この明 瞭な成長輪は急激な海水温の低下に伴って形成されることが明らかとなった。相模湾のダンベイキサゴは 主に海水温が低下する時期である秋季から冬季にかけて放精・放卵を行うことが知られている。つまり， 殻表面の明瞭な成長輪は放精・放卵によって形成されていると考えられる。これらの成長輪は同時期に採 集された個体にも同様に見られることから，本地域におけるダンベイキサゴは秋季に 1 回もしくは 2 回の 18 放精・放卵が生じていると推定された。しかしながら，分析個体の前半部分の δ O の値から春季もしく は夏季の前半にも放精・放卵が生じていると推定され，殻前半部分に放精・放卵に伴う明瞭な成長輪が形 成されないのは成長速度が速いためであると考えられる。本研究の酸素同位体比分析から推定される成長 速度は，幼貝部分において，先行研究の推定よりもわずかに速いことが明らかとなった。