Biogeography 18. 1–10. Sep. 20, 2016 Mating behavior of the dioecious snail Cerithidea rhizophorarum A.Adams,1855 ( Gastropoda; Potamididae) in the tidal flat of a mangrove forest Harumi Ohtaki, Kiyonori Tomiyama, Eiko Maki, Maya Takeuchi, Tatsujiro Suzuka & Saki Fukudome Department of Earth and Environmental Sciences, Graduate School of Science and Engineering, Kagoshima University, Korimoto, Kagoshima, 890-0065, Japan Abstract. The mating behavior of the dioecious snail Cerithidea rhizophorarum was observed in the field. Copulation was observed from mid May to mid September, with a peak from late June to late July. Continu- ation of copulation was confirmed by flag mark, pair was discovered on rounds every 15 minutes. The time pair parted from one another was considered the copulation termination time; both initial and termination times were recorded. Initial times in the daytime had one peak period about two and a half hours before low tide, while nighttime pairs starting copulation were especially frequent over three hours, with low tide oc- curring in the middle of that period. Copulation frequency in the daytime and nighttime was much the same, but the duration of copulation in the nighttime (mean 51.1minutes) was significantly longer than that in the daytime (26.7minutes). Because there was no significant correlation between shell width between pairs, the mating of C. rhizophorarum was assumed to be random with respect to shell width. Courtship acceptor snails were significantly bigger than initiator snails. By dissection of copulation pairs, 5.45% of the pairs were found to be between individuals having male reproductive organs. The difference in sex ratio between the population on the tidal flat area and on the trees in the summer was significant: the ratio of females in the population on the trees was higher than that on the tidal flat area. Key words: biogeography, copulation, mating behavior, mangrove, snail, Introduction maphroditic species can be divided between those that can self-fertilize and those that can not. Hence, Among molluscs, there are two types of mating molluscs have many divergences in reproduction systems: dioecism and hermaphroditism. Hermaph- pattern and may be a phylum which particularly has roditic snails can be divided into three types, namely, various reproductive patterns in an animal kingdom. simultaneous hermaphrodites, protandric hermaph- How would such varied reproductive strategies rodites and protogynous hermaphrodites. These di- evolve? vergent patterns of sex were found even in the same There are many reports of mating behavior in a classes. Bisexual reproduction (fertilization of sperm group of Hermaphrodites such as Pulmonata (Asami, and egg) is common in molluscs, and the pattern 1998) and Opisthobranchia (Yusa, 1996ab), but it of fertilization in most classes is internal, although is limited in dioecious group, such as Prosobran- external fertilization is found in some classes. Her- chia (Wells, 1983). Thus, understanding dioecious ——————————————————————— mating behavior would be one key to unlocking the *Corresponding author: [email protected] mysteries of evolution of various reproductive strat- − 1 − Mating behavior of manglove snail egies in molluscs. The dioecious prosobranch snail Cerithidea rhiz- ophorarum commonly inhabits tidal flats in eastern Asia. This species inhabits in tidal flat and is distrib- uted over the upper part of the intertidal zone, and they eat some alga species mainly. And, this species is oviparous, and the breeding season is the summer (Otaki et al., 2001a; Maki et al., 2002). In the tidal flat of the Atago River, Kiire-cho, Kagoshima, mat- ing by shell mounting was observed (Otaki et al., 2001a; Maki et al., 2002; Ando & Tomiyama, 2004; Takeuchi et al., 2008; Onoda et al., 2010; Fukudome & Tomiyama, 2013). Otaki et al. (2001b) also re- ported on this group’s mating behavior. As for their study, a mention of mating behaviour was important and was incomplete because statistics comparisons such as the sex ratio are not carried out again., but their report was incomplete. In this study, several aspects of the mating behav- ior of C. rhizophorarum were examined in the field, including duration of copulation both in the daytime Fig. 1. Map of study site. and the nighttime. (Powy, 1833), Clithon oualaniensis (Lesson, 1831) Materials and methods and Clithon faba (Sowerby, 1836), coexist on the tidal flat. 1. Study site This study was conducted on a tidal flat at the 2. Copulation frequency river mouth of the Atago River, which flow through Snail mating can be categorized as face to face or Kiire-cho, Ibusuki-gun, in Kagoshima-ken. The shell mounting (Asami, 1998). C. rhizophorarum mouth of the Atago River is located inside the Nisse- mates by shell mounting (Otaki et al, 2001a; Takeu- ki oil base, and joins the Yahata River at this point chi et al., 2008; Onoda et al., 2010). Otaki et al. (Fig. 1). A small mangrove forest dominated by (2001b) reported that copulating behavior was ob- Kandelia candel and Hibiscus hamabo, representing served from the middle of June to the middle of Au- the northernmost limit of mangrove distribution in gust and that the peak initial time of copulation was the West Pacific, covers this tidal flat. Several spe- before low tide. In the present study, the frequency cies of Gastropoda, such as Cerithideopsilla djad- of copulation was observed by viewing, and investi- jariensis (Martin, 1899), Cerithideopsilla cingulata gated by counting the number of all individuals and (Gmelin, 1791), Batillaria multiformis (Lischke, copulating individuals with the naked eye at the time 1869), Batillaria cumingi (Crosse, 1862), Clypeo- before low tide on the day of the spring tide. Obser- morus coralium (Kiener, 1841), Reticunassa festiva vations were carried out along the route (about 110 − 2 − Harumi Ohtaki, Kiyonori Tomiyama, Eiko Maki, Maya Takeuchi, Tatsujiro Suzuka & Saki Fukudome m) in the mid-intertidal zone. duration of copulation was estimated by checking Like Otaki et al. (2001b), we defined a snail that copulating pairs every 15 minutes between 22:00 approaches another and mounts its shell as the upper and 03:30 on 4 July, and between 9:45 and 17:45 on individual, and the snail that accepts courtship as 21 July. A headlamp covered with red cellophane the lower individual. After upper individuals were was used for night searches. During copulation, marked with an oily pen, copulating pairs were pairs don’t move. When a copulating pair was dis- brought to a laboratory. At the laboratory, the shell covered, the time and identification number of the width of each individual was measured and the in- pair were recorded. A flag marked with an identifi- dividuals were frozen After being thawed, the indi- cation number was stuck in the tidal flat near the pair viduals’ shells were broken and inside organs were to distinguish them from others. If a pair was found removed in order to distinguish sex by the difference to have terminated copulation or to have disappeared that the sex organs observed by a microscope have a at the next observation, the time of that observation sperm, or has an ovum. was recorded as the time of the end of copulation. 3. Daily pattern of copulation 4. Sex ratio of population on the tidal flat area and To examine changes of copulation frequency, on the mangrove forest area the number of all individuals including copulating In a small mangrove forest of the Atago River, individuals along the route were counted with the tree climbing behavior of C. rhizophorarum has naked eye every 1 hour between 11:00 and 16:00, been observed (Wakamatu & Tomiyama, 2000; and between 22:00 and 03:00 on 5 July (spring tide, Otaki et al., 2002). Whether there was a difference low tide 00:12, 12:34; high tide 05:46, 19:05), and between the sex ratio of the population on the tidal between 09:45 and 17:30 on 21 July (spring tide, flat area and in the mangrove forest area was ex- low tide 01:22, 13:42; high tide 06:59, 20:08). Fur- amined. Snails were randomly collected from the thermore, as with Otaki et al. (2001), the minimum tidal flat and mangrove forest areas (individuals who Fig. 2. Percentage of copulating individuals in the population from 9 May to 15 October. − 3 − Mating behavior of manglove snail crawl over the mud in the forest or climbing trees), Results with 28 and 30 individuals collected in investigation into approximately 40m section respectively. After 1. Copulation frequency collection, they were brought to the laboratory and Fig.2 shows seasonal changes in the frequency of distinguished by sex. copulation. There was no copulation pair on 9 May, but on 20 May. 1.3% of total individuals were ob- Fig. 3. The relationship between shell width (mm) of copulating pairs (upper snail and lower snail) and their frequencies. − 4 − Harumi Ohtaki, Kiyonori Tomiyama, Eiko Maki, Maya Takeuchi, Tatsujiro Suzuka & Saki Fukudome served copulating. The percentage increased sharply copulating pairs numbered 55 pairs. The mean shell to about 7% from 23 June to 21 July. On 4 August, width of upper and lower individuals was 10.85 ± the percentage rapidly decreased to 2.6%, and no 0.92mm (M ± SD; N = 55, Range = 9.0 - 13.0mm) copulating pair was observed on 15 October. and 11.72 ± 0.90mm (M ± SD; N = 55, Range = 9.6 - Fig.3 shows the relationship between shell width 13.9mm) respectively. There was no significant cor- of copulating pairs and these frequencies. Observed relation between the shell width of upper and lower Fig. 4. 4a-c. a: Percentage of copulating individuals in the population in the daytime on 4 July. b: Percentage of copulating individuals in the population in the daytime on 21July. c: Percentage of copulating individuals in the population in the nighttime on 4 July.