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Argonauta 4: 28-37 (2001)


Short Communication

An example of activity pattern of the estuarine snails, Clithon faba and Clithon oualaniensis (Gastropoda: Neritidae) in the Nagura Lagoon, Ishigaki Island.


 Shun-ichi Ohgaki
1-16-31 Mikonohama, Tanabe, Wakayama 646-0025, Japan

Genus Clithon of Neritidae is distributed in fresh or brackish water of the tropical or subtropical Indo-Pacific (Benthum Jutting 1956). On Ishigaki Island of the Ryukyu Islands, Clithon faba (Sowerby) and Clithon oualaniensis (Lesson) are often dense on intertidal sand-soil flat and pebble beaches in river estuaries. Budiman (1988) observed the behavior of Clithon oualaniensis during a low tide in the daytime in an Indonesian mangrove, although its activity pattern through all day period as well as that of C. faba is unknown. The purpose of the present study is to describe the activity patterns of C. faba and C. oualaniensis as a basic information on the behavior of this genera, and to compare the patterns between these congeneric species.

I observed Clithon faba and C. oualaniensis in the estuarine area of Nagura Lagoon on Ishigaki Island (24o23' N, 124o07' E). There, the two species are common on a muddy-sand flat extended around the lower reaches of the Nagura River, where pebbles, fragments of coral skeleton and dead molluscan shells are sporadically distributed. Two 50 x 50cm quadrats (Quadrat A and B) were set 80m upstream of Nagura-kobashi Bridge located 50m from the river mouth. These quadrats were 5m apart from each other and were both at the height of 110cm above datum. The mean low-water of neap tides was 105cm and the mean high-water of neap tides 114cm at Ishigaki City from July, 1988 to June, 1989, which means that the two quadrats were usually submerged at high tide and exposed at low tide. There was either of the two Clithon species in each of the two quadrats, Quadrat A including C. faba and Quadrat B having C. oualaniensis. At low tide, several shallow pools appeared within Quadrat A, whereas no pools were seen in Quadrat B. The size of C. faba (mean width of shell aperture±SD) was 7.2±0.2mm (n=295) and that of C. oualaniensis was 5.6±0.4mm (n=295) around the study site (random sampling at low tide on July 30, 1989). The two species were observed at 1-2h intervals for 24h from 10:00 a.m. on June 29 to 10:00 a.m. on June 30, 1989. The sky was clear or cloudy with no rain, and air temperature was between 27.0oC and 32.2oC during the study period. The day of observation was 3-4 days after the upper half moon, i.e., an intermediate condition between neap and spring tides. In each observation, number of the snails exposed completely on substratum (= exposed snails) and those buried partly were counted. The one that was crawling about or shaking the shell without shifting its position was recorded as moving. When a snail was attaching to the right side of another individual and kept still, those two were recorded as pairing. The percentages of moving snails and pairing snails in all the visible snails (i.e., exposed snails and partly buried ones) were calculated only when the number of visible snails were more than 10. The observations under water were made by snorkeling.

Figure 1 illustrates the activity patterns of Clithon faba and C. oualaniensis during 24h. The two species were most dense on the substratum right after the tide receded, regardless of day and night. The number of exposed snails decreased subsequently, and was least while the water covered the substratum. Their number under water was larger at night than in the daytime. The percentage of moving snails changed approximately in parallel to the number of snails exposed on substratum. The number of snails buried partly in sand was relatively large on rising or receding tides, in consistent with the field observation that the snails were on the way of burrowing into or crawling out of the sand on these occasions. The percentages of pairing snails were larger than 20% through low-tide periods in most cases in each species. In a supplemental observation around the study site at the daytime low tide on July 30, 1989, pairs in the two species were formed almost exclusively between conspecific individuals, even at sites where the two species were intermingled. In one case of C. oualaniensis, an elongated spermatophore was seen on the way of transport between the pairing snails when they were detached. Therefore, the pair formation in C. faba and C. oualaniensis seems to be, at least partly, a sexual contact within each species.

Figure.1
Figure 1. Activity patterns of Clithon faba and C. oualaniensis during 24h from 29 to 30, June, 1989. An upward wedge indicates the time of high tide (16:54, 137cm above datum; 3:57, 171cm) and a downward wedge the time of low tide (10:17, 45cm above datum; 21:56, 101cm; 11:18, 31cm). A half-toned area denotes dark period from the sunset at 19:56 to the sunrise at 5:24. Percentages of moving snails and pairing snails were calculated when the number of visible snails ('exposed on substratum' + 'partly buried') was more than 10.

Thus, C. faba and C. oualaniensis exhibited a common feature of activity that the snails appear at low tide and burrow into sand at high tide, regardless of day and night. Crawling on substratum at low tide seems to enable both species to feed and copulate. One notable difference between the two species is that the density of exposed C. oualaniensis decreased earlier than that of C. faba at low tide in the daytime (Fig. 1; 10:00-14:00, July 29). During this period, more snails of C faba were exposed on substratum in Quadrat A with small pools, compared to C. oualaniensis on the relatively dry surface in Quadrat B. On the other hand, at damp sites with pools around the study site, C. oualaniensis were still abundant on substratum during the same period. It is probable that the relatively dry surface in Quadrat B restricted the movement of C. oualaniensis, so that the observed discrepancy between C. faba in Quadrat A and C. oualaniensis in Quadrat B is not simply attributable to interspecific deference.

Abe (1934) reported that a soft-bottom snail, Batillaria multiformis appeared on the substratum at low tide and burrowed back into the sand before the tide covered its habitat. Budiman (1988) also found that the number of active snails on substratum was largest shortly after the tide receded and decreased through the low tide before submergence, burying themselves into substratum. These are consistent with the present result: C. faba and C. oualaniensis crawled on the sand at low tide and started to burrow before their habitats were covered by rising tide. Submergence itself does not seem to be the major cue to induce burrowing, and for the mechanisms of this behavior, reduction in surface moisture, as well as other factors such as an endogenous rhythm, should be considered.

Light and dark conditions does not seem to affect the activity patterns of Clithon faba and Clithon oualaniensis at the study site so intensively as tidal cycles, however, the snails burrowed earlier before submergence and less snails were on the substratum under water in the daytime than at night. Budiman (1988) mentioned that C. oualaniensis can reduce the mortality by visual underwater predators, as well as the risk of being dislodged by waves, by burrowing. One of the major predators for benthic invertebrates in the Nagura Lagoon may be a puffer, Chelonodon patoca (Hamilton) that is seen to intrude the Nagura Estuary at high tide and forage at the bottom actively. Shokita et al. (1988) reported that two species of Neritidae, although not Clithon, were found in the gut content of C. patoca from mangrove estuaries in Okinawa. The present result that C. faba and C. oualaniensis burrowed more promptly before submergence and disappeared more completely from the surface at daytime high-tide than at night high-tide seem to support the Budiman's hypothesis of the effect of visual underwater predators.

Based on the present study, the following explanation could be proposed for the behavior of the two species. C. faba and C. oualaniensis feed and copulate on the surface at low tide, but burrow into the substratum to avoid the risk of underwater predators and of being dislodged by water movement. Further investigations will enable more detailed analysis on the patterns and adaptive significance of the activity patterns of these species.

Acknowledgments: I am grateful to Dr. Naoya Abe and Dr. Takeharu Kosuge for reading the manuscript.


References

Abe N 1934 Ecological observations on Batillaria multiformis (Lischke) Sci. Rep. Tohoku. Imp. Univ. Ser. Biol. 8: 383-398.
Budiman A 1988 Aspect of ecology of Clithon oualaniensis (Gastropoda: Neritidae) on Sonneratia pioneer zone at Sosobok River, Kao Bay, Halmahera. in Biological Systems of Mangroves, a Report of East Indonesian Mangrove Expedition, 1986. eds. K Ogino & M Chihara, Ehime University, 59-65.
Benthem Jutting W S S 1956 Systematic studies on the non-marine Mollusca of the Indo-Australian Archipelago. V. Critical revision of the Javanese freshwater gastropods. Treubia 23: 273-284.
Shokita S, Amemiya T, Yoshida Y, Morishima H 1988 Fish fauna and their feeding habits in the mangroves of Okinawa. in Basic studies on the dynamics and conserbation of mangrove ecosystems, 61-76. Ministry of Education, Science and Culture, Japan (in Japanese).


要約：　沖縄県石垣島の名蔵川河口干潟で、カノコガイClithon fabaとヒメカノコガイClithon oualaniensisの活動パターンを調べた。1989年6月29日から30日の24時間、それぞれの単独生息域内50 x 50cmのコドラート内で観察したところ、両種とも、潮が引いて干潟表面が現れた直後に砂中から這い出して、底質上での密度が最も高くなり、干潮時を通じて活動し、満潮が近づいて上昇して来た水面が干潟を覆う直前に、ほとんどの個体が再び底質中に潜った。干潮時、全体の20-60％の個体にペア形成が見られた。これらの傾向は昼夜に共通していたが、満潮時に水没下で表面に出ている個体は、昼間より夜間の方が多かった。こうした両種の活動パターンについて、砂泥上での摂食、交尾の必要性と、水中捕食者の存在、満潮時に水流によって持ち去られる危険などが関与している可能性を指摘した。

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