Masayuki Saigusa

ADAPTIVE SIGNIFICANCE OF A SEMILUNAR RHYTHM IN THE TERRESTRIAL CRAB SESARMA

Larval release activities of the terrestrial crab Sesarma were observed 1.5 km upriver from the sea. The number of Sesarma haematocheir and Sesarma intermedium females releasing larvae peaked twice monthly, during the full and new moon periods. Larval release, at about dusk, coincided with high water at the nearby seacoast. Larvae of S. haematocheir and S. intermedium died quickly in fresh water. The semilunar rhythm of larval release gives the larvae, released just after high water of spring tides and around high water on the days following the full and new moons, a better chance of reaching the sea than otherwise. Sesarma dehaani did not reveal a clear semilunar rhythm and the time of day of larval release did not coincide well with high water. S. dehaani inhabits riverbanks and rice paddies near the sea, and its larvae have the highest tolerance to fresh water. These factors may account for its lack of synchronization with tides.

Larval release rhythm coinciding with solar day and tidal cycles in the terrestrial crab Sesarma - harmony with the semilunar timing and its adaptive significance.

This field study determined which cycle of a lunar day or tide coincides with the time of day of larval release in the terrestrial crab, Sesarma haematocheir. Observations of larval release were made at a riverside 100 m upriver from Kasaoka Bay in the Inland Sea of Japan where the tidal phase differs by several hours from that of the Pacific Ocean. The findings demonstrated that the timing of larval release coincided not with a lunar day cycle but with a local tidal cycle. The larval release pattern of the Kasaoka population showed a relatively strong correlation with tides when compared with the Izu population. This suggests that the Izu population pattern was transitional, going from a combined solar day and tidal pattern to a complete daily rhythm. The timing of incubation and larval release may be based on the following mechanisms: semilunar timing of incubation entrained by lunar cycle, and the time of day of larval release controlled by a combination of solar day and local tidal cycle. This study p...

Entrainment of Tidal and Semilunar Rhythms by Artificial Moonlight Cycles

A marked feature of the larval release activity of the terrestrial crab Sesarma haematocheir is its synchronization with the time of high water. This activity occurs only at night, so that the pattern of the tidal rhythm recurs at semi-monthly intervals. When adult specimens from Seto (Okayama Prefecture) and Shima (Mie Prefecture) populations were brought from the field into 24-h light-dark conditions in spring, the larval release occurred at night, but the overall activity pattern gave no indication of a tidal component. On the contrary, under simulated moonlight cycles the timing of release was strongly coordinated and exhibited a well-defined tidal component arranged at semi-monthly intervals. The phase difference between the evoked tidal rhythms of the two populations of 4-5 h was about equal to the phase difference of the tidal cycles in their natural habitats. Synchronization of larval release with the artificial moonlight cycle required more than 40 days of exposure. In addition to entraining the ...

Phase Shift of a Tidal Rhythm by Light-Dark Cycles in the Semi-Terrestrial Crab Sesarma pictum

The larval release activity of the semi-terrestrial crab Sesarma pictum was monitored for three-week periods under laboratory conditions of constant and cyclic light. Under conditions of constant dim light, the rhythm for the first ten days was unimodal (larval release just after the nocturnal high tide) and then became bimodal (no apparent synchrony with the tides or with other members of the population) for the remainder of the experimental period. On the other hand, in photoperiods similar to those in the field, the rhythm was maintained; the phase was bimodal and the timing of larval release was delayed 1-2 h from the predicted times of high water in the habitat. When the photoperiod was advanced or delayed, the tidal rhythm was phase-shifted accordingly. The photoperiod does entrain the release rhythm to bimodal tidal cycle. So the phase-shift of a tidal rhythm by 24-h LD cycles is a very difficult phenomenon to explain.

Control of Hatching in an Estuarine Terrestrial Crab I. Hatching of Embryos Detached From the Female and Emergence of Mature Larvae

Embryos of the terrestrial crab Sesarma haematocheir hatch simultaneously just prior to their release into water. Larval hatching occurs synchronously when the embryos are attached to a female, and the role of the female in this synchrony has been investigated. Clusters of embryos (200-2000 berries in each cluster) were detached from ovigerous females, and their hatching was compared with that of embryos attached to the females. Of the detached embryos in a cluster, either all hatched, or none hatched. A remarkable feature was that the success of hatching of these detached eggs depended upon the time of hatching of the eggs still attached to the female. Clusters of embryos that were detached from the female within 48-49.5 h of the projected time of larval release all hatched successfully, and swimming zoeas appeared. But embryos that had been detached from the female for longer periods did not hatch at all, though they were obviously alive. These results suggest a hatching process different from the embryonic development process. The female may trigger this process. In addition, detached eggs hatched later than eggs attached to the female, and their hatching was less synchronized. These observations suggest that the female not only initiates hatching, but also enhances the synchrony of hatching.

The tidal rhythm of emergence, and the seasonal variation of this synchrony, in an intertidal midge

The emergence of an intertial midge was investigated at a site on the coast of the Inland Sea of Japan. The population emerging at this site was drawn from a single species of the genus Clunio, probably C. tsushimensis. Emergence was not synchronized with the day-night cycle, but with the tidal cycle. Moreover, the pattern of synchrony changed with season. A bimodal phase appeared in midwinter; but the pattern of synchrony shifted gradually, during January and February, from morning low tides to afternoon low tides, and a unimodal phase appeared in March. This pattern--i.e., synchrony with afternoon low tides--lasted until early October. In mid-October, the synchrony shifted to the morning low tides. Only a brief bimodal phase appeared in autumn. The phase modality was clearly correlated with the height of tides; i.e., when the low waters in a day were very different in height, emergence was synchronized only with the lower one (April to December). During January and February, the higher low tide, as well as the lower low tide, recedes considerably. The exposure of the larval habitat at the higher low tide may stimulate emergence, resulting in bimodal phases in midwinter. But the unimodal pattern in March cannot be accounted for by a simple synchrony with lower low tide, or with exposure of the larval habitat to the air; the day-night cycle not only would be one of the zeitgebers of the tidal rhythm in every season, but also must participate in the expression of the unimodal phase in spring. Furthermore, the number of midges that emerged each day fluctuated with a semilunar cycle with the season. The phase of this rhythm would be shifted by water temperature.

Daily rhythms of emergence of small invertebrates inhabiting shallow subtidal zones: A comparative investigation at four locations in Japan

Many small invertebrates inhabit the shallow subtidal zone and some of them emerge at times into the water column. The daily timing of their emergence is affected by the day/night and tidal cycles, and shows various patterns of synchrony with these cyclical factors, depending on the species. To detect possible regional differences in their emergence patterns, sampling was carried out at four locations in Japan: a boreal sea (Akkeshi), a temperate sea (Sugashima), an inland sea (Ushimado) and a subtropical sea (Iriomote-jima). The emergence patterns of major taxa were examined by visual inspection and by two statistical methods (periodogram and autocorrelogram). The composition of the taxa collected by the pump system, mostly crustaceans, was similar in each location. The number of ‘taxa’ that emerged revealed a day/night rhythm in every location. This characteristic was clearest at Iriomote-jima and least clear at Sugashima. The daily fluctuation in the number of individuals in each taxon varied widely, from very clearly nocturnal to weakly diurnal patterns. In Iriomote-jima, the major taxa all showed well-demarcated nocturnal patterns, so these patterns were classified as either level N2 or N3 with regard to the degree of synchrony with the day/night cycle. With regard to the synchrony with the tide, the majority of patterns in all locations showed a ‘double-tidal interval’. Many patterns were slightly modified by the tidal cycle. These patterns were classified as level T1 or T2 with regard to the degree of synchrony with the tidal cycle. The synchrony with the tide was comparatively strong at Ushimado. The synchrony with day/night and tidal cycles varied even within the same species or closely related species. In benthic invertebrates, hiding or resting in the bottom substrates and swimming in the water column would occur alternatively. In planktonic animals, aggregation near the bottom and dispersal in the water column would occur alternatively. The daily timing of such activities may be synchronized with the day/night and tidal cycles to various degrees among species or populations, resulting in a wide variety of emergence patterns in subtidal small invertebrates. This type of behavior is not ‘daily (diel) vertical migration’; it should rather be called ‘daily emergence/dispersal’. Strong winds, rough waves and unknown seasonal factors would also affect emergence patterns. Furthermore, the transparency of the seawater may also strongly affect these patterns. Nocturnal patterns may be an adaptation to avoid vulnerability to sighted predators. Variation of synchrony with the tide indicates that by definition, the tidal rhythm can only be distinguished from the day/night rhythm. Hence, the daily patterns that are weakly modified by the tides (levels T1 and T2) should be called the tidal rhythms. As the period of such rhythms cannot be determined exactly by using statistical methods, lengthy field investigations and visual inspection of each pattern is essential to assess the influence of tides.

Control of Hatching in an Estuarine Terrestrial Crab. II. Exchange of a Cluster of Embryos Between Two Females

The eggs of an estuarine terrestrial crab, Sesarma haematocheir (akate-gani), are incubated by the female for about one month. In estuarine crabs larval hatching is synchronized with the nocturnal high tide. To investigate whether the female or the embryo controls the actual timing of the hatching, one cluster of embryos was detached from each of two ovigerous females and reciprocally transplanted. Hatching of the transplanted embryos was divided into the following three patterns according to the number of nights until either (or both) of the females released their larvae. In Pattern I, the transplanted clusters both hatched on the same night that the donor females released their larvae. In Pattern II, the hatching of one of the transplanted clusters was not controlled by the host female, whereas hatching of the other transplanted cluster was obviously induced. Finally, in Pattern III, not only the induction of hatching, but also the time of hatching, was controlled by the female. Hatching profiles of transplanted embryos transferred to aerated conditions indicated that hatching requires three nights, and that each embryo also has an endogenous rhythm for hatching. The female seems to play two roles in hatching: i.e., initiation of the hatching process, and enhancement of hatching synchrony in each embryo. A plausible hypothesis explaining the mechanism of induction and the synchronization of hatching is presented.

Emergence Rhythms of Subtidal Small Invertebrates in the Subtropical Sea: Nocturnal Patterns and Variety in the Synchrony with Tidal and Lunar Cycles

Abstract The subtidal zones near the shore are inhabited by many small invertebrates, including benthos and plankton. To characterize their emergence in the water column with regards to day/night, tidal, and lunar cycles, field investigations were carried out at the subtropical island (Iriomote-jima), Okinawa Prefecture. By use of two impeller pumps installed in both surface and bottom waters, invertebrates were sampled continuously for 23 days. Although most patterns were much the same between the surface and bottom waters, the abundance of animals was different between the two depths. A notable feature was that nocturnal patterns were very dominant. More than half of these patterns were not affected by the tidal cycle at all. In contrast, the pattern of Ericthonius sp. (Amphipoda) showed a clear synchrony with the nocturnal tide. Other patterns were weakly modified by the nocturnal tide (e.g. Propallene longiceps; Pantopoda). A pattern coincided with the lunar phase was only seen in Vargula hilgendorfii (Myodocopida). Most arthropods would hide in the bottom substrate, or would swarm under or near the lower pump in the daytime, and they would disperse in the water column at night. A variety in the synchrony with nocturnal tides strongly supports a notion that the tidal rhythm is only a variation of the day/night rhythm, rather than the hypothesis that both rhythms are present simultaneously in an animal. Statistical methods (autocorrelogram and periodogram) are used to demonstrate the tide-correlated component of the activity. However, these methods are not sufficient for this purpose; visual inspection of the pattern is very important.

Hatching of an estuarine crab, Sesarma haematocheir: from disappearance of the inner (E3) layer to rupture of the egg case.

Hatching of decapod crustaceans is characterized by the sudden rupture of the egg case. This study focused on the following two issues regarding the hatching mechanism of the estuarine terrestrial crab Sesarma haematocheir: (1) dissolution of the egg case, and (2) the site where the egg case breaks. The egg case comprises three layers: the outer two (E1 and E2) layers and the inner (E3) thin layer (0.2 microm in thickness). The outer layers showed no morphological changes upon hatching, but the inner layer (E3) was markedly digested. The digestion of this layer would enable the embryo to absorb ambient water via reverse peristalsis of the intestine, resulting in an increase of the volume. The egg case always ruptured perpendicular to the longitudinal axis of the embryo. In addition, breakage of the egg case occurred at the dorsal thorax of the embryo. The three major organs positioned at this area were (1) a sharp projection (dorsal spine), (2) an assemblage of muscles, and (3) a pair of secretory glands, each of which was about 30 microm in diameter. The dorsal projection is soft before hatching, and it is clear that the egg case does not break with the posterior expansion of this projection. The rupture instead appears to be caused by the expansion of the muscles arranged perpendicular to the body axis. In addition, some (unknown) factor might weaken the egg case just before hatching. The secretory glands may be a kind of rosette gland, but the role that this gland plays at hatching is not known. As a duct comes out from the center and enters the dorsal projection, some active substance may be released at the tip of this projection. However, immunochemical studies are not consistent with this substance being an ovigerous hair stripping substance (OHSS).