The giant cubozoan Morbakka virulenta was collected from the central part of the Seto Inland Sea, western Japan in October 2009, in order to observe its development and polyp formation. Fertilization occurred externally. Demersal fertilized eggs were obtained during the incubation of mature females and males kept at a temperature of ca. 17 to 21°C. From the two-cell stage onwards, fertilized eggs developed into blastulae within 4 h. The development of the blastulae stopped for 21 days after forming blastocysts. This formation of blastocysts is up to now unique within the Cnidaria. The planulae, which developed inside the cysts and lacked larval ocelli, this being characteristic for the Cubozoa, finally metamorphosed into polyps bearing only a single tentacle. The 16-tentacled-stage in polyps was reached about three months after this metamorphosis. Budding occurred in eight-tentacled polyps and swimming polyps were released nine days after the commencement of budding. These unique developmental features of M. virulenta may shed additional light on the evolution of life history strategies in the Cnidaria.

View full abstract

The role of the heterotrophic dinoflagellate Noctiluca scintillans in affecting the nutrient-environment and aquatic ecosystem was investigated in the neritic area of Sagami Bay, Kanagawa, Japan, from January 2002 to December 2006, based on abundance, intracellular nutrient content, excretion rate and response of phytoplankton (diatoms) to enrichment of nutrients extracted from N. scintillans cells. Seasonal variations in abundance and vertical distribution of N. scintillans were significantly related to the physical structure of the water column, water temperature, chlorophyll a and primary productivity. Intracellular nutrient contents, except for Si(OH)₄-Si, revealed clear seasonal fluctuations, which were significantly correlated to cell size variations. Thalassiosira rotula increased to higher cell abundances at higher concentrations of nutrients, which were extracted from N. scintillans cells. NH₄⁺-N and PO₄^3--P excretion rates were much higher during the first 1–3 h, and decreased rapidly with time. Daily NH₄⁺-N and PO₄^3--P supply by N. scintillans excretion was estimated to be on average 34.8% and 55.3%, especially 50.6–85.4% and 80.5–135.8% in April–July (monthly mean), of the daily N and P requirement for primary production, respectively. The large amounts of nutrients regenerated and released by N. scintillans excretion can increase the N and P concentrations in ambient seawater, especially in the upper layer in spring–summer, and consequently affect phytoplankton (diatom) abundance. These may exacerbate eutrophication in these waters by a mutually supportive relationship between phytoplankton and N. scintillans: bottom-up control (phytoplankton–N. scintillans) and nutrient supply by N. scintillans to phytoplankton through excretion.

View full abstract

The bivalve Meretrix lusoria secretes a mucous cord and drifts on tidal flats using the cord as a “sail.” To explain the drifting patterns of the clams in the field obtained in previous studies, conditions affecting cord secretion were examined in the laboratory. The results are summarized as follows: 1) clams over a wide size range (11–45 mm in shell length) secreted cords over a wide range of temperatures (10–30°C) with stimulation by air bubbling of the water and 2) cord-secretion activity depended on the season of collection, with maximum activity in spring and minimum in summer. In general, these results explained well the drifting patterns of the clams observed previously in the field.

View full abstract

Some animals show multiple behavioral responses after detection of chemical cues indicating a predation threat. Individuals of the high intertidal periwinkle, Littorina sitkana, showed two contrasting anti-predation responses, immobile cautiousness and active fleeing, against the predatory crab, Hemigrapsus sanguineus. This study examined the combined effects of chemical cues of predation threat (crab, crushed conspecific, mixed crab and crushed conspecific, and control), body size, and time (morning, mid-day and late-evening) on the anti-predation behaviors of L. sitkana. The cautious response of L. sitkana was strongest under the crab treatment, with the earliest fleeing response appearing when presented with the mixed chemical cue of crabs and crushed conspecifics. Time of day also affected the anti-predation responses, with a longer cautious response during late-evening than mid-day under control conditions, and no significant difference between late-evening and mid-day in response to the three types of chemical cues of predation threat. The fleeing response occurred more rapidly during late-evening than mid-day for the four types of chemical conditions; there was no significant difference between morning and mid-day under the control condition; and it occurred more rapidly during morning than mid-day under the three predation threat conditions. Larger individuals showed longer cautious responses for all conditions and remained submerged in seawater for longer under the three predation threat conditions. It is suggested that the level of predation risk may be assessed by L. sitkana and used to elicit the appropriate of the two contrasting anti-predation responses.

View full abstract

Plate culturing techniques are important in many biological experiments, though plate culturing of diatoms, especially Centrales species, is difficult. In this study, we tested the simple decantation method for dispersing cells on an agar plate with a finely dimpled surface for culturing the Centrales diatom Chaetoceros tenuissimus Meunier. The results showed that C. tenuissimus cells uniformly grew on the surface of a dimpled plate according to the pattern of each pocket, using the decantation method. In contrast, only a fraction of the cells survived and made colonies on a smooth-surfaced agar plate, even using the decantation method. Cell growth was not observed on either dimpled or smooth plates when cells were dispersed by daubing using a spreader. Since this new method is easy and inexpensive and cells grow well on the plate, it might be useful for studies on diatom biology.

View full abstract

Knowledge of movement patterns is essential in tailoring spatial conservation and management measures. Although populations of the coconut crab, Birgus latro, have been severely depleted in most habitats, there have been few serious attempts to manage the resources. Recently, spatial conservation (e.g. seasonal area closure) has been suggested as an effective tool for resource management. However, relatively little is known about coconut crab movements, especially reproductive migration. To determine the dynamics of the reproductive migration of coconut crabs, we investigated both male and female migration patterns during the reproductive season in Hatoma Island, southwest of Japan. Both females and males migrated between inland and coastal areas during the reproductive season. Observed female seaward and landward migrations were temporally coincident with their reproductive activities (egg extrusion and larval hatching). Observed temporal-spatial distribution of males would be shaped by temporal variations in expected future mating opportunities within the reproductive season and male reproductive strategy in response to these variations.

View full abstract