Species composition and vertical distribution of oncaeid copepods, which are potentially important prey for juvenile fish, were investigated in the Kuroshio Extension region, the NW Pacific, in April, August, November 1998 and February 2001. Samples were collected from 8 discrete layers in the epipelagic zone (0–200 m depth) using MOCNESS (0.064 mm mesh) during both day and night. Thirty-five oncaeid species were identified. ‘Oncaea’ (s.l.) zernovi and Spinoncaea ivlevi were numerically the dominant species comprising 20.0–48.2% and 15.2–26.8%, respectively, of adult oncaeid copepods in the epipelagic zone. Cluster analysis on all samples revealed that these were separated into three groups with discrete vertical ranges; the first one appearing in the 0–50 m depth surface layer in April and August and consisting mainly of Oncaea (s. str.), the second one located in the deepest layer and composed mostly of ‘O.’ zernovi and S. ivlevi with some mesopelagic species, and the third one located above the second one and having intermediate species composition. Species-specific vertical distributions indicate that most oncaeid populations shifted downward from August to November, when the thermocline remarkably descended. However, most Oncaea spp. did not show a downward shift with the thermocline, and were positively correlated to appendicularian abundances, suggesting that appendicularian houses, known to be oncaeid habitats and to provide food, were a possible factor affecting their vertical distribution. Niche partitioning, allowing coexistence of congeners, might be explained by differences in body size and distribution layers in Oncaea and by differences in distribution layer in Triconia.
Presence of Dinophysis spp., the main phytoplankton genus responsible for the occurrence of lipophilic biotoxins of the okadaic acid group (Diarrhetic Shellfish Poisoning; DSP) in Greek mussels, was recorded in the period 2003–2008 in five major mussel culture areas of Greece. Dinophysis spp. abundances were simultaneously studied with climatological parameters (air temperature, rainfall, wind direction and velocity) and the occurrence of DSP toxic episodes in mussels. Dinophysis spp. blooms occurred with a seasonal periodicity. Highest cell abundances of the bloom appeared steadily between late winter and spring, whereas occurrence of DSP toxicity in mussels followed a similar time pattern, with mouse bioassay-positive samples ranging between 10.2–28.1% of the total samples analysed. Maximum mean Dinophysis spp. counts coincided with air temperatures in the range of 10–20°C; abundances higher than 1,000 cells L-1 were more frequently recorded at air temperatures in the range of 4–20°C in combination with low rainfall and low velocity winds (<Beaufort scale 4), blowing from the north (NW, N and NE) and south (SE and S). To our knowledge, this is the first report to correlate the occurrences and abundances of potentially toxic Dinophysis spp. and climatological data for the majority of the shellfish production areas studied. Such observations may contribute to the clarification of the occurrence patterns of DSP toxic episodes, hence bettering the economical management of mussel production.
The small box jellyfish Copula sivickisi was collected from the Ryukyu Archipelago, southern Japan, in June 2011, in order to observe its early life history, including polyp formation and metamorphosis. Fertilization occurred internally. Fertilized eggs were packed in an embryo strand with nematocysts and released into the water from the female’s manubrium. Blastulae developed into planulae bearing about 30 larval ocelli within two days, and then settled and metamorphosed into primary polyps. Primary polyps developed into adult polyps within 40 days, and both polyp stages could actively detach to change location by creeping on the central part of their body. Adult polyps formed cysts at temperatures below 20°C, which when favorable conditions retuned (above 28°C), excysted from the capsules within a week. Budding occurred in adult polyps, and buds were released two days after the commencement of budding. Metamorphosis of a whole polyp into a single medusa occurred within 10 days. Juvenile medusae were distinguished from those of other cubozoans by the pattern of nematocyst warts on the exumbrella and the adhesive pads on the apex. The developmental features of C. sivickisi resemble most closely those of T. cystophora among the cubozoans. The similarities in all early life stages of both species support recent molecular results.
We examined the effects of salinity on diel vertical migration (DVM) of two coastal flagellates, Chattonella antiqua and Karenia mikimotoi, in 90-cm-high columnar aquariums. Experiments were performed with surface salinities from 5 to 32 and bottom salinity constant at 32. Cells of each flagellate were injected into the bottom of the aquarium at night and the vertical distribution of cells was monitored every 4 h for 36 h in one series of experiments, or twice daily (day and night) for 5 days in another. Ascent and descent started at approximately the same time in all water columns, indicating that difference in surface salinity does not substantially affect DVM rhythm in the two flagellates. During daytime, C. antiqua and K. mikimotoi transited haloclines with surface salinities ≥15 and accumulated at the surface, although K. mikimotoi required 2 days to transit the halocline with surface salinity of 20 and 4 days for surface salinity of 15. However, neither flagellate could transit haloclines with surface salinities of 5 or 10; instead they accumulated in the halocline during daytime. At night, most cells of both species accumulated at the bottom in all water columns, although the distribution of K. mikimotoi gradually expanded to the upper layers on successive days when surface salinity was 5 or 10. We demonstrated that low salinity in the surface layer blocks upward migration in two flagellate species and delays surface accumulation and weakens population synchrony of DVM at night in K. mikimotoi.
A wide variety of fish and invertebrates are associated with marine jellyfishes, and their interactions are diverse. We preliminarily investigated symbionts on two species of rhizostome jellyfishes collected from Leyte and Palawan Islands, the Philippines, in August 2013. The collected jellyfishes were Lobonemoides robustus Stiasny on both islands and Acromitus maculosus Light on Palawan Island. Lobonemoides robustus is commercially harvested on both islands, and seems to have been previously misidentified as Lobonema smithi Mayer. The associated fish juveniles on these hosts were identified as Alepes djedaba (Forsskål) and Carangoides equula (Temminck & Schlegel). Alepes djedaba is closely associated with many species of scyphozoan jellyfish broadly in the southeastern Asian waters. The associations of carangids with jellyfish are also reviewed.
Small species often dominate benthic communities in hypoxia-stressed sites, but the organizing process has rarely been investigated. Community assembly was investigated in northwestern Ariake Bay, which is exposed to hypoxia during summer. Sampling was performed 11 times between May 2010 and May 2012. The timing and duration of hypoxia differed between 2010 and 2011, which caused a different pattern of seasonal change in the community structure between the 2 years. The community was mainly composed of small species such as Theora fragilis, Veremolpa micra, and Sternaspis scutata at all times. The exception was the large arcid bivalve Scapharca kagoshimensis. This bivalve newly settled after hypoxia in 2010 and became a dominant species of the subsequent community, whereas its density decreased greatly after hypoxia in 2011. Interestingly, new settlement of arc shells did not occur in 2011 when hypoxia was relatively moderate compared with that in 2010. These results suggest that small species dominance at a hypoxia-stressed site cannot be attributable only to repeated hypoxic devastation and quick recovery of short-lived small species but also to the variability in natural recruitment of long-lived large species. Large species cannot easily recover if low recruitment years continue, even if hypoxic conditions are moderate. Therefore, the need for more attention to recruitment variability of large species is emphasized in striving to understand community assembly and efficiently restore coastal soft bottom communities that have deteriorated due to hypoxia.
Snails of the genus Batillaria are common on most muddy tidal flats throughout Japan. To conserve tidal flat ecosystems, it is essential to understand the ecology of the Batillaria snails. A previous mitochondrial (mt) cytochrome c oxidase subunit I (COI) gene sequence analysis indicated that two Batillaria species, B. flectosiphonata and B. multiformis, occur together on the tidal flats on Amami-Oshima, Japan. However, it is difficult to distinguish them by their morphological traits. In this study, a simple multiplex PCR (mPCR)-based method was developed to identify the two species without the need for DNA sequencing. The method allows the discrimination of species by the electrophoretic detection of species-specific DNA fragments of the COI gene amplified by mPCR. This method will enhance the ecological study of these Batillaria snails on Amami-Oshima.