Gnathiidae is a family of Isopoda, showing a distinct morphology and ecology. Species in the family have ten walking legs compared to 14 for isopods in general. The life cycle of the Gnathiidae is biphasic including a fish-parasitic larval phase and a non-feeding adult phase, whereas most other isopods have a monophasic life cycle with direct-developing young similar to adults in morphology and ecology. Although the ecological information on gnathiids has been described fragmentally, studies on the life cycles and population biology have increased dramatically since the 1980's. Larval biology including the host species, emergence pattern and predation risks during parasitism have also been studied from the view of fish disease and symbiotic interactions among fishes. Some authors reported the utilization patterns of benthic substrata and harem formation of adults therein. However, the ecological knowledge of gnathiids is based on a limited number of species of which the benthic habitats are known. Studies rarely identify larvae because of the lack, or insufficiency, of the taxonomic description of larvae of most species. The difficulty in identifying larvae has prevented detailed examination of ectoparasitic behaviour. Furthermore, gnathiid larvae emerging from the benthos or attaching to hosts have been investigated separately from benthic individuals. More efforts on taxonomy and the classification of larvae are necessary to elucidate the larval ecology and the integration of larval and adult biology.
Adults of Oithona atlantica, Oithona plumifera and Oithona similis occurred throughout the year in Toyama Bay (0-500 m), but in the surface layers (0-50 m), the adults of O. atlantica and O. plumifera were observed in Toyama Bay during different seasons. Based on the differences in adult distribution, O. atlantica nauplii were found in samples collected during spring (April to May) while O. plumifera were found in samples collected during autumn (September to October). Identification of O. atlantica and O. plumifera nauplii and comparisons of their morphological differences were based on this temporal separation. Nauplii of O. atlantica, O. plumifera and O. similis differed in their ratio of body height (maximum length of dorsal-ventral axis) to length (H/L). This characteristic ratio was common to all naupliar developmental stages and is therefore useful as a basis for distinguishing between naupliar stages of the three species. The population and community structure and densities of these species of Oithona were investigated in the surface water layer (0-50 m depth) of Toyama Bay from 1997 to 1999. Major population structural changes occurred in different sampling seasons, and may have been influenced by changes in hydrographic conditions, including temperature and water current. In winter and spring (from February to June), many O. similis and O. atlantica nauplii appeared, and the former nauplii were more dominant. Oithona similis nauplii are suggested to may be the most important species as prey for fish larvae.
Spatial variation in intertidal molluscan assemblage was investigated at 9 sites from outer to inner Tanabe Bay, central Japan. All the molluscan species were counted in 15, 50×50cm quadrats at each site. Cluster analysis and PCA detected three distinct groups of sites in terms of species composition, i.e., the site outside bay, the sites around the bay mouth, and those in the innermost area. Diversity of the molluscan species decreased as the topographical openness of the sites decreased. Toward the inner reaches, southern species (<35°N) and carnivorous species decreased, whereas northern species (>31°N) and filter-feeding species increased in terms of the number of species, quadrats, and individuals. A similar trend is shown also by a review of previous surveys around Tanabe Bay. Analysis of the environmental parameters shows that winter water temperature is lower and chlorophyll a concentration is higher at the bay head than at the bay mouth, whereas the difference in salinity is not distinct between the two areas. A positive correlation between winter water temperature and abundance of the southern species is detected both along the bay coast and over a long time-span at fixed localities. Such a spatio-temporal correlation and other evidence implies the controlling effect of temperature and trophic regime of bay water on the pattern of the molluscan assemblages along the bay coastline.
Distribution patterns of the three species of sand-burrowing mysids, Archaeomysis vulgaris, Archaeomysis japonica and liella ohshimai were investigated, and also the diel and tidal distribution patterns of the dominant mysid A. vulgaris were investigated in the surf zone of an exposed and mesotidal sandy beach at Fukiagehama, Kagoshima Prefecture, southern Japan. The morphodynamic state of the beach was an intermediate type with a low-tide terrace or ridge/runnel. Habitat zonation was recognized among the three species during the entire study period (May and October of 2002 and 2003). A. vulgaris occurred from the high-water shoreline to the low-water shoreline during the daytime, whereas A. japonica was observed off the low-water shoreline and I. ohshimai was found far from the longshore bar. Habitats of A. vulgaris shifted along the beach slope, depending on the tidal changes at the shoreline. However, at low water in May of 2002 and 2003, most individuals of A. vulgaris were distributed in the intertidal runnel and on the lowtide terrace, rather than off the low-water shoreline. A. vulgaris had the highest population density of the three species, and fluctuations in its total mean density in the surf zone were estimated at each tide in May of 2003. These ranged from 2,956 indiv. m-2 at low water at night to 172 indiv. m-2 at middle water during the daytime. Distribution patterns characteristic of A. vulgaris are ecologically interpreted in relation to beach morphology, water flow regimes in the surf zone, and avoidance of fish-predation pressure.
Heterocapsa circularisquama is the most noxious dinoflagellate to bivalves such as oysters, short-necked clams and pearl oysters because its red tides have caused major mass mortalities. In order to reduce the negative impacts of H. circularisquama, predictions of red tide occurrences are essential, and hence it is important to grasp its population dynamics. The population dynamics of H. circularisquama, however, have not yet been clarified throughout the year. To precisely monitor population dynamics, the fluorescent antibody technique is effective. We here report on H. circularisquama population dynamics monitored with the indirect fluorescent antibody technique (IFAT) using monoclonal antibodies. Samplings were carried out once a week in summer and twice a month in other seasons at 4 points in Ago Bay, Mie Prefecture, Japan, from April 2001 to March 2005. Direct counting of the cells was also performed using a normal optical microscope. Vegetative cells of H. circularisquama monitored with IFAT were generally detected from late spring (May) to late autumn, and the cell density increased (maximum 2.33×106 cells L-1) during summer. The lowest detection level was 1.33 cells L-1 with IFAT. In contrast, monitoring by counting with a common optical microscope often failed to detect H. circularisquama cells even at a density of 103 cells L-1 or more. Water temperature was significantly correlated to the abundances of H. circularisquama, and the cells were almost always observed at a temperature of 25°C or higher (summer season), and never detected at 10°C or lower (winter and early spring) in Ago Bay. In the waters of the bottom layer, H. circularisquama cells tended to be more abundant with lower dissolved oxygen (DO) values during summer. Thus, the seasonal distribution of H. circularisquama was clarified almost perfectly together with environmental factors in Ago Bay between spring and early winter.
Bacterial abundance determined with SYBR Green I and 4'6-diamidino-2-phenylindole (DAPI) methods were compared using the protocol for each method employing epifluorescence microscopy. This comparison, using seawater samples that were collected from Sagami Bay, revealed that the abundance determined with the SYBR Green I method was on average 1.6 fold greater than that with the DAPI method. A model 2 regression analysis showed the following relationship: Log SYBR Green I=-0.238+1.08 Log DAPI (r2=0.85, n=68, p<0.0001). These results suggest that when the method is switched between the SYBR Green I and DAPI methods, the difference in bacterial abundance determined by the two methods should be considered for a quantitatively consistent assessment of the bacterial biomass in the ocean.
A scyllarid lobster phyllosoma larva was observed attached to the siphosome of a prayid siphonophore while SCUBA diving in the Canary Islands. The phyllosoma was straddling the siphosome, which was still alive, with one pereiopodal endopodite per siphosomal cormidia and used its pereiopodal exopodites to pull the siphosome through the water. It did not detach, even after repeated disturbances, and it is proposed that the phyllosoma may be using the prayid siphosome as both food and for protection from predators.
A huge reclamation project is rapidly progressing in the Saemangeum area, on the western coast of South Korea. The total length of the reclamation dikes is 33km, with a reclamation area of 401km2. All dikes were finally completed in April 2006, and the tidal flats in this area have gradually dried out. In our present study, a crisis facing the unique molluscan fauna in this area, especially an unique bivalve species attached to the shell of Lingula anatina, is reported, and we claim that this constitutes an emergency for the conservation of the Saemangeum tidal flat.