To investigate the mechanisms influencing the spatial distribution of the toxic dinoflagellate Alexandrium tamarense in the Okhotsk Sea off Hokkaido, Japan, intensive field surveys were conducted at 34–37 stations in late July every year from in 2002–2007. Alexandrium tamarense occurred every year. However, the abundance of A. tamarense fluctuated year by year, with extremely low cell densities of A. tamarense in 2005. High abundances of A. tamarense were found frequently in the oceanic area of the surface low-salinity water (LSW, salinity≤32.5) and the mixed water (MW). Low abundances were found along the coastal area of the Soya Warm Current (SWC, salinity≥33.6) and in the dichothermal water (DTW, temperature≤2°C) in the layer of oceanic areas deeper than 30 m. The PO4-P concentration in each water mass was in the order DTW>MW>LSW>SWC and the lowest PO4-P concentration that occurred in the SWC is considered to be a potential limiting factor for the growth of A. tamarense. The reason for the low A. tamarense abundance in the DTW is considered to be the low water temperature and low light intensity. It is concluded that the water mass of LSW has favorable conditions for the growth of A. tamarense. The relative frequency of each water mass fluctuated every year and the results suggest that the frequency of occurrence of the LSW and MW is one of the most significant factors controlling the abundance of A. tamarense in the area.
Giant clams are simultaneous hermaphrodites and are assumed to ejaculate first and, after completely stopping ejaculation, release eggs. In the seed production method aimed at preventing self-fertilization, each adult clam is induced to ejaculate in a tank and then release eggs in another tank. Giant clams, however, have recently been suggested to continue ejaculation for a period after the beginning of egg release. The overlap between ejaculation and egg release might lead to self-fertilization in the tank used for egg release, especially for the eggs released just at the beginning of spawning. We examined the possibility of such self-fertilization for the giant clam Tridacna crocea and obtained three results. (1) In observations with the naked eye in a laboratory, 2 of 38 T. crocea simultaneously ejaculated and released eggs. (2) In a laboratory experiment, 1.5 to 80.0% of eggs released from each adult clam developed into D-shaped larvae without artificial cross-fertilization. Such development occurred more frequently for the eggs released earlier from each adult clam than for the eggs released later from the clam. (3) In observations at a hatchery, 2 to 94% of the eggs released from 4 of 5 adults were found to develop into D-shaped larvae without artificial cross-fertilization. The three results suggest that at least some T. crocea adults continue ejaculation for a period after starting spawning eggs, which causes self-fertilization.
Based on 3,484 specimens collected from a series of surveys, we revealed that three species groups of deep-sea whelks (Buccinidae) are distributed predominantly in the Sea of Japan (off Honshu Island, 200–1,900 m in depth). The first species group is composed of species of the genus Neptunea. Although N. intersculpta and N. constricta have been considered as representatives of the Sea of Japan, our mitochondrial (mt) 16SrDNA data showed no clear reason to recognize more than one taxonomic unit for them. The second is composed of Buccinum tsubai; three haplogroups off Honshu Island were reconfirmed using 16SrDNA sequences. The present study demonstrated two of these haplogroups distributed in the same region near the Noto Peninsula. The last group, made up of B. striatissimum and related taxa, is still problematic despite our present morphological and molecular analyses. In our tentative conclusion, two different species are widely distributed off Honshu Island, one is B. striatissimum inhabiting in shallower regions (< ca. 500 m in deep) and another is B. tenuissimum in deeper regions. To overcome the remaining uncertainty related to the genetic differentiation of these two species, further clarification of their taxonomic relationship is needed. Geographical distributions of these three species groups are detailed based on the present study.
To clarify favorable environmental conditions for the population development of Pfiesteria-like dinoflagellates (PLDs), laboratory experiments were carried out using isolated strains of Cryptoperidiniopsis brodyi, Luciella masanensis, and Pfiesteria shumwayae. We examined the availability of 12 strains of phytoplankters which frequently cause red tides in coastal waters as food for these PLD species. In addition, the effect of temperature and salinity on the growth of these PLD species was also investigated. Three PLD species were able to feed on the cryptophyte Rhodomonas sp., the raphidophyte Heterosigma akashiwo and the dinoflagellate Akashiwo sanguinea, but not on thecate dinoflagellates. Among the three species tested, C. brodyi utilized the raphidophytes, Chattonella ovata, C. marina, and C. antiqua; however, two other species, L. masanensis and P. shumwayae, could not utilize these raphidophytes as food. The growth rates of PLD species generally increased with an increase in temperature in the range of 15 to 30°C. Below 10°C, their growth was negative. While the growth response of the three species as a function of the temperature was similar, that as a function of the salinity was different. The salinity ranges required for positive growth were 5–15, 5–25, and 5–30, respectively, for L. masanensis, P. shumwayae, and C. brodyi. The bloom of PLDs likely depends on high abundances of available phytoplankters in brackish waters when water temperature is >15°C.
The estuarine mysid Mesopodopsis tenuipes (Crustacea: Mysida) was often found carrying the peritrich ciliate Zoothamnium duplicatum (Protozoa: Ciliophora) on its body. The prevalence and seasonality of this association were studied on the basis of a year-round survey conducted in the Merbok mangrove brackish water in north-western Peninsular Malaysia from March 2005 to February 2006. The frequency of mysids hosting ciliates varied from 0 to 57.3%, with an annual mean of 17.0%. Juveniles showed a significantly lower prevalence of ciliates than adult mysids. There was no difference in infestation prevalence between the sexes. An epibiotic renewal process could be assessed through observations in breeding females. The infestation prevalence and loads were appreciably lower in the females carrying earlier stages of embryos than in those with developed ones, indicating that zoothamnid ciliates may require two or more days to reach a saturation phase in terms of prevalence when ciliate reproduction is high. This, in turn, suggests that the high moulting frequency of the tropical mysid may inhibit high colonisation pressure by the epibiont. A periodic seasonal trend was not determined, although mysids in lower salinity conditions were more likely to carry epibionts than those at higher salinities.
The abundances and species compositions of chaetognaths were investigated on the northwestern coasts of the Persian Gulf in Khuzestan province (2004–2005). Highest mean chaetognath abundance was observed during the summer (324.7±97.6 indiv. m−3), while lowest values occurred during the winter (79.3±18.6 indiv. m−3). Six species were identified, Sagitta bedfordi, S. enflata, S. ferox, S. hexaptera, S. neglecta and Krohnitta pacifica. Sagitta ferox was most abundant. It represented 49.7% of total chaetognath numbers. Abundance variation throughout the year cycle seemed to be related to environmental factors. The correlation coefficients between abundance of chaetognaths and the water temperature were significantly positive but there was no significant relation for abundance and salinity. However, salinity values in the Persian Gulf are very high compared with other regions and perhaps pelagic chaetognaths species that inhabit this region are adapted to very high salinities of up to 45.