No.87(March,2015)
No.87(Maruch,2015)
A-515 Akira MIYAZONO
Ecology of the harmful dinoflagellate Alexandrium tamarense , a causative organism of the paralytic shellfish poison
in Funka Bay, Hokkaido（Doctoral thesis)
Contamination of paralytic shellfish poisons (PSP) to bivalves such as scallops, caused by blooms of the toxic dinoflagellate,
Alexandrium tamarense, is a serious economic hardship to the scallop aquaculture industry around Funka Bay where is an
important area for the scallop aquaculture in Hokkaido, Japan. It is empirically known that excystment of A. tamarense cysts in
sediments is important as inoculation for the blooms. However, there is a paucity of information on contribution of the
excystment on the bloom formation. For forecast of occurrence of PSP caused by A. tamarense , the present study was conducted
to understand the biological background of the mechanism of the blooms and a role of the cyst for bloom occurrences.
Long−term fluctuations of A. tamarense bloom and oceanographic parameters in southwestern Funka Bay were analyzed for
32 years during April 1982 to December 2012. Large bloom years (maximum cell number > 5000 cells L−1) were observed 3
times in 1982 − 1995, and no bloom years (maximum cell number < 100 cells L−1) and/or small bloom years (100 < maximum
cell number < 1000 cells L−1) were observed in 1996 − 2012. A. tamarense blooms fundamentally have three phases, i.e., the
primary population formation in March, the population growth in May, and the population disappearance in July. The bloom
period in 2000s shifted one month earlier than in 1980s and 1990s. Yearly increasing trends were confirmed in water temperature
in March and salinity in April. The environmental changes with these trends appear to affect the changes in period and
magnitude of the recent blooms. The pycnocline development for long period (almost two months) after the primary population
formation is thought to be important in the large bloom years (1984, 1989, 1994) indicating that vertical stability of water
column is an important factor for the bloom development.
The effects of temperature, light intensity and day length were investigated on the growth of an A. tamarense strain isolated
from Funka Bay with culture experiments. The Funka Bay strain of A. tamarense grew at 3℃ and temperature range of 10 −
18℃ was suitable for the growth, the strain appears to adapt to cold environments.
The growth parameters of the strain to light intensities showed higher values in the order of increasing water temperature from
3 to 10℃. The strain more adapted to low light intensities than other Honshu strains of A. tamarense . The growth rates were
strongly affected by day length in 5 and 10℃. These results suggest that the strain showed an adaptation to high latitude
environments.
The correlation between temperature and the excystment activity of A. tamarense cysts were examined with the field
observations and the culture experiments. The maximum excystment activity was recorded at water temperatures of above 10
℃. The temperature window (excystment activity > 50 % 10−days−1) of the cysts was found from 7 to 18℃ (possible > 18℃).
The excystment period was February − May and the dormant period was June − January, respectively. The seasonally of
excystment was observed both cyst populations in surface layers and deeper layers in bottom sediments. The excystment seasonally in the surface layer was explained by an innate dormancy and that in the deeper layer was explained by secondary
dormancy which was given to old cysts under quiescence with anoxia conditions.
The sequences of the blooms in large bloom years (1984, 1989 and 1994) were estimated with water temperature, light
conditions, growth parameters of the vegetative cells and of excystment characteristics. In 3 large bloom years, the combinations
of the extents of excystment and the growth of the vegetative cells were different in each year. The excystment and the growth
in 1989 were both active, the excystment was not so active but the growth was rather active in 1984, and the excystment was
active but the growth was fair in 1994. The observed vegetative cell numbers were results of processes of the excystment, cell
division and others. The excystment was considered evidently to contribute to the formation and maintain of the primary
population in 1989 and 1994. From these results, the most important role of the excystment of the cysts was concluded to be the
inoculation of vegetative cells into water columns, but the magnitude of the excystment is not so important to form blooms in
Funka Bay as compared to the growth of primary populations in water columns.
The vertical distributions of the cysts were examined in offshore sediment cores collected in Funka Bay. The cysts distributed
entirely in the sediments and subsurface maximum layers of the cysts abundances were observed in all the examined cores.
These subsurface maximum layers were thought to be formed in the same bloom.
Viabilities of A. tamarense cysts were examined from surface to 32 cm depth layer of the sediment core collected at the
deepest area in Funka Bay with the culture experiments. Some of the cysts from the sediment which deposited about hundred
years ago could germinate. This is the longest record of viability of A. tamarense cysts hitherto. A negative exponential
correlation was observed between the germination success and deposited period of the cysts. The viability was higher in the
newly deposited cysts. The cysts deposited hundred years ago had transparent protoplasm and thicker cell wall. From these
results, the old A. tamarense cysts in the subsurface sediments considered to have high tolerance ability to anoxia.
The ecological strategies of A. tamarense in Funka Bay were summarized as follow. The cysts in deeper sediments have a
high viability under anoxia conditions and are able to play a role as seed populations. The cysts have temperature window for
vigorous germination (7 − 18 ℃ and/or >18℃), but the excystment period and degree were limited when bottom water
temperature was 3 − 5℃ with the germination of small portion of cyst populations. The vegetative cells are able to grow under
low temperature (3 − 5℃) and low light intensity (I0:2.7 − 4.5 μmol photons m−2 s−1), and these features are helpful for
establishing primary populations in cold period. The large scale blooms which occur every several years and/or decades enlarge
abundance of cyst populations, however, small scale blooms or no bloom reduce the abundance of cysts in sediments.
The influence of long−term trends of water temperature and salinity was examined in western area of Funka Bay in relation
to formation of the blooms. The increasing trend of water temperature in March, and the increasing trend of salinity in April
were found and the former trend affects the scale of excystment and the latter does reduction in growth rate of the vegetative
cells, respectively. The key of the toxic blooms in Funka Bay is the success of the growth of the vegetative cells rather than
excystment. Thus if these trends continue, the magnitudes of the blooms are expected to be small and /or failure in the future
years.
A hundred cells L−1 of the vegetative cell density in water column is an indicator of the control of scallop shipments by
paralytic shellfish poisoning caused by A. tamarense blooms in Funka Bay. The vegetative cell density of 100 cells L−1 or more
in water columns can be attained by the direct excystment with higher bottom water temperature than the 32 year average (1982
− 2012) and the cyst density in May of over 400 cysts cm−3 in sediments. It is proposed that to monitor the cyst population in
every five years would contribute to predict the occurrences of toxin contaminations to scallops.
The density of viable cysts in subsurface layer (16 − 22 cm layer) is about 1.4 times magnitude of that in surface layer (0.5
− 5 cm layer) in sediments of offshore of Funka Bay. Tsunami resuspended cysts and selectively concentrated the cysts in
surface sediments in Funka Bay when the Tohoku Earthquake occurred in March 2011. These highly concentrated cyst
populations in surface sediments endangers the occurrences of the toxic blooms by A. tamarense. We need to pay more attentions
and to monitor A. tamarense in Funka Bay.
A-516 Kanji NAKAJIMA and Hiroo GOUDA
Seasonal change in body weight and intestine of Japanese sea cucumber（Apostichopus japonicus）inhabiting sea
waters around Soya in northern Hokkaido
The seasonal dynamics of body weight and intestine of sea cucumber, Apostichopus japonicus, caught in the sea around Soya
in northern part of Hokkaido were investigated. A total of 10 samples were surveyed every one to two months from March 2007
to April 2008. The weight of internal organs to body weight ratio and intestine length to body weight ratio were the lowest in
October. It is suggested the reset of internal organs happened in this season. The ratio of the body wall dry weight to body wall
wet weight was the lowest in the summer, and then it dramatically increased reaching the highest level between August and
September. These fluctuations indicate that chemical components in the body wall of sea cucumber decrease significantly. These
results revealed that sea cucumbers undergo dramatic changes in the volume of internal organs or the body wall moisture content
each year repeatedly.
A-517 Yoshiyuki TAKAYA and Nobukazu SATO
Influence of swimming on the chemical constituents of juvenile barfin flounder, Verasper moseri
We suggested a method for estimating the growth rate on the basis of the biochemical constituents of the liver or trunk in
juvenile barfin flounder. The method can be applied to predict the growth rate of the recaptured fish after its release in the field.
However, these experiments were conducted in the indoor tanks, where the experimental fish had little space for swimming
compared with the wild fish. Therefore, the growth rate estimated for several indices may be either too high or too low than that
in the wild fish. Change of the biochemical constituents when fish swim compulsorily was investigated and the influence of
swimming in fish on biochemical indices was confirmed.
A-518 Akiyoshi SHINADA and Koji MIYOSHI
Right period to determine the size of adductor muscle in bottom cultured Japanese scallop Mizuhopecten yessoensis in the coastal area of northeastern part of Hokkaido, Japan（Short paper）
Growth history of bottom cultured Japanese scallop Mizuhopecten yessoensis was investigated to reveal the correct period to
determine the size of adductor muscle in four−year−old individuals in the coastal area of northeastern part of Hokkaido, Japan.
Correlation analyses were conducted between the size values from August to October in four−year−old individuals, from October
to November in one− to three−year−old individuals, and from April to July in four−year−old individuals. Significant positive
correlation was found only between the size values from August to October and from April to July both in four−year−old
individuals. Because the adductor muscle cannot reflect the growth of the previous year due to shortage of food in fall and
winter, the size of adductor muscle in four−year−old individuals should be determined from April to July in a fishing year.
A-519 Yohei SHIMIZU , Takuma KAWASAKII , Shin-ichi TAKABATAKE , Toshiharu IWAI and Masakane YAMASHITA
spread of immunostaining method for investigating the larval distribution in the field（Technical report）