Seasonal fluctuations in the abundance of populations of marine cladocerans and their resting eggs in the Inland Sea of Japan

Seasonal fluctuations in the abundance of resting eggs of four species of marine cladocerans, Penilia avirostris Dana, Podon polyphemoides (Leuckart), Evadne tergestina Claus and Evadne nordmanni Lovén, were investigated in association with their planktonic populations in the central part of the Inland Sea of Japan for a period of over two years. Most numerous were the eggs of P. avirostris, followed by those of the species in the aforementioned order. There was a marked seasonal fluctuation in the number of eggs that was closely correlated with the occurrence of the planktonic population for each species. The egg-number was highest just before the planktonic population disappeared from the water column, and gradually decreased thereafter until a minimum value was reached shortly before the new planktonic population appeared in the next season. The pattern of fluctuation in number was found to be very similar among all four species, indicating strongly that the cladoceran populations in this warm temperate sea may persist either as plankton in warmer seasons or as benthic resting eggs in colder seasons. The possible causes of the decrease in egg density in bottom sediments are discussed.     

Growth and production of the inshore marine copepod Pseudodiaptomus marinus in the central part of the Inland Sea of Japan

The growth and production of the inshore marine copepod Pseudodiaptomus marinus was studied in the central part of the Inland Sea of Japan. The stage-specific growth rate was determined under controlled laboratory conditions by examining the length-weight relationship and development rates at various temperatures. The stage duration was short and constant from NII to CII, beyond which development was retarded. Males developed faster than females in CIV and CV. The specific growth rate was highest in copepodite stages followed by the nauplii and adult females (=egg production rate). The daily production of P. marinus was estimated from the stage-specific growth rate and stage-specific abundance in nature as the sum of the individual stages. The production changed seasonally with water temperature and population biomass. Daily production and biomass (P/B) ratios increased linearly with temperature. Total annual production was 20.7 mg C m^-3 yr^-1.     

Feeding habits and food requirements of some amphipods in the Black Sea

Food composition and food requirements of four amphipod species of the Black Sea — Dexamine spinosa (Mont.), Amphithoe vaillanti Lucas, Gammarellus carinatus (Rathke), Gammarus locusta L. — were studied and an attempt made to assess the quantitative composition of the food, daily rhythms of feeding, and daily food rations. The composition of the food of the above-named amphipods is similar and consists primarily of seaweeds. Daily feeding intensity is characterized by a distinct increase during the dark period (D. spinosa, A. vaillanti); however, in G. locusta feeding intensity increases in the morning. The daily food rations of amphipods vary greatly (from 1.7 to 360% of the specimens' body weight), depending upon factors such as the kind of food, age, sex, the physiology of specimens, and the water temperature. The total quantity of food eaten by the populations of these amphipods in the coastal zone of the Black Sea amounts to 4000 g organic matter per square metre within 1 year.     

Distribution and abundance of finless porpoise in the Inland Sea of Japan

Aerial sighting surveys were conducted in 2000 to evaluate the distribution and abundance of finless porpoise Neophocaena phocaenoides in the Inland Sea of Japan. We flew 60 north–south transects (2,218 km in total) at intervals of 6.43 km over the study area between 131°3′ and 134°59′ E. In total, 148 groups were detected by two observers. The average group size was 1.56 individuals. The effective strip half-width of each observer was estimated to be 107 m (coefficient of variation [CV] = 8.26%). Porpoise abundance was estimated at 7,572 individuals and the density was as low as 0.506 individuals/km² (CV = 17.3%). The sea is among the Japanese waters with the lowest density in spite of its favorable topographical conditions. In the western stratum of the study area, where the estimated density was 1.31 individuals/km², we observed a relatively regular distribution. In the central-eastern stratum, which had an estimated density of 0.208 individuals/km², we confirmed a clumped distribution that was restricted to inshore waters or near islands. No individuals were observed in waters between 132°51′ and 133°11′E, and between 133°43′ and 133°59′E, where sand dredging and other human activities have been active, suggesting that habitat fragmentation has occurred.     

Comparative phylogeography of marine cladocerans

We examined the population genetics of six species of marine cladocerans, using a ~600 bp fragment of the cytochrome oxidase subunit I gene sequence. Phylogenetic analysis revealed significant intraspecific, semi-allopatric phylogenetic breaks in four out of five species belonging to the Podonidae, supporting an ancient radiation and oceanic expansion for this group. By contrast, Penilia avirostris (Sididae) displayed no phylogeographic structure across a global sampling, suggesting a recent worldwide expansion. Our results also show a transoceanic distribution of identical or very similar haplotypes in several species of marine Cladocera, which may be interpreted as either natural transport or evidence of recent anthropogenic transport. If the latter is the case, marine cladocerans represent one of the first genetically documented cases of exotic or invasive marine zooplankton, likely an underreported group. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Sequence data from this article have been deposited with the EMBL/GenBank Data Libraries under accession nos. EU675871–EU675924.     

Distribution of marine cladocerans in the Indian Ocean

Cladocera of the Indian Ocean have been studied, on the basis of samples collected during the International Indian Ocean Expedition (IIOE). Out of a total of 1927 samples, 552 contained Cladocera. Although the distribution of this group is mainly coastal, one species, Evadne tergestina, has been found in oceanic waters also, where it may establish seasonal populations. Evadne spinifera was recorded off the Australian and South African coasts in the southern hemisphere, and in the Northeastern part of the Arabian Sea coastal waters in the northern hemisphere. Podon polyphemoides was reported from a single station in South African waters. Penilia avirostris was found in coastal waters throughout the Indian Ocean. Short-term distribution patterns of E. tergestina and P. avirostris are discussed. The possible value of this group as food for pelagic fishes has been indicated. This work makes clear that adequate sampling in space and time is required along the Continental Shelf of the Indian Ocean in order to obtain more comprehensive information on this group.Visiting Scientist at Indian Ocean Biological Centre, Cochin, India under UNESCO contract SC/2077/70-20579.     

Ontogenetic diel vertical migration of the planktonic copepodCalanus sinicus in the Inland Sea of Japan

The ontogenetic diel vertical migration of the planktonic copepodCalanus sinicus was investigated in the Inland Sea of Japan in summer 1988, when the water was thermally stratified with a thermocline of ca 5 °C between 35 and 45 m. Stage-specific differences in the diel vertical migration behavior ofC. sinicus were found. Eggs were spawned primarily within the surface-waters between midnight and dawn by ascending females, and sank gradually to deeper waters until they hatched into nauplii. Non-feeding nauplius stages (NI and II) were distributed throughout the water column, but the first feeding stage (NIII) performed an ontogenetic upward migration. NIV to VI and copepodite (C) stages I to III continuously aggregated in the phytoplankton-rich euphotic layer. However, the depth of the median CI to III populations descended as stage progressed. The onset of prominent diel vertical migration took place in CIV, and the amplitude of vertical migration increased with age, being maximal in adult females (CVI). Adult males (CVI), however, remained in the layer below 20 m, and did not migrate dielly. The ecological significance of ontogenetic diel vertical migration is discussed.     

Ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus in the Inland Sea of Japan

The ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus was investigated in the Inland Sea of Japan in June 1989, when the water column was thermally weakly stratified. Because of fewer eggs and less variation in their abundance, nocturnal spawning was not apparent. A pronounced upward migration occurred in NIII. NIII to CIII resided in the upper 20 m layer throughout the day, and from CIV on their median depths descended. CV and adult females underwent significant diel vertical migration, whereas adult males did not migrate. By integrating the results from the present study and those from our previous investigations (in August–September 1988, November 1988 and March 1989), we review seasonal variation in the ontogenetic diel vertical migration of C. sinicus. Spawning was largely nocturnal, reaching its maximum level around dawn, but spawning depth and fecundity changed seasonally. The distribution of pre-feeding stages, NI and NII, was similar to that of eggs. A pronounced upward migration always occurred in the first feeding stage, NIII, and late nauplii and early copepodites always resided in the food-rich upper layer, indicating that upward migration by NIII is feeding migration. As the stages progressed, they extended their vertical distribution range, and CV and adult females usually underwent diel vertical migration. However, the pattern and strength of this migration differed seasonally. Their day depths increased with the increase of relative biomass of planktivorous fish, indicating that predator avoidance induces their diurnal downward migration. High chlorophyll a concentrations in the upper layer (<15 m deep) relative to the lower layer (>20 m deep) amplified their diel vertical migrations. Diel vertical migration of C. sinicus is a phenotypic behavior.     

Ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus in the Inland Sea of Japan

The ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus was investigated in the Inland Sea of Japan in November 1988 and March 1989, when the water temperature was weakly stratified in a reversed manner. In both investigations a pronounced ontogenetic difference in vertical distribution was found. Spawning always occurred during nighttime, being confined to the upper 40 m water column in November but to the layer below 35 m in March. The distribution of pre-feeding nauplius stages, NI and NII, was more or less similar to that of the eggs. The first-feeding NIII performed a marked upward migration, and late nauplius stages (NIV to NVI) and early copepodite stages (CI and CII) continuously aggregated in the upper water column where phytoplankton was abundant. CIII to CVI (adult female and male) tended to disperse in the whole water column. In November, however, they avoided the upper 10 m strate during daytime and some individuals migrated upward to the surface during nighttime. In March, CV and CVI aggregated in the layer between 5 and 15 m deep in the daytime and migrated both upward and downward at dusk, resulting in homogeneous distributions during the nighttime.     

Long-term trends in abundance of cladocerans in the Central Baltic Sea

Long-term dynamics (1960-1997) of the cladoceran species Bosmina coregoni maritima, Evadne nordmanni and Podon spp. are described for the Gdansk Deep and the Gotland Basin (Central Baltic Sea). By using correlation analyses on seasonal time-series, the influence of temperature and salinity on the abundance of cladoceran species was investigated. A clear affinity to higher temperature was found for B. coregoni maritima in summer as well as for E. nordmanni and Podon spp. in spring. In addition to temperature, association tests with salinity revealed besides species-specific preferences, regional and temporal differences. Contrary to B. coregoni maritima, both other species were positively associated to salinity in summer and autumn in the Gdansk Deep. In the Gotland Basin only E. nordmanni was positively correlated to salinity in autumn. Differences in the response to hydrographic variables are possibly stage specific, i.e. between resting eggs and adults, or due to a different adaptation to the abiotic environment.     

The cladocerans of the Aegean Sea: Occurrence and seasonal variation

The little-known cladoceran population of the Aegean Sea was studied. One hundred and seventy-six planktonic samples were collected during 8 cruises in the Aegean Sea (4 in the northern part, 4 in the southern part). From these samples, the following 4 species of Cladocera were determined: Evadne spinifera Muller, Evadne tergestina Claus, Penilia avirostris Dana, Podon intermedius Lilljeborg. All species were found both in the north and in the south Aegean Sea, with the exception of P. avirostris, which was found only in the northern Aegean Sea. Its absence from the southern part is attributed to the higher salinities and the pelagic character of the area. With regard to abundance, E. spinifera is by far the commonest in this area; P. avirostris and E. tergestina could be characterised as frequent forms; P. intermedius is scarce. P. avirostris, E. spinifera and E. tergestina are thermophilic; their occurrence is more or less restricted to the warm season; P. intermedius is psychrophilic and appears only in winter.     

Annual life cycle of Chattonella spp., causative flagellates of noxious red tides in the Inland Sea of Japan

Effects of storage temperature on the dormancy and maturation of dormant cells of Chattonella spp. were investigated by the extinction dilution method using sediment samples collected in 1984 after the blooming in Harima-Nada. Seasonality of germinability in fresh sediments was also studied in 1985. A storage period of more than 4 months with a low temperature (11°C) was essential for the maturation to induce the germination capacity of dormant cells. The optimal temperature for germination (22°C) had no effects on the maturation. Using fresh Suo-Nada sediments, a marked seasonality of germinability was confirmed. It was weak from autumn to early winter, then strengthened gradually up to a high level, which was maintained between spring and early summer, and again decreased rapidly during summer. The annual life cycle of Chattonella spp., including vetetative and dormant phases, was summarized as follows: (1) vegetative cells in early summer originate from germination of dormant cells in sediments; (2) they form overwintering dormant cells during the summer season; (3) dormant cells spend a period of spontaneous dormancy until next spring; (4) the duration of post dormancy, an enforced one due to low temperatures, follows until early summer. The life cycle of Chattonella spp. is therefore well adapted to the temperature regime in temperate seas such as the Seto Inland Sea of Japan.     

Seasonal reproductive biology of the egg-carrying calanoid copepod Pseudodiaptomus marinus in a eutrophic inlet of the Inland Sea of Japan

In situ egg production of the egg-carrying calanoid copepod Pseudodiaptomus marinus was investigated in Fukuyama Harbor, a eutrophic inlet of the Inland Sea of Japan, at 3- to 5-d intervals for a year. This species reproduced throughout the year, and the adults showed a large abundance peak in June/July and a small peak in September/October. Females usually outnumbered males, comprising 61.4% of the annual mean. The composition of ovigerous females varied from 7.9 to 100%, with an annual mean of 55.7%. Adult prosome length was consistently large throughout winter and spring, and decreased with increasing temperature in summer and fall. Egg diameter varied from 98 to 121 μm, and was negatively correlated to temperature. The seasonal variation in clutch size (range: 15.1 to 38.2 eggs) was bicyclical, with peaks in May and December. The egg production rate of breeding females was low in January to March (mean: 2.3 eggs female⁻¹ d⁻¹), while it was constantly high from mid-May to early October (mean: 12.1 eggs female⁻¹ d⁻¹). The specific egg production rate for the breeding females was highly correlated to temperature; it increased linearly from 0.03 d⁻¹ at 9 °C to 0.27 d⁻¹ at 26 °C. Compared to other co-occurring copepods, the reproductive rate of P. marinus was lowest, which is one of the reasons why this species never dominates in this inlet. Received: 11 November 1996 / Accepted: 7 December 1996     

Synchronous recruitment and growth pattern of planktonic larvae of the amphioxus Branchiostoma belcheri in the Seto Inland Sea, Japan

Planktonic larvae of the amphioxus Branchiostoma belcheri were collected in the western part of the Seto Inland Sea, Japan, in order to document their seasonal occurrence, recruitment, and growth patterns. The larvae appeared from mid-July to early October and their size ranged from 0.6 to 5.5 mm. Three distinct cohorts were observed during the summers, indicating multiple, synchronous spawning within the population. The length increment pattern of a cohort demonstrates slow growth for at least a week during the early larval stage. July 10, 20, and 30 are the inferred dates when spawnings started to produce the three cohorts in 2001. Favorable temperature range for spawning was 21–23°C. Bias in population structure due to advection can be assumed to be negligible; therefore the length increment of the cohort, about 0.05 mm day⁻¹, is regarded as the larval growth rate after the initial slow-growth stage. The planktonic stage of the first cohort in 2001 is estimated to have lasted around 55 days. The relative proportions of the second and third cohorts in samples from surface and mid-depth waters suggest downward migration of the planktonic larvae, as they age.     

Feeding and absorption of the toxic dinoflagellate Alexandrium tamarense by two marine bivalves from the South China Sea

Paralytic shellfish poisoning (PSP) toxins can be accumulated by bivalves through the feeding process; therefore, knowledge on feeding and the assimilation of PSP-toxin-containing algae is critical to understand the kinetics of PSP toxins in these bivalves. In the South China Sea, it has been documented that the scallop Chlamys nobilis has a much higher PSP toxin burden than the clam Ruditapes philippinarum. Experiments were therefore carried out to assess whether the difference in toxin burden between these two species of bivalves was due to differences in feeding and absorption. In a mixed diet of Alexandrium tamarense (a PSP-toxin-producing dinoflagellate) and Thalassiosira pseudonana (a non-toxic diatom), the maximum clearance and filtration rates were about two times higher in the scallop C. nobilis than in the clam R. philippinarum. Furthermore, the clams produced pseudofeces at a lower cell density than the scallops. However, we found that the clams were unable to selectively exclude the toxic dinoflagellates by pseudofeces production. The scallop C. nobilis also possessed a greater ability to assimilate A. tamarense with a comparable carbon absorption efficiency to the diatom T. pseudonana. In contrast, the carbon absorption in the clam R. philippinarum was lower when feeding on A. tamarense than on the diatom. In general, the absorption efficiency decreased with increasing concentration of A. tamarense. Thus, it is likely that the higher PSP toxin levels in the scallops compared with clams can be partly explained by differences in their feeding and absorption behavior. Other processes, especially the biotransformation and biokinetics of PSP toxins, may also play a significant role in defining the inter-species differences in PSP body burden in marine bivalves.     

Population dynamics and production of the planktonic copepods in a eutrophic inlet of the Inland Sea of Japan. II. Acartia omorii

Population dynamics and production of the calanoid copepod Acartia omorii Bradford were studied from November 1986 to November 1987 in Fukuyama Harbor, a eutrophic inlet of the Inland Sea of Japan. This species was present in the plankton from October to July (temperature range: 8.9 to 24.3°C), with peaks in February-March and June. During this period, nine generations could be detected, for which the mean population egg production rate and midstage abundance of each life stage older than naupliar Stage (N) II were determined to trace survival. The population suffered extremely high mortality during the early life stages: on average only 2.5% of the eggs produced recruited into NII. This large loss is probably concentrated within the egg stage, due to predation, including cannibalism, by omnivorous copepods, in addition to sinking loss of eggs in the water column. However, the mortality from NII to copepodite Stage (C) V was negligible, indicating low predation pressure by large carnivores. The biomass of A. omorii showed marked seasonal variations in parallel with numerical abundance. The instantaneous growth rate of each stage increased exponentially with increasing temperature. The integrated production rate of A. omorii from 7 November 1986 to 21 July 1987 was 749 mg Cm^-3 or 5.62 g Cm^-2     

Population dynamics and production of the planktonic copepods in a eutrophic inlet of the Inland Sea of Japan. I. Centropages abdominalis

Population dynamics and production of the calanoid copepod Centropages abdominalis were studied from November 1986 to November 1987 in Fukuyama Harbor, in the central part of the Inland Sea of Japan. This species was present in the plankton during a cold-water period from November to June (temperature range: 8.9 to 21.1 °C), with a peak abundance (23 600 ind m^–3) in February. During this period, six generations could be detected, and each generation time agreed well with that predicted from food-satiated laboratory experiments, indicating that the natural population was not food-limited. The population suffered extremely high mortality during the period from egg to naupliar stage (N) II: only 0.02 to 4% of the eggs survived to NII. However, the mortality in stages older than NII was almost negligible. The growth rate of C. abdominalis increased exponentially with increasing temperature. Its biomass and production rate showed marked seasonal variations largely in parallel with numerical abundance. The estimated production between 7 November 1986 and 29 May 1987 was 355 mg C m^–3 or 2.66 g C m^–2, 95% of which occurred during February and March. The daily production rate to biomass ratio increased exponentially with temperature from 0.18 at 8.9°C to 0.37 at 19°C.     

“Seed bank” of coastal planktonic diatoms in bottom sediments of Hiroshima Bay, Seto Inland Sea, Japan

Abundance and temporal distribution of viable (able to germinate) resting stage cells of planktonic diatoms in bottom sediments have been investigated almost monthly during 1989 to 1992 in Hiroshima Bay, western part of Seto Inland Sea, Japan. The abundance of viable resting stages in bottom sediments was enumerated with the extinction dilution method (most probable number method, MPN). In bottom sediments of Hiroshima Bay, dominantly distributed species and/ or genera of the diatom resting stages were Skeletonema costatum, Chaetoceros spp. and Thalassiosira spp. Viable resting stages of these diatoms were densely distributed on the orders of 10³ to 10⁶ (MPN g⁻¹ wet sediments), and persisted in bottom sediments throughout the investigation period. Conversely, vegetative cells of these diatoms fluctuated remarkably in the water column and disappeared sporadically. Survival of the resting stages in a collected sediment sample was also determined with the MPN method, at different storage temperatures (5, 10, 15, 20, 25 °C). The survival test demonstrated that the diatom resting stages could survive in the dark for several months or years in sediments. Resting stages survived longer at the lower storage temperature, and the order of longevity was consistent within three diatoms (Chaetoceros spp. > Thalassiosira spp. > S. costatum) at each storage temperature. The present study suggests that these diatom resting stages in the coastal bottom sediments could serve as a “seed bank”, analogous to those of terrestrial plants. The seed bank would ensure the survival of diatoms within highly fluctuating coastal environments, while it would also be the source of sporadic and autochthonous diatom blooms in coastal waters. Received: 29 November 1996 / Accepted: 16 December 1996     

Feeding habits of a pelagic amphipod,Themisto japonica

Vertical distribution, chlorophylla (chla) and phaeopigment concentrations in the gut, and natural nitrogen isotope ratio ( ¹⁵N) were investigated for pelagic amphipodsThemisto japonica (Bovallius) collected from the Sea of Japan in July 1987. Differences in diel vertical migration behavior were clearly observed between small and largeT. japonica. Many small (<5 mm body length) amphipods appeared in the phytoplankton-rich shallow layers. Their gut pigment concentrations were higher (mean 0.52 ± 0.15µg chla g^–1 amphipod) than those of large amphipods (mean 0.33±0.14µg g^–1); this implies that the amphipods fed on a large amount of phytoplankton during the early stage of life. The ¹⁵N values of small amphipods were lower (5.7 to 6.3) than those of large amphipods (6.8 to 11.7), reflecting the lower trophic level of small amphipods compared to large ones. The ¹⁵N values for small amphipods were similar to those of herbivorous zooplankton. The amphipods' feeding behavior thus changes from herbivorous to carnivorous as they grow.