Population dynamics and production of the planktonic copepods in a eutrophic inlet of the Inland Sea of Japan. IV. Pseudodiaptomus marinus, the egg-carrying calanoid

Population dynamics and production of the egg-carrying calanoid copepod Pseudodiaptomus marinus were studied for a year in Fukuyama Harbor, a eutrophic inlet of the Inland Sea of Japan. This species was perennial, with a large numerical peak in June and small peaks in September/October and November/December. During the study period, at least 11 generations could be detected. For each generation, the stage-specific survival from egg to Copepodite Stage (C) V was determined; it was very high during early life stages (egg to NIII), and gradually decreased beyond. On average, 94% of eggs recruited into NIII, which is strongly contrasted with very high (>ca. 90%) mortality during the corresponding stages for free-spawning copepods, i.e. Acartia omorii, Centropages abdominalis and Paracalans sp. This demonstrates that the egg-carrying strategy has a great advantage to reduce mortality in egg stage. The biomass of this species showed marked seasonal variations largely in parallel with numerical abundance. The instantaneous somatic growth rate increased linearly with temperature. The population production rate was estimated as the sum of somatic growth of larval stages and egg production of adult females; the annual integration was 51.0 mg C m⁻³ yr⁻¹ or 0.38 g C m⁻² yr⁻¹. Received: 11 November 1996 / Accepted: 7 December 1996     

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.     

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.     

Population dynamics and production of the planktonic copepods in a eutrophic inlet of the Inland Sea of Japan. III.Paracalanus sp.

Population dynamics and production of the calanoid copepodParacalanus sp. were studied from November 1986 to November 1987 in Fukuyama Harbor, a eutrophic inlet of the Inland Sea of Japan. This species was perennial, with a large abundance peak in June/July and small peaks in September/October and November/December. During a year of investigation, 15 generations Gould be detected. For each generation, the mean population egg production rate and the mean daily midstage abundance front NIII to CV were determined to obtain a survival curve from egg to CV. The mortality was extremely high during the early life stages: on average only 7.1% of the eggs produced might survive into NIII. This high mortality might be caused by predation by sympatric omnivorous copepods, in addition to sinking loss of eggs from the waten column. The biomass ofParacalanus sp. showed marked seasonal variations largely in parallel with numerical abundance. The instantaneous growth rate of each developmental stage increased exponentially with temperature up to 20 °C, above which the rate was constant. The annual integrated production rate was 734 mg C m^–3 yr^–1 or 5.5 g C m^–2 yr^–1.     

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.     

Calanoid copepod eggs in sea-bottom muds. II. Seasonal cycles of abundance in the populations of several species of copepods and their eggs in the Inland Sea of Japan

The seasonal cycles of abundance of populations of dominant calanoid copepods in the water column and of their eggs recovered from the bottom sediment in the central part of the Inland Sea of Japan are described. The numbers of both copepods and eggs fluctuated markedly with season in an essentially similar pattern among the 6 species studied (Tortanus forcipatus Giesbrecht, Calanopia thompsoni A. Scott, Acartia erythraea Giesbrecht, A. clausi Giesbrecht, Centropages abdominalis Sato, C. yamadai Mori). The density of eggs in the sea bottom was highest shortly before the population of adults and late copepodids disappeared from the plankton; the numbers of eggs then gradually decreased until the appearance of the next planktonic population.     

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.     

Feeding habits of marine cladocerans in the Inland Sea of Japan

Natural food items of five species of marine cladocerans, Evadne nordmanni, E. tergestina, Penilia avirostris, Podon leuckarti and P. polyphemoides, were investigated in the Inland Sea of Japan between April 1986 and May 1987. Gut content examination with SEM (scanning electron microscopy) revealed that feeding was largely limited to centric diatoms and a few exceptions of pennate diatoms and dinoflagellates. No animal remains were detected, and some unidentified materials were also found. Phytoplankton smaller than 35 m in size (cell diameter in centric diatoms and longest dimension in others) was found most frequently in the gut of cladocerans. The role of grazing of marine cladocerans in trophodynamic pathways of the pelagic realm is discussed.     

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.     

Lunar influence on distribution of a calanoid copepod in the water column of a shallow,temperate estuary

Moonlight had a significant influence on the distribution of the copepodPseudodiaptomus hessei in the water column. Copepod samples were taken between March 1986 and March 1987 in the Sundays River estuary. South Africa. Late copepodid stages and adults showed vertical differences in abundance on moonlit nights, with more animals in bottom waters, but not on dark nights. Numbers of adults present in the water column were markedly lower on full moon nights compared with new moon nights. The strongest decrease in abundance was displayed by ovigerous females. These observed distribution patterns support published predation hypotheses.     

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.     

A comparative study of the inhibitory effect of diatoms on the reproductive biology of the copepod Temora stylifera

Egg production and viability in the copepod Temora stylifera (collected in the Bay of Naples, Italy in 1992) were strongly dependent on food type. A flagellate (Isochrysis galbana) diet induced the production of good quality eggs that developed to hatching. By contrast, two diatoms (Chaetoceros curvisetum, Phaeodactylum tricornutum) resulted in poor egg quality, with hatching success as low as 20% of total egg production. With the third diatom tested, Skeletonema costatum, females produced eggs for only 3 to 4 d, after which time they either became sterile or died. These results are discussed in relation to previous findings regarding the impact of the dinoflagellate Prorocentrum minimum and the diatom Thalassiosira rotula on the hatching success of T. stylifera eggs. Low egg viability was possibly not due to an absence of remating or a deficiency of some specific essential nutrient required for egg development but to the presence of inhibitory compounds blocking cell division during early copepod embryogenesis. This questions the traditional view that diatoms are an important food item regulating copepod secondary production.     

Spatial and temporal variability in egg production rates of the calanoid copepod Acrocalanus inermis

Egg production rate of the copepod Acrocalanus inermis was measured at 3 stations in south Kaneohe Bay, Hawaii in August 1977 and July–October 1979. The egg production rates were significantly variable spatially, but temporal variability over a time scale of weeks was larger than spatial variability. In three experiments, egg production rates were correlated with ambient concentrations of particulate matter, a crude estimate of food concentration. Most values for egg production rates ranged between 5 and 16.9 eggs female^-1 d^-1.     

Discriminate feeding of the calanoid copepod Acartia clausi in mixtures of phytoplankton and inert particles

Adult female Acartia clausi were allowed to feed in the laboratory on each of three algae, the small green alga Dunaliella tertiolecta (5.6 m ESD), the diatoms Thalassiosira decipiens (13.4 m ESD) and T. nordenskioldii (17.7 m ESD), singly or in mixtures with polystyrene beads (15.7 m ESD). The ingestion rate on beads was much lower than that on cells, even when the supply of beads was one order of magnitude higher than that of cells. Beads offered singly were not ingested. In the experiments with T. nordenskioldii, the more cells adult females ingested, the higher the ingestion rate on beads. In addition, a linear regression equation describing the relationship between bead concentration and ratio of ingested beads to ingested cells was highly significant. These results suggest that beads are ingested by chance in feeding bouts on T. nordenskioldii. The bead interference on the ingestion rate on cells was observed in the experiments with D. tertiolecta and T. decipiens, but not with T. nordenskioldii. In a mixture of beads and T. decipiens, juvenile A. clausi discriminated beads from cells more successfully than adults. The presence of beads did not affect the ingestion rate of juveniles on cells. Starved adult females tended to ingest more beads than well fed individuals. The possible effects of body size and hunger on the discriminate feeding of copepods are discussed.     

Early development and estimated life cycle of the mesopelagic copepod Pareuchaeta elongata in the southern Japan Sea

Life cycle of the mesopelagic copepod Pareuchaeta elongata was investigated combining laboratory rearing data on the eggs, nauplii, and early copepodites with field growth data on the late copepodites. Examination and incubation at near the in situ temperature (0.5°C) of egg sacs collected from the field indicated that the clutch size was 13 to 24 eggs (mean: 20), hatching time was 39.4d (mean), and hatchability was low (mean: 28.5%). The development time at 0.5°C was 2.4 d for Nauplius Stage 1 (N1), 4.6 d for N2, 6.2 d for N3. 7.4 d for N4, 7.1 d for N5, 20.8 d for N6, 36.7 d for Copepodite Stage 1 (C1) and 65.3 d for C2. From the numerical analysis of seasonal samples collected from the field, the development time was estimated as 1 mo for C3, 2 mo for C4 and 2.5 mo for C5. Thus, the egg hatching time plus the integrated development time by stage was 355 d or nearly 1 yr (i.e. span of one life cycle). Duration of the C6 (adult) was estimated as>2.5 mo. Combining the present data on development times of each stage with published data on the major spawning season (August to October) and ontogenetic migration, a schematic representation of the life cycle of this copepod was developed. Between-stage comparison of wet, dry, and ash-free dry weights of all developmental stages of preserved wild specimens revealed that there is almost no gain in weight during naupliar stages, and the greatest weight increment over the life cycle was during the C4 stage. The present results are compared with those for the same and related species living in other regions.     

Occurrence,viability and significance of resting eggs of the calanoid copepod Labidocera aestiva

Field and laboratory observations on the occurrence and viability of eggs of Labidocera aestiva Wheeler show that eggs of this temperate species which are laid in late fall remain viable on the bottom and hatch in late spring. These eggs thereby provide the means of repopulating temperate areas with this species after its winter disappearance from the plankton.