Calanoid copepod eggs in sea-bottom muds

We have found numerous eggs of neritic copepods in sea-bottom sediments. Eggs of 6 species of calanoid copepods: Tortanus forcipatus Giesbrecht, Calanopia thompsoni A. Scott, Acartia erythraea Giesbrecht, A. clausi Giesbrecht, Centropages abdominalis Sato and C. yamadai Mori are described and illustrated. The possibility that these eggs “rest” in the sediments, and their biological significance, are briefly discussed.     

Calanoid copepod eggs in sea-bottom muds. V. Seasonal changes in hatching of subitaneous and diapause eggs of Tortanus forcipatus

A warm-water neritic calanoid copepod, Tortanus forcipatus, has been found to lay two physiologically different types of eggs (subitaneous and diapause eggs), which are separately spawned in response to different seasonal environmental conditions. Subitaneous eggs are produced when the planktonic populations are at their numerical maxima, and are only shed until late summer. After peak spawning, true resting eggs are spawned which undergo diapause for 1 to 3 months. The eggs recover from their diapause around mid-winter, but hatching does not take place until the water temperature at the sea bottom exceeds 15°C in early summer.     

Calanoid copepod eggs in sea-bottom muds. IV. Effects of some environmental factors on the hatching of resting eggs

During cadmium uptake by the shore crab Carcinus maenas (L.), cadmium appears in both haemolymph and urine within 1 h, and comes into equilibrium with the haemolymp after some 60 h. The equilibrium concentration for haemolymph cadmium is always considerably less than that of the external medium, whether the latter is close to background level or at the artificially high concentration of 2.2 mg l^-1. Despite earlier evidence that cadmium entering the haemolymph is mainly bound to protein, current results show most of this to be freely labile and quickly displaced by cadmium taken up subsequently. There is some evidence to suggest that cadmium is passed on from the haemolymph to the hepatopancreas. A large proportion of whole-body cadmium is adsorbed onto the exoskeleton and gills. The comparatively rapid efflux of ¹⁰⁹Cd from loaded crabs over a period of 1 week is mainly due to desorption of this cadmium from the outer body surface.     

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.     

Calanoid copepod eggs in sea-bottom muds. III. Effects of temperature,salinity and other factors on the hatching of resting eggs of Tortanus forcipatus

The “resting” eggs of a marine neritic copepod, Tortanus forcipatus Giesbrecht, recovered from sea-bottom sediment were hatched in the laboratory. Hatching occurred at temperatures of 13° to 30°C, no eggs hatched at 10°C. Temperatures around 25°C were found to be optimal for hatching, although the range of optimal temperatures for hatching was approximately 5°C lower in eggs stored for 14 to 15 months than in those stored for 1 to 2 months. A wide range of salinity, from 18 to 54%S, was favourable for hachting. Eggs failed to hatch within the sediment mud, which suggests that they are in a state of dormancy in the mud. Hatching was successful under both light and dark conditions.     

Calanoid copepod,cladoceran, and rotifer eggs in sea-bottom sediments of northern Californian coastal waters: Identification,occurrence and hatching

In Ireland, mussels on exposed rocky shores constitute an interbreeding mixture of two forms of mussels,Mytilus edulis L. and the Mediterranean musselM. galloprovincialis Lmk. This paper presents an in-depth analysis, carried out between October 1984 and December 1986, of genetic variability at two partially diagnostic loci,Odh andEst-D, in two exposed-shore populations ofMytilus spp. in the west of Ireland. Significant differences at theOdh locus were observed in the genetic composition of adult mussels from different tidal levels. These differences were repeatable whether one was analysing replicate samples at a single point in time, samples collected at different points in time, i.e., in different years, or samples collected from different shores. Mussels recruiting to artificial substrates set out for a period of one month at different tidal levels at one of these sites were also observed to be genetically different; mussels higher up the shore exhibited higher frequencies of those alleles characteristically at high frequency inM. galloprovincialis for both theOdh andEst-D loci. Hence, the genetic differences observed in adult mussels are much more exaggerated in juveniles and are already apparent within the first month of benthic life. Possible reasons for the observed microgeographic differentiation are discussed. It is concluded that the observed genetic differences between mussels at different tidal levels arise either in the pelagic/attachment stage or very shortly after settlement.     

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.     

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     

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     

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.     

Seasonal study of planktonic copepods and their benthic resting eggs in northern California coastal waters

Few investigations have examined the occurrence of zooplankton resting eggs in the sea bed of waters deeper than 20 m. In this study the distribution and abundance of planktonic copepods and their benthic resting eggs in coastal waters off northern California, U.S.A., were determined and related to environmental parameters (temperature, salinity, depth, and sediment grain size). Sediment cores, net tows, and CTD profiles were obtained in April and October 1989, and February, April, and October 1990. Water depths in the study area ranged from approximately 60 to 120 m. The mean abundance of eggs was as high as 1.2×10⁵ m^-2 for Acartia clausi Giesbrecht and 1.9×10⁵ m^-2 for Tortanus discaudatus Thompson and Scott. These egg concentrations are comparable to those reported previously for shallower more protected regions. The abundance of eggs in the sediments decreased with increasing depth of the water column. For the region as a whole, eggs were least abundant in muddy sediments. The mean abundance of eggs in the sea bed also varied seasonally and annully. Benthic resting eggs of A. clausi were more abundant in April 1989 than in April 1990, and adults of the species were never found in the plankton samples. The lack of adults is not unusual since results of previous studies indicate that A. clausi is a cold-water species, and in this region water temperatures are colder in summer, than in winter, due to upwelling. Temperature and salinity data indicated that the upwelling season had commenced by the time of the April 1990, but not the April 1989 sampling. Thus, the reduced abundance of benthic eggs in April 1990 may have been due to egg-hatching in response to reduced temperatures. The results suggest that the presence of A. clausi in coastal waters off northern California is linked to recruitment from benthic resting eggs.     

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.     

Continuous Plankton Records: Seasonal cycles of phytoplankton and copepods in the North Atlantic ocean and the North Sea

Data from the Continuous Plankton Recorder survey of the North Atlantic Ocean and the North Sea are used to study geographical variations in the amplitude, duration and timing of the seasonal cycles of total phytoplankton and total copepods. It is shown that the distribution of overwintering stocks influences the distributions throughout the year. There is a relationship between the timing of the spring increase of phytoplankton and the amplitude of the seasonal variation in sea surface temperature. In the open ocean, the timing of the spring increase of phytoplankton corresponds with the spring warming of the surface waters. In the North Sea the spring increase occurs earlier, associated, perhaps, with transient periods of vertical stability, resulting in a relatively slower rate of increase. It is suggested that in the open ocean the higher rate of increase is under-exploited by copepods due to low overwintering stocks and longer generation times. Exceptionally early spring increases of phytoplankton off the west coast of Greenland and over the Norwegian shelf are probably associated with permanent haloclines. A high and late autumn peak of phytoplankton off the coast of Portugal may be associated with coastal upwelling.     

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. 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     

Distribution and abundance of ctenophores and their zooplankton food in the Black Sea. II. Mnemiopsis leidyi

The distribution of Mnemiopsis leidyi Agassiz, 1865 in the Black Sea was determined using plankton samples collected above the anoxic zone (maximum depth 200 m) in the summer, winter, and spring from 1991 to 1995. Distribution was patchy. Average biomasses of 15 to 500 g m⁻² were measured, and abundances varied from 10 to 180 ind m⁻². Biomass and abundance peaked in winter, and there was a secondary peak in the summer. The distribution of M. leidyi was correlated with hydrographic features in the Black Sea with higher concentrations in anticyclonic gyres. The centers of the two main cyclonic gyres generally had a low biomass of M. leidyi. From July 1992 to March 1995, the populations were largely offshore. M. leidyi were confined to the upper part of the mixed layer both day and night. Some individuals displayed a negative taxis to daylight and were concentrated below the thermocline at night. Smaller M. leidyi (1.5 to 2 cm) were present in the winter, and individuals reached maximum size in the summer. Although reproduction was continuous throughout the year, there were two distinct peaks: the larger peak in the summer and the smaller peak in the winter. Microscopic analysis of stomach contents showed that copepods and molluscs form their main diet. Received: 1 November 1997 / Accepted: 30 August 1999     

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.