Calanoid Eggs Research Articles

Effects of hypoxia on benthic eggs of calanoid copepods in the Southern Sea of Korea

Global warming is affecting the composition, structure, and function of marine ecosystems. The increase in hypoxic regions due to stratification is a major environmental problem worldwide. Off the southern coast of Korea, hypoxia occurs frequently in summer, and the area of water affected is gradually expanding. In this study, we investigated the effects of hypoxia on the eggs of copepods in the order Calanoida. Data on the distribution and abundance of eggs in benthic sediments were collected from 17 stations, using a piston core sampler (64 mm internal diameter, 50 cm length), from August 1 to 7, 2012. Significant variations in the distribution of calanoid eggs and the occurrence of abnormalities in egg development were found between stations. The abundance of eggs found in the sediments ranged from 0.004 to 2.389 × 106 eggs·m−2, with higher abundances identified in hypoxic than in normoxic areas. The proportion of abnormal eggs ranged from 0 to 92.7%. In particular, there were significantly more abnormal than normal eggs in areas where hypoxia occurred (p < 0.01). These results show that hypoxia can have a lethal effect on calanoid eggs and further affect population and community dynamics.

Summer and spring trophic niche of larval and juvenile Pleuragramma antarcticum in the Western Ross Sea, Antarctica

The aim of this paper is to contribute to the knowledge on the feeding habits of larvae and juvenile Pleuragramma antarcticum in the western Ross Sea. In summer, the diet of P. antarcticum postlarvae (8–17 mm) was dominated by calanoid eggs (35.5%), Limacina (32.1%) and tintinnids (17.6%), while the principal food of juveniles consisted mainly of copepods (98.2%), with Oncaea curvata being the most abundant (85.1%) and the most frequently consumed prey. The food composition of P. antarcticum postlarvae (24–29 mm), collected in spring, suggest that they fed actively under the sea ice. Stephos longipes, Harpacticus furcifer and Paralabidocera antarctica sea ice copepods represent, in all their different developmental stages, the most abundant biomass food in Terra Nova Bay in this period. Our results therefore suggest that the diet of younger Pleuragramma specimens shifted in prey composition from the first summer to the following spring. This study draws attention to the key role of the copepod, P. antarctica, in the food web of Terra Nova Bay.

Bioturbation changes the patterns of benthic emergence in zooplankton

We investigated the effects of two functionally different deposit feeders, the amphipod Monoporeia affinis and the bivalve Macoma balthica, on the benthic emergence of copepods (Acartia spp., Eurytemora affinis, Temora longicornis), cladocerans (Bosmina longispina maritima, Daphnia spp.), and rotifers (Synchaeta spp., Keratella spp., Notholca spp.) in the northern Baltic Sea. We performed 3.5‐week laboratory experiments in June‐July and in September‐October using surface sediment containing zooplankton eggs and various combinations of amphipod and bivalve densities. Both zoobenthic species increased egg mortality rates in Synchaeta spp. and in calanoids. Each amphipod consumed a maximum of one Synchaeta spp. egg and 31 calanoid eggs per day, while the bivalves consumed six Synchaeta and 143 calanoid eggs per day. Monthly consumption throughout the community matches or even exceeds the field densities of eggs. In summer, M. affinis decreased the daily emergence rate of Synchaeta spp. by 31‐53% and of M. balthica by 57%. The effect on calanoids was less severe: the emergence rate of Acartia spp. and E. affinis was significantly suppressed only by M. balthica (the reduction was 48% and 28% in Acartia spp. and in E. affinis, respectively). T. longicornis was the only species that benefited from macrofauna: together the two zoobenthic species resulted in a 2.7‐fold increase in the emergence of this calanoid. Bioturbation significantly influences the benthic emergence of zooplankton, with effects varying among species, and may thus affect the dynamics of pelagic populations.

Subitaneous eggs of freshwater copepods pass through fish guts: Survival, hatchability, and potential ecological implications

We measured the survival of subitaneous eggs of two calanoid (Eudiaptomus gracilis, E. graciloides) and two cyclopoid (Cyclops abyssorum, Macrocyclops albidus) freshwater copepods after they had been consumed by fish. Unexpectedly, over 80% of the calanoid eggs and 30–59% of the cyclopoid eggs were morphologically intact in fish feces. Subitaneous eggs of E. graciloides showed similar proportions of gut passage as dormant resting eggs. About 70–80% of the calanoid eggs and 35–50% of the cyclopoid eggs released nauplii within 3 d. Hence, a total of 50–70% of the calanoid eggs and 11–29% of the cyclopoid eggs survived ingestion and gut passage. Survival was slightly higher because of shorter gut passage time when the fish had been prefed natural plankton compared with hungry fish. We interpret digestion resistance of subitaneous eggs in copepods as an adaptation to fish predation on egg‐carrying females.

Composition, vertical distribution and age of zooplankton benthic eggs in the sediments of two contrasting estuaries of the Bay of Biscay

The effects of natural and anthropogenic factors on the vertical distribution of zooplankton benthic eggs of the sediments were analysed in the estuaries of Bilbao and Mundaka, both located in the Bay of Biscay. These estuaries differ largely in their pollution level, so in the estuary of Bilbao the human impact by physical perturbation (canalisation, dredging... ) and by introduction of pollutants (urban and industrial wastes) have been considerably greater than in Mundaka. Core samples were obtained in the intertidal soft sediments at the middle zone of each estuary using a hand core sampler 2 m length. Sediments grain size composition, and organic matter, chloropigments, lipid, carbohydrate and protein contents were analysed. Eggs were extracted from the sediment and their age was estimated by means of 210Pb and 137Cs analyses. Results showed that the benthic eggs abundance, composition and vertical distribution differed largely between estuaries. The total density of eggs recovered from the sediment was on average one order of magnitude higher in the estuary of Mundaka (105 eggs m−2) than in the estuary of Bilbao (104 eggs m−2), in which sediments anthropogenic organic enrichment was evident. Rotifer eggs dominated in Bilbao and calanoid copepod eggs in Mundaka, in agreement with the actual dominance of rotifers and calanoids in their estuarine zooplankton communities. In the estuary of Mundaka, benthic eggs and the majority of environmental variables were closely related to depth, calanoid eggs decreasing with depth while rotifer eggs increased. The results on 210Pb and the 137Cs allow us to know the date of the sediment layers, and to estimate at around 100 years the entire sequence analysed. Calanoid eggs dominated in recent sediments and rotifer eggs in older ones suggesting noticeable changes in the zooplankton community during the last century. In contrast, the age estimation was difficult in the estuary of Bilbao because 210Pb and 137Cs values indicated that more recent sediments were mixed with older sediments. This indicates that sedimentary processes are strongly affected by the human physical perturbation of sediments probably due to the periodical dredging and intense navigation in the estuary, and accounted for the rather homogeneous distribution of eggs with depth.

Seasonal occurrence and hatching of calanoid eggs in sediments of the northern Baltic Sea

The seasonal occurrence and hatching of benthic eggs of calanoid copepods were studied for 1 yr in the surface sediments at 2 sites (a 33 m deep archipelago area and a 42 m deep site in an enclosed bay) off the SW coast of Finland, northern Baltic Sea. Eggs were abundant at both sites (up to 4 and 6 X 106 eggs m-2). At the archipelago site, most eggs belonged to Acartia bifilosa and A. tonsa; at the bay site, eggs of Eurytemora affinis and Acartia spp. occurred. At the archipelago site, the egg numbers in the surface sediment followed closely the seasonal abundance of the planktonic Acartia spp. females. The eggs collected from the sediment were Incubated at temperatures corresponding to the in situ bottom temperatures. Hatching of the A. bifilosa eggs occurred throughout the year, but it was most intensive in autumn when water stratification broke and the deep water warmed up to 13°C. It is suggested that a large number of the A. bifilosa eggs sink to the bottom prior to hatching m shallow coastal areas of the Baltic Sea. Hatching of the benthic eggs occurs throughout the year and the rate of naupliar emergence from the sediments depends on benthc condtions and processes (e.g. temperature, sediment resuspension and bioturbation). At the bay site, where the surface water layer was hydrographically separated from the deep water, the coupling between the benthic egg abundance and the planktonic populations was not so obvious. The dominant species E. affinis carries its eggs in an egg sac unhl hatching, and probably only the diapause eggs, which are produced in autumn, fall to the bottom. A. tonsa was abundant in the water column at both study sites in autumn, even outnumbering other Acartia spp. in some samples. The eggs of the species only hatched in autumn, when the incubations were conducted at 10 to 13°C. It is probable that A. tonsa spends most of the year as benthic resting eggs In the northern Baltic Sea. In contrast, A bifilosa and E. affinis occurred in the plankton in winter at both study sites, though in low concentrations. They thus have 2 possible sources of recruitment when conditions are again favourable for population growth in spring: hatching of benthc eggs and reproduction by the overwintering population.

Seasonal occurrence and hatching of calanoid eggs in sediments of the northern Baltic Sea

Seasonal occurrence and hatching of calanoid eggs in sediments of the northern Baltic Sea

Hatching Nauplii of Planktonic Calanoid Copepods from Intertidal Estuarine Sediments

The dominant planktonic calanoid copepods in inshore and estuarine habitats in northern European waters; the Acartiidae, Centropagidae and Temoridae, produce eggs which survive and can remain dormant in the sediments (Marcus, 1996). Eggs of Eurytemora survived passage through the guts of fish larvae, in contrast with eggs of Pseudocalanus (Conway et al., 1994). Eggs of Paracalanus do not survive in sediments (Marcus, 1991). Lindley (1992) suggested that families within the superfamily Centropagoidea, including Eurytemora but not Paracalanus and Pseudocalanus, produce eggs which remain viable in sediments. Previous work on these eggs has been from subtidal sediments, although Naess (1996) showed that calanoid eggs survived in sediments in seasonally drained fish ponds.

Distribution and abundance of thaliaceans and copepods off the southeastern U.S.A. during winter

The abundance and distribution of planktonic copepods and tunicates were studied in relation to physical and biological variables from late January to early February 1990 on the middle and outer shelf off South Carolina, U.S.A. The inner and part of the middle shelf were vertically mixed throughout the study period, the middle and outer shelf only after passage of a storm on 4 and 5 February. Subtidal currents at the 40 m isobath were highly correlated with local alongshore winds, whereas at the 75 m isobath the currents showed a mixed response to wind and Gulf Stream events. Chlorophyll concentrations in the study area usually surpassed 0.5 and reached levels of 3 μg l −1. Zooplankton was characterized by high abundances of doliolids (Tunicata, Thaliacea) and relatively low concentrations of calanoid and cyclopoid copepods. When doliolids were very abundant the vertical distribution of doliolids was inversely related to those of the calanoids. We hypothesized that this inverse relationship could be the result of (a) doliolids removing food particles, thus reducing food particle concentrations, and through that calanoid reproduction rates; (b) doliolids ingesting calanoid eggs and nauplii, thereby reducing copepod recruitment; (c) active avoidance of doliolids by downward migration of copepods; and (d) vertical shear between the upper and lower part of the stratified water column. At stations on the middle shelf the abundance of photo- and heterotrophic nanoplankton was inversely related to the abundance of doliolids. The frequency of occurrence of thaliacea in neritic regions is evaluated in relation to hydrographic regimes and the availability of seed populations. From previous and present observations we assume that thaliacea due to their high rates of feeding, individual and population growth could impact zooplankton communities two-fold: indirectly, by reducing phytoplankton concentrations rapidly thus affecting reproduction of certain calanoid taxa, and directly, by consuming copepod eggs.

Calanoid resting eggs in the Baltic Sea: implications for the population dynamics of Acartia bifilosa (Copepoda)

Resting eggs of Acartia spp. (presumably A. bifilosa Giesbr.) at densities between 65 and 125 eggs cm-2 were found in sediment off the southern coast of Finland and represent the first observations of dormant calanoid eggs in the Baltic Sea. The highest egg densities were found at sediment depths between 4 and 8 cm. The hatching percentage varied between 0 and 90% in the different sediment layers and at different experimental temperatures (13, 16 and 19°C). Some nauplii emerged from the eggs extracted from sediment layers 9 to 10 cm beneath the surface layer and other nauplii emerged from eggs after 82 d of dark storage at 3°C. Light was not needed to trigger hatching; the nauplii were able to hatch in darkness when the eggs were resuspended in filtered seawater. Indirect evidence suggests that the marked spring peak of Acartia spp. nauplii seen in a monitoring data from the years 1973 to 1984 was derived from benthic eggs and not from spawning. The possible mechanisms for induction of hatching and the adaptive benefits of resting egg production for A. bifilosa are discussed.

Food and feeding ecology of postlarval and juvenile Pleuragramma antarcticum (Pisces; Notothenioidei) in the seasonal pack ice zone off the Antarctic peninsula

Stomach contents of 275 postlarval and 269 one year old juvenile Pleuragramma antarcticum caught in February 1982, March 1981 and November 1977 in the Bransfield Strait and adjacent waters were investigated. Juveniles in November 1977 fed mainly on calanoid copepods and Oithona spp. The principal food of postlarvae in February 1982 were Oncaea spp., eggs of calanoid copepods and tintinnids whereas the staple food of juveniles in February 1982 consisted of calanoid copepods of which copepodites of Calanus propinquus were most abundant. Eggs of Euphausia superba were frequently ingested by postlarvae and were the main component of the juvenile diet in the Antarctic Sound. Postlarvae in March 1981 preyed on Oncaea spp. and calanoid eggs. There was a shift from feeding on cyclopoid copepods by postlarvae to feeding on calanoids by juveniles. The food particle size increased as a function of fish length. Maximum prey size was controlled by mouth width up to at least 45 mm standard length. Prey selection was apparently food density dependent with an inverse relationship between food abundance and selective feeding. Density dependent size selection may have resulted in resource partitioning among postlarval and juvenile fishes in February. This specific feeding behaviour is likely to be an adaptation to the low zooplankton stocks of the high Antarctic shelf waters to minimize food competition among the youngest age classes. Predation on krill eggs indicated that the frequent association of one year old juvenile Pleuragramma antarcticum with the Antarctic krill is related to local spawning events.