Seasonal, Physiological and Genetic Functions in Antarctic Krill, Euphausia superba, at Different Latitudes in the Southern Ocean

Light regime and the endogenous timing system of Antarctic krill, Euphausia superba, are known to play important roles in the regulation of its seasonal cycles of growth, maturity, metabolic activity and gene expression. This dissertation aimed to investigate the effect of different latitudinal light regimes on the seasonal cycle of Antarctic krill. It comprised two main parts: a) a field study with the goal to characterize seasonal processes on the molecular level in Antarctic krill in different latitudinal regions (Publication 1), and b) a two-year laboratory study under constant food and temperature conditions that tested the effect of the latitudinal light regimes 52¡S, and 66¡S, and constant darkness on the seasonal cycle of Antarctic krill (Publication 2 & 3). Our results suggest that the seasonal timing system of Antarctic krill is highly plastic, thereby promoting regional acclimatization to different latitudinal light regimes. We further discuss that the internal 'photoperiodic-controlled' seasonal cycles in Antarctic krill may be adjusted by other environmental cues like temperature and food supply in the field

Light regime affects the seasonal cycle of Antarctic krill (Euphausia superba): impacts on growth, feeding, lipid metabolism, and maturity

Light regime is an important zeitgeber for Antarctic krill (Euphausia superba Dana, 1850), which seems to entrain an endogenous timing system that synchronizes its life cycle to the extreme light conditions in the Southern Ocean. To understand the flexibility of Antarctic krill’s seasonal cycle, we investigated its physiological and behavioural responses to different light regimes and if an endogenous timing system was involved in the regulation of these seasonal processes. We analysed growth, feeding, lipid content, and maturity in a 2-year laboratory experiment simulating the latitudinal light regimes at 52°S and 66°S and constant darkness under constant food level. Our results showed that light regime affected seasonal cycles of growth, feeding, lipid metabolism, and maturity in Antarctic krill. Seasonal patterns of growth, feeding, and maturity persisted under constant darkness, indicating the presence of an endogenous timing system. The maturity cycle showed differences in critical photoperiods according to the simulated latitudinal light regime. This suggests a flexible endogenous timing mechanism in Antarctic krill, which may determine its response to future environmental changes

Population genetic structure of Calanoides carinatus (Copepoda, Calanoida) in the eastern Atlantic Ocean and Benguela upwelling system

Molecular markers have a high potential to resolve genetic differentiation within species and populations in the pelagic realm where barriers to gene flow cannot be easily identified. In the present study Calanoides carinatus (Copepoda, Calanoida), an ecologically important compo- nent of African upwelling systems, was investigated aiming to (1) describe spatial patterns in the distribution of genetic variance, (2) identify potential barriers that may have shaped the ge- netic structure of the species and (3) look for cryptic species. Samples were obtained in the eastern Atlantic Ocean from the Iberian Peninsula to Namibia. Analysis of mitochondrial (cy- tochrome oxidase c subunit I; COI) and nuclear (citrate synthase; CS) marker genes revealed a genetically-cohesive population of C. carinatus along the west coast of Africa. Haplotype sharing of C. carinatus populations, relatively few private alleles and a prevalent shift in allele frequencies indicated ongoing gene flow within the study area. Significant genetic differentia- tion of C. carinatus populations was discovered between the northern and southern hemisphere which may point to a potential, but permeable barrier close to the equator, possibly triggered by discontinuity of ocean currents in the equatorial region. No vertical genetic structuring in C. car- inatus was observed among the active surface population and deep-living resting stages in the northern Benguela indicating that horizontal differentiation was more pronounced than vertical structuring of populations. The occurrence of three diverse sequences of mitochondrial COI and their absence within nuclear CS, rather suggests marker evolution of COI than a cryptic species

Baseline data of growth, feeding, lipid content and maturity analysis in Antarctic krill, Euphausia superba, during long-term photoperiodic controlled lab experiments

Light regime is an important Zeitgeber for Antarctic krill (Euphausia superba), which seems to entrain an endogenous timing system that synchronizes its life cycle to the extreme light conditions in the Southern Ocean. To understand the flexibility of Antarctic krill's seasonal cycle, we investigated its physiological and behavioural response to different light regimes and if an endogenous timing system was involved in the regulation of these seasonal processes. We analysed growth, feeding, lipid content and maturity in a two-year lab experiment simulating the latitudinal light regimes 52°S, 66°S and constant darkness under constant food level. Our results showed that light regime affected seasonal cycles of growth, lipid metabolism and maturity in Antarctic krill. Seasonal patterns of growth and maturity persisted under constant darkness indicating the presence of an endogenous timing system. The maturity cycle showed differences in development and critical photoperiods according to the simulated light regime. This suggests a flexible endogenous timing mechanism in Antarctic krill, which may determine its response to future environmental changes

Population genetic structure of Calanoides natalis (Copepoda, Calanoida) in the eastern Atlantic Ocean and Benguela upwelling system

The population genetic structure of Calanoides natalis (ex Calanoides carinatus; Copepoda, Calanoida), an ecologically important component of African upwelling systems, was studied in order to (i) search for potential cryptic species, (ii) describe spatial patterns in the distribution of genetic variance and (iii) identify potential barriers to gene flow. Samples were obtained in the eastern Atlantic Ocean from the Iberian Peninsula to Namibia. Analysis of mitochondrial (cytochrome c oxidase subunit I; COI) and nuclear (citrate synthase; CS) marker genes revealed a genetically cohesive population of C. natalis with a prevalent shift in allele frequencies. The discovery of a deep split solely present in the mitochondrial dataset does not point to cryptic speciation, but rather suggests the occurrence of nuclear mitochondrial pseudogenes or incomplete reproductive isolation upon secondary contact. Genetic differentiation between the northern and southern hemisphere was significant, which may point to a potential, but permeable barrier close to the equator. No vertical genetic structuring was detected in the northern Benguela implying that horizontal differentiation was more pronounced than vertical structuring. Retention mechanisms and the oxygen minimum zone did not have a strong impact on genetic differentiation of C. natalis in the Benguela region

Sampling information and phylogenetic trees for Calanoides natalis, link to supplementary material

The population genetic structure of Calanoides natalis (ex Calanoides carinatus; Copepoda, Calanoida), an ecologically important component of African upwelling systems, was studied in order to (i) search for potential cryptic species, (ii) describe spatial patterns in the distribution of genetic variance and (iii) identify potential barriers to gene flow. Samples were obtained in the eastern Atlantic Ocean from the Iberian Peninsula to Namibia. Analysis of mitochondrial (cytochrome c oxidase subunit I; COI) and nuclear (citrate synthase; CS) marker genes revealed a genetically cohesive population of C. natalis with a prevalent shift in allele frequencies. The discovery of a deep split solely present in the mitochondrial dataset does not point to cryptic speciation, but rather suggests the occurrence of nuclear mitochondrial pseudogenes or incomplete reproductive isolation upon secondary contact. Genetic differentiation between the northern and southern hemisphere was significant, which may point to a potential, but permeable barrier close to the equator. No vertical genetic structuring was detected in the northern Benguela implying that horizontal differentiation was more pronounced than vertical structuring. Retention mechanisms and the oxygen minimum zone did not have a strong impact on genetic differentiation of C. natalis in the Benguela region