ALBERT

Description: The euryhaline, eurytherm copepod Acartia tonsa plays a key role in many marine food webs as a grazer on phytoplankton and as prey species for commercially important planktivorous fishes. The different populations of this cosmopolitan species experience a wide range of environmental conditions. This study aims to elucidate the response of A. tonsa to changes in multiple environmental factors. The effects of temperature ( T ), salinity ( S ) and the interaction of both variables on recruitment processes were investigated to show potential influences of changes in environmental conditions on its life cycle and population dynamics. Estuarine conditions appeared to be optimal for reproduction, despite the marine origin of the population. Interactive effects of T x S were more pronounced when the population was exposed for a longer period of time to constant laboratory conditions. Potential resting eggs were identified based on morphology and hatching characteristics, and were mainly produced at high S and low T . Our results emphasize the importance of investigating the combined effects of different physical factors to gain a better understanding of potential changes in population dynamics resulting from climate-induced changes in key environmental factors.

Description: Detecting fluctuations in the species composition of bloom-forming jellyfish requires the ability to correctly identify each species in each developmental stage. We verified diagnostic morphological and molecular genetic characters to discriminate Cyanea lamarckii and Cyanea capillata from northern European waters. Intrusions in the subumbrellar muscle folds were present in all C. capillata 〉80 mm r-diameter (between opposite rhopalia tips), but absent in C. lamarckii . Clearly visible wart-like papillae on the central exumbrella were present in all C. lamarckii 〉10–80 mm r-diameter, but absent in C. capillata . Both morphological features were retained in formaldehyde-seawater (4%) preserved medusae which had shrunk by 12.8% (±2.7%) after 1 year of preservation. Our molecular genetic analyses demonstrated that fragments of mitochondrial cytochrome c oxidase subunit I (COI) and nuclear 18S rDNA clearly distinguished C. lamarckii from C. capillata , with intra- and inter-specific pairwise genetic distances of 0.0–1.5% and 15.5–17.0% (COI) and 0.0 and 0.2% (18S rDNA), respectively. The study revealed various bell colours in both species underlining that the identification based on the bell colours can result in misidentification. Our integrated taxonomic approach can help to correctly identify jellyfish species, which is fundamentally important for understanding the causes of jellyfish fluctuations and the development of jellyfish blooms.

Description: Hydromedusae are widespread and diverse representatives of the gelatinous zooplankton, but are often neglected because of their inconspicuousness and difficulties with identification. Here, we used an integrated approach combining both morphological and molecular genetic analyses of North Sea hydromedusae. Morphological identification was successfully carried out on living material, and preservation in 4% formaldehyde allowed re-examination of most morphological features. Ethanol and DESS were adequate fixatives for DNA analyses but led to distortion of morphological characters. In most cases, morphological species identifications were confirmed by molecular data (COI partial sequences) and the latter approach led to valid discrimination where morphological characters were insufficient. In comparison with 22 morphologically identified entities, COI analysis revealed 25 clades with a pronounced difference of ≥5.4% between intra- and inter-specific variability. Specimens morphologically identified as Obelia spp. were attributed to O. geniculata , O. dichotoma and O. longissima , while Clytia spp. were allocated to C. hemisphaerica and C. languida by the comparison to hydroid and medusa sequences retrieved from GenBank. Our results highlight the molecular approach as a powerful tool, extending the possibilities for valid species discriminations where morphological identification is difficult, for example, in species with a similar or identical morphology, in early life stages with insufficient identifying features and in linking different generations (hydroid and medusa). However, genetic analysis cannot replace morphologically based taxonomy in studies on species' population dynamics, physiology and ecology. Thus, most information is achieved by combining both methods in integrative studies using both morphological and molecular taxonomy.

Description: Four Paraeuchaeta species and three aetideids were frequently encountered along 51°30′S in the Atlantic sector of the Southern Ocean. Paraeuchaeta antarctica was most abundant close to the Antarctic Polar Front. Within the genera Paraeuchaeta and Gaetanus, congeners usually partitioned the water column. Euchaetidae had high lipid (≤37% dry mass, DM in adult females) and wax ester contents (≤22% DM). Fatty acid composition of Paraeuchaeta spp. was dominated by monounsaturated moieties, especially 16:1(n-7) and 18:1(n-9), while fatty alcohols were mainly saturated. Surprisingly, only the bathypelagic P. barbata contained moderate amounts of 20:1(n-9) and 22:1(n-11) fatty acids (≤14%) and high levels of the respective fatty alcohols (≤50%), generally considered trophic biomarkers for calanid copepods as prey. Thus, herbivorous calanid copepods seem to be a readily available prey source at bathypelagic depths, indicating that their seasonal vertical migration provides a “trophic shortcut” from primary production at the surface to the interior of the ocean. Aetideidae also contained substantial levels of total lipid (14–36% DM), but wax esters contributed only up to 12% DM in copepodite stages C5 of Gaetanus spp., whereas other stages of Gaetanus and Aetideopsis minor only contained ≤6% DM of wax esters. The fatty acid compositions of Aetideidae were more balanced with 16:0, 18:1(n-9), 20:5(n-3), and 22:6(n-3) as important components, indicating a generally omnivorous feeding behaviour.

Description: Spring is the season of most pronounced changes of biotic and abiotic parameters in boreal marine plankton communities. In order to establish a basis for the interpretation of trophic interactions and succession processes we monitored a set of ecological key parameters around the island of Helgoland (North Sea, German Bight) in 2004. We focussed our interest on the ability of copepods to utilize food during the development of the phytoplankton bloom when food quantity and quality are changing.Water temperature rose from 3 to 4°C in early March to more than 10°C at the end of May. Phytoplankton started to bloom in early April. The nitrogen content of seston increased concomitantly. The size of the dominant copepod species (Acartia clausii and Temora longicornis) increased from March to April. Their dry masses almost doubled. Variations in gross biochemical parameters and digestive enzyme activities were evident in both species. Grazing of females of either species on natural seston under in-situ conditions was selective.In spring 2004 phytoplankton bloomed comparatively late and may have been controlled by grazers. Food quantity and quality increased and enhanced the copepods physiological condition and reproductive success. The ability of copepods to efficiently utilize food was reflected by high digestive potentials. Species specific enzyme patterns indicate different traits of food utilization. The comprehensive knowledge about nutritional demands and catalytic potential may help to better understand the driving forces of food selection and, consequently, the succession of plankton communities.Acartia clausii, Temora longicornis, grazing, seston quality, digestive enzymes

Description: We investigated the nutritional condition of larval fish caught in daily ichthyoplankton hauls carried out from February to June 2004. We concentrated on larvae of dab (Limanda limanda) and lesser sandeel (Ammodytes marinus) in order to compare early life stages of iteroparous and nearly semelparous fish. Larvae were analysed for length, weight and their RNA/DNA ratios as a proxy for the condition of the larvae. The relationship between larval nutritional condition and larval size provided an indication of condition selective mortality due to a loss of poorly-conditioned larvae at larger size-classes. In larval sandeel, well-conditioned larvae were present in all size-classes, whereas in larval dab the maximum larval condition rose with size. Variability in both standard length and condition was high in the two species during their planktonic stage. Both species showed good nutritional condition in the early-mid season phase and declines in condition in late April. This was more pronounced in larval dab, which showed a higher dependency on feeding conditions than larval sandeel. Together these findings indicated for a more conservative strategy of early life stages of the nearly semelparous sandeel compared with the iteroparous dab.