Oithona similis (Copepoda: Cyclopoida)- a cosmopolitan species?

The present study investigated whether the cyclopoid copepod Oithona similis Claus 1866 is a cosmopolitan or a conglomerate of cryptic species. Adult and subadult females (C5 stages) of O. similis were closely examined morphologically and via DNA-barcoding from four study areas: the Arctic Ocean, the Southern Ocean, the North Sea and the Mediterranean Sea. Sampling was done during two expeditions with RV Polarstern in the Arctic Ocean (ARK XXIII-3, ARK XXV-1) and at one expedition in the Southern Ocean (ANT XXIV-2). Further samples from three stations in the North Sea and one station in the Mediterranean Sea were provided. Based on the shape of the rostrum, body size and the formula and structure of the outer setae of the exopodits of the swimming legs, five different morphotypes were identified: Oithona similis (Arctic Ocean, Mediterranean Sea, North Sea, Southern Ocean), O. atlantica (Arctic Ocean), O. frigida (Southern Ocean), O. nana (North Sea) and Oithona sp. (North Sea). Via CO1-sequencing in total eight different haplotypes of O. similis were found in this study: three in the Arctic Ocean, three in the Southern Ocean, one in the North Sea as well as in the Mediterranean Sea and one in the Mediterranean Sea. Only one haplotype was found in one than one sampling area. In addition to the number of haplotypes, this clearly indicates that O. similis is not a cosmopolitan but a conglomerate of cryptic species. Additionally to the Oithona similis groups, three other copepod species groups were identified morphologically as well as via sequencing: O. frigida in the Southern Ocean and in the North Sea O. nana (close to the island of Helgoland)and Oithona sp. (close to the island of Sylt). Oithona nana was chosen as the basis of a neighbor joining tree because it is not as closely related to O. similis as the other species are. Morphological differences regarding the appendages of the swimming legs of O. frigida and O. similis were obvious and were clearly reflected in the results of the CO1 sequences. The differences reflected in the appendage structures of the swimming legs were also obvious between O. similis and O. nana. Another haplotype named Oithona sp. shares the swimming leg appendage structure with O. nana, but has a bended rostrum like O. similis. The differentiation between these species is also clearly reflected in their position in the neighbour joining tree. Thus, O. similis and other Oithona species inhabiting the investigation areas can clearly be differentiated morphologically and genetically. The CO1- sequences of the Oithona similis haplotype containing individuals from two different places in the North Sea and the Mediterranean Sea differ from the sequences of the species sampled at the other regions. The fact that the same haplotype was found at different places in the North Sea as well as in the Mediterranean Sea shows that this species is widely distributed and might be quite flexible concerning environmental conditions. It is also possible that species of the genus Oithona are advected into the southern North Sea with Atlantic water Overall, almost no morphological differences were found within and between regions for individuals of the Oithona similis species groups from the Southern Ocean, the Arctic Ocean, the North Sea and the Mediterranean Sea. Exceptions are individuals from the Arctic Ocean that were described as Oithona atlantica. One aim of this study was to examine whether possibly existing cryptic species in the nominal O. similis either show no morphological differences or only very slight ones that make it impossible to differentiate between them morphologically. Since individuals that were described as Oithona atlantica prior to sequencing do not form an own haplotype, and as no other morphological differences within the O. similis individuals were found, this can be confirmed at least concerning the examined morphological characters

Global phylogeography of Oithona similis s.l. (Crustacea, Copepoda, Oithonidae) – A cosmopolitan plankton species or a complex of cryptic lineages?

Traditionally, many small-sized copepod species are considered to be widespread, bipolar or cosmopoli- tan. However, these large-scale distribution patterns need to be re-examined in view of increasing evi- dence of cryptic and pseudo-cryptic speciation in pelagic copepods. Here, we present a phylogeographic study of Oithona similis s.l. populations from the Arctic Ocean, the Southern Ocean and its northern boundaries, the North Atlantic and the Mediterrranean Sea. O. similis s.l. is considered as one of the most abundant species in temperate to polar oceans and acts as an important link in the trophic network between the microbial loop and higher trophic levels such as fish larvae. Two gene frag- ments were analysed: the mitochondrial cytochrome oxidase c subunit I (COI), and the nuclear ribosomal 28 S genetic marker. Seven distinct, geographically delimitated, mitochondrial lineages could be identi- fied, with divergences among the lineages ranging from 8 to 24%, thus representing most likely cryptic or pseudocryptic species within O. similis s.l. Four lineages were identified within or close to the borders of the Southern Ocean, one lineage in the Arctic Ocean and two lineages in the temperate Northern hemi- sphere. Surprisingly the Arctic lineage was more closely related to lineages from the Southern hemi- sphere than to the other lineages from the Northern hemisphere, suggesting that geographic proximity is a rather poor predictor of how closely related the clades are on a genetic level

First report of the planktonic copepod Oithona davisae in the northern Wadden Sea (North Sea): Evidence for recent invasion?

In October 2010, specimens of Oithona were taken from the List Tidal Basin in the northern Wadden Sea (North Sea) for a biogeographic study on Oithona similis. These specimens could not be assigned to O. similis or any of the other Oithona species known from the North Sea genetically. These specimens were identified as Oithona davisae Ferrari and Orsi 1984, a Northwest Pacific species, known as an invasive species from the Black Sea and the northwestern Mediterranean Sea. Recent sampling provided evidence that O. davisae is still present in the northern Wadden Sea and may thus now be a permanent plankton species

Oithona similis (Copepoda: Cyclopoida) - Ein Kosmopolit?

The present study investigated whether the cyclopoid copepod Oithona similis Claus 1866 is a cosmopolitan or a conglomerate of cryptic species. Adult and subadult females (C5 stages) of O. similis were closely examined morphologically and via DNA-barcoding from four study areas: the Arctic Ocean, the Southern Ocean, the North Sea and the Mediterranean Sea. Sampling was done during two expeditions with RV Polarstern in the Arctic Ocean (ARK XXIII-3, ARK XXV-1) and at one expedition in the Southern Ocean (ANT XXIV-2). Further samples from three stations in the North Sea and one station in the Mediterranean Sea were provided. Based on the shape of the rostrum, body size and the formula and structure of the outer setae of the exopodits of the swimming legs, five different morphotypes were identified: Oithona similis (Arctic Ocean, Mediterranean Sea, North Sea, Southern Ocean), O. atlantica (Arctic Ocean), O. frigida (Southern Ocean), O. nana (North Sea) and Oithona sp. (North Sea). Via CO1-sequencing in total eight different haplotypes of O. similis were found in this study: three in the Arctic Ocean, three in the Southern Ocean, one in the North Sea as well as in the Mediterranean Sea and one in the Mediterranean Sea. Only one haplotype was found in one than one sampling area. In addition to the number of haplotypes, this clearly indicates that O. similis is not a cosmopolitan but a conglomerate of cryptic species. Additionally to the Oithona similis groups, three other copepod species groups were identified morphologically as well as via sequencing: O. frigida in the Southern Ocean and in the North Sea O. nana (close to the island of Helgoland)and Oithona sp. (close to the island of Sylt). Oithona nana was chosen as the basis of a neighbor joining tree because it is not as closely related to O. similis as the other species are. Morphological differences regarding the appendages of the swimming legs of O. frigida and O. similis were obvious and were clearly reflected in the results of the CO1 sequences. The differences reflected in the appendage structures of the swimming legs were also obvious between O. similis and O. nana. Another haplotype named Oithona sp. shares the swimming leg appendage structure with O. nana, but has a bended rostrum like O. similis. The differentiation between these species is also clearly reflected in their position in the neighbour joining tree. Thus, O. similis and other Oithona species inhabiting the investigation areas can clearly be differentiated morphologically and genetically. The CO1- sequences of the Oithona similis haplotype containing individuals from two different places in the North Sea and the Mediterranean Sea differ from the sequences of the species sampled at the other regions. The fact that the same haplotype was found at different places in the North Sea as well as in the Mediterranean Sea shows that this species is widely distributed and might be quite flexible concerning environmental conditions. It is also possible that species of the genus Oithona are advected into the southern North Sea with Atlantic water Overall, almost no morphological differences were found within and between regions for individuals of the Oithona similis species groups from the Southern Ocean, the Arctic Ocean, the North Sea and the Mediterranean Sea. Exceptions are individuals from the Arctic Ocean that were described as Oithona atlantica. One aim of this study was to examine whether possibly existing cryptic species in the nominal O. similis either show no morphological differences or only very slight ones that make it impossible to differentiate between them morphologically. Since individuals that were described as Oithona atlantica prior to sequencing do not form an own haplotype, and as no other morphological differences within the O. similis individuals were found, this can be confirmed at least concerning the examined morphological characters

Sampling information, length measurements, DNA alignments, models and phylogenetics trees for Oithona similis s.l.

Traditionally, many small-sized copepod species are considered to be widespread, bipolar or cosmopolitan. However, these large-scale distribution patterns need to be re-examined in view of increasing evidence of cryptic and pseudo-cryptic speciation in pelagic copepods. Here, we present a phylogeographic study of Oithona similis s.l. populations from the Arctic Ocean, the Southern Ocean and its northern boundaries, the North Atlantic and the Mediterrranean Sea. O. similis s.l. is considered as one of the most abundant species in temperate to polar oceans and acts as an important link in the trophic network between the microbial loop and higher trophic levels such as fish larvae. Two gene fragments were analysed: the mitochondrial cytochrome oxidase c subunit I (COI), and the nuclear ribosomal 28 S genetic marker. Seven distinct, geographically delimitated, mitochondrial lineages could be identified, with divergences among the lineages ranging from 8 to 24%, thus representing most likely cryptic or pseudocryptic species within O. similis s.l. Four lineages were identified within or close to the borders of the Southern Ocean, one lineage in the Arctic Ocean and two lineages in the temperate Northern hemisphere. Surprisingly the Arctic lineage was more closely related to lineages from the Southern hemisphere than to the other lineages from the Northern hemisphere, suggesting that geographic proximity is a rather poor predictor of how closely related the clades are on a genetic level