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Two new species of the planktonic copepod genus Tortanus, subgenus Atortus, are described from inter- to sub-tidal water of South Andaman Island, India. The new species are assigned to the tropicus group sensu Othman (1987) and distinguished from the other members of the group by the characteristic morphology of the pair of posterior dorsolateral processes on the fifth pedigerous somite in the female and the antennule and leg 5 in the male. An analysis of previous records of the tropicus species group indicates their sporadic occurrence combined with high species richness in Southeast Asia, suggesting future discovery of even more species by sampling with finer geographic and habitat coverage.

This study reports on the planktonic copepod, Tortanus ( Atortus ) murrayi Scott A., 1909 from South Andaman, Northeast Indian Ocean. This is a new distributional record of murrayi species group of recticauda species complex from the Indian Ocean. T . ( A .) murrayi is a well-known species, yet, the taxonomic features of mouthparts and swimming legs were never previously illustrated and is achieved herein.

Two new species of the planktonic copepod genus Tortanus, subgenus Atortus, are described from inter-to sub-tidal water of South Andaman Island, India. The new species are assigned to the tropicus group sensu Othman (1987) and distinguished from the other members of the group by the characteristic morphology of the pair of posterior dorsolateral processes on the fifth pedigerous somite in the female and the antennule and leg 5 in the male. An analysis of previous records of the tropicus species group indicates their sporadic occurrence combined with high species richness in Southeast Asia, suggesting future discovery of even more species by sampling with finer geographic and habitat coverage. Deux nouvelles espèces du genre de copépode planctonique Tortanus, sous-genre Atortus, sont décrites des eaux inter- à sub-tidales de l'île Andaman du Sud, Inde. Les nouvelles espèces sont assignées au groupe tropicus sensu Othman (1987) et se distinguent des autres membres du groupe par la morphologie caractéristique de la paire de processus dorsolatéraux postérieurs situés sur le cinquième somite pédigère chez la femelle et par l'antennule ainsi que la patte 5 chez le mâle. Une analyse des mentions antérieures des espèces du groupe tropicus indique leur présence sporadique combinée avec une richesse en espéces élevée en Asie du Sud-Est, suggérant la découverte future d'encore plus d' espèces par un échantillonnage utilisant une couverture plus fine au niveau géographique et de l'habitat.

A new species of the copepod genus Tortanus, subgenus Atortus, is described from the coastal waters of Nha Trang, Vietnam. The new species is assigned to the tropicus species group sensu Othman (1987) and distinguished from the other members of the group by the morphology of the processes on the fifth pedigerous somite and the genital somite in the female, and the antennule and leg 5 in the male. An analysis of previous records of Atortus indicates highly sporadic occurrence, but a large number of species to be distributed in Southeast Asia. This obviously suggests future discovery of even more species, and also the need for sampling with better geographic and ecological coverage in this particular region. /// Une nouvelle espèce du genre de copépode Tortanus, du sous-genre Atortus, est décrite des eaux côtières de Nha Trang, Vietnam. La nouvelle espèce est attribuée au groupe d'espèces tropicus sensu Othman (1987) et se distingue des autres membres du groupe par la morphologie des processus situés sur le somite portant la cinquième patte et le somite génital chez la femelle, et par l'antennule et la cinquième patte chez le mâle. Une analyse des précédents signalements de Atortus indique une présence hautement sporadique, mais un grand nombre d'espèces réparties dans le sud-est asiatique. Ceci suggère clairement la découverte future de plus nombreuses espèces, et aussi la nécessité d'un échantillonnage réalisé avec une meilleure couverture géographique et écologique dans cette région particulière.

A new species of the planktonic copepod Tortanus (Atortus) (Calanoida: Tortanidae), T. (A.) magnonyx is described from the Seychelles, Mauritius and Madagascar. This is the sixth species of the Indian Ocean recticauda species group, of the Indo-West Pacific recticauda species complex, that has been described from the western Indian Ocean. The inshore areas where these copepods are found have been poorly surveyed, so the number of species found implies a high diversity.

ABSTRACT Tortanus (Boreotortanus), new subgenus, is established to accommodate the North Pacific/northwestern Atlantic species T. discaudatus and the taxonomy and ecology of the five subgenera in the genus Tortanus are discussed. Tortanus sensu stricto occurs in eutrophic waters in the Indo-West Pacific, Atortus in oligotrophic waters in the same region, Eutortanus in brackish waters in East Asia, Acutanus in eutrophic waters in the northwestern Atlantic, and Boreotortanus in eutrophic waters in the North Pacific and the northwestern Atlantic. Two new species are described: T. (Eutortanus) terminalis and T. (Acutanus) ecornatus. Phylogenetic relationships among the 5 subgenera were constructed based on cladistic and zoogeographical analyses. Boreotortanus may have originated in the Sea of Japan; Eutortanus in the East China Sea; Tortanus sensu stricto in the Indo-Malayan region. The final divergence is represented by the Atortus-Acutanus clade, which is characterized by reductions of elements on the antennule, the antenna, legs 2 and 3 of both sexes, and elongation of the left leg 5 of males. The ancestor of Acutanus could have colonized the northwestern Atlantic from the Indo-Malayan region through the Isthmus of Panama during the Middle Miocene to Pliocene. It appears that Atortus first succeeded in exploiting clear oligotrophic waters and in developing swarming behavior to avoid visual predators in such clear waters. These originally tropical copepods seem to have secondarily adapted to cold waters along the East Asian archipelagoes during the Pleistocene.

ABSTRACT Five species of Tortanus (Atortus) (Copepoda: Calanoida) were collected from shallow waters in Shijiki Bay, Shibushi Bay, and the Ryukyu Islands, South Japan, among which two are newly described. They are: T. (A.) digitalis, new species, T. (A.) erabuensis, T. (A.) longipes, T. (A.) rubidus, and T. (A.) ryukyuensis, new species. The subgenus Atortus is distributed mainly in tropical or subtropical coastal waters in the Indo-West Pacific region, and each species seems to have a restricted distribution. Twelve of the 16 known species of Atortus are assigned to 4 species groups based on morphological characters and distribution patterns. Direct observations by SCUBA diving revealed that some species of Atortus form swarms near the bottom, at least during the daytime.