Abstract
Two new species of Tortanus (Atortus), T. (A.) ampliramus and T. (A.) nishidai, are described from the northern Red Sea and the Gulf of Aden, respectively. These new species evidently belong to the recticauda species group in the recticauda species complex. The species group has a distinct distribution in the western Indian Ocean, extending from South Africa. The functional morphology of spermatophores of Tortanus is also discussed. A semelparous reproductive strategy may be employed in some species of Tortanus, whereas others adopt an iteroparous strategy as do most of centropagoideans. The formation of swarms was found exclusively in the subgenus Atortus which inhabits clear waters. Swarms of T. (A.) erabuensis may utilize swarms of another small copepod, Oithona oculata as prey.
