This work is a contribution to the study of geographically distinct populations of
Owenia fusiformis Delle Chiaje. It introduces new morphological criteria based on observation under a SEM of the anterior region (tentacles and thorax), and of the abdominal region (hooks and chaetae). Results of a morphological survey of preserved specimens identified as
Owenia fusiformis Delle Chiaje from 14 geographical areas were compared with material from the type area in the Mediterranean Sea, the 15
th site. A list of more than 40 characteristics, involving both hard and soft structures, was established. It was possible 1) to confirm the existence of 3 previously described species:
O. collaris Hartman, 1955,
O. johnsoni Blake, 2000;
O. gomsoni Koh Bhaud, 2001; 2) to announce the description of two new species from North Atlantic area:
O. polaris and
O. borealis Koh, Bhaud & Jirkov, 2003; 3) to describe in the present paper, two other new species:
Owenia petersenae from New Zealand area and
Owenia sp from the Persian Gulf; 4) to propose the reinstatement of two species:
O. brasiliensis Hansen, 1882 and
O. assimilis (Sars, 1851) which merit a new description. Variability was tested for different parts of one torus, for different segments of the body, for different specimens from a local area and over a regional geographic scale. As a whole, this work which includes data from 15 geographical sources, constitutes a study of the worldwide variability of the genus.