What structural element during development confirms *T. obesus* is in the Phylum Chordata?
Answer
A notochord
Placement within the Phylum Chordata is fundamentally defined by the presence of specific structural characteristics at some stage of the organism's development. For *Thunnus obesus*, this signifies that it possesses a notochord, which is a flexible rod providing structural support, alongside a dorsal nerve cord, pharyngeal slits, and a post-anal tail. These underlying features are crucial because they support the development of its highly streamlined and powerful physical architecture, which is necessary for its role as a powerful, highly mobile swimmer in the open ocean.
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