What physical feature aids in identifying a zebra shark regardless of its stage of pattern change?

The extreme length of the heterocercal tail fin.

While the body's pattern shifts dramatically from stripes in juveniles to spots in adults, serving as a visual clock of its life stage, there is a reliable anatomical constant for positive identification. This constant feature is the shark's tail structure. The zebra shark possesses a caudal fin where the upper lobe is so dramatically elongated that it nearly matches the length of the remainder of the body combined. This pronounced heterocercal shape, where the vertebral column extends far into the upper lobe, remains consistent throughout the shark's life cycle, offering a reliable clue for identification even when viewed from a distance or in poor visibility where the body spotting or striping might be hard to discern.