Each reef manta's ventral surface carries an irregular arrangement of dark spots between and behind the gill slits. The pattern is established early, does not drift with growth, and survives the scarring that most large rays accumulate, so a clear photograph functions as an identity card.
Catalogues built on this principle now hold tens of thousands of sightings, many contributed by recreational divers. Because the method needs no capture, it supports estimates of residency, movement between reefs, and reproductive interval, which for reef mantas is long: females typically produce a single pup, and not every year.
The method has limits often glossed over. A photograph only proves that an animal was somewhere at a moment, so absence from a catalogue is weak evidence of loss. Coverage is also biased towards dive sites, which is why aggregation counts are not the same as population counts.