Marine iguanas eat sea algae, then sneeze white salt. Why sneeze?

Answer: Nasal salt disposal

Nasal salt disposal ✓ — Nasal salt disposal is the mechanism. Marine iguanas graze algae in seawater, so salt comes in with the meal; glands linked to the nostrils concentrate and expel it. The white crust is not beach dust but a visible receipt for a rare ocean-feeding lizard lifestyle.

Predator alarm signal — Predator alarm signal would predict a social message, but the spray follows a chemical need. The iguana is not warning neighbors each time it snorts. It is pushing excess ions out after a salty meal, which turns a visible behavior into a body-chemistry clue.

Airways drying out — Airways drying out sounds like a diving explanation, yet the sources point to salt glands, not wet lungs. The fluid dries into salt because it is concentrated secretion. The animal is removing dissolved salt from blood after feeding, not clearing seawater from breathing passages.