Serpulid tubeworm

Photo: giancarlo_torre · CC BY-NC

Serpulid worms secrete hard calcareous tubes attached to cave walls and rock surfaces, extending feathery branchial crowns into the water column to filter phytoplankton and suspended particles. Inside Grotta Azzurra, the interplay of low light and bioluminescent blue-filtered sunlight makes these worms — with their vivid red, orange, and white plumes — among the most photogenic subjects in Italian cave diving. They retract instantly when approached, making patience the key technique for close observation.