Alex Mustard / Yellow tube sponge (Aplysina fistularis) on coral reef, diver exploring in background. Black Rock Drop Off, East End, Grand Cayman, Cayman Islands, British West Indies. Model released.

Alex Mustard / Yellow tube sponge (Aplysina fistularis) on a coral reef, with the camera rotated during the long exposure. East End, Grand Cayman, Cayman Islands. British West Indies. Caribbean Sea.

Alex Mustard / Spotlight goby (Gobiosoma louisae) looking out from inside a Yellow tube sponge (Aplysina fistularis), growing on a coral reef. Bloody Bay Wall, Little Cayman, Cayman Islands, British West Indies. Car...

Alex Mustard / Yellow tube sponges (Aplysina fistularis) growing on a Caribbean coral reef. Cayman Brac, Cayman Islands. Caribbean Sea.

Franco Banfi / Scuba diver and yellow sponges (Aplysina cavernicola) on Brioni Steamship wreck, Vis Island, Croatia, Adriatic Sea, Mediterranean. Model released.

Alex Mustard / Yellow tube sponge (Aplysina fistularis) top view of tubes with water swirling around them, Grand Cayman, Cayman Islands, British West Indies, Caribbean Sea.

Jeff Rotman / Yellow tube sponge (Aplysina fistularis) Red Sea, Egypt

Doug Perrine / Yellow tube sponge {Aplysina fistularis} with Yellowtail snappers {Ocyurus chrysurus Cayman Brac, Cayman Islands, Caribbean

Sergio Hanquet / Yellow umbrella slug (Tylodina perversa), two feeding on Golden sponge (Aplysina aerophoba). Tenerife, Canary Islands.

Sergio Hanquet / Yellow umbrella slug (Tylodina perversa) on Golden sponge (Aplysina aerophoba). Tenerife, Canary Islands.

Sergio Hanquet / Yellow umbrella slug (Tylodina perversa) feeding on Golden sponge (Aplysina aerophoba). Tenerife, Canary Islands.

Sergio Hanquet / Yellow umbrella slug (Tylodina perversa) on Golden sponge (Aplysina aerophoba). Tenerife, Canary Islands.