From the cramped conditions inside a school-bus-sized metal tube at the bottom of the ocean, scientist Chris Finelli traded his scuba gear for a T-shirt and bare feet, ready to talk about his latest mission: monitoring sponges on Conch Reef, more than three miles off the coast of Florida.

As coral populations disappear — they're down by 90 percent around Florida — other species are fighting for the newly freed up ocean real estate. The two top contenders to take over are sponges and algae…