But two years later, he said, the crabs have recovered, and then some. “The other day we were on that reef and someone said, ‘Have you ever seen so many blue-legged hermit crabs in your life?’ ” Mr. Kehoe said.

While acknowledging that some collectors are aware of the dangers of overfishing, Dr. Rhyne said there had been little scientific study of the blue-legged crabs and the hundreds of other species that are collected, including the 15 that make up about 90 percent of the catch. For example, with certain snails it is not known how long it takes for them to start to reproduce. If it is more than a year, then harvesting many of them from the same location year after year could be disastrous. There are many species that are probably not a concern, Dr. Rhyne said, but he added, “I don’t think anyone can use the word ‘sustainable’ when they don’t know enough about the animals.”

What is not in dispute is that the fishery has changed in the past two decades, coinciding with the rise in popularity of reef tanks. These aquariums include home or office tanks of a few gallons to several hundred gallons or more, and attractions like the 20,000-gallon coral reef tank at Atlantis Marine World in Riverhead, N.Y., considered one of the finest anywhere.

Jeff Turner, owner of Reef Aquaria Design of Coconut Creek, Fla., which builds and maintains large reef aquariums in homes, offices, hospitals and other institutions, says these are not mere decorations, but “an educational window into the sea.” The hobbyists and professional aquarists who undertake these projects, he said, “are concerned about the marine environment.”

The popularity of the tanks is reflected in a study of Florida fishery data by Dr. Rhyne, Michael Tlusty, director of the New England Aquarium, and others. As the researchers detailed in a paper earlier this year in the open-access journal PLoS ONE, the number of organisms collected from 1994 to 2007 increased by about 13 percent a year, on average, to about 8.8 million in 2007.

Over the same period, the types of invertebrates changed. In 1994, only 6 species among the top 15 were collected and sold for their ecological roles as cleaners, grazers, water filters and the like. The others were harvested for their ornamental value — because they look pretty in an aquarium — or as curios to be sold in shell shops and other stores. The most popular “working” invertebrates were turbonella snails, with about 175,000 collected.