Stockton Springs lobsterman Todd Ritchie has seen a lot of lobsters since he started lobstering almost 20 years ago in the early 1990s.

He had not, however, seen a lobster as blue as the one he pulled from the waters of Castine harbor Aug. 30 while lobstering from his boat, The Seacock.

Neither had the owners of Young’s Lobster Pound, where Ritchie brought his colorful catch on Aug. 31. The lobster was, appropriately enough, brought to Young’s in a blue-colored cooler.

Ritchie said while he had sometimes seen lobsters with patches of blue before, or ones that were a light shade of blue, he had never seen one so completely covered by such a brilliant shade of blue.

Ritchie said the plan for the one-and-a-quarter pound male lobster was for it to be kept at Young’s for a few weeks, where it would reside in a display tank for interested passersby to take a peek at. After that the hope is to find a home for it at an educational institution or a similar safe haven, such as the College of the Atlantic or University of Maine.

How unusual are blue lobsters? The exact answer to that question appears to depend on who you ask, but the common theme in those answers is that it is an unusual thing. In a fact sheet on lobsters, for example, the Gulf of Maine Research Institute reports that “blue lobsters occur once in every 3-4 million lobsters.”

A search for “blue lobster” in VillageSoup’s archives showed that a handful of other blue lobsters had been caught in the Midcoast region over the past decade. Here’s the scoop on those instances:

• In July of 2010, a blue lobster was caught by lobsterman Charles Flint in Knox County. The lobster, which was being stored in a tank at Ship to Shore Lobster in Owls Head, was headed for the Gulf of Maine Research Center in Portland, which had expressed an interest in having the lobster.

• In September of 2009, Friendship lobsterman Keith Simmons Jr. hauled up a three-pound lobster in a trap off Allen Island, which is off Port Clyde. Around the same time, the Maine State Aquarium in West Boothbay reported that it had a 10-pound blue lobster in its tanks, which had been caught off Phippsburg a month before.

• In August of 2009, 19-year-old commercial lobsterman James Morris pulled a blue lobster (and an orange-speckled lobster) from a trap in less than 30 feet of water in Port Clyde harbor.

• In October of 2006, lobsterman Eric Tweedie caught a light blue female lobster in a trap off Spruce Head. The article reported that the lobster had darkened somewhat in the days since it was first plucked from the ocean.

• In November of 2003, a Vinalhaven lobsterman caught a blue lobster while hauling traps for the James “Cobi” Driscoll Lobster Company. The lobster was brought to the mainland on Driscoll’s aptly named boat, Lobster.

• In October of 2003, Bill Reidy, who lobstered out of Islesboro, landed a 1.5-pound female blue lobster in the waters of West Penobscot Bay. That lobster was destined for a new home at the University of Maine, according to the article.

Why are some lobsters blue? Or, for that matter, why are some yellow, black, white or multi-colored? Diane Cowan, who writes a monthly column called “Ask the Lobster Doc” for the Commercial Fisheries News, answered that question in a January 2000 column titled, “A lobster of a different color.”

Cowan said some colormorphs — the scientific name for lobsters of a different color — can be attributed to genetic factors, while in other cases lighter or darker colors “can be influenced by diet, sunlight and bottom type.”

Lobsters, Cowan wrote, need to eat things that eat plants to get a certain pigment called astaxanthin, which in turn binds to a protein on their shell.

“The protein to which astaxanthin binds in natural-colored lobsters is blue in lobster shells and green in lobster eggs,” Cowan explained in her column. “When the astaxanthin is not in the diet, would-be natural-colored lobsters appear blue, because the blue pigment in the protein expresses itself.”