But consumers routinely pay for excess water when they buy the shellfish, according to a Boston Globe investigation of moisture content in scallops collected from 21 Massachusetts supermarkets.

The results revealed that some products sold as pure scallops — meaning they were not processed with additives to soak up moisture — contained water levels far exceeding industry standards. Seafood specialists say the findings suggest many scallops include such additives, but are packaged without any labeling to indicate the treatments.

Scallops are about 75 percent water when they are harvested from the ocean. But some brands appear to add more water, making customers pay for it instead of scallop meat.

For example, Target’s “wild caught all natural sea scallops” were 85 percent water, while New England Wild Jumbo Scallops at Trader Joe’s were 86 percent water, according to Globe testing conducted by an independent lab.

Looked at another way, the proportion of meat in these store-bought shellfish was about 40 percent less than in scallops pulled from the ocean.

Target Corp. said its scallops have no additives, and Trader Joe’s said it requires vendors to provide scallops that are not treated and are less than 83 percent water.

“We will be investigating this issue further, and if any of our suppliers are not meeting our quality standards, immediate action will be taken to rectify the situation,” said Alison Mochizuki, a spokeswoman for Trader Joe’s, a national grocery chain.

Some countries don’t permit water-added scallops to be sold. In Canada, for instance, scallops cannot have moisture content above 81 percent.

But the amount of water a natural scallop can contain has been debated here for years. The Food and Drug Administration, which oversees seafood labeling, once required scallops with more than 80 percent water to be labeled as “moisture added," and those with more than 84 percent water were deemed not suitable for sale.

The agency rescinded that policy in 2004 and now simply requires additives to be listed on labels. But the National ­Oceanic and Atmospheric ­Administration, which conducts a voluntary seafood inspec­tion program, decided to keep part of the guidelines abandoned by the FDA because of concerns over economic fraud.

Inspectors, according to ­NOAA’s handbook, are to ­assume that scallops with more than 83 percent moisture have been treated and should be properly labeled. NOAA said it expects to lower the permitted moisture level.

“Businesses are advertising wild-caught, naturally raised seafood that has really been treated and consumers are being misled and paying for water they shouldn’t,” said Beth ­Lowell of Oceana, a nonprofit group that combats seafood fraud.

For decades, American scallop processors, many of which are in New Bedford, soaked scallops in sodium tripolyphosphate (STPP), a solution that can help them retain moisture and preserve the products.

But consumer concerns about chemicals and a desire for all-natural products have led processors to seek alternatives that don’t use phosphates and can’t be as easily detected. Some scallop facilities employ full-time chemists who use various water-retention additives, adjusting the recipes for different retailers, according to industry officials. Regardless, any additives should be noted on the labeling.

Target could not explain how its all-natural scallops, which sold for $14.99 a pound in Westborough, tested as 85 percent water. Erin Conroy, a spokeswoman for the chain, would not identify Target’s supplier, but said its scallops do not contain additives.

“That’s a tremendous amount of water, and moisture content that high is an indication that there has been an additive involved,” said Laura ­Foley Ramsden, co-owner of ­Foley Fish, which supplies only all-natural, no-additive scallops to restaurants and supermarkets. The scallops it sells are typically landed with 77 percent water.

The Village Market in Roslindale was the only supermarket that admitted to selling fresh scallops treated to retain moisture, and they had a water content of 79 percent. Others, like the fresh sea scallops at Kam Man in Dorchester, were made up of 85 percent moisture, according to UL-STR, the independent lab in Canton that tested for the Globe.