The biologists, who discovered the bottle at about noon Wednesday, reported the finding to police, who brought it to the Contra Costa County coroner's office for analysis.
Medical examiners have confirmed that the specimen was a human fetus, Gagan said.
The bottle is about 4 inches tall and 2 inches wide on either side with about 3 ounces of some kind of liquid surrounding the specimen, Gagan said.
Coroner's investigators believe the liquid was a type of preservative, Gagan said, and are doing tests to determine what level of gestation the fetus was.
"There are scientific reasons why a specimen like this would exist, but it shouldn't have been where it was found," Gagan said.
Richmond police detectives are following up with the coroner's office to try to figure out how the bottle could have gotten on the beach, but so far investigators have no idea where it came from or how long it had been on the island before it was discovered. The finding is being investigated as an unexplained death, Gagan said.