On a clear, chilly day in January, professor George MacManus and a couple other marine scientists from the University of Connecticut’s Department of Marine Sciences at the Avery Point campus were taking their daily noontime walk around Jupiter Point in Groton when, in the middle of the Bayberry Lane boat launch parking lot, they encountered a large pile of bones, mixed in with dried seaweed and marsh grass wrack.

The bones appeared to be from a very large fish, the biggest individual vertebrae being over 2.5 inches long. No skull was found among the bones.

Was it a big tuna or other top predator in the food chain? Such a fish would probably not be caught close to shore by someone launching a boat from a trailer, and in the middle of winter to boot!

Although experts in fish anatomy could probably have identified the fish from its vertebrae, the group hit on a simpler plan. The bones had been picked clean, suggesting a long time since it was caught, but they noticed a small amount of spinal cord tissue adhering to one of the bones, allowing possible DNA analysis.

So MacManus took that vertebra to me because I am a marine biologist who has studied the genetic material of marine organisms for many years. He asked if I would be able to use genetic tools to find out the identity of the large fish bone.

I checked the bone and extracted DNA from spinal cord tissue. I then used a short length of DNA called a primer to make many copies of a segment called the mitochondrial DNA using the fish’s DNA as the original copy.

Since fish mitochondrial DNA sequences have been established like fingerprints for identifying different species, I compared the unknown bone’s mitochondrial sequence with many other fish DNA sequences reported in the GenBank database of the U.S. National Center for Biotechnology Information.

Amazingly, I found an almost perfect match to the DNA sequence of swordfish (Xiphias gladius). No wonder these bones are so big — swordfish can grow as long as 14 feet and weigh up to 1,400 pounds!

Although the fish had been identified, some mysteries remain. Swordfish are highly migratory, and although they can be found this far north and fairly close to shore in summer, Fishers Island Sound in winter would be a long way from home for this fish.

Also, though the bones were jumbled, they stayed together (except for the skull) during the long period when they would be decaying. One possibility is that someone caught this fish offshore some time ago, cleaned it, and tossed the carcass in a marsh, where it stayed intact as its remaining flesh decomposed.

Recent super high tides may have floated the bundle of bones out of the marsh and left it in the boat launch parking lot amid the seaweed and other flotsam. Whatever the answer to this mystery, the bones will have a second life as an educational tool because they have been placed in the UConn fish skeleton collection where future students can study them.

Next time, when you walk on a beach, look for large bones or other creature remains, and see what stories you can uncover.

Dr. Huan Zhang is an associate professor at the University of Connecticut at Avery Point and a marine molecular biologist.

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