There is a whale of a tale in Edgartown.

Marine mammal madness is what I call it. Earlier this week, I received a call about a few animals that have been swimming around Edgartown harbor. The caller thought that they were either dolphins or pilot whales. Either one would be a good sighting and would make for a nice article.

The next clue I received came from a Chappy Ferry favorite who confirmed that the animals were indeed pilot whales. Mystery solved, I thought, but not so fast.

The plot thickens from here, as I sought the true identity of the mystery mammal. I am a stickler for accuracy, so I called a few more folks. Definitively an Atlantic white-sided dolphin, I was told by one reliable source. Hmmm. Another call yielded just as emphatic an answer: they were pilot whales, he was certain.

This is definitely a fish story that is not about a fish.

Both Atlantic white-sided dolphins and pilot whales are members of the toothed whale family, animals that are air breathers, as opposed to fish, which respire through water-filled gills.

While there are some general outward resemblances between these two members of the whale family, there are more differences between the two than similarities.

Size matters when trying to differentiate between the two species. Dolphins are smaller, averaging 7 to 9 feet in length, while pilot whales can reach lengths of up to 20 feet. Even at pilot whales’ average size of 13 feet, they still are significantly larger than dolphins.

The company that they keep also differs. Pilot whales move in large schools that average 20 animals, although they can be seen singly or in pairs. They are sometimes seen with bottlenose dolphins and occasionally white-sided dolphins. White-sided dolphins hang out with finback and humpback whales, and their schools average around 50 animals. They too can occasionally be seen singly or in pairs.

Their body shapes and colors separate them as well. Pilot whales are almost all black, giving rise to their alias, blackfish. They do have a light anchor-shaped patch on their belly, but this is rarely visible as they do not usually breach or bowride so it is difficult to see.

White-sided dolphins, true to their name, have a narrow white patch on their side below their dorsal fin. This runs to the back of the animal and changes to a tan or yellow color as it reaches the tail. They also have a white belly, seen when they breach and bowride, common behavior in this animal.

The heads of both animals differ dramatically. The pilot whale has a blunt, bulbous head and has been called both a pothead and a melon head. The Atlantic white-sided dolphin has a pointed head and snout that narrows significantly at the beak.

Food is a giveaway, also. Although it isn’t practical to ask the beasts what they had for dinner, this information will give you a clue to their identity. Pilot whales prefer squid and eat them exclusively if they can, while Atlantic white-sided dolphins are fish eaters.

Since I didn’t get a glimpse of the animal seen in Edgartown harbor, I cannot positively identify them. Perhaps both were swimming around, since all of my sources are very reliable. The jury is still out, or at least in recess until the mystery visitors return. Like many an angler’s story, this is about the one that got away.

Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown.