Man-of-wars are 1 animal, not a mishmash of critters

Published September 17, 2016 in the “Ocean Watch” column, Honolulu Star-Advertiser ©2016 Susan Scott

At the beach, I recently ran into my neighbor, Marya, who, as usual, had a thoughtful marine animal question. She wondered how it is that a Portuguese man-of-war is a colony of individuals rather than one animal.

“I can’t picture how that works,” Marya said.

I can’t picture it, either, but we don’t need to because it’s an old idea. Besides that, the confusing colony statement detracts from the fact that the Portuguese man-of-war is one of nature’s most exquisite masterpieces.

A man-of-war starts life when a drifting sperm meets a drifting egg and grows into a larva, a flat bean-shaped thing with swimming hairs.

As it matures, the front end of the larva transforms into a blue, air-filled bubble topped with a fleshy pink sail that the creature trims to move at an angle to the wind. When strong tradewinds and storms overpower the little sails, man-of-wars get shipwrecked on our beaches.

While forming its float, the larva also produces, from below, three types of tentacles. One type contains sperm and eggs (man-of-wars are hermaphrodites). Another kind has long tentacles with stinging cells. The third tentacle type consists of hollow eating tubes.

As the creature sails around the ocean surface, its stinging tentacles troll below for small crustaceans and fish. When it gets a catch, the retractable tentacles reel it in to the eating tentacles, which wrap around the food.

Normally having openings invisible to the naked eye, the mouths of man-of-war eating tentacles can expand to three quarters of an inch. After spitting enzymes on the prey to externally digest it, the food tubes suck up the meal.

Most animals function similarly, having a nerve-directed team of genetically identical, but specialized, organs dependent upon each other to live and reproduce. But because the man-of-war grows its float and tentacles by budding (pooching out) from its larva, some biologists in the past declared that each pooch was an individual animal, even though each does a different job and none can live alone. It’s like saying our stomach is one person, our gonads another and limbs another.

Although the statement that each man-of-war is a colony rather than an individual is pervasive, it’s not written in stone. My favorite invertebrate zoology textbook authors write “ … that it sometimes seems more appropriate to consider the colony (of siphonophores, man-of-war and their clan) as one complex individual.” And the authors of my Monterey Bay Aquarium go-to book on gelatinous animals write that although it’s still debated, “ … most specialists now prefer to think of (siphonophores) as individuals with many well-integrated parts.”

Now that we can picture. Thanks, Marya, for another great question.