The Argonaut Octopus and the jellyfish

CREDIT | By Ned and Anna DeLoach; photos by Ned and Anna DeLoach

I don’t expect to see animals much larger than my thumbnail while I’m drifting on a night dive in the ocean. So I’m surprised when a translucent skirt of tentacles half the length of my arm breezes past. This creature is a behemoth in a sea of Lilliputians, so I kick hard to catch up while trying to figure out exactly what I’m after.

The form of a shelled argonaut octopus clutching a jellyfish takes shape just as the odd couple breaks the surface and bobs in a ring of ripples. Before I can catch my breath, the octopus dives, easily outdistancing me as it disappears back into the depths of Lembeh Strait with its hostage still in tow. An octopus consorting with a jellyfish — what’s that all about? It looks like another mystery to solve and a good one at that.

Back at the resort, I begin searching the internet. I start with a couple of overviews: Argonauts, four wide-ranging species of pelagic octopuses also known as paper nautiluses, are renowned for the females’ intricately beautiful shells. The popular collector’s items periodically wash ashore in large numbers.

As I read on, I learn that the famous shells are brooding chambers secreted and molded with the females’ two highly specialized lettuce-leaf-shaped arms. Back in the fourth century B.C., the creature captivated Aristotle, who suggested that the oddly flattened arms functioned as sails to power the octopus and its shell across the waves like the mythical crew of the Argo.
An octopus consorting with a jellyfish — what’s that all about? It looks like another mystery to solve and a good one at that.

The fanciful yarn, which ultimately gave the species their genus and family names, held true until the 1830s, when French seamstress-turned-self-educated-naturalist Jeanne Villepreux-Power took a closer look into the matter. She was quite a trendsetter who spent many hours on the banks of a bay in Messina, Sicily, observing argonauts and other marine species swimming around inside wooden holding tanks and glass boxes she designed. Among her long list of accomplishments, Villepreux-Power — lauded today as the originator of aquarium research and observation — proved that the females used their curious arms for constructing brooding chambers rather than sailing and paddling about the owners in shells stolen from other creatures.

I also get caught up on contemporary argonaut studies. A pair of Australian cephalopod researchers recently substantiated that argonauts dip air into their shells at the surface to maintain energy-conserving buoyancy at depth.

It is well past midnight, and I’m beginning to nod off when an article discussing the sacrificial sex life of free-swimming argonaut males, measuring less than a twelfth the size of females, snaps me awake. It appears that the high point in their short lives, and possibly the last thing they ever do, is leaving behind their wormlike fertilization arm inside the chamber of a female before swimming away to die. By a lucky coincidence, the article includes a photo of a thimble-sized male riding atop a jellyfish. I blink in disbelief; I photographed a similar image in Raja Ampat just the previous week but didn’t have an inkling the octopus might be an argonaut.

At this point Villepreux-Power reappears in the storyline. She was audacious enough to publicly propose that the arms found inside the females’ mantles were male reproductive organs, but the Victorian male science community couldn’t get their heads around such an unsettling assumption; instead they declared the organisms to be parasitic worms.

Finally, what I’ve been hoping for pops up: “Predation on jellyfish by the cephalopod Argonauta argo.” The paper, written by a trio of German scientists, chronicles their 1992 encounter with a female argonaut holding a jellyfish hostage. After swimming with the pair for an hour, the scientists transferred the specimens to an aquarium to study them in the tradition of Villepreux-Power.

It is well past midnight, and I’m beginning to nod off when an article discussing the sacrificial sex life of free-swimming argonaut males, measuring less than a twelfth the size of females, snaps me awake.

After taking measurements, they released the female but retained the hostage, still pulsing with life, for examination. Having been held securely by sucker-lined arms for an extended period, the detainee showed wounds channeling through its gelatinous bell and into five of its multiple stomach cavities. From the evidence, the authors speculated that the octopus hijacked the jellyfish to commandeer its food. On the other hand, the argonaut could have been hauling around the medusa to thwart predators with its hornet’s nest of stinging tentacles. Or both.
The next night we’re out in the blackwater again. Our guide finds a tiny octopus floating alone in the dark. The animal’s oversized siphon pegs it as a male argonaut. If I were giving out gold stars for cute, this little fellow would be awarded five. Even though I’m aware of his ultimate fate, I take his picture and wish him well anyway.

© Alert Diver Magazine Q1 2022/ DAN.org