Each morning, Shin Kubota, who is 60, visits Muronoyu, a simple onsen popular with the city’s oldest citizens that traces its history back 1,350 years. “Onsen activates your metabolism and cleans away the dead skin,” Kubota says. “It strongly contributes to longevity.” At 8:30 a.m., he drives 15 minutes up the coast, past the white beach, where the land narrows to a promontory that extends like a pointing, arthritic finger, separating Kanayama Bay from the larger Tanabe Bay. At the end of this promontory stands Kyoto University’s Seto Marine Biological Laboratory, a damp, two-story concrete block. Though it has several classrooms, dozens of offices and long hallways, the building often has the appearance of being completely empty. The few scientists on staff spend much of their time diving in the bay, collecting samples. Kubota, however, visits his office every single day. He must, or his immortal jellyfish will starve.

The world’s only captive population of immortal jellyfish lives in petri dishes arrayed haphazardly on several shelves of a small refrigerator in Kubota’s office. Like most hydrozoans, Turritopsis passes through two main stages of life, polyp and medusa. A polyp resembles a sprig of dill, with spindly stalks that branch and fork and terminate in buds. When these buds swell, they sprout not flowers but medusas. A medusa has a bell-shaped dome and dangling tentacles. Any layperson would identify it as a jellyfish, though it is not the kind you see at the beach. Those belong to a different taxonomic group, Scyphozoa, and tend to spend most of their lives as jellyfish; hydrozoans have briefer medusa phases. An adult medusa produces eggs or sperm, which combine to create larvae that form new polyps. In other hydroid species, the medusa dies after it spawns. A Turritopsis medusa, however, sinks to the bottom of the ocean floor, where its body folds in on itself — assuming the jellyfish equivalent of the fetal position. The bell reabsorbs the tentacles, and then it degenerates further until it becomes a gelatinous blob. Over the course of several days, this blob forms an outer shell. Next it shoots out stolons, which resemble roots. The stolons lengthen and become a polyp. The new polyp produces new medusas, and the process begins again.

Kubota estimates that his menagerie contains at least 100 specimens, about 3 to a petri dish. “They are very tiny,” Kubota, the proud papa, said. “Very cute.” It is cute, the immortal jellyfish. An adult medusa is about the size of a trimmed pinkie fingernail. It trails scores of hairlike tentacles. Medusas found in cooler waters have a bright scarlet bell, but more commonly the medusa is translucent white, its contours so fine that under a microscope it looks like a line drawing. It spends most of its time floating languidly in the water. It’s in no rush.

For the last 15 years, Kubota has spent at least three hours a day caring for his brood. Having observed him over the course of a week, I can confirm that it is grueling, tedious work. When he arrives at his office, he removes each petri dish from the refrigerator, one at a time, and changes the water. Then he examines his specimens under a microscope. He wants to make sure that the medusas look healthy: that they are swimming gracefully; that their bells are unclouded; and that they are digesting their food. He feeds them artemia cysts — dried brine shrimp eggs harvested from the Great Salt Lake in Utah. Though the cysts are tiny, barely visible to the naked eye, they are often too large for a medusa to digest. In these cases Kubota, squinting through the microscope, must slice the egg into pieces with two fine-point needles, the way a father might slice his toddler’s hamburger into bite-size chunks. The work causes Kubota to growl and cluck his tongue.

“Eat by yourself!” he yells at one medusa. “You are not a baby!” Then he laughs heartily. It’s an infectious, ratcheting laugh that makes his round face even rounder, the wrinkles describing circles around his eyes and mouth.

It is a full-time job, caring for the immortal jellyfish. When traveling abroad for academic conferences, Kubota has had to carry the medusas with him in a portable cooler. (In recent years he has been invited to deliver lectures in Cape Town; Xiamen, China; Lawrence, Kan.; and Plymouth, England.) He also travels to Kyoto, when he is obligated to attend administrative meetings at the university, but he returns the same night, an eight-hour round trip, in order not to miss a feeding.

Turritopsis is not the only focus of his research. He is a prolific author of scientific papers and articles, having published 52 in 2011 alone, many based on observations he makes on a private beach fronting the Seto Lab and in a small harbor on the coastal road. Every afternoon, after Kubota has finished caring for his jellyfish, he walks down the beach with a notebook, noting every organism that has washed ashore. It is a remarkable sight, the solitary figure in flip-flops, tramping pigeon-toed across the 400-yard length of the beach, hunched over, his floppy hair jogging in the breeze, as he intently scrutinizes the sand. He collates his data and publishes it in papers with titles like “Stranding Records of Fishes on Kitahama Beach” and “The First Occurrence of Bythotiara Species in Tanabe Bay.” He is an active member of a dozen scientific societies and writes a jellyfish-of-the-week column in the local newspaper. Kubota says he has introduced his readers to more than 100 jellyfish so far.