Working out how old a sea cucumber is sounds simple, but it is not. Sea cucumbers have no shell, no bones and no rings to count, and physical tags only stay on them for a few weeks or months. So researchers from the Australian Institute of Marine Science, the University of Sydney and Southern Cross University used DNA as the tag instead.
In June and December 2000 they took small skin samples from black teatfish (Holothuria whitmaei) at Bird Islet, near Lizard Island on the Great Barrier Reef. That gave them 76 different animals. In September 2024, three snorkelers searched within about 100 meters of a buoy placed on the same spot and sampled 52 sea cucumbers. Reading 12,772 genetic markers per sample, the researchers found that 31 of those 52 were animals already sampled in 2000. That is 60 percent, and since every one of them was a large adult back then, all 31 were at least 24 years old.
From how many survived, the researchers worked out an adult death rate of about 0.04 per year. Most estimates for adult sea cucumbers range from 0.3 to 1.5 per year. At such a low rate, the average lifespan comes to about 24 to 27 years, and one animal in a hundred would live to an estimated 109 to 126 years. Black teatfish are the first sea cucumber known to pass a century.
The second surprise was size. The recaptured animals had not grown at all. On average they were 21 percent lighter and 14 percent shorter than in 2000. Black teatfish do not split in two to reproduce, so that does not explain it. The researchers suggest the loss of hard coral on the reef may have affected them, but that is still an open question.
The finding matters for fishing. Black teatfish is one of the most valuable sea cucumbers, at roughly 300 US dollars per dried kilo, and it is listed as Endangered. Harvest models for tropical sea cucumbers assume much faster turnover, so the researchers say current catches are likely more than these slow, long-lived populations can replace.
