Sea cucumbers have long frustrated scientists attempting to answer a deceptively simple question: how long do they live? These soft-bodied echinoderms lack the hardened skeletal structures, such as otoliths in fish or shell growth rings in clams, that allow conventional ageing of marine animals. External tags, which work well on fish, are actively expelled by sea cucumbers and can leave painful lesions. As a result, claims about their lifespans rested largely on indirect evidence, chiefly growth models fitted to short-term mark–recapture measurements of animals of different sizes. Those models suggested multi-decadal lifespans, but sceptics in the fisheries modelling community argued that the inference was too uncertain to justify conservative harvest rules, sparking a genuine and sometimes heated scientific debate.

The new study, led by Steven W. Purcell of Southern Cross University’s National Marine Science Centre together with Clair Morton and Emma S. Smith, closes that evidentiary gap with a beautifully simple technique: photographic mark–recapture. In late October and early November 2012, the team surveyed two sites at Lizard Island, Mermaid Cove and a reef flat and lagoon between South Island and Palfrey Island, recording 138 individual leopardfish sea cucumbers. Each animal was photographed front-on, measured for length and width in situ with a clear ruler, and located with a handheld GPS. Many were also weighed on a boat after a short draining period. Crucially, both sites lie within a no-take Scientific Research Zone, meaning the animals were protected from fishing throughout the entire study period.

The identification method relies on the species’ striking colouration. Bohadschia argus, a large sea cucumber whose adults commonly exceed 30 centimetres in length, bears distinctive eye-spots across its body, appearing either as brown spots on grey or mauve spots on grey. Because the spots are arranged in patterns as unique as fingerprints, the researchers could match individuals between surveys. To guard against false positives, a match was only confirmed when seven eye-spots in a row could be aligned between the 2012 and 2024 photographs, a threshold that, assuming spots occur at random positions on other animals, yields a false positive rate of less than one percent. Near matches were independently checked by a second author, and animals with obviously different colour schemes or spot densities were quickly excluded.

When the team returned on 28 and 29 February 2024, they found and photographed 102 individuals at the same sites. Four of them matched animals photographed 11.3 years earlier: three at Palfrey Lagoon and one at Mermaid Cove. The spot patterns of the matched individuals had proved remarkably stable. On average, for every 63 eye-spots that persisted from 2012 to 2024, only six new spots appeared and two disappeared, and this ratio did not differ significantly among the four recaptures. Some spots changed shape slightly, grew larger or smaller, merged with neighbours or detached from clusters, but the overwhelming majority, 89 percent, remained identifiable. This durability of spot patterns over more than a decade is itself a valuable discovery, because it validates photographic identification as a reliable long-term tool for studying ‘unmarkable’ soft-bodied invertebrates.

The growth data are equally revealing. Body length alone proved misleading: the longest animal in 2012 was actually slightly shorter in 2024, echoing previous reports that large sea cucumbers can shrink over time. But when the researchers applied the bidimensional SLW index, the square root of the length multiplied by width, which compensates for the compensatory widening of animals as they contract, growth appeared more consistent. Over eleven years, the four animals increased in size by only 2.5 to 14.5 percent of their initial dimensions, equivalent to a modest 0.2 to 1.2 percent per year. As in earlier short-term studies of B. argus and related species, the two smallest individuals grew the most, reinforcing a consistent pattern: small tropical sea cucumbers grow fastest, while large ones grow very slowly or even shrink. The authors caution that slow growth is not universal across tropical holothuroids, since smaller species such as Holothuria atra and H. scabra can grow far more rapidly, but it appears to be characteristic of the large-bodied species that dominate commercial fisheries.

The longevity estimate follows from combining the new recaptures with a previously published Gompertz growth model for the species. Based on their estimated body weights in 2012, the four recaptured animals were already roughly 10 to 22 years old when first photographed. Adding the 11.3 years that followed yields a minimum lifespan of at least 33 years. Because age at first maturity for B. argus can be approximated at about 26 centimetres body length, using the closely related B. vitiensis as a proxy, the model suggests these animals do not mature until around nine years of age. The midpoint between age at maturity and maximum age therefore puts the generation length at a minimum of 21 years, and possibly considerably more, particularly since long-lived echinoderms such as the red sea urchin, which can exceed 100 years, show no reproductive senescence and remain fertile throughout their lives.

These numbers matter far beyond academic curiosity. Generation length is a central parameter in the IUCN Red List assessment of extinction risk and in CITES listing proposals, where population declines are evaluated over a timeframe of three generations or ten years, whichever is longer. Eleven sea cucumber species are already classified as Vulnerable or Endangered on the Red List due to fishing-driven declines. Tropical sea cucumbers are harvested in more than 100 countries, largely to supply the luxury dried seafood markets of Asia, and many of these fisheries follow notorious boom-and-bust trajectories. Yet harvest strategy models, including those applied on the Great Barrier Reef, have sometimes assumed young ages at maturity and high natural mortality rates, liberal parameters that critics argue bias outputs toward less conservative sustainable harvest estimates. For B. vitiensis, for example, an age at maturity of just three years and a natural mortality rate of 0.73 per year were previously assumed. The new evidence, from a species slightly larger than B. vitiensis, shows those assumptions are untenably optimistic for large tropical holothuroids.

The study also delivered a striking picture of long-term movement. Using GPS waypoints from 2012 and 2024, the researchers calculated that the four recaptured animals had displaced, on average, 61.7 metres over the 11.3-year period, with individual displacements ranging from 6.5 to 131 metres. Remarkably, one individual was found less than seven metres from where it had been recorded more than a decade earlier, while another had moved 131 metres, shifting from a deeper sandy area to the inner reef flat at Mermaid Cove. All recaptures were located close to the reef edge, mirroring the distribution of the wider population. This mix of home-ranging and nomadic behaviour within a single population has implications for marine protected area design, since sedentary individuals gain long-term protection within reserves, while more mobile animals may help scattered populations avoid the mate-finding Allee effects that threaten reproduction when fishing thins densities.

The authors are careful to note the limitations of their study: search effort differed between the two surveys, not all habitats were covered in 2024, and some animals may have moved beyond the search area or changed their spot patterns beyond recognition, so the recapture rate was not used to estimate mortality. Even so, the core conclusion stands unshaken. Multi-decadal longevity in a commercially harvested coral reef sea cucumber is now empirically proven, not merely modelled. The researchers argue that fishery managers should assume tropical holothuroids are generally long-lived and slow-growing unless robust evidence shows otherwise, and that the findings justify a re-evaluation of B. argus on the IUCN Red List. As the species grows in commercial importance across the Indo-Pacific, and as related Bohadschia species face similar pressures, this eleven-year act of photographic patience offers a sobering message: the animals being scooped from tropical reefs are not the fast-turnover commodities some models assumed, but slow, long-lived residents whose populations, once depleted, may take generations to return.

Subject of Research: Longevity and growth of the tropical sea cucumber Bohadschia argus determined by an eleven-year photographic mark–recapture study on the Great Barrier Reef.

Article Title: Eleven-year mark–recapture of tropical sea cucumbers proves long lifespans

Article References: Purcell, S. W., Morton, C., & Smith, E. S. (2026). Eleven-year mark–recapture of tropical sea cucumbers proves long lifespans. Coral Reefs. https://doi.org/10.1007/s00338-026-02948-2

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02948-2

Keywords: sea cucumbers, Bohadschia argus, longevity, mark–recapture, photographic identification, echinoderms, coral reefs, growth, generation length, fisheries management, IUCN Red List, Great Barrier Reef