The Negligible Senescence of Sea Urchins

Aging as we understand it is almost a universal phenomenon in animals. Clearly there is something advantageous in evolutionary terms in having disposable individuals carry the immortal germline forward in time. One possibility is that aging is an emergent property of the fact that selection pressure is always going to fall more heavily on younger individuals, and thus evolution favors change in the direction of biological systems that are highly effective in youth but fall apart later on. Resources directed towards long-term maintenance subtract from resources directed towards immediate reproductive success. It is a brutal zero-sum race to the bottom, driven by the mortality of predation and a hostile environment. Younger individuals contribute more to the fitness of a species, because fewer of them have been eaten or otherwise removed from the picture.

Another view is that immortal species do have certain advantages in certain situations, and will emerge over time in any period of stability. They will vanish in eras of environmental change or hardship, however, outcompeted by species that age, as aging makes them more likely to adapt successfully. This viewpoint predicts the present situation, in which there exist only a very few species that appear not to age (as for hydra), or to age negligibly to various degrees (lobsters, sea urchins, naked mole-rats, and so forth). But at root, these evolutionary theories are all based on models and hypotheses, and thus prone to shift in and out of favor over time. Proof is hard to come by in this field.

Some varieties of sea urchin are among the small number of species that show very few signs of aging. Like many of their negligibly senescent peers they are capable of proficient regeneration, and the details of their aging (or lack of said) is in fact quite poorly studied in comparison to what is known of mammalian biochemistry in unusually long-lived species such as naked mole-rats. Even simpler data can be poorly characterized: maximum life span is an entirely speculative number for many sea urchin species, for example. It is only known that the number is quite large.

Senescence and Longevity of Sea Urchins

Echinoids, known as sea urchins, are a relatively small class of marine invertebrates with just over 1000 extant species. Historically, sea urchins served as model organisms in developmental biology. Later on, their properties were expanded to studying the innate immune system. Recently, the sea urchin was suggested as a novel model for studying longevity and senescence. Sea urchins are organisms of great lifespan diversity; some of which show extreme longevity. A noteworthy example is the red sea urchin, Mesocentrotus franciscanus, which has been confirmed to live well over 100 years with some specimens reaching 200 years. Conversely, the green sea urchin, Lytechinus variegatus, has an estimated maximal life expectancy of only four years. The lifespan diversity between different sea urchin species and the extreme longevity that some species achieve raises questions about their aging process. Do sea urchins age? Are there any indications of aging?

Aging in many organisms is accompanied by the complex mechanism of senescence, which involves a substantial number of biological processes which have different characteristics, such as genomic instability, telomere shortening, mitochondrial dysfunction, loss of proteostasis, stem cell exhaustion, and changes in intracellular communications. In some multicellular organisms, these processes can be so slow to the point where they might be considered negligible. Organisms that fit the criteria for negligible senescence display no noticeable increase in age-specific mortality or decrease in reproduction rate with age, as well as no noticeable weakening in their physiological capacity or disease resistance. Sea urchins grow indeterminately and reproduce throughout their entire adult life.

The lack of age-associated telomere shortening has been observed in both long-lived and short-lived sea urchins. Analysis from several adult M. franciscanus samples indicated continuous telomerase expression and maintenance of telomeres. Lifelong telomerase activity was also reported in another species of sea urchin, Echinometra lucunter. Even though telomere shortening has been suggested to be a tumor-protective mechanism and despite neoplasia occurring in diverse species of marine invertebrates, neoplasms are rarely seen in sea urchins.

Sea urchins do not fit within the classic understanding of biological aging. Members of this class are among the oldest animals on earth and it is apparent that the hallmarks of aging do not apply in their case. Considering the lack of senescence and sequencing revealing a genetic relation to humans, it is clear why researchers suggest the sea urchin is a novel model for studying aging. However, the research on sea urchins from that point of view is relatively new. At the end of the last century, even the centenarian sea urchin M. franciscanusm was thought to live just above 30 years. It was only in 2003 that carbon-14 dating exposed evidence of nuclear weapon testing from the 1950s in tissues of M. franciscanus and thus confirmed its exceptional lifespan. Further, work from 2012 was, to the best of our knowledge, the first and only global approach study on age-related gene expression in sea urchins. Since the evidence of negligible senescence is similar across short- and long-lived sea urchins, the mechanism of their mortality remains poorly understood. Therefore, further research is required.

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