the immortal life of a hydra

Students often get to look at hydras – tiny, fresh-water members of the group that includes sea anemones, jellyfish, corals, and the Portuguese man’o’war. All these cnidarians have a simple body-plan: two layers of true tissue with a jelly-like layer between them, a sac-like gut with a single opening that acts as both mouth and anus, and the characteristic stinging cells – cnidocytes – that give the taxon its name. And many of them rely on endosymbiotic algae for their survival, using some of the sugars that the algae produce by photosynthesis. The image below shows part of a hydra’s tentacle – you can see not only its green algal symbionts, but also a halo of discharged stings.

I’ve talked about hydras in class, when we’ve been discussing the main animal phyla – but  there’s something really unusual about them that I didn’t know until now. It seems that most of their cells remain in an undifferentiated state ie they are stem cells. This means that they can differentiate into the other types of cells that comprise their little bodies. According to this article in Biographica, there’s quite a turnover – the cells making up the animal’s tentacles, the column of its body, & the ‘foot’ that holds it to a submerged leaf or pebble can be completely replaced within a week or so. According to researcher David Martinez,

There are no old cells … Even differentiated cells are recently differentiated cells.

This comment’s based on a research paper published in 2016 by Martinez and several colleagues (Schlaible, Scheuerlein, Danko et al. 2016). They begin the paper by noting that

Senescence, the increase in mortality and decline in fertility with age after maturity, was thought to be inevitable for all multicellular species capable of repeated breeding.

In other words, once you hit maturity, aging is a given. Apparently the trajectory of these declines varies in some species, but it can be hard to study in detail, partly (the authors comment) due to a scarcity of older individuals. However, hydras can be kept in the lab & studied in quite large numbers, and so the research team studied 2,256 Hydra from 2 related species. Their hypothesis:

that Hydra mortality and fertility are constant over age,

something they tested by

following large populations under controlled conditions for extended periods that greatly exceed the life expectancy of Hydra in the wild.

In order to tell whether individual hydras were deteriorating as they aged, the team looked at their reproduction. As they comment rather dryly in the paper, observing deaths alone isn’t helpful because any one individual can die just once. Instead, they looked at how often the animals produced offspring by budding, for these creatures usually reproduce asexually. Each organism produced by budding is a clone of its parent. Schlaible & his colleagues define a group of genetically-identical clones as a ‘genet’, and point out that the age of such a clone can exceed the age of any individual living at a particular point in time. This is important, because

[f]rom the perspective of evolutionary theories of senescence, the genet is subject to natural selection.

The genets used at the beginning of this study ranged in age from less than a year to more than 33 years.

The researchers found that death rates in all their groups of Hydra were very low, and independent of the age of the genet. That is, an old clonal line had the same probability of death as a young one. They commented that, if the mortality rates they observed were representative,

such low levels of mortality imply that 5% of individuals would still be alive after between 494 and 3,376 years.

These little creatures are up there with bristlecone pines in terms of longevity – at least, in lab conditions. In the field, an average adult Hydra may last only a few weeks, due to the impact of predation, food supply, disease, and extremes of temperature.

As far as reproduction was concerned, the cohorts of Hydra in this study tended to show an increase in fertility with age, although for individual animals budding rate was essentially constant over the 8 years of the study provided conditions were constant. Changes in the lab environment, such as the amount of food provided, changed budding rate – this increased in proportion to food supply.

Schlaible & his colleagues conclude that the sustained very low mortality and continued fertility in Hydra may be an evolutionarily stable strategy:

evolution pressures that favour high levels of maintenance and repair in Hydra together with a capacity for regeneration and for preventing deterioration may have jointly favored the Hydra‘s life history of nonsenescence.

There’s a lot more to the humble Hydra than initially meets the eye.

R.Schlaible, A.Scheuerlein, M.J.Danko, J.Gampe, D.E.Martinez & J.W.Vaupel (2016) Constant mortality and fertility over age in Hydra. PNAS 112(51): 15701-15706. doi: 10.1073/pnas.1521002112

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