Curio Vol 2 | Issue 5

This isn’t a story about gigantic, multi-headed, mythical beast — the kind that the Greek demigod Heracles overcame in one of his twelve labours. It is however a story about animals with the same name, but on a much smaller scale.

Hydra are a genus of fresh water animals that are tiny. How small? These creatures are usually around 10mm in length, are tubular, symmetrical, gel-like and have a mouth opening that is usually surrounded by tentacles they use to feed.

The hydra family (there are around 30 different species) are predators that feed on worms, insect larvae, larval fish and small crustaceans. Their tentacles are coated with highly specialised stinging cells called nematocysts, that shoot dart-like threads into their prey and paralyse them with neurotoxins. While these nematocysts are harmless to humans, some larger fish have been known to be stung to death by repeated attacks.

Many factors of a hydra’s biology are curious, and certainly beyond the abilities that humans possess.

Hydra have two ways of reproducing; the slightly more prevalent form is asexually over traditional sexual reproduction. This becomes rather confusing when you consider that some hydra are hermaphroditic (having both female and male sex organs) whilst others aren’t.

Sexual reproduction happens when female eggs form on the outside and sperm is released into the water for fertilisation. These eggs become hard when fertilised and are released off the mother hydra this way.

For hermaphroditic hydra, asexual reproduction simply happens by a small hydra growing from the side of its parent and then detaching to live independently.

These unique procreative properties aside; scientists have also long been interested in hydra species due to their natural ability to regenerate. If a hydra is dissected into several sections, a new hydra will generate from each severed piece.

Additionally hydra do not appear to die of senescence (old age).

Hydra belong to the larger family (phylum) of Cnidaria, which are symmetrical invertebrates with tentacles (~10,000 species). Cnidaria encapsulates species such as coral, sea anemones and jellyfish.

It is widely believed that hydra and another microscopic species of jellyfish (Turritopsis dohrnii) — (5mm in length) are biologically immortal, due to their ability to disobey normal laws of cellular senescence.

In many animals, cell states change through division, with this reaching a maximum limit of around 50. This phenomenon was investigated by Leonard Hayflick in 1965 and was later known as the Hayflick Limit — after which, mortality is ultimately reached.

Without getting overly scientific, each time a cell divides/replicates, the telomere (a segment of DNA) becomes shorter and shorter in length. In effect, this triggers DNA to damage itself — a scientific way of describing the aging function.

Uniquely, the telomere in hydra DNA never get shorter when it divides; one of the reasons why Hydra do not die from old age (not in the traditional sense anyway. Scientists hypothesise if they had to put a number to it, Hydra have a lifespan close to 1,400 years)

Though hydra do possess this incredible propensity to survive, this doesn’t mean that they are immune to death. Hydra only occur in freshwater and have a low tolerance to pollution.

Scientists are yet to adequately explain how telomere lengths in hydra remain the same, nor how hydra never mature into adult-states.

Perhaps it is a mystery that something so small, imperceptible and simple has such a survival instinct, defying what we as humans have come to understand the laws of nature and finite existence. The hydra’s enduring skill for life is unparalleled. They were here long before humans walked this earth. And the chances are they will be around long after us too.

Curio is released weekly on Mondays. Words and Illustrations by Rob Lee.