Today is a big milestone for me, but one that I easily could have missed if I wasn’t paying attention*. It is exactly 10,000 days since I was born. Not sure what to make of this occasion because it has been a very long time since I’ve thought about my age in units any smaller than whole years. In my experience, it’s normally only young children who will tell you their ages to the nearest quarter of a year—but this does also seem to be a habit that people pick-up again in their twilight years. Nevertheless, as this milestone approached, it made me consider those creatures whose lifespans are only accurately measured in days, and not years.
The mayfly is famed for having an adult lifespan of a single day. In fact, the 2,000 or so species of mayfly that exist worldwide—which all belong to a group called Ephemeroptera (from the Greek for ‘short-lived wing’)—can have adult lifespans that range from a few minutes to a few days. Nevertheless, throughout their short lives, adult mayflies only have one thing on their minds: the need to breed!
Typically mayflies will emerge en masse once a year, in late spring or early summer, and each one has only a few hours to find a mate, breed, and lay its eggs before it dies. Adult mayflies do not even stop to eat during this time, and couldn’t even if they wanted to—their mouthparts are useless and the guts are filled with air. Eating is the job of the immature mayfly or nymph, which hatches from an egg laid in the water by an adult mayfly. A nymph will spend about a year submerged feeding on microscopic algae and other aquatic plants: after which it will swim to the surface, moult and spread their wings ready to continue the lifecycle. Thus although the adults spend only a fleeting time in the sun; mayflies actually live for much longer, hidden as nymphs in lakes, rivers and streams.
Another, perhaps less well-known, animal genuinely does have a lifespan measured in days. The gastrotrichs are microscopic worms that live in aquatic environments, and can complete their lifecycles within 3 – 10 days. These tiny creatures have a transparent body and flat underside covered in cilia—miniscule protrusions from cells that sweep back-and-forth—allowing it to crawl across surfaces underwater. Gastrotrichs do not have a respiratory or a circulatory system; but do have a simple nervous system that responds to touch, whilst some have primitive ‘eye spots’ meaning that they can also respond to light.
Gastrotrichs hatch from eggs, and grow to reach sexual maturity within three days. Unlike most animals, developing gastrotrichs will undergo a fixed number of cell divisions, with all mature individuals of a given species having the exact same number of cells (something that is also observed in some of my favourite little critters the tardigrades). Gastrotrichs are hermaphrodites, having both male and female reproductive organs, although some species have lost their sperm-producing cells and switched to solely asexual reproduction. Fertilised eggs are kept with the body of the worm, until they are released through a rupture in the worm’s body wall. Gastrotrich eggs will then hatch within one to four days and the whole lifecycle can repeat, again and again.
Gastrotrichs are often ranked within the top three most abundant groups of microscopic invertebrates in aquatic environments: and in some freshwater habitats there can be as many as 150 individual gastrotrichs per 10 cm². As such in spite of their short lifespans, gastrotrichs are, in a way, remarkably successful—and I can’t help being amazed to think that these critters have easily passed through a thousand generations in my lifetime.
*If anyone else is interested in discovering important milestones in their life too - just have a look here!