Yesterday we went across to Tauranga to see my in-laws. It was a terrible day for driving; the forecast was for periods of heavy rain & it was pouring down when we arrived. My father-in-law had emptied the rain gauge that morning (23mm, he said), & by 1pm it was back up to 80mm & rising. There was a lot of surface flooding on the road coming home, the roadside drains had turned to foaming brown torrents, and many paddocks were more like pools, with cattle standing close to hedges to avoid at least some of the driving rain.
And seeing those pools my thoughts turned (as they do) – to rotifers.
Rotifers are rather cool little microscopic animals. Each rotifer has less than 1000 cells in their tiny body, and some of them are smaller than single-celled protozoa. They’re found in fresh water just about everywhere (although there are some marine species as well) & will quickly colonise temporary ponds. (Hence the drift of my thoughts.) If you look at some of the rotifer images here you’ll see that while their shapes may differ they all have one feature in common: a crown of beating cilia at one end of that small rounded body. The name ‘rotifer’ reflects the fact that in many rotifers – the bdelloid rotifers – the beating cilia give the impression of a rapidly spinning rotor, or wheel.
I talk a bit about these fascinating wee animals in some of my lectures, on account of their reproductive habits. Why? Because many species (& all of the bdelloid rotifers) reproduce asexually, & have done so for a very long time indeed. "No sex for 30 million years" announced the headline of a rotifer story in our local paper – unfortunately this headline sat next to a photo of the researcher studying them… (My colleague had to put up with a bit of teasing & the journalist concerned was mortified, muttering at length about the iniquitous sub-editor who’d thought up that little gem!)
There’s only one gender in the bdelloid rotifers, & that’s female. Lacking males, they reproduce by parthenogenesis. Each female produces what are known as amictic eggs, which develop without fertilisation to produce daughters that are effectively clones of their mother. Because every individual in the population is capable of producing offspring, such populations can grow in size very rapidly.
Other species have the best of both worlds, combining parthenogenesis & sexual reproduction in quite a complex cycle. A female can produce 2 types of eggs that both develop parthenogenetically – one into females & the other into males, followed by a round of sexual reproduction. Apparently the latter type is produced in response to a change in the environment. This makes a lot of sense, in evolutionary terms. While the environment is relatively constant, producing offspring that are all clones of the parent means that they’ll all be fairly well-adapted to that environment (otherwise, the original ‘mother’ rotifer wouldn’t have survived in the first place). But a changed environment may place different demands on the organism. Producing males in response to such a change allows for genetic recombination, producing a range of individuals some of which may be able to survive in their new situation.
Speaking of changed situations – I was intrigued to discover that bdelloid rotifers can also enter a state called ‘anhydrobiosis’: their tissues dehydrate & in this condition they can survive for some considerable time until rehydrated again (eg after heavy rainfall refills temporary ponds). This is a relatively common strategy, but my favourite anhydrobiotic organism has to be the water bear 🙂
This image is of a normal, fully hydrated water bear, or tardigrade. As the name suggest, these cute little critters live in fresh water, crawling around on plants. But in the fully dessicated, anhydrotic state they’d win any survival contest, I think: they can cope with being frozen to around -273oC or heated to more than 100oC, exposed to pressures equivalent to 6000 atmospheres or to high doses of radiation, or even the vacuum of space. And then they’ll bounce back to their normal cuddly state on rehydration. The ultimate Survivor of the animal kingdom, methinks.