27 October 2005

The thing from the bird bath


No, it's not a bird, it's a rotifer! Specifically, a bdelloid rotifer (possibly Philodina acuticornis).

This animal is a representative of a phylum of microscopic aquatic invertebrates known as the Rotifera. The identifying characteristic of them is the corona, the two ciliated disks surrounding their mouths. Despite being tiny―this particular individual was about 0.4 mm long―they have an almost complete set of organs, including a pair of jaws (the trophi), a brain, a dorsal antenna, a stomach (dark red in these pictures), a cloaca and toes ending their feet. This species, and many others, even has a pair of eyes that are visible as orange specks above its brain on the back of its head. The questions of why such a tiny animal would need seemingly binocular vision and what it can actually see are, of course, open to endless speculation.


The animal on the left is feeding with its corona fully open. The beating of the cilia along the edges of the corona continuously pulls into the rotifer’s mouth water along with its food suspended in it: bacteria, algae and tiny organic debris. The animal on the right has closed its mouth and withdrawn its corona into its head; a ciliated sucker-like proboscis, the rostrum, is now at the front. The rotifer is crawling, like an inchworm, by alternately attaching the toes at the end of its foot and the rostrum on the substrate. Its antenna, an organ of unknown function, is sticking up from the top of its head.

Bdelloid rotifers live in ponds, lakes, creeks, wet soil, mosses, leafy lichens, bird baths and any other place where water may accumulate even intermittently. They are one of the main consumers of bacteria in waste water treatment plants. The individual who posed for my pictures did come from the bird bath in my backyard (and went down the toilet to the local WWTP).

Bdelloid rotifers (class Bdelloidea), with about 350 or so known species, have one characteristic that makes them stand out among all other animals: they are the largest group of animals that reproduce exclusively by parthenogenesis1-3. Every individual bdelloid is a female that produces unfertilized eggs from which more females hatch and so on. There is no published record of a male bdelloid having ever been observed.

Another characteristic that they share with some other microscopic creatures, such as tardigrades and nematodes, is that they can survive complete desiccation for long periods. Before I switched to snails, I worked with bdelloids for several years. I was really fascinated (I still am) with their ability to remain alive in a dry state. In a series of experiments4, I demonstrated that they could be stored alive at -20 °C and <1% humidity for at least up to 18 months.

Every November I empty out the bird bath, dry it and bring it inside. Every April I put it back out, fill it and invariably the same bdelloids return. I will, one day, figure out their secret.

1. Judson, O.P. & Normark, B.B. 1996. Ancient asexual scandals. Trends in Ecology & Evolution 11:41-46.
2. Welch, D.M. & Meselson, M. 2000. Evidence for the evolution of bdelloid rotifers without sexual reproduction or genetic exchange. Science 288:1211-1215.
3. Birky, C.W. 2004. Bdelloid rotifers revisited. Proceedings of the National Academy of Sciences 101: 2651-2652. pdf
4. Örstan, A., 1998. Factors affecting long-term survival of dry bdelloid rotifers. Hydrobiologia 387/388:327-331.―Reprints available! E-mail your postal address to me (please put reprint in the subject line). Sorry no pdfs.


pascal said...

Heh, my wife was just watching an educational video about rotifers. Amazing critters.

Any chance their eggs hide in your bird bath over winter? Although there have to be ways they got there in the first place. Perhaps via birds?


Some probably survive dry over winter. They are probably also brought by birds, rain, wind, leaves...