Hidden Lives - the Movie

As I announced over a year ago, I have a website featuring the protists (protozoans and algae) that one can find in freshwater (ponds, reservoirs, drains...) in Singapore. The pages are illustrated with pictures (photomicrographs) and videos, and are organized as a guidebook (inspired by the Singapore Science Centre nature guidebooks that I grew up with).

With my collection of videos, though, I wanted to put them together into a documentary-style film, and I finally found time to do it this summer.



Hidden Lives (SD) from brandon seah on Vimeo.

This was made with iMovie '08, with music sourced from ccMixter. Feel free to share, with credits! Read more about the motivation here.

How the bean got its twist

Legumes are among the most diverse and successful families of plants in the world. In the Neotropical rainforests, the dominant tree species are mostly legumes (unlike in Southeast Asia, where dipterocarps predominate).

Their defining feature are their eponymous fruits, which appear to have a myriad ways of breaking up to release the seeds within. Some drop to the ground and shatter, like the tubular pods of Cassia:

Broken Cassia fistula pod exposing seeds

... while others twist and contort as they dry out to present seeds to dispersers, like Acacia:

Dried up and twisted Acacia auriculiformis seed pods

Legumes are certainly not the only plants with structures that change their shapes as they desiccate. They're not the only organisms to do so, either (think of mushrooms with caps that curl upwards as they mature to release their spores). However, their ubiquity and convenient size makes them a good subject for study and experimentation.

A team of physicists and mathematicians from Israel have recently figured out the rules behind the seed pod's twist, using a legume Bauhinia variegata as the model (paper in Science behind paywall). The basic idea is that the pod wall is made of an anisotropic material, that is, its material properties are not uniform but depend on the direction that it's being manipulated in. In this case, the anisotropy results from the orientation of fibres in the wall. Expansion or shrinkage tend to happen transversely to the aligned fibres. It's also a composite material, being made from at least two layers of wall sandwiched together. These two layers, however, have their fibres aligned in different orientations. As a result, when they dry out, they want to shrink in different directions. This conflict results in a deformation of the wall that produces a helical pattern.

Ray the Rotifer!

While gazing through the microscope at yet more pond water, I was surprised to see this swim past my field of view (click for larger versions)....

It is evidently some kind of rotifer (according to Maxine Mowe) with part of its cuticle expanded into lateral 'wings'. The animal is able to open and close these wings, and even flap them somewhat.

Do you think that there's some resemblance to the chaps below?

The resemblance may be misleading, though. The rotifer is about a quarter of a millimeter across, whereas these skates and rays can measure about a meter or more. The way that water behaves at microscopic scales is very different from the macroscopic scale that we (and these fishes) live in. For zooplankton, the dominant property of water is its viscosity, as opposed to its inertia. Where a manta ray would coast along for several meters after a flap of its mighty wings, this winged rotifer would instead come to an immediate halt. So what are those wings for, on the tiny rotifer? Looks like it's something for an enterprising student to find out.