The Smallest Whirlpool Ever

Many amazing new species were named today — several moths, an Andean tarantula, and ten Arctic Ocean sponges, to name a few. Choosing one to feature on this site was a challenge, but finally I settled on Metacystis similis, a new species of single-celled protist (Zhang et al. 2015). I probably could have gotten more people to read this article if I had featured moths or tarantulas, but that isn’t the point of this blog. Every living thing, no matter how small or seemingly mundane, has an amazing story to tell. Metacystis it is.

Metacystis recurva, with cilia-covered "mouth" facing the top right corner. Photo by William Bourland, licensed under CC BY-NC 3.0.

Metacystis recurva, with cilia-covered “mouth” facing the top right corner. Photo by William Bourland, licensed under CC BY-NC 3.0.

First, what is a protist? Animals, plants and fungi evolved from single-celled ancestors hundreds of millions of years ago. These ancestors are survived by an astounding diversity of single-celled relatives, which we collectively call protists. Algae are protists, as are amoebas. Many protists are parasitic, and Plasmodium, the one that causes malaria, kills hundreds of thousands of people every year.

Metacystis is far more benign. It is a filter feeder, engulfing smaller cells (such as bacteria) and other particles that drift through the water. The new species was found in the East China Sea, near Shanghai. Others have been discovered in freshwater habitats, and at least one is known to live in waste-water treatment plants (Arregui et al. 2010). That might sound like a less-than-ideal place to live, but at least bacterial prey are easy to find.

The Metacystis cell is roughly bottle-shaped, with a large, rounded “base” narrowing to a neck at the top. The neck is topped by an opening, the mouth, where food is drawn in.

The beating cilia around a Metacystis mouth. Photo by William Bourland, licensed under CC BY-NC 3.0.

The beating cilia around a Metacystis mouth. Photo by William Bourland, licensed under CC BY-NC 3.0.

All of the important cell parts — the nucleus, DNA, mitochondria, and so on — are kept safely tucked away in the base of the cell. Metacystis can attach itself by the base to a rock, seaweed, or the inner wall of a sewer. Here it lives as a filter feeder or, for the more dramatic, an ambush predator.

While it waits, the neck extends out into the water. The mouth is surrounded by a ring of tiny hair-like appendages called cilia. When Metacystis senses food, it beats the cilia in a wave-like pattern. This creates a water current — or, if you like, a swirling vortex of death — which pulls the hapless prey to its doom in the cell.

Once inside, anything Metacystis captures is quickly digested. If food is abundant, the protist sticks around, but if not, it can simple detach from its substrate and find a new place to live.

Cited:

Arregui L., B. Perez-Uz, A. Zornoza, and S. Serrano. 2010. A new species of the genus Metacystis (Ciliophora, Prostomatida, Metacystidae) from a wastewater treatment plant. Journal of Eukaryotic Microbiology 57(4): 362-368.

Zhang X., D. Ji, Q. Zhang, and C. Li. 2015. Description and phylogeny of a new prostomatid, Metacystis similis nov. spec. (Protista, Ciliophora) from the East China Sea. Zootaxa 4033(4): 584-592.

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