Today we started covering a couple of examples of pseudocoelomate phyla, Rotifera and Nematoda.
 |
| Left: Philodina gregaria (Rotifera: Bdelloidea - Micrographia) Right: Stephanoceros fimbriatus (Rotifera: Monogononta - Water Bear web base) |
Rotifera, the "wheel bearers", are an interesting phylum of microscopic or nearly microscopic, mostly freshwater organisms, typically with a cephalic structure called crown, bearing a series of cilia that beat metachronally, giving the impression that a wheel is spinning. Such behavior is used by most members of this phylum to feed and swim.
We studied their main external anatomical features, feeding and swimming/"walking" behavior, their mode of osmoregulation, and their nervous system.
Then we had an overview of the main groups, Digonatans (Seisonids and Bdelloids) and Monongonontans.
When discussing the bdelloids we emphasized the fact that they reproduce parthenogenetically and no males of this group have ever been observed.
They can also undergo cryptobiosis when environmental conditions change drastically. They go into a desiccate dormant state, in some groups a cyst, that can be rehydrated months to years later, depending on the group. Cysts can be exposed to extreme conditions and still be able to rehydrate. Examples are exposure to -200º C for a few days, or exposure to nearly absolute zero temperatures (0.008º K) for a few seconds.
Monogonontan rotifer populations are composed exclusively by females, but males have been observed. When environmental conditions become stressful eggs are produced by meiosis (instead of the normal mitotic process), allowing for the birth of haploid males and females. These reproduce producing diploid eggs that are very resistant to adverse conditions, and will hatch when the stress has receded.
A bdelloid rotifer feeding
A monogonont rotifer feeding
Nematodes, unsegmented "roundworms" are the most ubiquitous and abundant animals on earth. Insects may be the most diverse (so far), but in terms of number, nematodes take the prize. They are so abundant (free living and parasitic) that it is said that "if everything on Earth were to disappear except the nematodes, the outlines of everything would still be visible: the mountains, lakes and oceans, the plants and the animals would all be outlined by the nematodes living in every habitat." (I do not know the original source of such statement.
We discussed a cross section of their internal anatomy, their digestive and nervous systems.
 |
| A (morphologically) typical nematode Nova Scotia nematodes |
No comments:
Post a Comment