Friday, December 10, 2004
Water bears are tiny creatures that can be seen only with a microscope but they are so strange I couldn't resist telling their story. I had never heard of them until Tom Brownrigg drew my attention to a Mass Wildlife article on Red Oak Lichens (Journal No. 3, 2003). The lichens were interesting but the article included a page about water bears that live on, under, or near lichens and I decided to go bear hunting.
Name: Water bears are also called moss piglets. They have eight legs but are not classified with Arachnids (spiders, ticks and mites) in the phylum Arthropoda. They have their own phylum, Tardigrada, and are known as tardigrades. The phylum name comes from tardi — meaning slow — and grado — meaning walker. It's a good descriptive name because despite living in water, water bears don't swim — they just slowly stumble around with a lumbering gait.
When and where found: Water bears live in ponds and damp places from the tropics to the poles. They are often found in lichens and mosses. I looked for them for several months in numerous samples of lichens and mosses before finding my first one sometime this summer in a clump of moss growing on a rock in my backyard. This week I took another sample of moss from the same rock, and found another one. The moss is Hypnum imponens and looks like a miniature fern (a subject for another day).
Hunting for water bears: It is too hard to find the little critters in among the moss so you soak the moss in fresh clean water for a few hours and then drain off the water and discard it. Then take the sodden moss and squeeze the residual water into a clear glass or petri dish. Systematically scan the water in the dish under the microscope at 40X. You will see lots of other tiny animalcules like nematodes and rotifers. The water bear is unmistakable.
Identifying characteristics: The chubby body and the eight stubby legs identify the water bear. Imagine a dachshund with two extra pairs of legs, no ears and no tail, and you will have a good idea of the proportions. They range in size from 0.1mm to 1.2mm long. The references say that water bears can be different colors like gray, bluish, brown, red, yellow, orange, green, or tan. The two that I have watched under the microscope were almost colorless. When they have been dislodged from the moss, the little legs flail around quite rapidly for something to grasp. A close examination of the mouth parts, the claws, and the skin with a higher-powered compound microscope is needed to identify water bears at the genus and species level. There are over 400 species. The tardigrade web site at Illinois Wesleyan University, listed in the references, has a good video clip of a water bear (which looks a lot like mine), as well as a key for species identification.
Hardy tardigrades: Tardigrades are famous for being able to survive hideously extreme conditions using a process called cryptobiosis. They convert themselves into desiccated barrel-shaped objects called "tuns" and hang out in a state of near-zero metabolism until conditions are favorable. Tuns can be revived after exposure to temperatures approximating absolute zero and as high as 150 degrees Celsius; they can tolerate x-ray radiation at 1000 times the lethal dose for humans; they can withstand 6000 atmospheres of pressure; water bears have been recovered from tuns in a museum moss specimen known to have been 120 years old.
References: Red Oak Lichens, by Roger Monthey, Constance Stubbs and Stephen Sharnoff in Mass Wildlife, Journal No. 2, 2003; Pond Water Zoo by Peter Loewer (this book is in the Gleason Library); Guide to Microlife, by Kenneth Rainis and Bruce Russell; web site of Tardigrade facts at http://www.iwu.edu/~tardisdp/tardigrade_facts.html.
Please feel free to write the Biodiversity Corner on any living thing large or small that interests you. The only requirements are that it exists in the wild and was seen in Carlisle. Send a note to Kay Fairweather at 392 School Street, Carlisle MA 01741 or to firstname.lastname@example.org
© 2004 The Carlisle Mosquito