WHAT WE DO NOT KNOW ABOUT TARDIGRADES

It is generally believed that tardigrades in the cryptobiotic state can be carried on the winds. This would account for their worldwide distribution. Tardigrades have been found on remote volcanic islands where dispersion could only have been by wind or birds. The debate is supported by circumstantial evidence and awaits direct proof. What is not understood is why some apparently suitable microhabitats are not inhabited and why tardigrades may be more common in temperate and polar regions than in the tropics.

Do animals that undergo cryptobiosis have fewer generations? Does cryptobiosis allow tardigrades to inhabit microenvironments that most other animal groups would find too harsh? Are the most advanced tardigrades the ones that have been in the most stable environment for the longest time? From what did tardigrades evolve?

Eighty percent of the tardigrades described are Eutardigrades. Is this the actual makeup of the phylum or result of the collections of the last 200 years being concentrated in terrestrial mosses and lichens? Are there more species to be found? Will there be more new marine tardigrades because the salt water environment is less studied and harder to sample?

We know little of how tardigrades go about living normally. What do tardigrades do when active in the moss? Tardigrades have been observed eating nematodes, rotifers, and each other, but how often and how many? How do they find food or each other? Are they drawn to or away from light, cold, heat, oxygen or CO2? Are tardigrades positively or negatively associated with their microscopic neighbors: nematodes, rotifers, and springtails? How do they fit into the micro ecosystems? What are the effects of pollutants on tardigrades? Comparisons of tardigrades from moss and lichen samples from pristine localities and those exposed to air, chemical, or thermal pollution has not been done.

Our records of world distribution are based on only a few hundred observations (McInnes 1994). Most of those collections are from Europe. The other continents have very few records. The reports of tardigrades in the United States are scarce and scattered (Figure 12). Patterns of distribution within localities and affinities for or against particular environments or elements of the habitat are only now beginning to emerge. Almost any verified collection will add to our knowledge of distribution.