While I was reading about earthworms recently, I got to thinking about ice worms, which I’d heard about over the years, but I wanted to learn more. This opened up a whole new ‘can of worms’, so to speak, because the term ‘ice worm’ has been applied to more than one kind of beast.
Let’s start close to home, with the North American glacier ice worm (there is reportedly an Asian one too, in the Himalayas, but I have found no good information about that one). Our glacier ice worms live only in the maritime glaciers of the coast ranges of the North Pacific, from Oregon to Alaska. Sometimes the populations are quite dense, with hundreds or even thousands of worms per square meter, but because they don’t like sunlight, they are seldom seen by most folks. However, the high densities of these worms, coupled with their digestive processes and eventual decomposition, probably contribute a rich source of nutrients to downstream ecology.
These worms are usually dark colored and very small, usually half an inch to an inch and a half long. They are intolerant of warm temperatures (over about 5 degrees Celsius). Their enzymes denature and their bodies just fall apart. But they love the cold—they have antifreeze proteins that keep ice crystals from forming and tearing apart their tissues. But not too cold—only a few degrees below freezing. They burrow around between ice crystals and come to the surface to feed on algae and bacteria and any other tiny edible morsels. Sometimes they are seen in meltwater pools and streams on the glacier surface, wiggling around and possibly feeding there. Occasionally investigators see writhing bundles of many ice worms in meltwater pools and conjecture that bundling is part of the reproductive process. They are probably hermaphroditic (both male and female), but it still takes two (or more) to tango.
Glacier ice worms were discovered in 1887 on the Muir Glacier over in Glacier Bay. More recent work has shown that there are three principal genetic lineages, probably originating from an aquatic ancestor several million years ago, in or near the St. Elias range. From there, they extended north to the Chugach range and, later, to the south. Researchers note that each of the three lineages corresponds to a major mountain region that offers elevations over 4300 meters (St. Elias, Chugach, Cascade), which may have provided refugia when glaciers retreated. During periods of glaciation, they could migrate through the glaciers and icefields, although at a rate of about three meters per hours, this would take some time. Nevertheless, that may be how they got to the Chugach. How they got to the southern mountain ranges is not clear, however. One conjecture is that they were carried by birds.
There are two known outlier populations whose distribution is something of a mystery: One on Vancouver Island, which is genetically related to the populations in south-central Alaska, and one in Oregon, well separated from the rest of the southern populations by a gap that was never glaciated. How did these outliers get to their present locations, so isolated from their nearest relatives? Birds, again?
Curiously, there is a big gap in their coast distribution: they have never been found on suitable glaciers between Skagway and Petersburg, including, of course, our local Mendenhall Glacier. The reason for the gap is not well understood, but some researchers tentatively suggest that when the Pleistocene glaciers reached the sea coast here, the tall mountains on the Alexander Archipelago may have blocked some of the warming and wetting effect of maritime air on low-elevation coastal glaciers. At the same time, the Pleistocene icefields at higher elevations could have been subject to cold, dry winds from the east. Together, these climatic effects may have created unsuitable conditions for ice worms—and they are still absent.
Glacier ice worms are distantly related to earthworms, belonging to the same general group known as oligochaetes (for the relatively few bristles on their bodies).
Very different indeed are some other little beasts that are also called ice worms. These small worms belong to another taxonomic class, called polychaetes (many-bristled). Known as methane-hydrate ice worms, they live in burrows near the surface of methane hydrate mounds that form when methane gas molecules bubble up and get trapped in a lattice of water molecules at low temperatures (just above freezing) and high pressure. Under these conditions, the hydrate can form a stable solid structure (like ice). Until they get buried by sediment, they are colonized by a variety of small organisms, including these worms.
Methane-hydrate ice worms are usually whitish or pinkish, and, unlike many other worms, the sexes are separate. They probably eat bacteria and other tiny organisms. They have elaborate appendages all along their sides; the action of their swimming appendages may in fact create their burrows by eroding the surface of the mounds. Observers note that each burrow usually houses one full-sized worm or several small ones. These weird worms were only discovered in 1997 (in the Gulf of Mexico), so their biology is little known.