Another visit to muskeg ponds

…worlds of their own

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Photo by Katherine Hocker

The little ponds that dot many of the local muskegs are worlds of their own, with a variety of inhabitants. On a September tour of a dozen or two of these ponds at mid elevation, we found several small beasts of interest.

Almost every pond had water striders skating along in the surface film on their four long walking legs; the rear pair is used mainly for steering while the other two provide propulsion and some steering. Each foot created a dimple in the water surface, so on the bottom of the pond the shadow of the strider looked like some strange rig on four round pontoons. Striders can walk on water because their feet have hairs that resist wetting and trap tiny water bubbles, allowing them to stay on the surface. I’ve read that if the foot does get wet, the strider may drown, but water-resistant hairs on the body help prevent this. Sometimes there were two smaller dimple-shadows at the front, perhaps created when the strider touched the surface with its short front legs, which are used for prey capture. Striders, like other true bugs, have piercing, sucking mouthparts; they stab their insect prey, caught on the water surface, and suck it dry.

Eggs are laid on the water surface too, or on floating objects; juvenile striders have a rounded body shape, unlike the long, narrow adult. Developmental rates depend on environmental temperatures; it may take two months or more for an egg to reach adult status. If ponds become too crowded, striders generally can move from pond to pond by flying. They can overwinter as adults, sheltering under leaf litter, bark fragments, or rocks, presumably near their ponds.

Striders are preyed upon by birds, less commonly by fish or frogs. Sometimes they cannibalize each other.

Many ponds were home to water boatmen, which swim using two pairs of hind legs. The conspicuous legs that extend out to the side like oars are the sturdy back legs, the middle legs assist in locomotion and also function for hanging onto rocks and vegetation. Boatmen move quickly, zigzagging up and down and side to side. Research in British Columbia suggested that small developmental differences (in foot length and number of hairs) between the right middle leg and the left one had significant negative effects on body condition and survival; asymmetry of these legs was a detriment to foraging, apparently. I wonder what might be the consequences of asymmetry in the driving legs in the back!

Water boatmen eat all sorts of tiny organisms, scooped up by the front feet, which are broad and fringed with hairs. Although they are true bugs, the standard piercing-sucking mouthparts are reported to be more adapted to piecing and rasping than sucking; small organisms can be ingested whole, and then ground up in the mouth and throat. Boatmen breathe air, by capturing a bubble at the surface and carrying it with them as they dive. Adult boatmen can sing, like crickets, but the method of doing so seems to vary: some reports say the bug scrapes part of a leg over a rough part of the head, other reports say the male rubs his penis over rough spots on his abdomen. Adults reportedly can fly, engaging in mating flights at the end of the summer; they lay their eggs underwater, on aquatic plants. Presumably, the eggs overwinter, hatching in spring; adults may overwinter too.

Down on the bottom of many of the ponds were short, tubular objects that only announced that they might be alive when the objects began to crawl. These were caddisfly larvae, in protective cases of sand grains, conifer needles, or bits of leaf and grass, all held together with silk spun by the larvae. There is a mind-boggling diversity of caddisflies, in different taxonomic families, but this family (which contains multiple species) is quite common around here. The larvae shred dead vegetation or scrape the biofilm (a conglomeration of bacteria, fungi, algae, and such) from the surfaces of rocks and vegetation; stream-dwelling species may gnaw on salmon carcasses. Larvae commonly overwinter, remaining active even at low temperatures, and metamorphose to flying adults in the next spring, summer, or fall; some species may sometimes spend more than one winter in the larval stage.

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Two caddisflies fighting over a case. Photo by Bob Armstrong

Case-building caddisflies like the ones we saw are eaten by fish and birds. We have often watched American Dippers grab an inhabited case and then shake and pound it until the case breaks away from the succulent larva inside.

We saw a couple of diving beetles and a lone damselfly larva, too. Several of the ponds had colonies of strange little turrets on the bottom: each turret seemed to have a layer of brown, fluffy stuff surrounding a vertical tube less than half and in high. I have no idea what could have made these.

Several of the ponds are very deep, well over the tops of the ordinary rubber boots of anyone foolish enough to step in. These ponds have thick layers of loose, flocculent material (algae and what else?) that hide the true bottom of the pond but probably provide nice hiding places for some inhabitants.

It was probably too late in the season to hope to find bladderworts, which I have seen in muskeg ponds elsewhere in Juneau. This wispy plant captures tiny aquatic organisms with minute traps that snap shut over the prey. It would be interesting to learn how often a prey item is captured—and perhaps compare the ‘hunting success’ of bladderworts with our other insectivorous plants (sundews, butterwort). Ah well, perhaps next year!?

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