Each Fall the Augsburg Ecology class takes a trip to Minnehaha Creek, a few miles south of Augsburg College, to study stream invertebrates (stream animals without backbones). Our goal is to compare the abundance and diversity of invertebrates in areas with different stream bed types (e.g. smooth sandy bottoms vs. rough rocky bottoms) and different flow rates. Though we usually find some crustaceans and annelid worms (leeches and other aquatic worms), the most common stream invertebrates are insects, and most of these insects are immature stages of species that will leave the stream and fly when they mature to adulthood. In some of the lower-flow areas we often find damselfly nymphs such as the one shown here. Damselflies are in the insect order Odonata (which also includes the dragonflies) and this order includes a number of different families of damselflies. This particular nymph belongs to the family Calopterigidae, a family that was the subject of a recent Photo of the Month story (see the Black-winged Damselfly story for more information and for photos of what this nymph will grow into).

When we first caught this nymph I was puzzled by its fuzzy appearance. Upon closer examination it turned out to be covered with a dense growth of colonial, stalked ciliates (Vorticella sp., or some similar stalked ciliate). This undoubtedly provides the nymph some degree of camoflauge, but I don't know whether or not these ciliates are harmful to the nymph (I suspect these ciliates are harmless). However, I can say with some certainty that this burden is not permanant, for these ciliates will be shed with the nymph's exoskeleton the next time it molts.

Like other arthropods, this damselfly nymph has an exoskeleton that is shed (molted) periodically as it grows. Before molting, an arthropod develops a new, larger exoskeleton under its older, smaller one. Just prior to molting the animal is packed tight into it's exoskeleton, bulging at the seams, so to speak. After molting, the new exoskeleton expands to its larger size, but it is occupied by the same animal that lived in the older, smaller exoskeleton. Consequently, though an insect looks larger just after molting, and the linear dimensions of its head, legs, and other body parts do in fact increase by quantum leaps with each molt, the insect is sort of skinny just after molting. As it eats and grows, it gradually plumps up and fills out its new exoskeleton .

Though molting is typically described as a way around the limitations on body expansion imposed by a relatively rigid, unstretchable outer body covering, this month's photo suggests an additional benefit for aquatic arthropods. Molting gets rid of organisms that colonize the surfaces of exoskeletons. Pretty much any solid surface in the water will be colonized by aquatic life of some sort. In freshwater habitats the colonizing organisms are typically microorganisms and algae, but in the ocean there is a whole host of sessile organisms that colonize surfaces. Indeed, exoskeletons of marine arthropods such as crabs and shrimp often become colonized by sea life such as algae, tube worms, or other sessile invertebrates, but periodic molting gets rid of these hitchhikers. Some marine arthropods molt throughout life, whether they are still increasing in size or not, but in some crab species molting stops once the crabs reach maturity. Such crabs become increasingly weighed down, a problem that can eventually kill them.

Damselfly nymph covered with colonial, stalked ciliates.

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Links

A quick Web search yields many web sites with wonderful photos of Vorticella and other stalked ciliates. Here are just a few:

http://www.micrographia.com/specbiol/protis/cili/peri0100.htm

http://www.microscopy-uk.org.uk/mag/artjan02/telotroch.html

http://protist.i.hosei.ac.jp/PDB/Images/Ciliophora/Vorticella/index.html