From Observation to Theory with Resurrected Rotifers

Abstract

... I took some of this dry sediment, which I had taken out of the leaden gutter and had stood almost two days in my study, and put a little of it into two separate glass tubes, wherein I poured some rain water which had been boiled and afterwards cooled.... ... I examined it, and perceived some of the Animalcules lying closely heaped together. In a short time afterwards they began to extend their bodies, and in half an hour at least a hundred of them were swimming about the glass.... The preceding kinds of experiments I have many times repeated with the same success, and in particular with some of this sediment, which had been kept in my study for above five months, and upon pouring on it rain water, which had been boiled, and afterwards cooled, I saw in a few hours time many of the Animalcules before described. --Quoted from Keilin, 1959 [FIGURE 1 OMITTED] These words, written by Antony van Leeuwenhoek in 1702, are the first report of the revival of desiccated organisms--those minuscule yet complex animals known as rotifers (Figure 1). Swimming across microscope fields propelled by their whirling crowns of cilia, creeping along surfaces like inchworms, drawing hoards of microbes into ciliary currents and crushing them in their pulsating jaws, they have fascinated observers for more than 300 years. And van Leeuwenhoek's report, importantly, gives more than just his discovery; it describes the careful process he used to obtain it. When repeated in biology labs today, there are gains for both teachers and students. For teachers, a common difficulty when introducing scientific thinking is finding ways to engage the participation of the students. Because most scientific problems require at least some specialized knowledge to formulate and solve, it can be hard to go beyond merely discussing the steps of the scientific method and illustrating them with the common textbook examples. This article offers one solution to this conundrum: A laboratory exercise involving the revival of desiccated rotifers collected from local habitats. The microscopic observations are readily done, and reasoning is based mainly on everyday learning acquired without formal training. Students gain practice in scientific thinking, experience in experimental design, a taste of biological discovery, and a touch of utter amazement. Arrival, Survival & Revival To briefly summarize the ecology and physiology of the inhabitants of temporary aquatic habitats ... these organisms may either be highly mobile transients that arrive when water appears and leave before the habitat dries (mosquitoes, for example), or they may be permanent residents able to survive through periods of dryness (Williams, 1987). The focus here is on the latter. Initially, the permanent residents must have dispersed from elsewhere. In most species this occurs not by active locomotion, but passively via the air, flowing or splashing water, or attachment to mobile animals (Gregory, 1973). Algal cells, for example, are widely dispersed after being picked up by the wind from dry soil, ponds, or lake shores; they are also transported by ducks, mosquitoes, dragonflies, and other animals that frequent water (Williams et al., 1994). Rotifers have been collected from rain and airborne dust (Ricci, 2001). In short, these organisms are highly mobile and therefore very widespread. But our appreciation of this often falls short, unfortunately, because of the limitations of our unaided eyes. Survival through periods of dryness occurs in numerous ways. Some species avoid water loss, either by burrowing down to where moisture remains or by encasing themselves in protective structures. Others can survive the loss of most of their internal water; they enter a state of suspended animation known as anhydrobiosis. (This term is from the Greek for life without water.) This is widespread among kingdoms and phyla, being found among bacteria, cyanobacteria, protozoans, algae, fungi, lichens, mosses, vascular plants, nematodes, crustaceans, insects, tardigrades, and rotifers (Potts, 1994; Schuster, 1979; Evans, 1958; Parker et al. …