Science Tasks (Document 2 of 2) MOISTURE-RELATED HABITAT PREFERENCES IN ISOPODS

PROJECT DESIGN PLAN

Isopods -- also known as "sowbugs" or "pillbugs" -- are usually mistakenly thought of as insects. In reality they are the only terrestrial species of crustacean, and are evolutionarily more related to crabs, shrimp and lobster than any kind of "bug."

This evolutionary relationship to so many aquatic species -- and the dearth of land crustaceans besides the isopods -- raises interesting questions about the isopod choice of habitat.

Crustaceans obviously have a system of underwater respiration using gills. Isopods also have these gills but do not live underwater: they are predominantly found in moist damp environments (beneath a rotting log).

But could isopods live underwater if necessary, or are their gills fully adapted to land life now?

I proposed an experiment to determine habitat preferences in isopods. It would offer a representative sampling of isopods the chance to choose between separate habitats.

In order to gauge levels of moisture preference, these would be presented in three forms: dry (sandy), moist (potting soil), and aqueous (mud-puddle).

My initial hypothesis is that -- like other animals in which respiration occurs not through the absorption of oxygen in the atmosphere but rather through the oxygen dissolved in water, such as earthworms or newts -- isopods will be susceptible to drowning, and will therefore exhibit an equal aversion towards pooled water as they will towards dry sand.

2. RELEVANCE OF TESTABLE QUESTION

Because isopods are considered an agricultural pest -- and can do significant damage to crop plants or garden plants -- it is relevant to determine their habitat preferences.

Information gathered could help future pest-management strategies for these particular species, or simply to limit their geographical spread.

If the hypothesis holds true, a moat would be sufficient to contain isopod populations.

3. LITERATURE REVIEW

For the purposes of this experiment, I relied mainly upon two standard texts which treat the biology of the isopod: Gibb and Oseto (2006) and Robinson (2005).

Gibbs and Oseto identify the respiratory methods of isopods: "breathing involves paired gills on the lower hind part of the body. Because the gills must remain moist, sowbugs cannot withstand drying." (p. 117)

Gibbs and Oseto do not note whether the gills function in water, but they do indicate the close relation between isopods and aquatic species like "crabs, lobsters, shrimps" (p. 107).

Robinson (2005) describes the respiratory apparatus of the terrestrial isopod species:

The anterior abdominal appendages usually bear functional gills in aquatic forms. The terminal abdominal appendages are enlarged and elongate. In terres- trial species respiration is by means of tubelike air-channels or pseudotracheae. These open to the exterior by a single pore which lacks a spiracular closing device possessed by other arthropods. Some respiration takes place through the moist surface of the integument. The primitive pseudotracheae and the absence of a layer of cuticular wax expose isopods to desic- cation. Isopods usually remain on or in damp soil or other moist habitats, and are active at night when humidity is high. They often occur in small to large numbers as a means of reducing evaporation and maintaining water balance. (p. 426)

4. EXPERIMENTAL DESIGN

A representative sample of 100 isopods (sowbugs) was placed on a Plexiglas strip, 12 inches by 1 inch, with raised walls at the 1-inch-wide ends.

On either side of this Plexiglas strip were placed two plastic tubs containing the habitat environments.

Three habitat tubs were filled: one with ordinary sand (collected from a playground sandbox), one with moist potting soil, and one with approximately half potting soil and half water, to create a mud puddle with substantial depth of water above the soil level.

The isopod population was timed at 5-minute intervals to see if they exhibited any marked preference for the dry, moist, or wet environments when compared to one another.

Three separate trials were conducted. The...

I wished to isolate one aspect of that environment -- its higher level of moisture -- to see if the bug's sensory apparati were attuned to detecting moisture levels and indicating a preference. Considering that they breathe through gills, which by definition can only absorb oxygen through a water solution and not from the atmosphere, this seemed like a fairly obvious thing to ask.
SEQUENCE OF EVENTS: This experiment was conducted in a windowless underground room (properly an oubliette) which formerly served as my bedroom in early childhood; it is maintained at 70 degrees Fahrenheit and tends to be relatively dry. The temperature of the sand was maintained at 70 degrees Fahrenheit as well.

The temperature of the puddle and the damp earth were consistently lower than 70 degrees, ranging from 65 to 68 degrees -- possibly within the margin of error but presumably due to the effect of evaporation.

Decision was made not to allow this natural effect to continue, as possibly being the sensory clue that allows the sowbug to negotiate its environment. This would mimic as closely as possible an abstracted form of the natural environment in which the pillbug finds itself.

Sow bugs would be considered to have left the habitat when they were at least over the rim of their starting surface. In point of fact no bug who did travel in the direction of the puddle traveled all the way into the water, but those who crossed the rim to cling to the side of the central portion closer to the water itself were judged to have "entered" that portion of the habitat. They would be counted over the course of a half hour at five-minute intervals because previous observation of sowbugs in the wild had indicated that if they weren't able to scurry out of the light in thirty minutes, they were dead.

TOOLS, TECHNOLOGIES, AND MEASURING UNITS: Plexiglas was most appropriate for the problem tested because of its high level of moisture resistance and ability to conform to prevailing room temperature. It would therefore be a neutral presence. Light sources were maintained to minimize temperature disruption.

Of course the design of an experiment is a key to its success because otherwise there is no way to isolate the genuinely relevant data. By providing the sowbugs with a situation where the only difference were moisture and a preference for earth included with it, or a wetter aquatic environment, we could see if their reputation as a crustacean might indicate an atavistic lust for life under the sea.

This particular experiment can be replicated by anyone with the requisite equipment, including the Plexiglas housing, ordinary topsoil, a few dozen sowbugs, and an oubliette.

VARIABLES: Room temperature was maintained as a constant; the variation in temperature of the habitat choices was permitted to remain as it would exist in the wild. Lighting was sufficient to prompt the sowbugs to move but not to alter temperature of the experiment.

THREAT REDUCTION TO INTERNAL VALIDITY. Since this experiment was designed to test preferences for environment based upon moisture levels, the same soil was used for the two separate aquatic trials.

DATA COLLECTION:

The time period for data collection was established based on the relative crawling speed of the sowbug. Conducted under light, which I assumed based on prior observation would motivate them to move, . The experiment was designed for simple computation of percentages by using sowbugs in multiples of ten. Photographs were taken at the time intervals and were counted on the basis of the photograph to minimize errors.

5. EXPERIMENTAL DATA

TIME ELAPSED ENVIRONMENTAL PREFERENCE

Trial 1. SANDY vs. MOIST EARTH

5 minutes 36% 64%

10 minutes 30%…