Adaptations in the ventilation process

in Larval stoneflies (Acroneuria sp.)

Mike White

Biology Department, St. Lawrence University, Canton, New York 13617

Introduction

Respiration is the process in which an organism supplies oxygen to its tissues and removes carbon dioxide. For respiration to occur, an organism must be able to obtain oxygen from the surrounding environment. Oxygen must come in contact with the respiratory surfaces for gas exchange to occur between the organism and the environment. The mechanism which assures a supply of oxygen to the respiratory surfaces is called ventilation.

Aquatic organisms must derive their oxygen from an oxygen poor medium. The oxygen concentration of water is significantly less then the concentration in air of a terrestrial environment. The oxygen concentration in water varies with temperature and the turbidity of the medium. Colder temperatures and increased turbidity allow for a higher oxygen concentration present in the water. Streams generally have high dissolved oxygen (DO) concentrations. The constant motion of the water and the generally colder temperature of streams cause more oxygen to dissolve in the water.

Larval stoneflies (Acroneuria sp.) are aquatic and are commonly found in lotic environments (streams) clinging to rocks in areas with significant water currents. The current is important for the stonefly larvae because it acts to ventilate the gills by continuously bringing fresh water over them. Under ordinary circumstances, the stream current alone works to ventilate the organism. However, it has been observed that when placed in water of varying DO concentrations or with little current, the stonefly has demonstrated a behavior that resembles a pushup. This is thought to be a behavioral adaptation which aides in the ventilation process. The motion of the body during the pushups creates a current which supplies the gills with fresh water that contains a higher DO level. Left uncirculated, the oxygen levels in the water surrounding the organism will be depleted.

The goal of this lab was to determine if DO concentration had an effect on the pushup rate of the stonefly. We hypothesized that as DO concentration decreased, pushup rates of the organism would increase.

Materials and Methods

A beaker was prepared with 800 mL of 16°C water that was taken from the Grasse River near Canton, NY. The temperature of this beaker was maintained at 16°C by placing it in a larger beaker which contained a water and ice mixture.

We placed one stonefly into the beaker of water along with a small stone for the organism to attach to. The stonefly was also taken from the Grasse River near Canton. The fly was allowed approximately five minutes to acclimate to the new environment before the DO concentration was manipulated.

The DO concentration was first raised to 8.7 ppm with an aerator which fed air through a small hose into the water. After a two minute acclimation period, the number of pushups the organism did was counted over a five minute period. Next, the DO level was lowered using nitrogen gas. A small hose fed the nitrogen gas from a gas cylinder to the water until the DO level reached 5 ppm. The same acclimation and counting periods were followed. Finally the DO was lowered once more with nitrogen gas to a level of 3 ppm. The organism was allowed to acclimate and the number of pushups were counted over a five minute period. A DO meter was used to determine DO concentrations.

Five other research groups involved in this project were responsible for running the experiment each with three different DO levels. This was done to get pushup rates for a wide variety of DO concentrations between approximately 1 ppm and 9 ppm. These groups followed the same techniques described above. Each group used a separate stonefly. In all, 18 pushup rates where counted at 17 different DO levels, one being a duplicate.

Results

Pushup rates were determined at 17 different DO levels by six groups of researchers. The number of pushups performed by the stonefly organism over a five minute period varied from a low of six (DO: 8.5 ppm) to a high of 383 (DO: 4 ppm) (Table 1). There is some correlation between DO levels and pushups (regression = -.284) but this relationship was not significant (correlation test; p = .1992).

Table 1. Pushup rates for stonefly larvae at varying dissolved oxygen levels.

The number of pushups were counted over a five minute period.

The pushup rates for individual stoneflies varied depending mostly on which three DO levels it was subjected to. Stonefly 1 was tested at 1.9, 5.5 and 8.5 ppm. Stonefly 2 at 1, 4, and 7.1 ppm. Stonefly 3 at 6, 8, and 9 ppm. Stonefly 4 at 1.02, 3.09 and 7.07 ppm. Stonefly 5 at 2.4, 5.3 and 8.5 ppm. Stonefly 6 at 3,5,and 8.7 ppm.

Figure 1. Pushup rate vs. DO levels (ppm) for each individual stonefly. Trendlines show the linear trend of pushup rates compared with DO levels. Each stonefly was tested at diffferent DO levels so the graphs do not have common scales.

Discussion

When looking at the data for all of the stoneflies as one group (Table 1) there appears no overall trend toward decreasing pushup rates with rising DO levels. This is most likely linked to differences in the individuality of the organisms (body size, morphology, metabolism etc.) and their unique response to the DO level. For this reason the data for each stonefly was graphed on a separate panel.

All of the stoneflies but number 2 and 6 show a similar relationship between DO level and pushups: as DO levels rise, the number of pushups the organisms do go down (see Fig 1). Stonefly number 2 showed a very low pushup rate at the lowest DO level. This probably occurred because the DO level was lowered too rapidly to such an extreme level which seemed to be too much of a shock for the organism. The organism appeared to pass out at this level and was only revived when the DO was raised. Even after acclimation to the 1 ppm DO level, the stonefly did not seem to fully recover giving us a very low reading at the 1 ppm DO level. Stonefly number 6 had a higher pushup rate at 8.7 ppm than it did at 5 ppm. There is no obvious reason why this was true unless the DO reading was inaccurate or the organism was shocked by the changing DO concentrations.

Linear trendlines were added to each of the six panels of Figure 1. Linear trendlines were used because they most accurately depicted the average of the points. Trendlines for stoneflies 1,3,4 and 5 support our original hypothesis that the number of pushups that a stonefly organism does increases as the DO level of the water decreases (inverse relationship). Thus it appears that this data also supports the idea that pushups act to bring fresh water into contact with the gills (ventilation). This seems to be in contrast to results of similar experiments carried out in years past. According to Baldwin (pers. comm.) previous data has shown that there is a bell shape curve relationship between pushup rate and DO level instead of an inverse relationship represented in our data.

The regression between pushup rate and DO concentrations was calculated (r = -.284) for all of the stoneflies together. The regression for each individual stonefly was not calculated because with only three degrees of freedom, I did not feel that it would be an accurate measurement.

This lab experiment would most likely have more conclusive data if multiple stoneflies had been subjected to more DO levels. For example if 30 stoneflies were tested at each whole number DO increments between 1 and 9 ppm then a more accurate trend would be seen. Additionally, using this many stoneflies will allow for more scrutiny of the results by various significance test i.e.. correlation test. Another important change that would benefit the accuracy of the experiment is the rate at which the DO levels are changed. We manipulated the DO concentration very rapidly which most likely stressed the organisms. With a more gradual change of DO levels the animal would display a more natural behavior. A final point that needs to be discussed is the control. In this experiment we did not have a control to compare our results to. A control could be set up similar to the other beakers in the experiment, with the same water volume and temperature. A stonefly and small stone would be placed in the beaker and left undisturbed for a period of time. Then, the pushup rate of this organism could be counted for the five minute period to determine if the stonefly exhibits the pushup behavior if the DO levels are not manipulated.