All stoneflies were on the rock substrates and 

 were alive after 96 hr with no abnormal behavior 

 patterns, abdominal distention, or signs of stress; 

 only one Acroneuria was observed with a few bub- 

 bles on it. All were apparently unaffected after 

 8 days, but on day 11 one Acroneuria was at the 

 surface and was unable to get to the bottom because 

 of gas bubbles. Its body was partially distended and 

 had external bubbles among the gills. On the 12th 

 day one Acroneuria in Tank 1b and one in Tank 1c 

 were at the water surface and their bodies were 

 partially distended. Bubbles buoyed them to the sur- 

 face and they became trapped in the aquaria cor- 

 ners. This would not have occurred in streams 

 where substrate would be more suitable than the 

 slick glass surface. However, they would be more 

 vulnerable to predators in nature as they would 

 float to the surface more frequently because of their 

 additional buoyancy. All stoneflies were alive and 

 apparently unharmed after 12 days' exposure at 

 125% saturation. 



Test 9 The stonefly species Acroneuria californica 

 was tested at 135%, 120%, 115%, and 100% total dis- 

 solved gas saturation. Ten late instar nymphs were 

 tested for 11 days at 12°C at each gas level. 



No effects on stoneflies were observed after 

 96 hr at 120%, 115%, and 100% saturation. A few 

 bubbles were observed on the stoneflies, but caused 

 no difficulties. At 96 hr the gas in Tank 1, 120%, 

 was reset to 135% and left at that level for the 

 remaining 7 days of the test. One stonefly molted 

 successfully at 100 hr when the gas (130%) was in 

 transition between 120% and 135%. 



The stoneflies appeared unaffected after 48 hr 

 at 135%. However, after 120 hr of exposure, two 

 Acroneuria in cage A had distended bodies but were 

 active when disturbed. Two stoneflies in cage B 

 also had distended bodies and one was having dif- 

 ficulty moving normally. Controls were normal and 

 showed no signs of GBD. Stoneflies were held at 

 115% for 10 days without any observable effect. 

 Those at 135% were more sluggish than the controls 

 and those at 115% and had bubbles adhering to 

 their external body wall. Two stoneflies in cage A, 

 135%, were full of air bubbles; the bubbles were 

 concentrated in the body fluids of the thorax at the 

 base of the legs and gills. Their bodies were fully 

 distended, appearing expanded like balloons. A 

 small midge living on one stonefly was filled with 

 air, also looking like a tiny balloon. Two stoneflies 

 in cage B, 135%, also had bubbles in the body fluids, 

 and their bodies were distended to the capacity of 

 the body wall. Bubbles were visible through the 

 body wall at the base of the legs, the gills, and also 

 scattered throughout the body fluids, such as in the 

 mandibles, etc. 



The stoneflies in 135% were removed from the 

 supersaturated water after 7 days' exposure and 

 placed in control water. After 4 hr no bubbles could 

 be found in the body fluids and the insects were no 

 longer distended, although they were still somewhat 

 sluggish. They were able to recover from short 

 periods of relatively high levels of supersaturated 

 water. Thus stoneflies are much less susceptible to 

 gas-supersaturated water than are fish, especially 

 salmon and trout, but the floating and unnatural 

 buoyancy may be important in increasing drift and 

 predation. 



DISCUSSION 



Insects and crayfish were more tolerant of gas 

 supersaturated water than any of several fish spe- 

 cies tested (Nebeker, unpublished data). Daphnia, 

 although able to withstand short exposures, were 

 unable to avoid the problem of food blockage by air 

 in the gut and died at levels similar to those that 

 affected young salmon. Only one instance was 

 observed where an air bubble was seen in the body 

 fluid of Daphnia, and the heart was never observed 

 to be impaired by emboli. The circulatory system of 

 Daphnia is so simple that the problem of capillary 

 blood-vessel blockage by air, crucial to fish, was 

 nonexistent. The open circulatory systems of the 

 insects and crayfish, though more complex than 

 that of Daphnia, are relatively simple compared to 

 fish and appeared to be the main reason for the 

 greater tolerance of invertebrates to supersatura- 

 tion. The external exoskeleton apparently prevents 

 much of the surface injury and secondary infections 

 that are common to fish exposed to supersaturated 

 water (Nebeker and Brett, 1975). 



The few reports of GBD in invertebrates that 

 have been found are generally incidental observa- 

 tions of bubbles on or in the tissues, and little or no 

 quantitative gas data are given. Gorham (1901) 

 reported that scallops, hydroids, and squids showed 

 signs of GBD. Evans and Walder (1969) used the 

 shrimp Crangon crangon to study bubble formation 

 under decompression because its transparent exo- 

 skeleton allowed any bubbles formed to be immedi- 

 ately seen. The shrimp, when subjected to 400 

 kg/cm 2 for 10 min and then removed, were not as 

 active as before exposure but rapidly recovered 

 without apparent harm. The stoneflies in the pres- 

 ent study responded similarly, although exposure 

 conditions and purposes were quite different. 

 Hughes (1968) reported gas bubble disease in lob- 

 sters when exposed to water supersaturated by air 

 leaking into the hatchery water supply, but no gas 

 levels were given. The occurrence of GBD in three 

 species of bivalve molluscs was described by 

 Malouf et al. (1972), but dissolved gas levels again 

 were not given. Massive blisters were formed on the 



Effects on Freshwater Invertebrates 63 



