Waterworks Archive

Fish Lateral-Line Response is Possible Water Pollution Monitoring System

by Hong Y. Yan
School of Biological Sciences
University of Kentucky

It has been well documented that mining activities could have severe impacts on the water quality of watershed areas surrounding mines. The results of mining impacts can be observed from acid water drainage, sediment in the creek, soil erosion and heavy metals pollution. These types of large-scale deleterious impacts have been reported in east-central Kentucky where strip mining is common. Among all the pollutants, cadmium (Cd) is one of the heavy metal pollutants of major concern, largely due to its nondegradable bioacculumation in the food chains. Cadmium is considered capable of altering aquatic trophic levels for centuries. However, evaluating possible impacts of cadmium pollution has been a major challenge to scientists.

Traditionally, physical and chemical methods have been used to monitor both long-term and short-term cadmium concentration changes around mining sites. In addition, biological assays are also used to evaluate its biological effects on aquatic animals. From published literature, we know that cadmium could cause the following physiological impacts on fish: vertebral alterations (spinal deformities), increased respiratory rates, histopatho-logical effects on gastrointestinal, renal and gill tissues, blood cells abnormalities, ionic concentration changes of plasma, reproductive failure and changes of predator avoidance behavior. Except for short-term, acute lethal concentration studies such as 96-h LC50 (lethal concentration killing 50 percent of the group), sublethal concentration studies generally require long-term (from months to years) exposure to cadmium by fish to monitor the impacts. A fast but highly-sensitive assay method is needed to evaluate physiological impacts of sublethal concentrations of cadmium (or other pollutants) on fish.

With grant support from Kentucky Water Resources Research Institute, a new assay method using electrophysiological responses from lateral line nerves of fathead minnows is being developed to monitor possible impacts of cadmium exposure on fish. The lateral line system of fish is one of major mechanosensory systems used by fish to detect water movements and transient pressure changes in the vicinity of a fish. The receptor cells of the lateral line system are distributed on the surface of scales and inside the lateral line canals (Figure 1). These sensory cells are in constant contact with aquatic media. Therefore, any pollutant existing in the media even at sublethal concentrations could have an immediate impact on the lateral line system and alter its function. A dysfunctioning lateral line system means the affected fish could not perceive vibrational signals around it, especially under low visibility conditions (such as high turbidity or darkness) when visual system are not functioning. The consequences are that fish could not always detect approaching predators or prey, and they could not form schools to evade predators. These conditions would make it difficult for the fish to survive.

The system developed in this study includes first exposing the test fish (a fathead minnow) to a Cd concentration of 450 µg/l for 24 hours. Then the fish is anesthetized and secured in a recording chamber where oxygenated water is constantly passed through the gill surface. A silver wire electrode is then hooked to an exposed lateral line nerve. A miniature shaker is positioned above the caudal region of the fish. Computer programs are used in conjunction with a function generator to create sinusoidal vibrations in the minishaker. The vibrations perceived by the sensory cells of the lateral line system generate electric currents (generally called compound action potentials) that are picked up by the electrode. The signals are then passed through a preamplifier to a computer for data storage and analysis. The schematic diagram of Figure 2 shows the arrangement of the system used in the study.

Typical compound action potentials generated by a healthy fathead minnow (control group) exposed to 80 Hz (cycle/second) vibrational signals are shown in Figure 3. Before the onset of the vibrational signals, the spontaneous firing rate of the lateral line at resting stage is minimal (Figure 3-A). However, with the onset of vibrations (Figure 3-B), the firing rates of lateral line nerves increase. The amplitude of the compound action potentials also increases (Figure 3-A). Action potentials are generated throughout the 400 ms period of stimulation.

When a test fish is exposed to Cd (450 µg/l) for 24 hours (experimental group), the spontaneous firing rate of the lateral line ceases to exist (Figure 3-C. Compare the amplitude scale on this figure with Figure 3-A and note the difference). No action potentials are generated when 400 ms duration vibration signals are given. It is obvious from the examples shown here that measuring electrophysiological changes of the lateral line is a fast and highly-sensitive assay method to measure the effect of cadmium exposure on fish. More importantly, the protocol can be easily applied to other pollutant assay studies.

Last modified: November 28, 1995

Copyright © 1995 University of Kentucky - Kentucky Water Resources Research Institute