Running Head: NEUROETHOLOGY
Proposed research activity using escape mechanism in crayfish
Bioinvasion has been a major contributor to threat on biodiversity. This is especially so in freshwater where crayfish exist as invaders and threatened native species. Humans have played a large role in introduction of crayfish foreign to habitats with other species of crayfish which causes decline of the native crayfish species. Given that crayfish make up the freshwater largest invertebrates, they play an important role as preys, consumers and disturbance agents. Any changes in their composition will widely impact the freshwater community. According to Capelli (1982) one of the successful species of crayfish to be integrated in other habitats is the Orconectes rusticus which has successfully invaded streams and lakes and replaces other native species. This invasion by O. rusticus that follows displacement of ecologically similar species does not have a relation to the common forces of displacement like parasitism, predation or habitat change (Butler & Stein, 1985). Due to this, researchers have come up with hypotheses on the successful invasion of O. rusticus that include their larger size results to lower predation rates, interspecific shelter competition superiority, different behavior for avoiding predators, higher growth rate, and superiority in food competition interspecifically and interference of reproduction. There is a high probability that a combination of these hypotheses may give the reason why the O. rusticus is a successful invades and displacer of the native crayfish species. O. rusticus invasion has happened over a wide variety of habitats in different physical and biological conditions which signifies that different mechanisms are involved during invasion in different geographical areas. Most evidence used to support the different ways used by O. rusticus is from laboratory experiments that can support only a few conditions like predators and physical conditions (Garvey et al., 1994).
The excitability of the crayfish escape mechanism is not fully predictable due to the fact that it is evoked by a descending inhibition triggered during restraint or feeding called tonic inhibition. Tonic inhibition inhibits lateral giant (LG) neurons postsynaptically which is one form involved in escape mechanism. The inhibition takes place at the LG dendrites which are far from the initiating zone whereas postsynaptic inhibition occurs due to recurrent inhibition which prevents new escape mechanisms from starting while another one is in progress. Tonic inhibition is activated especially when the crayfish is restrained making it hard to bring forth escape maybe to hold until when a flip might succeed in freeing the animal.
The proposed evidence for this study include both laboratory and field experiments to test the hypothesis that predation differences among the crayfish species promotes replacement of native species by O. rusticus. The predation rate could be influenced by their ability to initiate or switch off tonic initiation that enables the larger crayfish to employ an escape mechanism that is thorough than small size crayfish. The smaller crayfish species would be prone to predators than larger ones and that if size is controlled, mortality rates would be similar in crayfish species.
Materials and methods
Laboratory experiments will be used to compare predation rates of native crayfish species Orconectes propinquus, Orconectes obscures and non-native species O. rusticus by smallmouth bass Micropterus dolomieu. Field experiments using tethering will also be used to complement the laboratory experiments. Water depth factor will be included since water depth can influence predation rates. Tethering will be done using monofilament fishing line and cyanoacrylate cement to restrict movement of crayfish. Tethering as a factor that can affect predation will also be tested to determine whether the ability to escape from tethers among the crayfish species is similar to the fact that size determines the rate of predation where smaller species would be hard to escape from predators because they lack strength to do so. To test this cages will be used Thirty trials will be done using all these species in two studies. Predator susceptibility which is caused by the characteristics of species will be tested using crayfish of equal size. This is because smallmouth bass are size specific which can result in different rates of predation. Crayfish to be used in this experiment will be obtained from local streams and would be fed for the entire duration of experiments with lettuce and fish food. Smallmouth bass would be fed with crayfish until a day prior to the trial. Control experiments where only crayfish will be present will also be done.
Another setting to determine if the size of O. rusticus in comparison to O. propinquus and O. obscures influences the tonic inhibition will be done using two kinds of preparations, intact and semi intact preparations. Intact preparation will be used to describe tonic initiation at behavioral and network level where animals will be immobilized with rubber bands while in water and the recording and stimulation electrodes will be used to identify LG action. Semi intact preparation will involve cutting the crayfish to expose the ganglions and the LG. Stimulation of the LG will be via platinum electrodes which will be used to induce recurrent inhibition and the current pulses measured in a circuit.
It is expected that all crayfish in control experiments will be present to show that any crayfish found missing during the experiments will be accounted as having been eaten by smallmouth bass. It is also expected that predation rate for the O. propinquus and O. obscures will be higher than that of O. rusticus by smallmouth bass. Crayfish eaten by smallmouth bass should be of smaller size than those that survive. O.rusticus crayfish sizes are higher than that of native species where smaller sizes will be vulnerable to predation. In contrast, mortality rate of the native crayfish species is expected to be higher than that of the non-native. Tethering raises predation rate because the results should indicate that the smallmouth bass consumed more crayfish when they were tethered in comparison to when they were free. Also the smaller the crayfish, the better it is to escape from predators when in a cage because it can navigate through the small wires of the cage as compared to larger sizes of crayfish.
In relation to size, experiments done on other water species indicate that smaller sizes of the species are more likely to be predated upon than bigger ones. This should also be true in this experiment to indicate that O. rusticus is able to escape predation in non-native habitats because it is larger in size than the native crayfish species. In tonic inhibition studies it is expected that restraining induces tonic inhibition that affects transmission in LG which after some time induces the escape mechanism. Reduced transmission to LG is higher in O. propinquus and O. obscures in comparison with O. rusticus to indicate that the bigger the crayfish the weaker the tonic inhibition and the stronger the LG transmission that initiates escape. This is because tonic inhibition inhibits the LG postsynaptically and this is effective in smaller size crayfish because tonic inhibition is able to suppress weaker stimuli in smaller crayfish as compared to stronger stimuli in larger crayfish.
However, taking size into account such that the sizes of both native and non-native species are similar, predation rate is equal because the smallmouth bass is able to predate on smaller size crayfish despite the type of species. The behavioral patterns of O. rusticus could also play a huge role in escaping predation because the predators in non-native habitats are not used to its escape mechanisms and so they tend to target those species they are familiar with. Behavioral patterns could also account as to why the mortality rate of the native species is higher than of non-native species. This is because native species are adapted to their habitat and they know how to escape from the predator and also how to survive in that environment while non-native species are not used to using their escape mechanism in that kind of environment thus they rarely survive for long. When tethering is used to test susceptibility of prey to predators in laboratory and field experiments, results are not expected to be similar. This is due to the fact that in laboratory setting the experiment is measuring survival rate of the crayfish against one type of predator while in the field experiment the survival rate of crayfish in a sea of different predators is being measured. Survival rate in the field experiment is thus lower than in laboratory experiment. In some experiments it is indicated that tethered crayfish have a higher surviving rate than those not tethered ones because smallmouth bass does not eat tethered crayfish. Research done to compare how water depth plays a role in predation indicates that in deep areas, size specificity is highly applicable than in shallow waters when confined. In shallow waters, predation is because of terrestrial predators.
Potential pitfalls in this experiment could result from tethering of crayfish which could increase the vulnerability of crayfish to other predators that normally would not be able to capture them if they are free while some predators do not eat tethered crayfish maybe because they consider immobilized crayfish dead or defective. When tethered during field experiments it would be difficult to determine what kind of predator captured the crayfish because they will be many. In some cases the predator may be unable to capture any crayfish and die as a result of starvation. To eliminate this, the smallmouth bass would have to be removed from the experiment and given food so that they may not starve. Some crayfish may also die during the experiment but not due to predation which may alter the results. It is important that in laboratory experiments, adequate shelter is provided for the crayfish and smallmouth bass for it to portray a real habitat because if few shelters are present, results will be compromised because it will be easy to spot the prey or predator.
This study will contribute to future experiments where research can be conducted using different predators to eliminate the factor of species specificity. It will also enhance understanding larger size crayfish are able to escape predation in comparison to smaller species in relation to inhibition of their LG. Researchers will also be able to come up with new hypotheses as to why O. rusticus continues to invade and displace native crayfish species.
Butler . M. J and Stein, R.A (1985) An analysis of the mechanisms governing species
replacements in crayfish, Oecologia, 66, 168-177.
Capelli, G. M (1982). Displacement of northern Wisconsin crayfish by Orconectes rusticus
(Girard). Limnology and Oceanography. 27, 741-745.
Garvey, J. E, Stein R. A and Thomas H. M (1994). Assessing how fish predation and
interspecific prey competition influence a crayfish assemblage. Ecology. 75, 532-547