The velocity of the image expansion on the observer's retina, measured as the angular velocity of the moving borders.Ī neuron that is comparatively much larger than the other neurons in the tissue. The size of an object measured in terms of the angle it subtends on the observer's retina. In the absence of a burrow, the behavioral response to an increasing visual risk is to first freeze, followed by running directly away from the threat and, ultimately, raising the claws and pointing them towards the menace ( Fig. 1B). Neohelice can also be found wandering in areas without burrows. This run ends at the burrow entrance from where, if the level of risk continues to increase, the crab will retreat into its burrow ( Hemmi, 2005 Hemmi and Pfeil, 2010 Fathala and Maldonado, 2011 Hemmi and Tomsic, 2015). However, if the risk to the crab increases because the predator continues to approach, a second strategy consisting of a ‘home run’ is initiated. Upon perceiving an approaching object, the crab's first observable response is to freeze, a strategy that is likely adopted to increase the crab's chances of remaining undetected by the predator, and which also helps to stabilize the image and improve visual information ( Hemmi and Tomsic, 2012). The two species were found to respond to the dummy predator in a similar manner both react to an approaching object when its apparent size (see Glossary) is less than 2 deg, such that it is perceived by one or two ommatidia (see Glossary Hemmi and Pfeil, 2010 Hemmi and Tomsic, 2015). After extensive studies in the laboratory (see below), field studies began to be performed on Neohelice using the same approach ( Fig. 2A). Visually elicited escape behavior has been extensively studied in fiddler crabs in the field, by using moving dummies to simulate predator stimuli (e.g. Similar to fiddler crabs ( Hemmi, 2005), Neohelice crabs protect themselves from predators by digging individual burrows ( Fig. 1A,C) to which they run and where they will hide if they assess a risk to be great enough ( Fathala and Maldonado, 2011). They spend long periods of time out of the water, where they are preyed upon by gulls and other birds ( Fig. 1B,D). Like fiddler crabs, for which the behavioral aspects of predator avoidance have been extensively investigated in the field, Neohelice crabs live in a mud flat environment and form dense populations ( Fig. 1A, Fig. 2A). Neohelice granulata is a robust running grapsid crab, reaching 36 mm across the carapace. Here, we describe similarities and differences in the results obtained between the field and the laboratory, discuss the sources of any differences and highlight the importance of combining the two approaches. Moreover, field studies have led to the discovery of a robust visually guided chasing behavior in Neohelice. In addition, behavioral analyses performed in the natural environment reveal a more complex picture: crabs make use of much more information than is usually available in laboratory studies. This work, in combination with neuroanatomical and electrophysiological studies, has allowed the identification of various giant neurons, the activity of which reflects most essential aspects of the crabs' avoidance performance. Analyses of the behavioral responses to visual stimuli in the laboratory have revealed the main characteristics of the crab's performance, such as the continuous regulation of the speed and direction of the escape run, or the enduring changes in the strength of escape induced by learning and memory. In this Review, we describe current knowledge – acquired through both laboratory and field studies – on the visually guided escape behavior of the crab Neohelice granulata. Studies in the laboratory alone, however, can lead to a biased interpretation of an animal's behavior in its natural environment. Naturally occurring avoidance behaviors, in particular, can be easily and reliably evoked in the laboratory, enabling their neurophysiological investigation. They require fast reactions controlled by comparatively straightforward neural circuits often containing giant neurons, which facilitates their study with electrophysiological techniques. Predator avoidance and prey capture are among the most vital of animal behaviors.
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