Individual neurons can be multifunctional within a species. Some of those functions are shared across species, whereas others are not. The pattern of use and reuse of homologous neurons in various forms of swimming and other behaviors further demonstrates that the composition of neural circuits influences the evolution of behaviors.
Behavior and neural mechanisms can be considered to represent two different levels of biological organization (Lauder, 1986, 1994; Striedter and Northcutt, 1991; Rendall and Di Fiore, 2007). Nevertheless, the evolution of behavior and the evolution of neural circuits underlying behavior are intertwined. For example, it has been suggested that the properties of neural circuits affect the evolvability of behavior); the evolution of particular behaviors could be constrained or promoted by the organization of neural circuits (Airey et al., 2000; Bendesky and Bargmann, 2011; Carlson et al., 2011; Katz, 2011; Yamamoto and Vernier, 2011). Darwin and the early ethologists recognized that behaviors, like anatomical features, are heritable characters that are amenable to a phylogenetic approach (Darwin, 1876; Whitman, 1899; Heinroth, 1911; Lorenz, 1981). The use of behavioral traits to determine phylogenies has been validated several times (Wenzel, 1992; De Queiroz and Wimberger, 1993; Proctor, 1996; Stuart et al., 2002), and the historical debates about homology and homoplasy of behavior have been thoroughly reviewed (Lauder, 1986, 1994; Wenzel, 1992; Foster et al., 1996; Proctor, 1996; Rendall and Di Fiore, 2007). Examining the neural bases for independently evolved (i.e., homoplastic) behaviors within a clade could provide insight into fundamental aspects of neural circuit organization. However, it is difficult enough to determine the neural basis for behavior in one species. Doing this in several species with quantifiable behaviors is even more challenging.
Studies of the neural bases of swimming behaviors in the Nudipleura (Mollusca, Gastropoda, Opisthobranchia) offer such a possibility. These sea slugs exhibit well differentiated categories of swimming behaviors, and their nervous systems have large individually identifiable neurons, allowing the neural circuitry underlying the swimming behaviors to be determined with cellular precision.
Here we will summarize what is known about the phylogeny of Nudipleura, their swimming behaviors, and the neural circuits underlying swimming. We will also provide data comparing the roles of homologous neurons. We find that neural circuits underlying the behaviors of the same category are composed of overlapping sets of neurons even if they most likely evolved independently. In contrast, neural circuits underlying categorically distinct behaviors use nonoverlapping sets of neurons. Furthermore,