Pheromones and Behavior

Chemical communication is widely used by crustaceans, for example, in sexual interactions, larval release, and planktonic settlement. However, we know the identity of very few of the molecules involved. In this chapter, I introduce pheromones and contrast them with signature mixtures. Pheromones are molecules that are evolved signals, in defined ratios in the case of multiple component pheromones, which are emitted by an individual and received by a second individual of the same species, in which they cause a specific reaction, for example, a stereotyped behavior or a developmental process. Signature mixtures are variable chemical mixtures (a subset of the molecules in an animal’s chemical profile) learned by other conspecifics and used to recognize an organism as an individual (e.g., lobsters, mammals) or as a member of a particular social group such as a family, clan, or colony (e.g., mammals, desert woodlouse Hemilepistus reaumuri, ants, bees). A key difference between pheromones and signature mixtures is that in all taxa so far investigated it seems that signature mixtures need to be learnt (unlike most pheromones). These signature mixtures may be best thought of as cues. Pheromones evolve frommolecules which give a selective advantage to the receiver and signaler. The evolution of pheromones is facilitated by the combinatorial basis of the olfactory system found in crustaceans and other animals. In crustaceans, some pheromones are also detected by the distributed chemosensory system. Crustaceans have great potential as model organisms for chemical communication research, in particular now that the Daphnia pulex genome has been sequenced. 2.1 Introducing Chemical Ecology Like other animals, crustaceans live in a chemosensory world full of chemical information and signals from conspecifics, prey, and predators (Fig. 2.1). To a human T.D. Wyatt (*) Department of Zoology, The Tinbergen Building, South Parks Road, Oxford, OX1 3PS, UK e-mail: [email protected] T. Breithaupt and M. Thiel (eds.), Chemical Communication in Crustaceans, DOI 10.1007/978-0-387-77101-4_2, # Springer Science+Business Media, LLC 2011 23 it may be strange to think of a world of smells underwater (as we experience smells in air) but chemical communication is widely used by aquatic animals including crustaceans. However, the types of molecules used by crustaceans for communication underwater are likely to be different from those used in terrestrial environments by most insects and mammals. For aquatic animals, solubility of signal molecules is the equivalent of volatility for airborne messages. The division of chemical senses in vertebrates into taste and smell does not work well for crustaceans and other invertebrates. Like vertebrates, crustaceans have olfactory receptor neurons (ORNs) (these are also known as olfactory sensory neurons) with one end exposed to the chemical world outside the animal and the other leading to the brain. These ORNs in the aesthetasc sensilla of crustaceans approximate to vertebrate “olfaction” as the ORNs project to glomeruli in the crustacean olfactory lobe, analogous to the organization of the brain in vertebrates (Caprio and Derby 2008; Derby and Sorensen 2008; Schmidt and Mellon, Chap. 7). Long distance sex pheromones and chemical cues for social interactions tend to be processed by the olfactory/aesthetasc pathways, for example, spiny lobster responses to conspecific urine signals (Horner et al. 2008). However, crustaceans also have “distributed chemoreception” (Schmidt and Mellon, Chap. 7) which goes beyond vertebrate taste. Distributed chemoreceptors are typically packaged with mechanosensors into sensilla over other parts of the body and have a nonglomerular organization in the brain/ganglia. While distributed chemoreception does include the equivalent of taste, with contact chemoreception for food for example, it also includes other chemical senses such as control of antennular grooming and the coordination of mating and copulation (Schmidt and Mellon, Chap. 7). Schmidt and Mellon point out that integrated inputs from sensilla in both the olfactory and distributed chemoreceptor systems may participate together in the control of complex behaviors, including associative odor learning. As you will see in the rest of this book there is already an impressive understanding of crustacean chemical communication – but really we are just at the beginning of the exploration. In the coming years, as demonstrated in the chapters Fig. 2.1 Just as peacock males display their fan tails, the male lobster (left) displays with pheromones in the directed jet he sends toward the female. Drawing by Jorge A. Varela Ramos (Picture inspired by the mouse pheromone “peacock tail” in Penn and Potts 1998) 24 T.D. Wyatt