Introduction to Pheromones and Related Chemical Cues in Fishes

Chapter 1 Introduction to Pheromones and Related Chemical Cues in Fishes Peter W. Sorensen University of Minnesota, St. Paul, USA 1.1 CHEMIcAL INFORMATION TRANSFER IN FISH Information transfer between fishes, the largest and most diverse group of vertebrates, has long been both of practical importance and a source of wonder given the evolutionary, ecological, and economic importance of this group. Chemicals play an especially significant role in this process, presumably because they function well in vast dark spaces, can encode a great deal of information, are readily soluble, and are inherently “honest.” This book specifically addresses how and why chemical information is transferred between fishes of the same species. First I define some basic terminology to promote clarity and then I introduce some other terms, types of chemical cues, and principals along with the chapters which discuss them. Information transfer between different species is not explicitly addressed except how it might occur as part of transfer within a species. This section intro- duces these terms and issues, and then where more information on them can be found in the book. 1.2 TERMINOLOGY 1.2.1 Overview Because the terminology used to describe conspecific cues has been used in many ways since the term “pheromone” was first coined 50 years ago (Karlson and Luscher, 1959), I suggest and define some terms in this introductory section to provide clarity. Definitions were chosen for practical reasons and to be consistent with those used by researchers outside the world of fishes. Emphasis is placed on recent work by Tristram Wyatt (2003, 2010). Authors were asked to consider the terminology suggested herein, but not necessarilyCOPYRIGHTED to use it if their opinions differed. MATERIAL Information transfer between members of different species (kariomones) is not directly addressed in this book. 1.2.2 Pheromones Following Tristram Wyatt (2010), a “pheromone” may be defined as: “molecules that are evolved cues which elicit a specific reaction, for example a stereotyped behavior and/or development process in a conspecific.” This definition is closely based on the original definition by Karlson and Luscher (1959). Key elements are that pheromones are “evolved cues” (stimuli whose production is in some Fish Pheromones and Related Cues, First Edition. Edited by Peter W. Sorensen and Brian D. Wisenden. © 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc. 1 0002208140.indd 1 12/4/2014 3:17:47 PM 2 Fish Pheromones and Related Cues way adaptive), “species-wide” (used by all members of the species), and that some type of innate recognition is implied. Nevertheless, responses to pheromones can be conditional on context, and/or internal state. Similarly, although pheromone composition (pheromones may be single com- pounds or mixtures) may vary slightly among individuals, this variation is expected to reflect physiological state (e.g., dominance) and not individual eccentricities or identity. In other words, I believe that the information contained in pheromones should be equally relevant to all individuals of the species (e.g. ‘shared’ by all members of that species). This type of rigidity appears to be associated with some type of specialized neural mechanisms. In contrast, organismal chemical cues that vary between individuals of the same species (either because of their chemical structures and/or because they are learned) should not considered to be pheromones but ‘related conspecific cues.’ Identification of pheromones in fish is difficult and requires isolation of the released chemical(s) and proof that it/they elicit a specific adaptive response via some type of innate (neural) mechanism. This is not to say that learning cannot be involved but it should presumably be highly prescribed so that all members of the species recognize the same cues. Indeed, only a handful of fish pheromones meet these criteria these species include goldfish (Carassius auratus) hormonal sex pheromones, sea lamprey (Petromyzon marinus) migratory and sex pheromones, the Atlantic salmon (Salmo salar) sex pheromone, and perhaps the reproductive cues used by the African catfish (Clarias gariepinus). Nevertheless, there is a great deal of circumstantial evidence that pheromones are commonly produced and used by most, if not all, fishes. Their functions are diverse and include conspecific recognition, recognition of reproductive state, and the presence of injured conspecifics. These functions will be reviewed in Section 1.3; but first, a few terms that are commonly used to describe the pheromones in fish and other species are defined. Primer Pheromone A priming pheromone is a conspecific chemical cue that drives an adaptive developmental or otherwise wholly physiological response in an exposed conspecific. All members of a species in the same physiological state should typically be similarly affected. Examples include hormonal cues that drive endocrine changes in exposed conspecifics and alarm cues that change growth characteristics of exposed conspecifics. Releaser Pheromone A releasing pheromone is a conspecific chemical cue that drives a rapid, adaptive, and innate behavioral response in a conspecific. All members of a species in the same physiological state should typically be similarly affected. Examples include hormonal metabolites that drive sexual arousal. Pheromones may have both releasing and priming effects. Pheromone Mixture Pheromones may comprise mixtures of chemical cues that may act on their own or synergistically. A “blend” is very specific type of mixture that requires that multiple components be present in very specific ratios for the mixture to have activity. Although commonly described in insects, no examples appear to exist in fish. By contrast, a “complex” is mixture of pheromonal components that can assume different functions depending on mixture composition. Ratios are not necessarily of primary importance but the overall composition is. Recent studies suggest that complexes in goldfish (see chapter 2) may include nonhor- monal components that encode species identity and hormonal components that encode sexual condition. 1.2.3 Signature Mixtures Fish, like all other vertebrates, release and learn to recognize conspecific chemicals for various purposes such as individual and kin recognition. Following Wyatt (2010), the term “signature mixture” is used for non-phermonal but related cues that can be defined as “variable chemical mixtures which are released by organisms and learned by other conspecifics to recognize individual or a member of some type of 0002208140.indd 2 12/4/2014 3:17:47 PM Introduction to Pheromones and Related Chemical Cues in Fishes 3 social group.” The term “signature” implies some level of individuality; unlike pheromones, these cues are not anonymous and they are learned. The manner in which they are learned need not be prescribed. Further, their composition is typically complex and variable. In fishes, as in mammals, these cues appear to be commonly used to mediate recognition of individuals within social hierarchies or perhaps other aspect of special value such that they have been subject to recent stress. In mammals, these cues often appear to be genetically based on the major histocompatability complex (MHC) that codes genetic identity, but this possibility has not been fully resolved in fishes. A signature mixture is expected to be highly context dependent and may change with diet and other environmental factors. Recognition appears to require combinatorial responses of broad elements of the olfactory system. Signature mixtures may be found together with pheromones. Identification of a signature mixture requires isolation of the released chemical(s) and proof that it elicits an adaptive response that is learned. Several examples have been described in fish. For example, Bryant and Atema (1987) show that diet influences production of odors associated with social hierarchy in the bullhead catfish, Ictalurus nebulosus, and that amino acids change in urine. Fish have also been show to readily learn to recognize the odors of conspecifics that have been attacked (Chivers and Smith, 1998). These functions will be reviewed in Section 1.3. Many ornamental odors such as those reviewed by Lynda Corkum and Karen Cogliati in Chapter 4 may fit this definition. 1.2.4 Other Definitions Relevant to this Book A few other definitions associated with production and detection of conspecific chemical cues and signals are defined to promote clarity. Some of these definitions have also been the subject of consid- erable controversy which is not discussed here as they are defined largely for operational reasons. Cues refer to any stimulus that elicits a sensory response in an animal’s sensory system. Signals are a prescribed set of cue(s) whose chemical identity has been influenced by evolutionary processes and may thus be considered to be specialized. Communication may be defined as the exchange of adaptive information (e.g. signals) between two conspecifics. The olfactory sense is the chemosensory component of the cranial nerve 1 (i.e., taste and common chemical sense are not included). It is also known as the sense of smell. An odorant is a molecule that binds with olfactory receptor(s) and stimulates the olfactory sense. An odor is an identifiable suite of odorant(s) that an animal’s olfactory system can discriminate. Fish are chordates with gills and fins that spend most or all of their live in water. (This book will address jawless, cartilaginous, and boney fishes.) 1.3 FUNCTIONS SERVED BY PHEROMONES AND RELATED CUES 1.3.1 Overview Pheromones and related conspecific cues are known and defined by their biological function(s). Although these functions are diverse, they can be placed into five broad categories as outlined below. Some of these categories are not mutually exclusive, and presumably others may still await discovery. 1.3.2 Alarm Cues As with other organisms, fish have evolved to recognize and respond to stimuli associated with the risk of predation, of which chemicals released by injured conspecifics are one (Chivers and Smith, 1998; Brown, 2003; Wisenden, Volbrecht, and Brown, 2004; Ferrari et al., 2010).

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