RESEARCH ARTICLE Assessing acoustic communication active space in the Lusitanian toadfish Daniel Alves 1, *, M. Clara P. Amorim 2 and Paulo J. Fonseca 1 ABSTRACT The active space of a signal is an important concept in acoustic communication as it has implications for the function and evolution of acoustic signals. However, it remains mostly unknown for fish as it has been measured in only a restricted number of species. We combined physiological and sound propagation approaches to estimate the communication range of the Lusitanian toadfish’s (Halobatrachus didactylus) advertisement sound, the boatwhistle (BW). We recorded BWs at different distances from vocalizing fish in a natural nesting site at ca. 2–3 m depth. We measured the representation of these increasingly attenuated BWs in the auditory pathway through the auditory evoked potential (AEP) technique. These measurements point to a communication range of between 6 and 13 m, depending on the spectral characteristics of the BW. A similar communication range (ca. 8 m) was derived from comparing sound attenuation at selected frequencies with auditory sensitivity. This is one of the few studies to combine auditory measurements with sound propagation to estimate the active space of acoustic signals in fish. We emphasize the need in future studies for estimates of active space to take informational masking into account. KEY WORDS: Information masking, Fish, Communication range, AEP technique, Auditory evoked potential, Boatwhistle INTRODUCTION Acoustic communication is a widespread phenomenon across vertebrates (Bradbury and Vehrencamp, 1998), as well as other taxa (e.g. insects; Hedwig, 2014) and it is used in a great variety of contexts such as advertisement, courtship, spawning, agonistic interactions, competitive feeding or disturbance (Bradbury and Vehrencamp, 1998; Hedwig, 2014). To be effective, acoustic signals produced by the sender must be correctly perceived by the receiver (Bradbury and Vehrencamp, 1998). In some behavioural contexts, such as mate attraction, it is advantageous for the emitter to maximize its communication range, i.e. the area/volume around an individual where communication with conspecifics can occur (Clark et al., 2009). However, in close- range interactions, acoustic signals with decreased active space may also evolve (e.g. Reichard and Anderson, 2015). Independently of sound source characteristics (e.g. amplitude level), the communication range of an acoustic signal will be limited by the environmental sound propagation properties and ambient noise conditions that will act as an ‘acoustic filter’ (Fine and Lenhardt, 1983). Therefore, the effective communication distance has important implications for the evolution and function of acoustically mediated behaviour (Bradbury and Vehrencamp, 1998). This parameter has been studied in a variety of terrestrial animals (e.g. insects – Kostarakos and Römer, 2010; anurans – Kuczynski et al., 2010; birds – Brenowitz, 1982; reptiles – Todd, 2007; mammals – de La Torre and Snowdon, 2002) but poorly addressed in aquatic animals such as fish (e.g. Radford et al., 2015). In general, underwater acoustic communication has received less attention than terrestrial acoustic communication, probably due to technical difficulties. Most studies on the communication range of aquatic animals have been carried out with cetaceans (e.g. Janik, 2000; Sirović et al., 2007; Tervo et al., 2012). Although teleost fish are considered the largest group of vocal vertebrates (Ladich, 2004), communication range estimation in this group is so far restricted to a small number of species in shallow water conditions (e.g. Fine and Lenhardt, 1983; Myrberg et al., 1986; Mann and Lobel, 1997; Lugli and Fine, 2003; Locascio and Mann, 2011; Ghahramani et al., 2014; Holt and Johnston, 2015; Radford et al., 2015). Sound propagation is reduced in shallow waters, where low frequency sounds, such as most fish vocalizations (Amorim, 2006), are strongly attenuated with distance (Bass and Clark, 2003; Mann, 2006). Depending on the species, estimated ranges vary from a few centimetres to tens of metres (Amorim et al., 2015). The Lusitanian toadfish Halobatrachus didactylus (Bloch and Schneider 1801) is a member of the family Batrachoididae that inhabits coastal waters and estuaries (Roux, 1986). It is a benthic species with an unusually rich vocal repertoire (Amorim et al., 2008) that produces sounds in both reproductive and agonistic contexts (dos Santos et al., 2000; Vasconcelos et al., 2010). During the breeding season, males aggregate in nesting areas close to the substrate and produce advertisement calls – the boatwhistle (BW) – to attract mates (Jordão et al., 2012; Vasconcelos et al., 2012). The BW is the most commonly produced acoustic signal in this species throughout the year (Amorim et al., 2006, 2008, 2010). Halobatrachus didactylus has been used in both behavioural (e.g. Vasconcelos et al., 2010; Ramos et al., 2012; Conti et al., 2015) and physiological (e.g. Vasconcelos and Ladich, 2008; Vasconcelos et al., 2011a,b) studies, making it an excellent model species for the assessment of active space of acoustic signals in fish. Here, we aimed to estimate the communication range in the Lusitanian toadfish using complementary physiological and sound propagation approaches. MATERIALS AND METHODS Auditory evoked potential (AEP) technique Test subjects Lusitanian toadfish were collected in the Tagus estuary (Portugal) from trawling by local fishermen during the months of December 2013 to February 2014. After collection, fish were transported to the laboratory at the University of Lisbon (Portugal), where they were kept in aerated 80 l stock tanks equipped with protein skimmers, Received 18 November 2015; Accepted 4 February 2016 1 Departamento de Biologia Animal and cE3c - Centre for Ecology, Evolution and Environmental Changes, Faculdade de Ciências, Universidade de Lisboa, Lisbon 1749-016, Portugal. 2 MARE – Marine and Environmental Sciences Centre, ISPA-Instituto Universitá rio, Lisbon 1149-041, Portugal. *Author for correspondence (dbalves@fc.ul.pt) 1122 © 2016. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2016) 219, 1122-1129 doi:10.1242/jeb.134981 Journal of Experimental Biology