Sensory ecology acts as the interface between processes occurring within organisms and those occurring between organisms and their environment (Weissburg 2005). Fishes have a variety of sensory receptors that enable them to glean information from their surroundings (Atema et al. 1988). Among these receptors, the inner ear is associated with balance and sound detection (Popper and Fay 1993, Popper and Lu 2000). Usually, fishes are classified as hearing generalists if they can detect sound frequencies no greater than 1 to 1.5 kHz; they are classified as hearing specialists if they can detect sound frequencies greater than 1.5 kHz (Popper et al. 2003). Morphologically, the inner ear of teleostean fishes is essentially formed by three semicircular canals and otolithic organs (sacculus, utriculus and lagena), within which are located the otoliths (sagitta, lapillus and asteriscus, respectively) (Assis 2003, 2005, Cermeño et al. 2006). The otoliths are acellular concretions of calcium carbonate and other inorganic salts developing over a protein matrix (Carlström 1963, Blacker 1969, Degens et al. 1969) and in close association with the sensorial macula (Platt and Popper 1981, Lychakov and Rebane 2000, Schulz-Mirbach et al. 2011). The otoliths, especially the sagittae, play an important role in inner ear functions (Platt and Popper 1981, Popper and Fay 1993, Popper and Lu 2000). Previous studies have indicated that the size of the sagittae is an adaptive factor associated with sensitivity to sound (Myrberg 1980, Montgomery and Pankhurst 1997, Paxton 2000, Cruz and Lombarte 2004). Fishes with large otoliths produce sounds and show highly developed intraspecific acoustic communication (Luczkovich et al. 1999, Holt 2002). These characteristics enable them to live in coastal and deep environments where visual and light communications are less important (Deng et al. 2011, 2013). Moreover, it has been reported that females can use the auditory sense to detect and locate vocalizing males during the breeding season and can change their hearing sensitivity depending on their reproductive status (e.g. Winn 1967, Sisneros and Bass 2003).

Many fishes vary morphologically among habitats. The variations depend on hydrostatic conditions, visibility, intraspecific competition, buoyancy and predation (Robinson and Wilson 1994, Jonsson and Jonsson 2001). Ecomorphology tries to understand how the ecology and evolutionary processes of an organism are related to its morphology (Luczkovich et al. 1995, Wainwright and Bellwood 2002). Most ecomorphological studies are focused on feeding mechanisms (Wainwright et al. 2001, Collar and Wainwright 2009) and locomotion patterns (Robinson and Wilson 1994, Pakkasmaa and Piironen 2000) because these factors may play a role in shaping the patterns of abundance and habitat distribution in fishes (Mittelbach 1984, Wainwright 1996). However, this scientific discipline has also been applied in otolithology because certain characteristics of otoliths (e.g. sulcus area, depth of the sulcus, sulcus area:otolith area ratio or shape) vary according to environmental, ontogenetic, phylogenetic and ecological factors (e.g. Nolf 1985, Lombarte 1992, Lombarte and Lleonart 1993, Paxton 2000, Gauldie and Crampton 2002, Volpedo and Echeverria 2003, Lombarte and Cruz 2007, Tuset et al. 2010, Reichenbacher et al. 2007, Lombarte et al. 2010, Teimori et al. 2012). However, it is not known how otolith shape variability affects hearing ability (Popper and Lu 2000, Popper et al. 2005).

The snappers (Lutjanidae) are a group of circumtropical fishes comprising 23 genera and 123 species (Froese and Pauly 2011). Twelve species of snappers have been identified along the Iranian coasts of the Persian Gulf and the Oman Sea (Assadi and Dehgani 1997, Valinassab et al. 2010). Ecologically, snappers play an important role in near-shore systems, including mangroves, seagrass beds and freshwater streams, and in open-water habitats, inside or around reefs (Aiken 1993, Appeldoorn and Meyers 1993, Cervigón 1993, Baisre 2000, Claro et al. 2001). These habitats play different roles in development and life history by serving as daytime refuges, feeding nurseries and/or nesting areas for many species, including snappers. They also offer pre-recruits and juveniles abundant food resources, less competition with adults and less predation (Druzhinin 1970, Thayer and Chester 1989, Nagelkerken et al. 2001, Cocheret et al. 2003). Recently, Sadighzadeh et al. (2012) demonstrated that otolith shape descriptors and morphometrics are useful for discriminating among Lutjanus species in the Persian Gulf. In this study, a novel methodology for analysing otoliths based on outline sections is developed.

The S:O ratio and otolith size are related to the hearing capabilities of marine fishes (Gauldie 1988, Montgomery and Pankhurst 1997) and ecological factors such as depth distribution, fish mobility and differences in food and spatial niches (Lombarte 1992, Aguirre and Lombarte 1999, Tuset et al. 2010). Our results stressed the relevance of the sagittal otolith characteristics to the ecomorphological characteristics, showing otolith shape patterns associated with functional and ecological factors.

Several species groups of snappers are recognized on the basis of morphology and external colouration, e.g. ‘blue-lined’, ‘black spot’ complex, ‘yellow-lined’ or ‘red-lined’. These groups are congruent with phylogenetic evolution (Miller and Cribb 2007). The fishes living in shallower water have acquired a tendency to be yellowish with stripes and form aggregations to avoid large predators. They also have larger eyes and bright colour patterns favouring visual communication. The otoliths are small, most likely to avoid the background noise produced by rough seas (Paxton 2000, Volpedo and Echeverria 2003, Cruz and Lombarte 2004). In contrast, species inhabiting deeper or dimly illuminated waters have a darker colouration. Many are solitary, exhibit territorial behaviour, and possess larger otoliths (Volpedo and Echeverria 2003, Cruz and Lombarte 2004, Lombarte et al. 2010). This ecological pattern was clearly noted in the species studied, illustrating the relationship of morphology and external colouration vs. otolith size. Thus, the snappers of the ‘black spot’ complex and the ‘yellow-lined’ group (L. ehrenbergii, L. fulviflamma, L. lutjanus and L. russellii), which inhabit shallow waters (Druzhinin 1970, Kuiter and Tonozuka 2001), showed the highest S:O ratio and the smallest otolith size. The clade containing the ‘red-lined’ and ‘blue-lined’ snappers (L. argentimaculatus, L. erythropterus, L. malabaricus, L. lemniscatus, and L. rivulatus), which live in deeper or dimly illuminated waters and have a dark colouration (Allen 1985), showed the lowest S:O ratio and highest otolith size. L. johnii has characteristics common to both groups. Although it should have been closer to the ‘black spot’ species complex, it is genetically closer to L. erythropterus (Miller and Cribb 2007).

Species inhabiting environments with a limited visual field can increase their hearing capabilities (Lombarte and Fortuño 1992, Deng et al. 2013). The development of the ostial area of the sulcus acusticus region of the sagittal otolith is correlated with an increase in the proportion of horizontally oriented sensory hair cells (Popper and Coombs 1982, Ramcharitar et al. 2006), which may help to detect directional acoustic stimuli and to locate prey (Popper and Fay 1993). Moreover, the species that use environments with dim illumination tend to show increases in otolith size and adopt non-visual communication (acoustic or chemical) (Paxton 2000, Cruz and Lombarte 2004). Our study showed a relationship between the variations in the antero-dorsal area of the sagittal otolith of snappers and fish behaviour. A flattened shape was observed in L. rivulatus, which forages during the day, whereas extensive development of the antero-dorsal area was found in L. argentimaculatus, a species that is active at night (Martínez-Andrade 2003). Accordingly, we infer that L. argentimaculatus, L. erythropterus, L. malabaricus and L. lemniscatus (all ‘red-lined’) should be adapted to dim light conditions or nocturnal activity; L. ehrenbergii, L. fulviflamma (‘black spot’ complex), L. lutjanus (‘yellow-lined’) and L. rivulatus (‘blue-lined’) should be more active during the day; whereas L. russellii and L. johnii (‘black spot’ complex) should show a nocturnal-diurnal dichotomy. Thus, the diel activity rhythm facilitates coexistence between competitors extending beyond the effects of adaptation to different behavioral strategies and feeding habitats (Colmenero et al. 2010, Fox and Bellwood 2011, Azzurro et al. 2013).

The results presented here demonstrate that wavelet analysis is a very useful mathematical procedure for ecomorphological studies in addition to its use in species discrimination (Parisi-Baradad et al. 2005, 2010, Sadighzadeh et al. 2012). The identification of otolith zones with high morphological variability implies that information on shape of the whole otolith may not be necessary for the identification of stocks or species or for ontogenetic or ecomorphological studies. These findings constitute a novel approach to species discrimination. Finally, discrimination of the activity of fishes will be essential for a better understanding of ecosystem functioning and the ecological roles played by fish species (Pulcini et al. 2008, Colmenero et al. 2010, Meakin and Qin 2011, Aguzzi et al. 2013).