INTRODUCTIONTop

The order Lophiiformes, commonly known as anglerfishes, is a diverse group of benthic and pelagic species inhabiting shallow to deep-sea waters. This order comprises approximately 358 extant species in five suborders (Pietsch and Grobecker 1987Pietsch T.W., Grobecker D.B. 1987. Frogfishes of the world: systematics, zoogeography, and behavioral ecology. Stanford University Press, 420 pp., Nelson et al. 2006Nelson J.S., Grande T.C., Wilson M.V.H. 2006. Fishes of the world. John Wiley and Sons, New Jersey, 707 pp.): Lophioidei, Antennarioidei, Chaunacoidei, Ogcocephaloidei and Ceratioidei. Phylogenetic studies reveal that Lophioidei, the most primitive group, evolved independently of the remaining groups (Caruso 1985Caruso J.H., 1985. The systematics and distribution of the lophiid anglerfishes: III. Intergeneric relationships. Copeia 4: 870-875., Pietsch and Grobecker 1987Pietsch T.W., Grobecker D.B. 1987. Frogfishes of the world: systematics, zoogeography, and behavioral ecology. Stanford University Press, 420 pp., Pietsch and Orr 2007Pietsch T.W., Orr J.W. 2007. Phylogenetic relationships of deep-sea anglerfishes of the suborder Ceratioidei (Teleostei: Lophiiformes) based on morphology. Copeia 2007: 1-34. , Miya et al. 2010Miya M., Pietsch T.W., Orr J.W., et al. 2010. Evolutionary history of anglerfishes (Teleostei: Lophiiformes): a mitogenomic perspective. BMC Evol. Biol. 10: 58.). Although some lophiiform morphological features are similar (Caruso 1983Caruso J.H. 1983. The systematics and distribution of the lophiid anglerfishes: II. Revisions of the genera Lophiomus and Lophius. Copeia 1: 11-30., Pietsch and Orr 2007Pietsch T.W., Orr J.W. 2007. Phylogenetic relationships of deep-sea anglerfishes of the suborder Ceratioidei (Teleostei: Lophiiformes) based on morphology. Copeia 2007: 1-34. ), body shape differs between clades: dorso-ventrally flattened in Lophioidei (with rhomboidal head) and Ogcocephaloidei (with triangular or circular head) (Caruso 1985Caruso J.H., 1985. The systematics and distribution of the lophiid anglerfishes: III. Intergeneric relationships. Copeia 4: 870-875., Ho and Shao 2008Ho H.C., Shao K.T. 2008. The batfishes (Lophiiformes Ogcocephalidae) of Taiwan, with descriptions of eight new records. J. Fish Soc. Taiwan 35: 289-313.), laterally compressed in Antennarioidei (Pietsch and Grobecker 1987Pietsch T.W., Grobecker D.B. 1987. Frogfishes of the world: systematics, zoogeography, and behavioral ecology. Stanford University Press, 420 pp., Arnold and Pietsch 2012Arnold R.J., Pietsch T.W. 2012. Evolutionary history of frogfishes (Teleostei: Lophiiformes: Antennariidae): A molecular approach. Mol. Phylogenetics Evol. 62: 117-129. ) and globose in Chaunacoidei (Ho and Ma 2016Ho H.C., Ma W.C. 2016. Revision of southern African species of the anglerfish genus Chaunax (Lophiiformes: Chaunacidae), with descriptions of three new species. Zootaxa 4144: 175-194.). In Ceratioidei, species are characterized by specific morphologies adapted to mesopelagic and bathypelagic lifestyle, which has led to their rapid diversification (Miya et al. 2010Miya M., Pietsch T.W., Orr J.W., et al. 2010. Evolutionary history of anglerfishes (Teleostei: Lophiiformes): a mitogenomic perspective. BMC Evol. Biol. 10: 58.). In general, lophiiforms are opportunistic (non-selective) ambushers, luring their prey by raising and moving the illicium, a modified first dorsal-fin spine with a terminal esca (bait) (Pietsch and Grobecker 1987Pietsch T.W., Grobecker D.B. 1987. Frogfishes of the world: systematics, zoogeography, and behavioral ecology. Stanford University Press, 420 pp., Afonso-Dias 1997Afonso-Dias I.M.D.S.B.R.P. 1997. Aspects of the biology and ecology of anglerfish (Lophius piscatorius) off the west coast of Scotland (ICES sub area via). Ph.D. thesis, Univ. Aberdeen, 192 pp.). The Ogcocephaloidei species seem to be more adapted for the capture of small demersal prey (durophagy) such as gastropods, small crustaceans and polychaetes (Gibran and Castro 1999Gibran F.Z., Castro R.M.C. 1999. Activity, feeding behaviour and diet of Ogcocephalus vespertilio in southern west Atlantic. J. Fish Biol. 55: 588-595., Nagareda and Shenker 2008Nagareda B.H., Shenker J.M. 2008. Dietary analysis of batfishes (Lophiiformes: Ogcocephalidae) in the Gulf of Mexico. Gulf Mexico Sci. 26: 28-35.). Indistinctly, all species are considered top-predators where the capture efficiency is favoured by a jet-propulsive locomotion, which is produced through pore-like gill openings behind the pectoral fin (Pietsch 1981Pietsch T.W. 1981. The osteology and relationships of the anglerfish genus Tetrabrachium with comments on lophiiform classification. Fish. Bull. 79: 387-419.).

The Indian Ocean, and especially the region around the Andaman and Nicobar Islands, is characterized by their rich deep-sea fishery resources (Venu and Kurup 2002Venu S., Kurup B.M. 2002. Distribution and abundance of deep-sea fishes along the west coast of India. Fish Technol. 39: 20-26., Jayaprakash et al. 2006Jayaprakash A.A., Kurup B.M., Sreedhar U., et al. 2006. Distribution, diversity, length-weight relationship and recruitment pattern of deep-sea finfishes and shell fishes in the shelf-break area off southwest Indian EEZ. J. Mar. Biol. Assoc. India 48: 56-67., Hashim 2012Hashim M. 2012. Distribution, diversity and biology of deep-sea fishes in the Indian EEZ. Ph.D. thesis, Cochin Univ. Sci. Technol. India, 131 pp., Sumod 2018Sumod K.S. 2018. Deep-sea eels (Teleostei: Anguilliformes) of Indian EEZ: Systematics, distribution and Biology. Ph.D. thesis, Cochin Univ. Sci. Technol. India, 474 pp., Rajeeshkumar 2018Rajeeshkumar M.P. 2018. Deep-sea anglerfishes (Pisces-Lophiiformes) of the Indian EEZ: Systematics, distribution and Biology. Ph.D. thesis, Cochin Univ. Sci. Technol. India, 307 pp.). Recent experimental surveys have reported 22 lophiiforms (Rajeeshkumar 2018Rajeeshkumar M.P. 2018. Deep-sea anglerfishes (Pisces-Lophiiformes) of the Indian EEZ: Systematics, distribution and Biology. Ph.D. thesis, Cochin Univ. Sci. Technol. India, 307 pp.). This eco-evolutionary scenario necessarily implies a high interspecific phenotypic variability leading to coexistence or segregation of species. It is known that this phenotypic variability is linked to multiple extrinsic (Colborne et al. 2013Colborne S.F., Peres-Neto P.R., Longstaffe F.J., et al. 2013. Effects of foraging and sexual selection on ecomorphology of a fish with alternative reproductive tactics. Behav. Ecol. 24: 1339-1347., Aguilar-Medrano et al. 2016Aguilar-Medrano R., Frederich B., Barber P.H. 2016. Modular diversification of the locomotor system in damselfishes (Pomacentridae). J. Morphol. 277: 603-614.) and genetic factors (Pietsch and Orr 2007Pietsch T.W., Orr J.W. 2007. Phylogenetic relationships of deep-sea anglerfishes of the suborder Ceratioidei (Teleostei: Lophiiformes) based on morphology. Copeia 2007: 1-34. , Miya et al. 2010Miya M., Pietsch T.W., Orr J.W., et al. 2010. Evolutionary history of anglerfishes (Teleostei: Lophiiformes): a mitogenomic perspective. BMC Evol. Biol. 10: 58., Arnold 2015Arnold R.J. 2015. Evolutionary Relationships of the Enigmatic Anglerfishes (Teleostei: Lophiiformes): Can Nuclear DNA Provide Resolution for Conflicting Morphological and Mitochondrial Phylogenies? Ph.D. thesis, Univ. Wash. U.S.A., 83 pp.), avoiding direct competition for feeding resources (Bellwood et al. 2010Bellwood D.R., Klanten S., Cowman P.F., et al. 2010. Evolutionary history of the butterflyfishes (f: Chaetodontidae) and the rise of coral feeding fishes. J. Evol. Biol. 23: 335-349., Frederich et al. 2016Frederich B., Olivier D., et al. 2016. Trophic ecology of damselfishes. In: Frederich B., Parmentier E (eds), Biology of Damselfishes, CRC Press, pp. 153-167.). For example, the distribution range or temporal segregation in the behavioural activity could play a key role in the coexistence for many sympatric species (Carothers and Jaksić 1984Carothers J.H., Jaksić, F.M. 1984. Time as a niche difference: the role of interference competition. Oikos 42: 403-406., Seehausen et al. 2008Seehausen O., Terai Y., Magalhaes I.S., et al. 2008. Speciation through sensory drive in cichlid fish. Nature 455: 620-626., Foster et al. 2015Foster K., Bower L., Piller K. 2015. Getting in shape: habitat-based morphological divergence for two sympatric species. Biol. J. Linn. Soc. 114: 152-162.), as occurs between Lophius budegassa and L. piscatorius on the continental shelf and upper slope of the Mediterranean Sea. Both species have similar prey preferences (Preciado et al. 2006Preciado I., Velasco F., Olaso I., et al. 2006. Feeding ecology of black anglerfish Lophius budegassa: seasonal, bathymetric and ontogenetic shifts. J. Mar. Biol. Assoc. U.K. 86: 877-884., Bohórquez-Herrera 2015Bohórquez-Herrera J., Cruz-Escalona V.H., Adams D.C., et al. 2015. Feeding ecomorphology of seven demersal marine fish species in the Mexican Pacific Ocean. Environ. Biol. Fish. 98: 1459-1473.), but they have developed sensory specialization in eye and otolithic organs that allows L. budegassa to be more active at night, whereas L. piscatorius is more active during daytime (Hislop et al. 2000Hislop J.R.G., Holst J.C., Skagen D. 2000. Near-surface captures of post-juvenile anglerfish in the North-east Atlantic-an unsolved mystery. J. Fish Biol. 57: 1083-1087., Colmenero et al. 2010Colmenero A.I., Aguzzi J., Lombarte A., et al. 2010. Sensory constraints in temporal segregation in two species of anglerfish, Lophius budegassa and L. piscatorius. Mar. Ecol. Prog. Ser. 416: 255-265. ). Thus, sensory (visual and hearing) and morpho-functional features of an organism can be used to discern and understand the ecological segregation among species (Lombarte 1992Lombarte A. 1992. Changes in otolith area: sensory area ratio with body size and depth. Environ. Biol. Fish. 33: 405-410., Arellano et al. 1995Arellano R.V., Hamerlynck O., Vincx M., et al. 1995. Changes in the ratio of the sulcus acusticus area to the sagitta area of Pomatoschistus minutus and P. lozanoi (Pisces, Gobidae). Mar. Biol. 122: 355-360., Tuset et al. 2016Tuset V.M., Otero-Ferrer J.L., Gómez-Zurita J., et al. 2016. Otolith shape lends support to the sensory drive hypothesis in rockfishes. J. Evol. Biol. 29: 2083-2097.). Overall ecomorphological studies on the ecology of lophiiforms are scarce (Carlucci et al. 2009Carlucci R., Capezzuto F., Maiorano P., et al. 2009. Distribution, population structure and dynamics of the black anglerfish (Lophius budegassa) (Spinola, 1987) in the Eastern Mediterranean Sea. Fish. Res. 95: 76-87. , Colmenero et al. 2010Colmenero A.I., Aguzzi J., Lombarte A., et al. 2010. Sensory constraints in temporal segregation in two species of anglerfish, Lophius budegassa and L. piscatorius. Mar. Ecol. Prog. Ser. 416: 255-265.), and there are none for the species inhabiting the Indian Ocean.

The aim of this work is to understand better the coexistence of the most common benthic species of lophiiforms occurring at the Andaman and Nicobar Islands (Rajan and Sreeraj 2013Rajan P.T., Sreeraj C.R. 2013. Fish fauna of Andaman and Nicobar Islands: a review. In: Venkataraman K., Sivaperuman C., et al. (eds), Ecology and Conservation of Tropical Marine Faunal Communities. Springer, Berlin, Heidelberg, pp. 231-243., Balakrishnan et al. 2008Balakrishnan M., Srivastava R.C., Pokhriyal M. 2008. Biodiversity of Andaman and Nicobar Islands. Biobytes 3: 9-12., Hashim 2012Hashim M. 2012. Distribution, diversity and biology of deep-sea fishes in the Indian EEZ. Ph.D. thesis, Cochin Univ. Sci. Technol. India, 131 pp., Rajeeshkumar et al. 2016Rajeeshkumar M.P., Jacob V., Sumod K.S., et al. 2016. Three new records of rare deep-sea Anglerfishes (Lophiiformes: Ceratioidei) from the Northern Indian Ocean. Mar. Biodivers. 46: 923-928., 2017Rajeeshkumar M.P., Meera K.M., Hashim M. 2017. A New Species of the Deep-Sea Ceratioid Anglerfish Genus Oneirodes (Lophiiformes: Oneirodidae) from the Western Indian Ocean. Copeia 105: 82-84., Ho et al. 2016aHo H.C., Meleppura R.K., Bineesh K.K. 2016a. Chaunax multilepis sp. nov., a new species of Chaunax (Lophiiformes: Chaunacidae) from the northern Indian Ocean. Zootaxa 4103: 130-136.): Chaunax apus Lloyd, 1909 and C. multilepis Ho, Rajeesh & Bineesh, 2016 (Chaunacidae), Halieutaea coccinea Alcock, 1894 and Malthopsis lutea Alcock, 1891 (Ogcocephalidae), and Lophiodes lugubris (Alcock, 1894) (Lophiidae). To this end, we characterized the sagitta otolith (henceforth otolith) morphology for each species that may be essential for building marine food webs (Lombarte et al. 2010Lombarte A., Palmer M., Matallanas J., et al. 2010. Ecomorphological trends and phylogenetic inertia of otolith sagittae in Nototheniidae. Environ. Biol. Fish. 89: 607-618., Tuset et al. 2008Tuset V.M., Lombarte A., Assis C.A. 2008. Otolith atlas for the western Mediterranean, north and central eastern Atlantic. Sci. Mar. 72S1: 7-198., 2010Tuset V.M., Piretti S., Lombarte A., et al. 2010. Using sagittal otoliths and eye diameter for ecological characterization of deep-sea fish: Aphanopus carbo and A. intermedius from NE Atlantic waters. Sci. Mar. 74: 807-814., 2018Tuset V.M., Olivar M.P., Otero-Ferrer J.L., et al. 2018. Morpho-functional diversity in Diaphus spp. (Pisces: Myctophidae) from the central Atlantic Ocean: Ecological and evolutionary implications. Deep Sea Res. I 138: 46-59.), analysed the morphometric relationships of otoliths with fish length as an indirect factor of the range of spatial distribution in depth of the species (Tuset et al. 2010Tuset V.M., Piretti S., Lombarte A., et al. 2010. Using sagittal otoliths and eye diameter for ecological characterization of deep-sea fish: Aphanopus carbo and A. intermedius from NE Atlantic waters. Sci. Mar. 74: 807-814. , Colmenero et al. 2010Colmenero A.I., Aguzzi J., Lombarte A., et al. 2010. Sensory constraints in temporal segregation in two species of anglerfish, Lophius budegassa and L. piscatorius. Mar. Ecol. Prog. Ser. 416: 255-265. , Nazir and Khan 2019Nazir A., Khan M.A. 2019. Spatial and temporal variation in otolith chemistry and its relationship with water chemistry: Stock discrimination of Sperata aor. Ecol. Freshwater Fish 28: 499-511.) and obtained functional traits as an indicator of ecological strategies and to detect the degree of functional niche overlapping between species (Gatz 1979Gatz A.J. 1979. Community organization in fishes as indicated by morphological features. Ecology 60: 711-718., Sibbing and Nagelkerke 2001Sibbing F.A., Nagelkerke L.A.J. 2001. Resource partitioning by lake Tana barbs predicted from fish morphometrics and prey characteristics. Rev. Fish. Biol. Fish. 10: 393-437., Karpouzi and Stergiou 2003Karpouzi V.S., Stergiou K.I. 2003. The relationships between mouth size and shape and body length for 18 species of marine fishes and their trophic implications. J. Fish Biol. 62: 1353-1365., Wainwright et al. 2007Wainwright P., Carroll A.M., Collar D.C., et al. 2007. Suction feeding mechanics, performance, and diversity in fishes. Integr. Comp. Biol. 47: 96-106.).

MATERIALS AND METHODSTop

Data collection

Specimens were collected during the deep-sea fishery exploratory surveys of the Fishery Oceanographic Research Vessel (FORV) Sagar Sampada (71.5 m LOA: 2285 hp) (Cruise no 349) in Andaman and Nicobar waters in April 2016 using a High-Speed Demersal Trawl -crustacean version (HSDT-CV) at a towing speed of 2.5 to 3.5 knots. Eight stations were surveyed (one operation at each station) along the continental margins of the Andaman and Nicobar Islands (7.29-13.76°N and 92.14-93.11°E) at depths ranging from 300 to 650 m (Fig. 1). The locations were scanned using a SIMRAD EK60 echo sounder before trawling operations and stations were selected on the basis of the suitability of the grounds for trawling. The fishing operations were carried out from 6 am to 6 pm depending upon the weather conditions.

The lophiiforms were identified following standard identification keys (Alcock 1891Alcock A.W. 1891. Natural history notes from H.M. Indian Marine Survey Steamer “Investigator” Ser. II, No. 1. On the results of deep-sea dredging during the season 1890-91. Ann. Mag. Nat. Hist. 6: 16-34., 1894Alcock A.W. 1894. Natural history notes from H.M. Indian Marine Survey Steamer Investigator’- No. 11. An account of a recent collection of bathybial fishes from the Bay of Bengal and from the Laccadive Sea. J. Asiat. Soc. Bengal 58: 115-140., Rajeeshkumar et al. 2016Rajeeshkumar M.P., Jacob V., Sumod K.S., et al. 2016. Three new records of rare deep-sea Anglerfishes (Lophiiformes: Ceratioidei) from the Northern Indian Ocean. Mar. Biodivers. 46: 923-928., Ho et al. 2016aHo H.C., Meleppura R.K., Bineesh K.K. 2016a. Chaunax multilepis sp. nov., a new species of Chaunax (Lophiiformes: Chaunacidae) from the northern Indian Ocean. Zootaxa 4103: 130-136., bHo H.C., Kawai T., Satria F. 2016b. New records of the anglerfish family Lophiidae (Order Lophiiformes) from Indonesia. Acta Ichthyol. et Piscatoria 46: 77-85. ). Only non-damaged adult fishes were selected for meristic and morphological measurements and to extract the otoliths. The catch per unit effort (CPUE) and the spatial distribution of each species along with their geographical positions are given in the Figure 2.

Otolith morphology and morphometry

Otoliths were collected and washed with distilled water to remove exogenous matter, dried and kept in plastic vials for further analysis. Otoliths from the right side of each fish were oriented with the inner side (sulcus acusticus) uppermost on a slide in order to digitize their form using a microscope (S8APO Camera, Leica DFP-425). Otolith length (OL, mm), height (OH, mm), area (OA, mm²) and perimeter (OP, mm) were measured using ImageJ with magnification depending on otolith size. Otolith weight (OW, mg) was obtained using an electronic balance (Metler Toledo, ML 503) (see descriptive values in Appendix 1). The morphological characteristics of each species were described following Tuset et al. (2008)Tuset V.M., Lombarte A., Assis C.A. 2008. Otolith atlas for the western Mediterranean, north and central eastern Atlantic. Sci. Mar. 72S1: 7-198. .

Fish body morphological data

Sixteen morphological variables were measured on each specimen using a Vernier calliper (0.1 mm precision): total length (TL), standard length (SL), eye diameter (ED), mouth opening (MO), head depth (HD), eye height (EH), pectoral fin base (PFB), pectoral fin insertion, pectoral fin length (PFL), pectoral fin surface (PFS), caudal peduncle depth (CPD), caudal fin surface (CFS), caudal fin depth (CFD), body depth (BD), body length, body width (BW), mouth height (MH) and mouth width (MW) (Fig. 3). From these measurements, the following 11 ecomorphological attributes correlated with foraging, manoeuvrability and locomotion were selected for detailed studies. The formulas for estimating the functional traits are given in italicized letters.

– Oral gape surface (Osf)=(MW×MH)/(BW×BD), which indicates the nature/size of the prey that can be captured. A large oral gape allows feeding on a wide size range including large prey (Karpouzi and Stergiou 2003Karpouzi V.S., Stergiou K.I. 2003. The relationships between mouth size and shape and body length for 18 species of marine fishes and their trophic implications. J. Fish Biol. 62: 1353-1365.).

– Oral gape shape (Osh)=MH/MW, which defines the method for capturing food items. A greater width allows species to capture highly mobile prey and have a more aggressive behaviour (Karpouzi and Stergiou 2003Karpouzi V.S., Stergiou K.I. 2003. The relationships between mouth size and shape and body length for 18 species of marine fishes and their trophic implications. J. Fish Biol. 62: 1353-1365., Wainwright et al. 2007Wainwright P., Carroll A.M., Collar D.C., et al. 2007. Suction feeding mechanics, performance, and diversity in fishes. Integr. Comp. Biol. 47: 96-106.).

– Oral gape position (Ops)=MO/HD, which shows the feeding position in the water column. The position of the oral gape influences the retention of prey during ingestion (Kumar et al. 2017aKumar K.V.A., Tuset V.M., Manjebrayakath H., et al. 2017a. Functional approach reveals low niche overlap among common deep-sea fishes from the south-eastern Arabian Sea. Deep Sea Res. I 119: 16-23., Villéger et al. 2017Villéger S., Brosse S., Mouchet M., et al. 2017. Functional ecology of fish: current approaches and future challenges. Aquat. Sci. 79: 783-801. ).

– Eye size (Edst)=ED/HD, which defines the prey detection efficiency. It also influences the feeding rhythms (nocturnal vs diurnal) and predator avoidance and indicates the availability of light in the microhabitat (Boyle and Horn 2006Boyle K.S., Horn M.H. 2006. Comparison of feeding guild structure and ecomorphology of intertidal fish assemblages from central California and central Chile. Mar. Ecol. Prog. Ser. 319: 65-84., Bellwood et al. 2014Bellwood D.R., Goatley C.H.R., Brandl S.J., et al. 2014. Fifty million years of herbivory on coral reefs: fossils, fish and functional innovations. Proc. R. Soc. B 281: 20133046.).

– Eye position (Eps)=EH/HD, which displays the vertical position in the water column. High values indicate dorsally located eyes (Watson and Balon 1984Watson D.J., Balon E.K. 1984. Ecomorphological analysis of fish taxocenes in rainforest streams of northern Borneo. J. Fish Biol. 25: 371-384., Ribeiro et al. 2016Ribeiro M.D., Teresa F.B., Casatti L. 2016. Use of functional traits to assess changes in stream fish assemblages across a habitat gradient. Neotropical Ichthyol. 14: e140185.).

– Body transversal shape (Bsh)=BD/BW, which indicates the vertical position of the fish in the water column as well as hydrodynamic efficiency (Villéger et al. 2017Villéger S., Brosse S., Mouchet M., et al. 2017. Functional ecology of fish: current approaches and future challenges. Aquat. Sci. 79: 783-801. ).

– Caudal peduncle throttling (Cpt)=CFD/CPD, which shows the caudal propulsion efficiency through the reduction of drag (Webb 1984Webb P.W. 1984. Body form, locomotion and foraging in aquatic vertebrates. Am. Zool. 24: 107-120., Zhao et al. 2014Zhao T., Villéger S., Lek S., et al. 2014. High intraspecific variability in the functional niche of a predator is associated with ontogenetic shift and individual specialization. Ecol. Evol. 4: 4649-4657.).

– Fin surface ratio (Fsr)=(2×PFS)/CFS, which indicates the type of propulsion between caudal and pectoral fins. Higher values denote a swimming driven by pectoral fins, whereas lower values correspond to a greater caudal fin propulsion (Mouillot et al. 2013Mouillot D., Graham N.A., Villéger S., et al. 2013. A functional approach reveals community responses to disturbances. Trends Ecol. Evol. 28: 167-177., Zhao et al. 2014Zhao T., Villéger S., Lek S., et al. 2014. High intraspecific variability in the functional niche of a predator is associated with ontogenetic shift and individual specialization. Ecol. Evol. 4: 4649-4657.).

– Fin surface to body size ratio (Fsb)= ((2×PFS) + CFS)/(π/4×BW×BD), which indicates the acceleration and/or manoeuvring competence. Higher values indicate prolonged sustained swimming speed and fitness, which positively influence endurance, acceleration and manoeuvring capacities (Zhao et al. 2014Zhao T., Villéger S., Lek S., et al. 2014. High intraspecific variability in the functional niche of a predator is associated with ontogenetic shift and individual specialization. Ecol. Evol. 4: 4649-4657., Kumar et al. 2017aKumar K.V.A., Tuset V.M., Manjebrayakath H., et al. 2017a. Functional approach reveals low niche overlap among common deep-sea fishes from the south-eastern Arabian Sea. Deep Sea Res. I 119: 16-23.).

– Aspect ratio of the pectoral fin (ArPF)= PFL²/PFS, an indicator of swimming ability, which helps sustained swimming. Longer pectoral fins favour sustained swimming speed (Watson and Balon 1984Watson D.J., Balon E.K. 1984. Ecomorphological analysis of fish taxocenes in rainforest streams of northern Borneo. J. Fish Biol. 25: 371-384., Casatti and Castro 2006Casatti L., Castro R. 2006. Testing the ecomorphological hypothesis in a headwater riffles fish assemblage of the rio São Francisco, southeastern Brazil. Neotropical ichthyol. 4: 203-214.).

– Aspect ratio of the caudal fin (ArCF)=CFD²/CFS, which indicates the caudal fin use for propulsion and/or direction. A higher ratio produces the maximum thrust (Webb 1984Webb P.W. 1984. Body form, locomotion and foraging in aquatic vertebrates. Am. Zool. 24: 107-120., Bridge et al. 2016Bridge T.C. Luiz O.J., Coleman R.R., et al. 2016. Ecological and morphological traits predict depth-generalist fishes on coral reefs. Proc. R. Soc. B 283: 20152332. ).

To estimate the functional traits, the morphological data were standardized to remove the allometric effect using the total weight (Mouillot et al. 2005Mouillot D., Mason W.N., Dumay O., et al. 2005. Functional regularity: a neglected aspect of functional diversity. Oecologia 142: 353-359., Kumar et al. 2017aKumar K.V.A., Tuset V.M., Manjebrayakath H., et al. 2017a. Functional approach reveals low niche overlap among common deep-sea fishes from the south-eastern Arabian Sea. Deep Sea Res. I 119: 16-23.). The allometric relationship between morphological data (X) and body mass (M) is X=aM^b, where ‘b’ varies with species. The effect of body mass was eliminated by using the residuals of the common within-group slopes of linear regressions for each component of body mass.

Statistical analysis

The Kolmogorov-Smirnov and Levene tests were used to check normality of the data distributions and variance homogeneity, respectively. The intraspecific variability was analysed considering the fish size-otolith measurement relationships as a tool in the feeding ecology to estimate fish size and biomass (Kumar et al. 2017bKumar K.V.A., Nikki R., Oxona K., et al. 2017b. Relationships between fish and otolith size of nine deep-sea fishes from the Andaman and Nicobar waters. North Indian Ocean. J. Appl. Ichthyol. 33: 1187-1195., cKumar K.V.A., Deepa K.P., Hashim M., et al. 2017c. Relationships between fish size and otolith size of four bathydemersal fish species from the south eastern Arabian Sea, India. J. Appl. Ichthyol. 33: 102-107.) and the otolith relative size as a resemblance to fish habitat and depth distribution (Lombarte and Cruz 2007Lombarte A., Cruz A. 2007. Otolith size trends in marine fish communities from different depth strata. J. Fish Biol. 71: 53-76.). For the first analysis, the relationships between otolith morphometric variables (OL, OH, OA, OP, OW) were described using the allometric power equation (Y = aX^b) (Huxley 1924Huxley J.S. 1924. Constant differential growth-ratios and their significance. Nature 114: 895-896.). Measurements were converted into logarithmic values (log10) to identify and exclude possible outliers in the data (Froese et al. 2011Froese R., Tsikliras A.C., Stergiou K.I. 2011. Editorial note on weight-length relations of fishes. Acta Ichthyol. et Piscatoria 41: 261-263.). Regression parameters a and b were estimated by the least square regression method, where b represents the constant of differential growth rate (Froese 2006Froese R. 2006. Cube law, condition factor and weight-length relationships: history, meta-analysis and recommendations. J. Appl. Ichthyol. 22: 241-253. ). An analysis of covariance (ANCOVA) was performed to compare the regression slopes between species, treating the species as the main factor and fish size (SL) as a covariate. Specific difference was analysed using a post-hoc Tukey-HSD test. In the second analyses, the otolith measurements were standardized for each species by removing the effect of allometry (Lleonart et al. 2000Lleonart J., Salat J., Torres G.J. 2000. Removing allometric effects of body size in morphological analysis. J. Theor. Biol. 205: 85-93.). Different relative sizes were estimated for each otolith morphometric variable using the following criteria (Lombarte and Cruz 2007Lombarte A., Cruz A. 2007. Otolith size trends in marine fish communities from different depth strata. J. Fish Biol. 71: 53-76.): ORi= (otolith variable)i SL^b, with b=1 for OL, OH and OP variables, b=2 for OA and b=3 for OW. An ANOVA was conducted for each variable on the relative size to test differences in the averages among species. A post-hoc test (Dunn’s test) was performed to elucidate the pairwise comparison of relative otolith sizes (Pohlert 2014Pohlert T. 2014. The pairwise multiple comparison of mean ranks package (PMCMR). R package, 27 pp.). All statistical analyses were performed in PAST (PAlaeontologicalSTatistics, version 3.26) (Hammer et al. 2001Hammer O., Harper D.A.T., Ryan P.D. 2001. PAST: Paleontological Statistic software package for education and data analysis. Paleontol. Electron. 4: 4. ).

To order species in the functional space, a principal component analysis (PCA) based on the correlation matrix of the functional traits was performed. The choice of which principal components to interpret was based on a broken-stick model, which constructs a null distribution of eigenvalues and compares it with observed ones (Collar and Wainwright 2006Collar D.C., Wainwright P.C. 2006. Discordance between morphological and mechanical diversity in the feeding mechanism of centrarchid fishes. Evolution 60: 2575-2584., Villéger et al. 2011Villéger S., Novack-Gottshall P.M., Mouillot D. 2011. The multidimensionality of the niche reveals functional diversity changes in benthic marine biotas across geological time. Ecol. Lett. 14: 561-568.). Our hypothesis of significant difference among the species and Bonferroni’s correction for post-hoc pairwise multiple comparisons were tested using multivariate analysis of variance (MANOVA) (Marcus 1993Marcus L.F. 1993. Some aspects of multivariate statistics for morphometrics. In: Marcus L.F., Bello E., et al. (eds), Contributions to morphometrics. Monog. Mus. Nac. Cienc. Nat. 8: 95-130., Layman et al. 2005Layman C.A., Langerhans R.B., Winemiller K.O. 2005. Body size, not other morphological traits, characterizes cascading effects in fish assemblage composition following commercial netting. Can. J. Fish. Aquat. Sci. 62: 2802-2810., Marrama and Kriwet 2017Marrama G., Kriwet J. 2017. Principal component and discriminant analyses as powerful tools to support taxonomic identification and their use for functional and phylogenetic signal detection of isolated fossil shark teeth. Plos ONE 12: e0188806.).

The degree of functional niche overlap among species was performed using a non-parametric kernel density function (NO_K) (Mouillot et al. 2005Mouillot D., Mason W.N., Dumay O., et al. 2005. Functional regularity: a neglected aspect of functional diversity. Oecologia 142: 353-359., Mason et al. 2008Mason N.W., Lanoiselée C., Mouillot D., et al. 2008. Does niche overlap control relative abundance in French lacustrine fish communities? A new method incorporating functional traits. J. Anim. Ecol. 77: 661-669., Geange et al. 2011Geange S.W., Pledger S., Burns K.C., et al. 2011. A unified analysis of niche overlap incorporating data of different types. Methods Ecol. Evol. 2: 175-184. ):

$$N{O}_{Kw}(i,j)=\frac{1}{{\displaystyle {\sum }_{t=1}^T{W}_t}}{\displaystyle \sum _{t=1}^T{w}_{t}N{O}_K(i,j,t)}$$

(1)

NO_K (i, j, t) is the niche overlap between species i and j for the trait t, T is the number of functional traits and wt is the weighting parameter, which is calculated as:

$$w_t=\frac{1}{2}+{\displaystyle \sum _{t=1}^T(1-\frac{r_{tl}^2}{2})}$$

(2)

r_tl is the Pearson correlation coefficient between traits t and l over all five species selected for the study. To understand the niche differences between the anglerfishes, permutation tests were performed to assess whether the observed niche overlap was significantly low based on the potential distribution of niche overlap values (Mouillot et al. 2005Mouillot D., Mason W.N., Dumay O., et al. 2005. Functional regularity: a neglected aspect of functional diversity. Oecologia 142: 353-359., Mason et al. 2008Mason N.W., Lanoiselée C., Mouillot D., et al. 2008. Does niche overlap control relative abundance in French lacustrine fish communities? A new method incorporating functional traits. J. Anim. Ecol. 77: 661-669., Geange et al. 2011Geange S.W., Pledger S., Burns K.C., et al. 2011. A unified analysis of niche overlap incorporating data of different types. Methods Ecol. Evol. 2: 175-184. ). Pseudo-values were calculated through randomly permuting species types in the corresponding data set for more than 1000 runs followed by computing the distribution of the average niche overlap for the null model to create the statistical null distributions. A Bonferroni adjustment of type I (Quinn and Keough 2002Quinn G.P., Keough M.J. 2002. Experimental Design and Data Analysis for Biologists, Cambridge University Press, Cambridge, 558 pp.) was performed for the multiple comparisons. Density functions available in R (R Development Core Team 2017R Development Core Team. 2017. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria.) were used to calculate niche overlap and for the subsequent null model tests. We followed the source code provided by Geange et al. (2011)Geange S.W., Pledger S., Burns K.C., et al. 2011. A unified analysis of niche overlap incorporating data of different types. Methods Ecol. Evol. 2: 175-184. for the above analysis in the R environment.

RESULTSTop

Otolith anatomical description

All species shared otolith features such as dorsal lobes and the lightly marked sulcus acusticus, with a well-defined crista inferior (Fig. 4). The otoliths of Chaunacidae (C. apus and C. multilepis) are characterized by a sulcus acusticus with undifferentiated ostium and cauda referred to as archaesulcoid. Indeed, they maintain an oval shape throughout growth with a smoothed and deep convex ventral margin. The dorsal margin has a variable number of lobes depending on species. In general, C. apus have more lobes (5 to 7) that are less angled than in C. multilepis. In Ogcocephalidae species, otoliths show a stronger differentiation in shape: H. coccinea has a semi-circular pattern (in the largest specimens), with a high number of deep lobes (6 to 10), some irregularities on the dorsal margin, a smooth, convex ventral margin, a rounded to angled anterior margin, and an angled end at the posterior margin for the largest specimens, providing an oblong shape. In contrast, M. lutea has an oval shape, with a sinuous to lightly lobed dorsal margin (3 to 6 lobes) and a smooth, shallow, convex ventral margin, and the anterior margin is oblique, lacking a rostrum. In both species the sulcus acusticus is archaesulcoid, mesial and ascendant, with an oval ostium (poorly defined) and a cauda smaller than the ostium. In particular, the sulcus acusticus of M. lutea is placed in an inframedian position. Finally, the otolith of L. lugubris (Lophiidae) is characterized by a semi-circular to oblong shape (in the largest specimens), with a sinusoidal ventral margin and a deeply lobed (6 to 9) irregular dorsal margin, a blunt anterior margin and an undefined rostrum and pointed end of the posterior margin of the largest specimens. The sulcus acusticus is a homosulcoid type, with oval ostium and cauda.

Interspecific variability in the otolith morphometry

All otolith morphometric variables showed a statistically significant relationship with fish length for all species (Table 1, Appendix 2). However, otolith length and weight were the best variables correlated with fish size (r² ranges from 0.740 to 0.936 for OL, and between 0.708 and 0.959 for OW). The other variables showed a high intraspecific variation, and even attained very low values in the otolith height (r²=0.287) for M. lutea and the otolith perimeter (r²=0.243) for H. coccinea (Table 1). The ANCOVA exhibited no differences between species in the slopes of relationships SL-OH (F= 0.879, df=4, p=0.482) and SL-OA (F=2.158, p=0.085), but it indicated interspecific variability for the SL-OL (F=4.764, df=4, p=0.002), SL-OP (F=2.705, df=4, p=0.039) and SL-OW (F=6.787, df=4, p<0.001) relationships (Appendix 3). In particular, the slope (b) for the SL-OW relationship was higher in M. lutea than in C. apus-H. coccinea, and higher in L. lugubris than in H. coccinea. In fact, H. coccinea and L. lugubris also varied for the SL-OP and SL-OL relationships, and L. lugubris also showed differences with C. apus for the latter.

The ANOVA tests revealed significant differences for all relative variables (OA_R, F=166.2, df=4, p<0.05; OL_R, F=120.1, df=4, p<0.05; OH_R, F=309.8, df=4, p<0.05; OP_R, F=46.6, df=4, p<0.05; OW_R, F=124.1, df=4, p<0.05). Pairwise comparison using Dunn’s test (Bonferroni, p<0.05) indicated significant inter-species differences, except between Chaunax spp. (Fig. 5, Table 2). The highest interspecific variability was obtained for OH_R and the lowest for OP_R. No particular clustering was noted between species. In general, L. lugubris showed the lowest values for all relative sizes and C. apus the highest ones.

Comparing the functional niches

The first five PCA axes explained 97.9% of the total variance and the first three explained 93.8%. The PC1 axis alone contributed 63.7% of the total variance and was mainly correlated with Fsb (r= 0.868) (Appendix 4). The positive values represented species with a more dorso-ventrally flattened body and higher swimming capabilities (M. lutea, H. coccinea and L. lugubris) versus species with higher body depth and lesser swimming abilities (C. multilepis and C. apus) (Fig. 6). The PC2 axis (19.1% of variance) was related to propulsion and acceleration capabilities (ArCF, r=0.893), showing a similar pattern in all five species. The PC3 axis (10.9% of variance) was mainly related to swimming performance (Arcf, r=–0.834). The remaining PC scores (4 to 11) cumulatively explained 6.2% of the variance and were related to locomotion traits (Appendix 4). MANOVA confirmed the occurrence of significant differences among these deep-sea anglerfishes (Wilk’s Lambda=00.0023, F_44,258.3=22.88, p<0.001). The pairwise comparisons among species using sequential Bonferroni correction indicated significance differences among all species (p<0.001) (Appendix 5).

The functional traits Ops, Edst and Eps showed the highest interspecific differences, whereas Osf, Cpt and Fsr showed the lowest (Table 3, Fig. 7). The overall niche overlap ranged between 0.32 for C. apus-M. lutea and 0.65 for H. coccinea-L. lugubris. The species with highest niche partitioning was M. lutea due to its differentiation in the variables such as Osf, Ops, Edst and Eps. The analysis revealed significant differences between species, with M. lutea having a more differentiated functional niche, and both species of Chaunax showed more resemblance between them (Table 4). In any case, the findings indicated that functional niches did not overlap among the common five anglerfishes from the Indian Ocean.

DISCUSSIONTop

Most studies performed on deep-sea fish species from Indian waters have focused on taxonomy and biology (Karuppasamy et al. 2008Karuppasamy P.K., Balachandran K., George S., et al. 2008. Food of some deep sea fishes collected from the eastern Arabian Sea. J. Mar. Biol. Assoc. India 50: 134-138., Sreedhar et al. 2013Sreedhar U., Sudhakar G.V.S., Meenakumari B. 2013. Length-weight relationship of deepsea demersal fishes from the Indian EEZ. Ind. J. Fish. 60: 123-125., Kumar et al. 2016Kumar K.V.A., Thomy R., Deepa K.P., et al. 2016. Length-weight relationship of six deep-sea fish species from the shelf regions of western Bay of Bengal and Andaman waters. J. Appl. Ichthyol. 32: 1334-1336., 2018Kumar K.V.A., Thomy R., Hashim M., et al. 2018. Length-weight relationships of 11 deep-sea fishes from the western Bay of Bengal and Andaman waters, India. J. Appl. Ichthyol. 34: 1048-1051.), and only few have analysed interspecific competition (Narayani et al. 2015Narayani S., Venu S., Kumar M.A. et al. 2015. Ecomorphology of the feeding characteristics in selected reef fishes from south Andaman Islands: a preliminary study. J. Mar. Biol. Oceanogr. 4: 1-7., Kumar et al. 2017aKumar K.V.A., Tuset V.M., Manjebrayakath H., et al. 2017a. Functional approach reveals low niche overlap among common deep-sea fishes from the south-eastern Arabian Sea. Deep Sea Res. I 119: 16-23.). The present study delved into this matter by analysing the differences in the sensory capability and functional niche of most common anglerfishes inhabiting these waters. In this context, our findings revealed a strong environmental adaptation of sagitta otolith shape to the depth distribution of species, confirming the ecomorphological pattern proposed by Colmenero et al. (2010)Colmenero A.I., Aguzzi J., Lombarte A., et al. 2010. Sensory constraints in temporal segregation in two species of anglerfish, Lophius budegassa and L. piscatorius. Mar. Ecol. Prog. Ser. 416: 255-265. for Lophius spp. from the Mediterranean Sea. Moreover, the dissimilarity between the functional niches indicated a low interspecific niche overlap. Finally, no phylogenetic influence was inferred from the morpho-functional features analysed, as occurs in other fish species (Tuset et al. 2010Tuset V.M., Piretti S., Lombarte A., et al. 2010. Using sagittal otoliths and eye diameter for ecological characterization of deep-sea fish: Aphanopus carbo and A. intermedius from NE Atlantic waters. Sci. Mar. 74: 807-814. , 2018Tuset V.M., Olivar M.P., Otero-Ferrer J.L., et al. 2018. Morpho-functional diversity in Diaphus spp. (Pisces: Myctophidae) from the central Atlantic Ocean: Ecological and evolutionary implications. Deep Sea Res. I 138: 46-59., Kéver et al. 2014Kéver L., Colleye O., Herrel A., et al. 2014. Hearing capacities and otolith size in two ophidiiform species (Ophidion rochei and Carapus acus). J. Exp. Biol. 217: 2517-2525., Schwarzhans 2014Schwarzhans W. 2014. Head and otolith morphology of the genera Hymenocephalus, Hymenogadus and Spicomacrurus (Macrouridae), with the description of three new species. Zootaxa 3888: 73 pp.), although a greater number of taxa should be necessary for this purpose.

The relative size of fish otoliths tends to increase with depth (Lombarte and Cruz 2007Lombarte A., Cruz A. 2007. Otolith size trends in marine fish communities from different depth strata. J. Fish Biol. 71: 53-76.), improving their hearing capacities to compensate for the limitation in visual communication (Lychakov and Rebane 2000Lychakov D.V., Rebane Y.T. 2000. Otolith regularities. Hear. Res. 143: 83-102. , Paxton 2000Paxton J.R. 2000. Fish otoliths: do sizes correlate with taxonomic group, habitat and/or luminescence? Philos. Trans. R. Soc. Lond. B. 355: 1299-1303., Tuset et al. 2018Tuset V.M., Olivar M.P., Otero-Ferrer J.L., et al. 2018. Morpho-functional diversity in Diaphus spp. (Pisces: Myctophidae) from the central Atlantic Ocean: Ecological and evolutionary implications. Deep Sea Res. I 138: 46-59.). However, this trend is reversed due to carbonate under-saturation below 1000 m depth (Wilson 1985Wilson Jr. R.R. 1985. Depth-related changes in sagitta morphology in six macrourid fishes of the Pacific and Atlantic Oceans. Copeia 4: 1011-1017., Lombarte and Cruz 2007Lombarte A., Cruz A. 2007. Otolith size trends in marine fish communities from different depth strata. J. Fish Biol. 71: 53-76.). This ecomorphological pattern was found in the present study: Chaunax spp. and M. lutea, characterized by a wide bathymetric distribution (200-700 m; Ho et al. 2016aHo H.C., Meleppura R.K., Bineesh K.K. 2016a. Chaunax multilepis sp. nov., a new species of Chaunax (Lophiiformes: Chaunacidae) from the northern Indian Ocean. Zootaxa 4103: 130-136., Rajeeshkumar 2018Rajeeshkumar M.P. 2018. Deep-sea anglerfishes (Pisces-Lophiiformes) of the Indian EEZ: Systematics, distribution and Biology. Ph.D. thesis, Cochin Univ. Sci. Technol. India, 307 pp.), had a greater relative otolith size in the area, height and weight; L. lugubris, the shallowest species (<250 m; Alcock 1894Alcock A.W. 1894. Natural history notes from H.M. Indian Marine Survey Steamer Investigator’- No. 11. An account of a recent collection of bathybial fishes from the Bay of Bengal and from the Laccadive Sea. J. Asiat. Soc. Bengal 58: 115-140., Ho et al. 2016aHo H.C., Meleppura R.K., Bineesh K.K. 2016a. Chaunax multilepis sp. nov., a new species of Chaunax (Lophiiformes: Chaunacidae) from the northern Indian Ocean. Zootaxa 4103: 130-136., Rajeeshkumar 2018Rajeeshkumar M.P. 2018. Deep-sea anglerfishes (Pisces-Lophiiformes) of the Indian EEZ: Systematics, distribution and Biology. Ph.D. thesis, Cochin Univ. Sci. Technol. India, 307 pp.), had a smaller relative otolith size; and H. coccinea, which can inhabit over >1000 m (Rajeeshkumar 2018Rajeeshkumar M.P. 2018. Deep-sea anglerfishes (Pisces-Lophiiformes) of the Indian EEZ: Systematics, distribution and Biology. Ph.D. thesis, Cochin Univ. Sci. Technol. India, 307 pp.), also reached low values for some relative otolith sizes. Certainly, the set of relative otolith indices did not follow the same trend, which may be due to the high irregularity of sculpture of the dorsal margin in anglerfishes (see more examples in AFORO website, http://aforo.cmima.csic.es/; Lombarte et al. 2006Lombarte A., Chic Ò., Parisi-Baradad V., et al. 2006. A web-based environment for shape analysis of fish otoliths. The AFORO database. Sci. Mar. 70: 147-152.; present study). It is known that this variability occurs at inter- and intraspecific levels and is a disadvantage for the automated separation of stocks (example in Cañás et al. 2012Cañás L., Stransky C., Schlickeisen J., et al. 2012. Use of the otolith shape analysis in stock identification of anglerfish (Lophius piscatorius) in the Northeast Atlantic. ICES J. Mar. Sci. 69: 250-256.) and for the identification of species. Moreover, it would explain the low coefficients of determination and the interspecific similarity obtained in the slope value (b) of some morphometric relationships. Although some studies have demonstrated a morpho-functional correlation between the otolith and fish body shapes (Volpedo et al. 2008Volpedo A.V., Tombari A.D., Echeverría D.D. 2008. Ecomorphological patterns of the sagitta of Antarctic fish. Polar Biol. 31: 635-640., Mille et al. 2016Mille T., Mahe K., Cachera M., et al. 2016. Diet is correlated with otolith shape in marine fish. Mar. Ecol. Prog. Ser. 555: 167-184., Tuset et al. 2018Tuset V.M., Olivar M.P., Otero-Ferrer J.L., et al. 2018. Morpho-functional diversity in Diaphus spp. (Pisces: Myctophidae) from the central Atlantic Ocean: Ecological and evolutionary implications. Deep Sea Res. I 138: 46-59.), we found no evidence that the morphometry, relative otolith size and sculpture of the otolith margins were associated with the fish body morphotypes (globose versus dorso-ventrally flattened) or had any phylogenetic meaning in anglerfishes.

Given that common anglerfishes from the Indian Ocean had different functional niches and can coexist in some bathymetries, the slight variations in their functional traits suggest that functional variability is linked to competence for similar resource requirements (theory of limiting similarity, MacArthur and Levins 1967MacArthur R., Levins R. 1967. The limiting similarity, convergence, and divergence of coexisting species. Am. Nat. 101: 377-385.), as occurs in other fish groups such as cichlids (Winemiller et al. 1995Winemiller K.O., Kelso-Winemiller L.C., Brenkert A.L. 1995. Ecomorphological diversification and convergence in fluvial cichlid fishes. In: Luczkovich J.J., Motta P.J., et al. (eds), Ecomorphology of fishes. Springer, Dordrecht, pp. 235-261.), labrids (Wainwright et al. 2002Wainwright P.C., Bellwood D.R., Westneat M.W. 2002. Ecomorphology of locomotion in labrid fishes. Environ. Biol. Fish. 65: 47-62.), butterflyfishes (Bellwood et al. 2010Bellwood D.R., Klanten S., Cowman P.F., et al. 2010. Evolutionary history of the butterflyfishes (f: Chaetodontidae) and the rise of coral feeding fishes. J. Evol. Biol. 23: 335-349.), notothenids (Lombarte et al. 2010Lombarte A., Palmer M., Matallanas J., et al. 2010. Ecomorphological trends and phylogenetic inertia of otolith sagittae in Nototheniidae. Environ. Biol. Fish. 89: 607-618. ), rockfishes (Ingram 2011Ingram T. 2011. Speciation along a depth gradient in a marine adaptive radiation. Proc. R. Soc. B 278: 613-618.), damselfishes (Frederich et al. 2016Frederich B., Olivier D., et al. 2016. Trophic ecology of damselfishes. In: Frederich B., Parmentier E (eds), Biology of Damselfishes, CRC Press, pp. 153-167.) and lanternfishes (Tuset et al. 2018Tuset V.M., Olivar M.P., Otero-Ferrer J.L., et al. 2018. Morpho-functional diversity in Diaphus spp. (Pisces: Myctophidae) from the central Atlantic Ocean: Ecological and evolutionary implications. Deep Sea Res. I 138: 46-59.). Anglerfishes with a dorso-ventrally flattened body (M. lutea, L. lugubris and H. coccinea) were characterized by a higher swimming efficiency in relation to species with globose body (Chaunax spp.). However, unlike M. lutea and H. coccinea, both L. lugubris and Chaunax spp. attract their prey with an angling apparatus (or illicium), which has a bait (esca) in the case of Chaunax spp. (Pietsch and Grobecker 1987Pietsch T.W., Grobecker D.B. 1987. Frogfishes of the world: systematics, zoogeography, and behavioral ecology. Stanford University Press, 420 pp., Armstrong et al. 1996Armstrong M.P., Musick J.A., Colvocoresses J.A. 1996. Food and ontogenetic shifts in feeding of the goosefish, Lophius americanus. J. Northwest Atl. Fish. Sci. 18: 99-103. , Ho et al. 2016aHo H.C., Meleppura R.K., Bineesh K.K. 2016a. Chaunax multilepis sp. nov., a new species of Chaunax (Lophiiformes: Chaunacidae) from the northern Indian Ocean. Zootaxa 4103: 130-136.). This bait facilitates a predator behaviour based on slow movements by waiting for the potential prey very close to the mouth, whereas the greater swimming ability of L. lugubris would indicate the possibility of capturing prey more actively (i.e. at a greater distance from its prey).

Overall, anglerfishes with higher swimming capability and oral gape surface (e.g., L. lugubris and H. coccinea) seem to ingest more mobile and larger prey, including fishes (Zhao et al. 2014Zhao T., Villéger S., Lek S., et al. 2014. High intraspecific variability in the functional niche of a predator is associated with ontogenetic shift and individual specialization. Ecol. Evol. 4: 4649-4657., Kumar et al. 2017aKumar K.V.A., Tuset V.M., Manjebrayakath H., et al. 2017a. Functional approach reveals low niche overlap among common deep-sea fishes from the south-eastern Arabian Sea. Deep Sea Res. I 119: 16-23.), whereas those with lesser swimming abilities or a smaller oral gape select crustaceans and gastropods as the main potentially preys (Gibran and Castro 1999Gibran F.Z., Castro R.M.C. 1999. Activity, feeding behaviour and diet of Ogcocephalus vespertilio in southern west Atlantic. J. Fish Biol. 55: 588-595. , Karuppasamy et al. 2008Karuppasamy P.K., Balachandran K., George S., et al. 2008. Food of some deep sea fishes collected from the eastern Arabian Sea. J. Mar. Biol. Assoc. India 50: 134-138., Nagareda and Shenker 2008Nagareda B.H., Shenker J.M. 2008. Dietary analysis of batfishes (Lophiiformes: Ogcocephalidae) in the Gulf of Mexico. Gulf Mexico Sci. 26: 28-35.). Although the theory on the resource partitioning among the species in deep-sea habitats is essentially based on prey size and swimming capacity near the bottom (Papiol et al. 2013Papiol V., Cartes J.E., Fanelli E., et al. 2013. Food web structure and seasonality of slope megafauna in the NW Mediterranean elucidated by stable isotopes: relationship with available food sources. J. Sea Res. 77: 53-69., Kumar et al. 2017aKumar K.V.A., Tuset V.M., Manjebrayakath H., et al. 2017a. Functional approach reveals low niche overlap among common deep-sea fishes from the south-eastern Arabian Sea. Deep Sea Res. I 119: 16-23.), species can also differentiate their feeding rhythms (nocturnal or diurnal). The ability to be more active at night is based on a higher sensory sensitivity from visual and hearing capabilities (Warrant 2004Warrant E. 2004. Vision in the dimmest habitats on earth. J. Comp. Physiol. A 190: 765-789., Schmitz and Wainwright 2011Schmitz L., Wainwright P.C. 2011. Nocturnality constrains morphological and functional diversity in the eyes of reef fishes. BMC Evol. Biol. 11: 338., de Busserolles et al. 2013de Busserolles F., Fitzpatrick J.L., Paxton J.R., et al. 2013. Eye-size variability in deep-sea lanternfishes (Myctophidae): an ecological and phylogenetic study. PLoS ONE 8: e58519., Sadighzadeh et al. 2014Sadighzadeh Z., Otero-Ferrer J.L., Lombarte A., et al. 2014. An approach to unraveling the coexistence of snappers (Lutjanidae) using otolith morphology. Sci. Mar. 78: 353-362.). Colmenero et al. (2010)Colmenero A.I., Aguzzi J., Lombarte A., et al. 2010. Sensory constraints in temporal segregation in two species of anglerfish, Lophius budegassa and L. piscatorius. Mar. Ecol. Prog. Ser. 416: 255-265. concluded that the eye size reflected the nocturnal phenotype between Lophius spp. from the Mediterranean Sea. Our findings suggest a similar behavioural ability in M. lutea and L. lugubris in relation to the remaining species.

In conclusion, anglerfishes have evolved functionally towards different ecological strategies to live in low-energy habitats. Hence, morpho-functional traits seem to be good ecological predictors for explaining the coexistence of species. Functional traits associated with feeding habits, locomotion and manoeuvrability help us to understand the ecology of these species (Bridge et al. 2016Bridge T.C. Luiz O.J., Coleman R.R., et al. 2016. Ecological and morphological traits predict depth-generalist fishes on coral reefs. Proc. R. Soc. B 283: 20152332., Kumar et al. 2017aKumar K.V.A., Tuset V.M., Manjebrayakath H., et al. 2017a. Functional approach reveals low niche overlap among common deep-sea fishes from the south-eastern Arabian Sea. Deep Sea Res. I 119: 16-23.) and to predict their niches (Mouillot et al. 2005Mouillot D., Mason W.N., Dumay O., et al. 2005. Functional regularity: a neglected aspect of functional diversity. Oecologia 142: 353-359., Mason et al. 2008Mason N.W., Lanoiselée C., Mouillot D., et al. 2008. Does niche overlap control relative abundance in French lacustrine fish communities? A new method incorporating functional traits. J. Anim. Ecol. 77: 661-669., Zhao et al. 2014Zhao T., Villéger S., Lek S., et al. 2014. High intraspecific variability in the functional niche of a predator is associated with ontogenetic shift and individual specialization. Ecol. Evol. 4: 4649-4657.). The eyes seem to be crucial for the differentiation of their feeding activity and the otolith for their hearing capabilities (Colmenero et al. 2010Colmenero A.I., Aguzzi J., Lombarte A., et al. 2010. Sensory constraints in temporal segregation in two species of anglerfish, Lophius budegassa and L. piscatorius. Mar. Ecol. Prog. Ser. 416: 255-265.).

ACKNOWLEDGEMENTSTop

The authors express their sincere thanks and gratitude to the Secretary of the Ministry of Earth Sciences (MoES), New Delhi and the Director of the Centre for Marine Living Resources and Ecology (MoES), Government of India, for supporting the work and providing the facilities onboard FORV Sagar Sampada for the sample collection. We are very grateful to the chief scientists, fishing master, fishing hands and all participants of FORV Sagar Sampada for their excellent cooperation during the cruise (Cr. No. 349). We also express our sincere thanks to William Watson (NOAA) for the critical evaluation of the manuscript, which certainly improved its quality. The editorial assistance from N Rajendran (CMLRE) is also thankfully acknowledged. The study was carried out as part of the in-house project “Resource Exploration and Inventorisation Systems” under the Marine Living Resource Programme of CMLRE, MoES. The financial, technical and logistical support from CMLRE is wholeheartedly appreciated. This is CMLRE contribution no 117.

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