REPORT Do ecological traits of low abundance and niche overlap promote hybridisation among coral-reef angelfishes? Federico Vitelli 1 • Glenn A. Hyndes 1 • Benjamin J. Saunders 2 • David Blake 1 • Stephen J. Newman 3 • Jean-Paul A. Hobbs 2 Received: 20 September 2017 / Accepted: 16 May 2019 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Hybridisation among species of coral-reef fish was previously considered to be rare. However, recent studies have revealed that hybridisation is prevalent in coral-reef fish, highlighting the need to understand the causes of this process. The angelfishes (family Pomacan- thidae) have the greatest proportion (* 30%) of hybri- dising species to date, with 26 species reported to hybridise. The aim of this study was to examine ecological factors (rarity of parent species and niche overlap) that promote hybridisation in terrestrial environments and to test these factors in the marine environment by examining hybridising angelfishes at Christmas Island (Indian Ocean). Three species of pygmy angelfish (Centropyge flavissima, C. eibli, and C. vrolikii) and their hybrids were investigated to address three objectives: (1) to determine temporal and spatial patterns in abundance and whether these patterns are linked to environmental factors; (2) to test for over- lapping patterns in habitat use; and (3) to test for over- lapping patterns in diet. Based on 14 yr of surveys, C. flavissima was abundant (4.53 individuals per 250 m 2 ± 0.66), whereas C. eibli, C. vrolikii, and all hybrid combinations were consistently rare (average abundance \ 0.3 per 250 m 2 ± 0.03). Parent species and their hybrids were more abundant at 20 m depth compared to 5 m. All species and their hybrids had similar patterns of abundance around Christmas Island, with significantly high abundances evident at the most sheltered sites. Parent species and their hybrids also had similar diets that com- prised a mix of green, red, and brown algae. The rarity of parent species, their niche overlap, and the haremic reproductive strategy likely promote hybridisation in angelfishes at Christmas Island. This study provides empirical evidence that hybridisation in reef fishes con- forms to terrestrial-based hypothesis, and thus advances our understanding of the processes underlying hybridisa- tion in coral-reef systems. Keywords Centropyge Á Christmas Island Á Diet Á Reef fish Á Habitat overlap Á Hybrids Introduction Hybridisation, through the interbreeding between two genetically distinct populations or species (Harrison 1993), can potentially increase or decrease species diversity (Seehausen 2004, 2006). Hybridisation has been docu- mented for at least 10% of animal species and 25% of plant species (Mallet 2007) and is common in terrestrial and freshwater environments (Arnold 1997). Hybrids usually exhibit characteristics from each parent, and although some hybrids are unfit, other hybrids are able to survive to reproduction (Mallet 2007) and can play a vital role in the evolution of plants and animals (Barton 2001; Wissemann 2005). For example, hybrids can colonise unoccupied Topic Editor Morgan S. Pratchett & Federico Vitelli fjvitelli@gmail.com 1 Centre for Marine Ecosystem Research, School of Sciences, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia 2 Department of Environment and Agriculture, School of Science, Curtin University, P.O. Box U1987, Perth, WA 6845, Australia 3 Western Australian Fisheries and Marine Research Laboratories, Department of Primary Industries and Regional Development, Government of Western Australia, P.O. Box 20, North Beach, WA 6920, Australia 123 Coral Reefs https://doi.org/10.1007/s00338-019-01816-6