Ecology of hybridisation in angelfishes at Christmas Island

Post by Federico Vitelli

Reference: Federico Vitelli, Glenn A. Hyndes, Benjamin J. Saunders, David Blake, Stephen J. Newman, Jean-Paul A. Hobbs  Do ecological traits of low abundance and niche overlap promote hybridisation among coral reef angelfishes?

Hybridisation or the process of mating organisms of different varieties or species to create a hybrid has been traditionally considered rare in coral reef fish. However, recent studies have revealed that hybridisation is quite common in this group, and have highlighted the importance of understanding the causes of this process. Since the majority of fish hybrids in the marine environment have been reported from coral reef ecosystems, this is particularly an issue in conservation programs for these systems. Determining the factors that facilitate hybridization provides an understanding of how marine fishes overcome the barriers of assortative mating, as well as predicting how hybrids will cope with changing environmental conditions, as is being seen in coral reef systems with climate change. The angelfish (family Pomacanthidae) have the greatest proportion (~30%) of hybridising species with 26 species reported to hybridise. Three of these hybrids have been observed in heterospecific harems where they interbreed and provide a unique opportunity to examine the ecological factors promoting hybridisation in coral-reef fishes.

This study aimed to examine a range of ecological factors that are considered to promote hybridization in terrestrial environments, and test these factors in the marine environment by studying hybridising angelfishes at Christmas Island (Indian Ocean). In collaboration with the Department of Fisheries of WA, Parks Australia and Curtin University, we studied temporal and spatial patterns in abundance of the three parent species from the genus Centropyge (C. flavissima, C. eibli and C. vrolikii) and their hybrids, which have been reported from Christmas island. We tested for overlapping patterns in habitat use and diet. Based on 14 years of surveys (2002-2015), C. flavissima was abundant, whereas C. eibli, C. vrolikii and all hybrid combinations were consistently rare. Parent species and their hybrids were more abundant at 20 m of depth compared to 5 m. All species and their hybrids had similar patterns of abundance around Christmas Island with significantly higher abundance recorded at the most sheltered sites. In addition, all parent species were recorded in similar microhabitats characterised by massive corals, encrusting and turf algae (see Figure 1).

Figure 1 Mixed species harem: two individuals of Centropyge flavissima (bottom left) swimming with a C. eibli (top right). The formation of mixed harems can facilitate hybridisation by increasing the chances of encounters between individuals of different species. Photo credit: Tane Sinclair-Taylor

Spatial and taxonomic patterns in abundance were consistent across surveyed years. Parent species and their hybrids also had similar diets that were comprised of a mix of green, red and brown algae. These findings help support that the rarity of parent species and niche overlap aid in promoting hybridisation in angelfishes at Christmas Island. This study provides empirical support that hybridisation in reef fishes conforms to terrestrial-based theories, and thus advances our understanding of the concept in coral reef systems. This research is ongoing, with the next aim to determine differences in the fitness of the three parent species and hybrids in terms of growth rates and histology, with final results expected in 2017.

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