Date of Award
Doctor of Philosophy
School of Science
Professor Glenn Hyndes
Dr Stephen Newman
Professor Euan Harvey
Field of Research Code
The angelfishes (family Pomacanthidae) have the greatest proportion (~30%) of hybridising species, compared to other families of reef fishes, with 26 species implicated in hybridisation. However, very little is known about hybridisation in angelfish, especially in terms of fitness of the hybrids and possible ecological consequences. Hybrids between three species (C. flavissima, C. eibli and C. vroliki) in the genus Centropyge have previously been reported from Christmas island, where these have been observed in heterospecific harems and interbreeding. This provides the unique opportunity to examine the breakdown in assortative mating in marine fishes. The broad aim of this study was to determine causes and consequences of hybridisation in angelfish at Christmas Island (Western Australia). To achieve this, the abundances, habitat and fitness of the three angelfish parent species and their hybrids have been investigated. Based on 14 years of surveys, C. flavissima was abundant (4.53 per 250 m2 +/- 0.66), whereas C. eibli, C. vrolikii, and all hybrid combinations were consistently low in abundance or rare (average abundance < 0.3 per 250 m2 +/- 0.03). Parent species and their hybrids displayed high niche overlap, with all being more abundant at 20 m depth compared to 5 m, and showing similar diets that comprised a mix of green, red, and brown algae. Thus, rarity of parent species and niche overlap would help to promote hybridisation in angelfishes at Christmas Island.
The relative fitness of angelfish hybrids against the parent species was evaluated by comparing key life history traits. Both the von Bertalanffy growth coefficient (k) and the asymptotic length (L∞) of the hybrid C. flavissima × C. eibli did not differ from the parent species C. flavissima. In comparison, C. eibli tended to grow at a slightly faster rate but reached a smaller size than the hybrid and C. flavissima, potentially increasing its reproductive success. Both females and males of the hybrid C. flavissima × C. eibli presented similarly developed gonads and showed all stages of oocytes and spermatocytes development to the parent species. The presence of post-ovulatory follicles indicated possible spawning activity (and fertility) of the hybrid. The level of fitness of the hybrids at Christmas Island was similar to their parent species, and could explain their persistence at this location.
Angelfish hybrids formed harems with all parent species and the species composition of the harems reflected the underlying patterns of abundance for the parent species. The total number of fish and number of males did not differ between different types of harems (mixed and pure). Harems comprised an average of ~4 fishes, generally with one male, but some harems had 2 males, which were found in each type of harem. Almost one third of the dominant males in the mixed harems were hybrids, while ranks two to six were occupied by similar percentages of hybrids. The ability of the angelfish hybrids reaching similar ranks as both males and females of parent species and being in similar sized harems in mixed harems, combined with them displaying similar growth and size, indicates that their reproductive output would be similar to the parent species C. eibli and C. vrolikii. However, since C. flavissima was far more abundant and was the only species to form mono-specific harems, its reproductive output would be larger than the other two species and its hybrid.
This study provides empirical evidence that hybridisation in reef fishes conforms to terrestrial-based theories, and thus advances our understanding of the processes underlying hybridisation in coral-reef systems. Given the variable changes in environmental conditions occurring in the ocean, hybridisation and introgression could prove beneficial if it can provide new genotypes that increase the adaptive capacity of hybrids and/or parent species.
Chapter 3 is not included in this version of the thesis.
Vitelli, F. (2019). Causes and possible consequences of hybridisation in angelfishes at Christmas Island. Retrieved from https://ro.ecu.edu.au/theses/2197
Available for download on Tuesday, May 05, 2020