Reproductive isolation among Acropora Species (Scleractinia: Acroporidae) in a marginal coral assemblage

Abstract

Hybridization was proposed as being an important source of evolutionary novelty in broadcast-spawning reef-building corals. In addition, hybridization was hypothesized to be more frequent at the periphery of species' ranges and in marginal habitats. We tested the potential for hybridization in 2 ways: observations of the time of spawning and non-choice interspecific fertilization experiments of 4 sympatric Acropora species in a non-reefal coral assemblage at Chinwan Inner Bay (CIB), Penghu Is., Taiwan. We found that colonies of more than 1 species rarely released gametes at the same time, thus limiting the opportunities for cross-fertilization in the wild. On the few occasions when different species released gametes in synchrony, interspecific fertilization in experimental crosses was uniformly low (the proportion of eggs fertilized ranged 0%-4.58% with a mode of 0%), and interspecific-crossed embryos ceased development and died within 12 h after initially being fertilized. Ecological and experimental analyses indicated that reproductive isolation exists in these 4 Acropora species even though they have the opportunities to spawn synchronously, suggesting that hybridization is not very frequent in this marginal coral habitat at CIB.