Possible Natural Hybridization of Two Morphologically Distinct Species of Acropora (Cnidaria, Scleractinia) in the Pacific: Fertilization and Larval Survival Rates

Abstract

Natural hybridization of corals in the Indo-Pacific has been considered rather rare. However, field studies have observed many corals with intermediate interspecific or unusual morphologies. Given that the existence of F1 hybrids with intermediate interspecific morphologies has been proven in the Caribbean, hybrids may also inhabit the Indo-Pacific and occur more frequently than expected. In this study, we focused on two morphologically different species, Acropora florida and A. intermedia, and performed crossing experiments at Akajima Island, Japan. Results showed that these species could hybridize in both directions via eggs and sperm, but that fertilization rates significantly differed according to which species provided eggs. These results are similar to those reported from the Caribbean. Although all embryos developed normally to the planular larval stage, the developmental processes of some hybrid embryos were delayed by approximately 1 h compared with conspecific embryos, suggesting that fertilization occurred 1 h later in interspecific crosses than in intraspecific crosses. More successful hybridization could occur under conditions with low numbers of conspecific colonies. Additionally, a comparison of survival rates between hybrid and intraspecific larvae revealed that intra- and interspecific larvae produced from eggs of A. florida survived for significantly longer than those produced from eggs of A. intermedia. Considering these data, under specific conditions, hybrids can be expected to be produced and survive in nature in the Pacific. Furthermore, we identified one colony with intermediate morphology between A. florida and A. intermedia in the field. This colony was fertilized only by eggs of A. florida, with high fertilization rates, suggesting that this colony would be a hybrid of these two species and might be backcrossed.

The New Systematics of Scleractinia: Integrating Molecular and Morphological Evidence

“…However, data from such studies are limited in terms of taxonomic and geographic coverage. Crosses have been tested for a variety of Acropora species and interspecific fertilisation observed in several combinations (Willis et al 1997;Hatta et al 1999;Fukami et al 2003;Isomura et al 2013). Nevertheless, interspecific fertilisation rates tend to be lower than intraspecific ones (Wei et al 2012), allowing species boundaries to be defined (Willis et al 2006).…”

Interspecific Hybridization May Provide Novel Opportunities for Coral Reef Restoration

“…Hybrid survival and size were similar to that of the maternal purebred species, although it exceeded purebred values in some occasions/conditions. Maternal effects have previously been shown for survival of interspecific hybrid larvae from an A. florida × A. intermedia cross (Isomura et al, 2013), and for thermal tolerance and gene expression levels of intraspecific A. millepora hybrid larvae from a higher and lower latitude cross (Dixon et al, 2015). It is unclear whether the observed fitness for the F1 hybrids from our study is due to nuclear or cytoplasmic maternal effects.…”

“…Our results indicate a prezygotic barrier has not evolved to maintain reproductive isolation of these two species either, and that hybridization may occasionally occur in nature. Lower fertilization in one direction in Acropora hybrid crosses, as was observed for hybrid LT, is not uncommon Isomura et al, 2013). The likelihood of A. palmata eggs being fertilized by A. cervicornis sperm, for instance, is smaller than the likelihood of A. cervicornis eggs being fertilized by A. palmata sperm .…”

“…∧ Symbiodinium uptake was assessed before treatment commenced, hence recruits were not under treatment conditions. cross in Japan (Isomura et al, 2013). This delay, however, was only limited to one early time point and no aberrant development was observed in the hybrids at any time point.…”

“…Interspecific hybridization among Acropora spp. has been shown to occur in experimental crosses, with varying degrees of prezygotic barriers (Willis et al, 1997;Van Oppen et al, 2002;Fogarty et al, 2012;Isomura et al, 2013). Among multiple pairs of Acropora spp.…”