Estimation of the reproductive timing of two genetically different lineages of the blue coral <i>Heliopora coerulea</i> (Pallas, 1766) around Sekisei Lagoon

Abstract

The blue coral (Heliopora coerulea, Pallas 1766) is a reef-building coral, which is recently been classified as a threatened species. Recent population genetic analyses have found two genetically distinct, cryptic lineages of this coral (types A and B). However, the ecological differences between them, as well as the drivers of this speciation, are still unclear. The aim of this study was to examine the possible differentiation in the reproductive timing of the two lineages, as a plausible cause of speciation. We subjected both formalin-and ethanol-preserved samples of the coral, collected from sympatric H. coerulea populations, for tissue and genetic analyses. Samples were collected every one or two months, from June 2014 to September 2017, around Sekisei Lagoon, southwest Okinawa, Japan. We also conducted field observations of the coral's reproductive behavior (larval brooding) in mid-June and July. As a result we could observe larval brooding behavior from mid-to late-July in type B; however no such behavior was observed in type A during the observation and sampling period. After examining a total of 468 histological samples, we found that the diameters of the oocytes and spermaries peaked from May to June in type A, whereas in type B they peaked from June to July, after which the spermaries disappeared or diminished. Our study demonstrates that the two H. coerulea lineages, under the same environmental conditions, exhibit different reproductive timings. This gap in reproductive timing, which possibly stemmed from endogenous differences, might have promoted their genetic differentiation and speciation into two distinct lineages.

Phylogeography of Blue Corals (Genus Heliopora) Across the Indo-West Pacific

“…Colonies of Heliopora sp. group (HC-A) are relatively small and more abundant outside the well-developed fringing reef, where water temperature is lower, while colonies of H. coerulea group (HC-B) can be larger, forming micro-atolls, and dominating inner reef habitats where water temperature is higher (Yasuda et al, 2010;Taninaka et al, 2018), providing evidence of niche differentiation which further support the hypothesis of ecological speciation between these two groups.…”

“…have different reproductive timing: the reproductive timing of Heliopora sp. group (HC-A) and H. coerulea group (HC-B) in Japan (Saito et al, 2015: Taninaka et al, 2018 and the Philippines (Villanueva, 2016) differ by almost one month. Heliopora hiberniana and H. coerulea colonies in northwestern Australia also have been found to have different reproductive timings even when found in sympatry (Richards et al, 2018).…”

“…Previous studies have also reported intermediate growth forms sometimes rendering it difficult to distinguish even between HC-A (Heliopora sp. group) and HC-B (H. coerulea group) possibly due to infrequent hybridization and phenotypic plasticity in the field (Yasuda et al, 2014;Taninaka et al, 2018). Therefore, not all colonies can be clearly classified into the Heliopora groups based on field observation alone.…”

“…Study from Yasuda et al, 2014 hypothesized that there are at least two genetically distinct lineages (HC-A and HC-B) of H. coerulea in the Kuroshio Current region that can also be distinguished by different gross morphologies (small branch for HC-A and flat shapes for HC-B, Yasuda et al, 2014Yasuda et al, , 2015Iguchi et al, 2019). Subsequent observations (Saito et al, 2015;Villanueva, 2016) and histological examination (Taninaka et al, 2018) revealed that the reproductive timing of the two lineages is different by almost one month even under similar environmental conditions. However, the phylogeographical relationships among HC-A and B in the Kuroshio Current regions, and with H. hiberniana and H. coerulea in northwestern Australia are still unknown.…”