Reproductive isolation and genetic differentiation of a scleractinian coral Mycedium elephantotus

Sexual reproduction and genetic variation of the scleractinian coral Mycedium elephantotus at Maoao (MO), Yenliao Bay, northern Taiwan, and at Tiaoshi (TS), Nanwan Bay, southern Taiwan, were studied from 1990 to 1996. M. elephantotus is a hermaphroditic broadcast spawner with an annual reproductive cycle. Gametogenesis of the MO population began in January, and spawning occurred in July-August. In the TS populations, there were 2 reproductive groups with different timing of gametogenesis and spawning. Gametogenesis of the first group started in November, and spawning occurred in April or May, while gametogenesis of the second group started in March and spawning occurred in August or September. The temporal reproductive isolation of the 2 groups was possibly the byproduct of adaptation. Allozyme electrophoresis was applied to study the genetic variations among regional coral populations and the 2 reproductive groups. Significant genetic variations were detected among the MO and TS populations and between the 2 reproductive groups at TS, but no fixed differences were found. The MO population demonstrated larger genetic distances with TS populations (Nei’s unbiased D = 0.229 and 0.165, for the first and second reproductive groups respectively) reflecting the effect of isolation by distance. Significant genetic variation and moderate genetic distance (D = 0.045) between the 2 reproductive groups at TS indicate the effect of genetic differentiation due to reproductive isolation in sympatric populations. The lack of morphological differences and the moderate genetic distance between the 2 reproductive groups suggest recent diversification within the taxon. The presence of 2 reproductive groups of M. elephantotus in Nanwan Bay may represent the initial stage of sympatric speciation in marine environment or a secondary contact of allopatric populations.