Cryptic Species of Mugil cephalus in the Northwestern Pacific

Three cryptic species (NorthWestern Pacific cluster: NWP1, NWP2 and NWP3) of Mugil cephalus have been recently identified using several genetic markers and have been found to be living in sympatry in the northwestern Pacific. Spatial and temporal spawning isolation were the assumed mechanism which inhibited their gene flow. However, the spawning period and location has only been found for NWP1. In this study, the spawning periods of the different M. cephalus species were estimated using the temporal change in the gonardosomatic index (GSI) of 402 individuals from August 2009 to March 2010 in the Kaoping River estuary. After species identification using rapid PCR screening, 2 major M. cephalus species (NWP2 and NWP3) were found to be sympatric and occupied more than 95% of the estuary.  The time-series GSI changes of these 2 warm-water cryptic species suggested that the GSI of NWP2 increased to 10% during September and October and then disappeared from the estuary probably to spawn in the offshore waters during November. NWP3, on the other hand, is a tropical species with longer spawning period. Its GSI increased to 20% from October until February of the next year. The spawning period of NWP2 is isolated from its temperate sister species, NWP1, which only spawned from middle December until early January, have GSI as high as 29.8%. Although the spawning period of NWP3 overlapped with NWP1, we didn’t find any NWP3 individuals in the spawning ground of NWP1 for three years (2005-2007). This suggested that spatial and temporal spawning isolation prevented the gene flow among 3 sympatric M. cephalus cryptic species in the northwestern Pacific.