hybridization is a controversial topic in coral reef ecology 
 1 , 
 2 . while small numbers of hybrid embryos can be produced in a few species 
 in vitro 
 , the evidence for hybrids in the field is often equivocal because the genetic techniques used for corals can not distinguish between hybridization and incomplete lineage sorting 
 .
in fact , only one of the over 1300 species in the order is generally accepted to be unequivocally of hybrid origin : 
 acropora prolifera 
 1 , 
 5 . nonetheless , hybridization is often invoked as a source of evolutionary novelty in the order 
 scleractinia 
 6 , 
 7 . here , we report an incidental observation on the potential for hybridization between two closely related scleractinian corals species in the family fungiidae , 
 ctenactis echinata and 
 ctenactis crassa .
these species are sympatric , often dominating large multi - specific assemblages of fungiid corals throughout the central indo - pacific 
 .
colonies of these species are superficially very similar ( 
 figure 1a and b ) but can readily be distinguished by the shape of the costal dentitions 
 .
both species are gonochoric , that is each colony is either male or female , and reproduce by broadcast spawning , releasing gametes into the water column for fertilization 
  ( 
 figure 1c and d ) . at our study site on sesoko island ( 2638'13.00"n ; 12751'56.24"e ) ,
okinawa , japan , spawning occurs following the full moons from july to august 
 .
furthermore , both species release gametes at the same time 
  and consequently there is the potential for hybridization . in the days before the predicted date of spawning in july 2013 , we collected four colonies of 
 c. echinata and six colonies of 
 c. crassa , to produce larvae for other experiments .
coral species in the family fungiidae , such as these colonies of 
 fungia repanda , are gonochoric broadcast spawners : each individual releases either eggs ( 
 c ) or sperm ( 
 d ) into the water column where fertilization takes place ( arrows indicate gametes ) . while the species are relatively easy to identify , determining the sex of each individual prior to spawning is impossible without destructive sampling to expose the gametes .
consequently , we placed each individual in a separate 20 l bucket containing sea water in the open air at approximately 20:00 h in order to sex each individual once gametes had been released .
on the night of 27 july between 22:30 and 23:30 h three 
 c. echinata and five 
 c. crassa spawned revealing that the three spawning 
 c. echinata were female , while four 
 c. crassa were females and one was a male .
the size of the eggs of each species at the time of release was distinct with a range in maximum diameter of 244266 m in 
 c. echinata and 133155 m in 
 c. crassa .
in contrast to earlier work on 
 c. echinata 
 , we saw no symbiotic algae in the eggs of either species .
we collected approximately 5000 eggs from the three 
 c. echinata females and mixed them with sperm from the 
 c. crassa male .
as a positive control we mixed eggs from the four 
 c. crassa females with the 
 c. crassa sperm .
approximately 100 eggs were observed under a stereo - dissecting microscope for cleavage , indicating fertilization , every 2 to 6 h over the next 24 h. at no point did we observe cleavage in the cross between species indicating that no hybrid embryos were produced .
in contrast , over 90% of 
 c. crassa eggs in the positive control were fertilized within 2 h. we conclude that despite synchrony in the time of gamete release between these two closely related sympatric species there appears to be strong pre - zygotic mechanism to avoid hybridization . while our observations are preliminary and in only one direction ( i.e. we did not cross 
 c. echinata males with 
 c. crassa females ) we predict that hybridization between these species is unlikely .
this observation adds to a growing body of evidence indicating strong pre - zygotic barriers to hybridization in many scleractinian corals 
 11 
 13 .
 hybridization is often cited as a potential source of evolutionary novelty in the order 
 scleractinia . 
 while hybrid embryos can be produced 
 in vitro , it has been difficult to identify adult hybrids in the wild . here , we tested the potential for hybridization between two closely related species in the family fungiidae . 
 we mixed approximately 5000 eggs of 
 ctenactis echinata with sperm from 
 ctenactis crass . 
 no hybrid embryos were produced . 
 this observation adds to a growing body of evidence for pre - zygotic barriers to hybridization in corals and challenges the claim that hybridization is a major source of evolutionary novelty in the order . 
Observation