Suzuki MM, Nishikawa T and Bird A.

J Mol Evol. 2005 Nov;61(5):627-35. Epub 2005 Oct 4.

Genomic approaches reveal unexpected genetic divergence within Ciona intestinalis.

C. robusta sperm can fertilise C. intestinalis eggs. In 3 reciprocal crosses, C. int sperm was less efficient (40~70%) and in 2 crosses, it was ver inefficient (~5 %). Hybrids developed to young adults in laboratory. Reproductive isolation estimated to have started 30 MY ago. Ideal temperature for breeding differs of 5 °C.