The highest diversity of opisthobranchs is in the tropical regions, especially in the triangle formed by Papua New Guinea, the Philippines, and Indonesia. The number of species decreases dramatically in cold and temperate waters; nevertheless, some species have been reported in polar regions such as Antarctica and the Arctic Ocean. Many species have large geographic ranges because they have planktonic larvae, which can be transported long distances by ocean currents.

Costa Rica has two well-defined zones of opisthobranch fauna: one in the Caribbean, the other in the Pacific. There has been no contact between opisthobranchs from these areas since the closing of the Isthmus of Panama about 3.5 million years ago. Yet, some species are found in both bodies of water. Many of these, such as Stylocheilus striata and Aplysia parvula, are distributed worldwide in tropical waters. A few others, such as Flabellina marcusorum and Glossodoris sedna, are found only in this area.

Although there are not many species in common along the two coasts of Costa Rica, there are several cases of "sister species"; that is to say they share a common ancestor. This suggests that before the closing of the Isthmus of Panama both zones shared the same fauna.

The Pacific Coast of Costa Rica, the scope of this page, belongs to the Panamic Biogeographic region, which extends from 25ºN (Southern Baja California, Mexico) to 6ºN (Northern Peru). While it is quite possible to find the same species of opisthobranch throughout this region, species composition varies regionally, especially near the boundaries. These variations are more pronounced during years with abnormal climatic events such as El Niño, which affect the distribution patterns of species.

As of this printing, approximately 25 species have been found exclusively from the Pacific Coast of Costa Rica; nevertheless, this does not indicate that these species are endemic, because the lack of sampling in other areas. More sampling of the Panamic region is essential to discard or affirm claims of endemism.

Finally, it should be mentioned that while the distribution of opisthobranchs, in some cases, has been affected by human activities, there are no reports of artificially introduced species in Costa Rica.