Flabellina marcusorum grows to a maximum length of 25 - 30 mm and has a mainly translucent pink or orange body colour. Elongate rhinophores, foot corners and cerata are tipped with an opaque white (or sometimes opaque yellow), with a broad, purple band beneath. The posterior face of the rhinophores may have up to one hundred papillae. The oral tentacles are elongate and thin, and are longer than the rhinophores.
Flabellina marcusorum is know to occur in two disjunct populations in the waters of central and south America: one group on the west coast of central America in the eastern Pacific Ocean, and the other on the east side of the continent in the Caribbean and in the western Atlantic Ocean down to Brazil, and as far east as the Galapagos Islands
This species is found in shallow waters in the subtidal and intertidal zones at depths of less than 200 metres.
- "Flabellina marcusorum | eBivalvia". Ebivalvia.lifedesks.org. http://ebivalvia.lifedesks.org/pages/80101. Retrieved 2012-07-05.
- "Malacolog 4.1.1: Western Atlantic Mollusk Species Database at The Academy of Natural Sciences". Malacolog.org. http://www.malacolog.org/search.php?nameid=9542. Retrieved 2012-07-05.
- "CDF Galapagos Species Checklists - Flabellina marcusorum". Checklists.datazone.darwinfoundation.org. http://checklists.datazone.darwinfoundation.org/marine-invertebrates/mollusca/flabellina-marcusorum-gosliner-kuziriam-1990/. Retrieved 2012-07-05.
- "WoRMS - World Register of Marine Species - Flabellina marcusorum Gosliner & Kuzirian, 1990". Marinespecies.org. http://www.marinespecies.org/aphia.php?p=taxdetails&id=224326. Retrieved 2012-06-04.
- Rudman, W.B.. "Flabellina marcusorum". The Sea Slug Forum. http://www.seaslugforum.net/factsheet.cfm?base=flabmarc. Retrieved 2012-07-05.