Marcus & Marcus, 1960
A. 3mm. On Avrainvillea nigricans in 0.5m. Treasure Cay, July 3, 1992. B. 1.5mm. On an algae-covered rock in the surf zone, ocean side of Whale Cay, July 1, 1996. Rodger Bunnell, collector. C. 2.5mm. On Cladophora in 0.5m, Guana Cay, February 19, 2004. All locations in Abaco, Bahamas. Photos: Colin Redfern.
Marcus's original description from Brazil describe a predominantly black animal with white along the anterior edge of the foot, and a white lines up the outer edge of each rhinophore to the white tip. The cerata are also white-tipped. There is a large white patch around each eye, which touch in the midline. Redfern's animals from the Bahamas show a white edge to the foot and a considerable degree of white spotting over the body. Redfern suggests that it doesn't grow much larger than 3mm long and although it is normally associated with it algal food Avrainvillea nigricans, it is sometimes found on Cladophora.
• Clark, K.B. (1984) New records and synonymies of Bermuda opisthobranchs (Gastropoda). The Nautilus, 98(2): 85-97.
• Marcus, Ev. & Marcus, Er. (1960) Opisthobranchs from American Atlantic warm waters. Bull. Mar. Sci. 10(2): 129-203.
• Redfern. C., 2001. Bahamian Seashells: a Thousand Species from Abaco, Bahamas.
Rudman, W.B., 2004 (March 1) Costasiella nonatoi Marcus & Marcus, 1960. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/costnona