Alcha evelinae Marcus, 1949

Known distribution. Widespread in tropical and subtropical coastal zones: Brazil (Marcus 1949); California (USA) (Karling & Schockaert 1977); Mombasa (Kenya): McKenzie Point and English Point, at 6 m depth on Thalassia hemprichii, partly covered by the epiphyte Syringodium isoetifolium (Jouk & De Vocht 1989).

New localities in the Indian Ocean. McKenzie Point, Mombasa (Kenya) on Thalassia from some shallow tide pools with sandy sediment at the stairs near the Four Seasons Hotel (mid-eulittoral) (30/09/1991).

New localities outside the Indian Ocean. Mazatlan (Mexico), algae from a tide pool near some rocks near hotel Valentino (04/12/1996).

Material. The lectotype (a sectioned specimen) and two paralectotypes (two whole mounts) (SMNH). One whole mount and three sectioned specimens from California (SMNH). All the material of Jouk & De Vocht (1989) (more than 25 whole mounted specimens). Studies on live animals, one whole mount and one sagitally sectioned specimen from the new locality in Kenya. One whole mount from Mexico.

Remarks. Habitus and internal organisation largely correspond with the description by Karling & Schockaert (1977). Some animals were more uniformly blue, without showing clear separated bands of pigment. In the newly found specimens from Kenya and Mexico, the length of the complicated prostate stylet type III is 42 µm and 35 µm respectively, which correspond with the range found in literature: 33–53 µm (Marcus 1949; Karling & Schockaert 1977; Jouk & De Vocht 1989). We observed a large amount of sperm stored in the male atrium, which was not mentioned in earlier descriptions.

The female system is more or less as described by Karling & Schockaert (1977): a strong muscular female duct type I proximally splits into two enlarged sperm containing vesicles. Distally from these sperm-containing vesicles, the two oviducts enter the female duct. We did not observe the insemination ducts (see Karling & Schockaert 1977; Artois & Schockaert 2005). However, in the Californian specimens these insemination ducts are clearly present. These structures are very delicate and difficult to see, and we presume their presence in the Kenyan specimen.