Okenia cf. amoenula ( Bergh, 1907 )

Okenia cf. amoenula ( Bergh, 1907) View in CoL

( Fig. 1 View FIG )

Idaliella amoenula Bergh, 1907: 80-82 , pl. 8, figs 6-11.

MATERIAL EXAMINED. — Luanda Province. Punta das Palmeirinhas , 9°09’S, 12°58’E, II.1987, 1 specimen 6 mm preserved length. GoogleMaps

DISTRIBUTION. — Okenia amoenula has only been reported from South Africa ( Gosliner 1987a). This paper is the first reference for this species to Angola.

DESCRIPTION

The background body colour is opaque white in the living animal. In the centre of the dorsum there are a few reddish spots and short lines of the same colour ( Fig. 1 View FIG ). The oral tentacles are white and have four yellow tips each. The dorsum has, on each lateral margin, ten short papillae with yellow pigmentation on the apex. All over the dorsum there is a network of spicules. The rhinophores are white, non-retractile, and ringed with 18 lamellae. There are 11 unipinnate (the largest one is bipinnate) branchial leaves white in colour, having the rachis yellow (in the distal region edged with two red lines). The tail has a yellow line in the centre with few red spots.

REMARKS

The single specimen collected was badly preserved, making dissection and investigation of the internal anatomy impossible. In the process of removing the foregut the radula was lost. For that reason a positive identification of the specimen could not be done.

Bergh (1907) described Okenia amoenula ( Bergh, 1907) in the binomen Idaliella amoenula Bergh, 1907 , as being a white animal with three crimson lines on the dorsum, which joined together in the anterior part, and a crimson patch on the anal area. The dorsal papillae were reddish and yellow, the branchial leaves had a crimson colour, and the rhinophores were yellow. Our specimen, as well as Bergh’s, has a white background colour, red and yellow pigmentation on the lateral papillae, and the branchial leaves are also reddish (with two red lines). However, our specimen shows a few differences in colour pattern from Bergh’s material (white rhinophores instead of yellow, white branchial leaves instead of crimson), and with that described by Gosliner (1987a) from South Africa (dorsum with more red pigmentation in Gosliner’s specimen and yellow lateral papillae, but also with red spots), which could be caused by differences in size. Otherwise they are identical.

Okenia amoenula has been considered by Schmekel (1979) as a synonym of Okenia mediterranea (Ihering, 1886) . Okenia mediterranea , like our material of O. amoenula , has a yellow line in the centre of the tail with some red spots, and the rhinophores are white. However, our specimen lacks tubercles in the centre of the dorsum, which is a definitive character that separates the two species ( Valdés & Ortea 1995).

Okenia amoenula is similar in colour to Okenia hispanica Valdés & Ortea, 1995 . Both species have a white background colour, a white tail with a yellow line, and yellow pigmentation on the gill. However, they are clearly distinguishable by several external features. The dorsum of O. hispanica has two large pink patches, just behind each rhinophore, and another one on the anal area, the rhinophores are pink, and the oral tentacles are white, whereas in O. amoenula the dorsum is white with red spots or lines (lacking large patches), the rhinophores are white or yellow and the oral tentacles are white with four small yellow spots. Other differences include the absence of tubercles on the dorsum of O. hispanica and the radular morphology, already compared by Valdés & Ortea (1995).