Phyllidia koehleri Perrone, 2000

Yonow, Nathalie, 2012, Opisthobranchs from the western Indian Ocean, with descriptions of two new species and ten new records (Mollusca, Gastropoda), ZooKeys 197, pp. 1-130 : 47-48

Phyllidia koehleri Perrone, 2000 Fig. 19, Plates 64, 65

Phyllidia koehleri Perrone, 2000: 89, figs. 1-4 (Maldives).

Material.

Maldives: 11 × 6 mm pres. (NY-142, orange with black triangles), Fulidhoo House Reef, Felidhoo Atoll, 20 m depth, 12 January 1991, leg. N Yonow; 14 × 7 mm pres. (SH-65, "bright yellow" with black ring), Maaya Tila, Ari Atoll, 16 m depth, August 1991, leg. S Harwood; 17 × 7 mm pres. ("yellow with black" ring), Hans’ Place, North Malé Atoll, 23 m depth, 3 November 1994, leg. SG Buttress & RC Anderson; photographs only of two individuals together, 10 mm approx. lengths (yellow with black ring), 12 m depth, July 1992, J Hinterkircher.

Description.

Phyllidia koehleri is a small species (the largest recorded size is 17 mm preserved length, above), bright yellow or orange, with a central longitudinal black line and a black ring, solid or broken, encompassing the anus and rhinophores. Body oval with single raised ridge behind each rhinophore, either side of central black line. Apart from two strong central ridges and tiny single tubercles covering skirt, the mantle is smooth. Broad central black band extending from behind the rhinophores to the anus, black ring then encloses central area (Plate 64); colour in life of these specimens yellow. Sometimes, as in one specimen, the black ring is broken into dashes which form bases of black triangular rays extending to margin (Plate 65); colour of this specimen orange but both patterns occur in both colours in images on the internet. There may be a few black spots in the marginal band and the orange form appears more tuberculate than the yellow ringed form. Rhinophores same colour as body, yellow or orange depending on body colour, extremely long and thin, bearing little more than 10 diagonal lamellae. Anal papilla translucent, extending from raised rim at junction of two ridges very near the posterior border, surrounded by some small tubercles. Ventrally, foot white in life, but hyponotum and gills yellow or orange in life. Anterior margin of foot notched, head white, and large oral tentacles triangular, grooved on outer margins, tipped with orange in life (Fig. 19A-C).

Internally, all specimens are similar, and illustrated here (Fig. 19D-F). The pharyngeal bulb is conical and slightly asymmetrical. Its posterior surface is glandular, the ventral projections a little longer than the dorsal ones. The oesophagus is very long, exiting the bulb posteriorly and bending anteriorly to pass through the nerve ring sitting on the bulb.

Distribution/Remarks.

This small species appears to be endemic to the Maldives; a dozen photographs on various websites are recorded only from the Maldive archipelago. Colour variation appears standard: either animals have a black ring or they have black triangles, and both patterns occur in yellow and orange. Although they appear rather different, they are consistent in external (two strong ridges, scattered marginal tubercles, rhinophores with few diagonal lamellae) and internal (asymmetrical buccal bulb, a long oesophagus bending anteriorly before passing through the nerve ring) morphologies.