Opisthobranch of the Week Data
Platydoris formosa is considered to be very rare in the waters of Okinawa's main island. The featured animal is the only individual seen here by RFB, however, Yukinobu Adachi has a photo taken of P. formosa, also from Okinawa's main island. The above featured animal was collected during a mid-morning SCUBA excursion from an area of mixed sand and coral rubble; the animal came tumbling out amid a cloud of silt as a large slab of rubble was overturned. The color of this Okinawa animal is perhaps a bit atypical, but certainly well within the range of color seen in the species. On Erwin Koehler's Opisthobranchs From Around the World, there are a series of photo links to the species illustrating some of the color variations found within the species. Also, Marshal & Willan (1999, fig. 142, p. 223) depict a nearly all blood-red specimen.
This species is very similar in appearance to P. cruenta, however in that species there are obvious brown lines on the dorsum, unlike P. formosa, which has no brown lines but rather the presence of fine brown specks. Both of these species have red patches of pigment.
The following description of Platydoris formosa is taken from the description of external morphology from Dorgan, Valdes and Gosliner (2002):
External morphology. The body shape is flat and wide. The dorsum is entirely covered with small caryophyllidia. The rhinophoral and branchial sheaths are elevated. The branchial sheath has six branchial lobes, with the anterior and posterior ones being larger than the others. The gill is composed of six tripinnate branchial leaves. The perfoliate rhinophores always have 37 lamellae. The background colour of the dorsum is white cream. There are numerous small, light brown red dots covering the dorsum. There are also large pale cream irregular, ramified areas and large, rounded or oval dark red blotches randomly distributed on the dorsum. Generally, these red blotches are larger near the centre of the body. A few black spots occur between the rhinophores and anterior to the branchial sheath in the material examined. The rhinophoral and branchial sheaths are whitish, with irregular dark brown spots. The branchial leaves are cream with dark lines running along the rachises. The rhinophores are uniformly red, with the tips somewhat lighter. The ventral side is white and has many large, bright orange red spots, which are faded and often not at all evident in the preserved animals. These spots are larger and densely concentrated near the foot, where some of them can be fused together. There are also very small, dark brown to black spots on and around the foot.
As of late-March, 2003, the following four described species of Platydoris are known from Okinawan waters: