Opisthobranch of the Week Data
I've not personally seen Phyllidiella nigra here in the waters of Okinawa's main island and Atsushi Ono mentions (pers. comm.) seeing only the above single featured animal in the Keramas.* Atsushi of course considers the species to be very rare in the Keramas. Atsushi found the above featured animal amid a bit of silt on a sandy bottomed bay in seven meters.
This phyllidiid is one of four species of described Phyllidiella found in Okinawan waters as of early-July, 2005. The four Okinawan species are:
The following description of P. nigra is taken from Brunckhorst (1993):
External morphology. Live specimens ranged in length from 22-63 mm and mean size was 36 mm. Phyllidiella nigra is broad and oval in shape, and convex dorsoventrally (cf. flattened). Dorsal colouration consists of a black background and dark pink to red tubercles. The notal tubercles are most commonly single and isolated (although occasionally two or three coalesce), have black bases and, their sides and apices are dark pink to red. They are evenly distributed over the dorsum. Individual tubercles are quite tall (up to 3 mm high) with relatively straight sides and a rounded summit. Tubercles around the mantle margin are smaller. There is no continuous pale edge to the mantle. The pitch black rhinophores are rounded apically and each rhinophoral clavus possesses 19-21 lamellae (specimens greater than 30 mm).
Ventrally, the hyponotum and gills are dark grey. The foot sole is uniformly grey. The sides of the foot have a darker grey band. The oral tentacles are broad, black and cylindrical with rounded tips.
Anatomy. Typical for the genus Phyllidiella. The anterior portion of the foregut is black. The pharyngeal bulb is quite broad when retracted. The intestine is also broad and it has black longitudinal striations on its surface. The reproductive organs are cream in colour except for the bursa copulatrix which is orange brown. The prostatic vas deferens is long. The penis is short and slightly bulbous. The penial spines are large (35-40 µm in vertical height) and have a broad pitted base and a long slender stem with a rough surface. The spines stand upright, are gently recurved distally, and their tip is slightly hooked.
Distribution. Phyllidiella nigra is usually encountered on intertidal reef areas. It is known from throughout the Indo west Pacific region. It appears to be less common in the eastern Indian Ocean.
Remarks. Phyllidiella nigra is easily distinguished from conspecifics by its tall, rounded, dark pink to red tubercles which are evenly distributed (not clustered) over the dorsum. The bases of these tubercles are black. Other characteristic features include: the entirely black, rounded oral tentacles; the black anterior part of the foregut; the broad, black, striated intestine; and the large, upright, broad based penial spines which have a hooked apex.
* Keramas = the Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha.