Opisthobranch of the Week Data
The following species description is taken from Marshall & Willan (1999):
This very small sacoglossan has a smooth body with a rounded head and foot corners. The rhinophores are very long, cylindrical, and they taper evenly. The cerata are arranged in two rows. The body is pale green heavily patterned with dark green. The cerata are dark green from the internal diverticulum and flecked with white. The rhinophores are pale green and patterned like the body. The foot is pale green with a dark green dendritic pattern.
Because Placida dendritica is common in temperate waters all round the world (Bleakney 1989) there have been numerous studies on its ecology. These studies have suggested that a complex of sibling species might exist (Jensen 1990; Trowbridge 1995: 123).
Gosliner (1987) points out that P. dendritica is found in the South African material most commonly associated with Elysia viridis on several species of green algae, including Codium spp., Bryopsis sp., as well as Halimeda cuneata.
Cynthia Trowbridge (2000) in a message posted on the Sea Slug Forum comments on the possible species complex associated with this animal:
The species is common throughout SE Australia, including Tasmania, around Melbourne (Port Phillip Bay and Western Port), and Sydney. I am using molecular techniques to amplify and sequence mitochondrial genes of this "species" or potential "species complex" throughout the world. I have collected animals from Scotland, Ireland, Australia, New Zealand, Oregon, and Japan to do my molecular work. If any colleagues from South Africa, the Mediterranean, South America, or the east coast of the U.S. had animals, I would gladly pay expenses and a collection fee to include animals from those areas.
In terms of why I think there are more than one species grouped under the name, I have studied the ecology of this "species" pretty intensely since about 1984 and there are some unusual anomalies in the chemical ecology, physiology, behavioural ecology, etc. Such anomalies would be easier to explain if this was a species complex.
Placida dendritica is currently unknown from the waters of Okinawa's main island and Atsushi Ono reports (per. com.) seeing only two animal in the Keramas (the above 5mm animal as well as a 3mm individual). The above featured animal was found and photographed on the rope of a buoy by Atsushi in February of 1999 from Agonoura Bay, Zamami Island, Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha. Atsushi has kindly furnished the above image of this 5mm animal and is used here with his permission.
The only other Placida species known from Okinawan waters at this time is P. cremoniana.