Opisthobranch of the Week Data
Bornella hermanni is considered to be very rare on Okinawa as I have seen and collected a total of only two individuals over a period of many years of diving in OKinawan waters. The two animals were collected from different locations in relatively deep water during March of 1997, 28 days apart.
Bornella hermanni is one of three described species of Bornella currently found on Okinawa; the other two species are B. anguilla and B. johnsonorum. When disturbed, these attractive bornellids swim with a lateral undulation of the body are superficially similar in appearance to each other, but the large paddle-like extra-branchial processes of B. anguilla readily separate it from Bornella hermanni. Like other members of the family, B. hermanni is a hydroid predator and both of the collected individuals were found in dense colonies of feather-like hydroids.
The following description of Bornella hermanni is taken from Bill Rudman's Sea Slug Forum (Rudman, W.B., 2009):
The elongate general body shape is typical of the genus, with well-developed rhinophore stalks, paired dorso-lateral processes and oral tentacles elaborated into digitate oral lobes. The animal is a translucent cream colour with a broken network of red-orange lines on the back and sides of the body and scattered subepidermal opaque white granules which are sometimes aggregated in irregular patches. The colour pattern can extend onto the rhinophore stalk and the dorso-lateral processes. The lobe-like oral tentacle papillae, the branches of the rhinophore sheaths and the dorsolateral processes are translucent or pigmented white, sometimes with some yellow-orange pigmentation.
The rhinophore sheaths are tall and at the upper edge of each sheath are three long, narrow anterior and anterolateral papillae and a taller, posterior sail-like papilla, which bears two or three secondary papillae. Behind the rhinophores there are usually three pairs of dorsolateral processes, followed by two unpaired processes in the dorsal midline. The first and second pair of the dorso-lateral processes have three or four branches, one larger at the center (which can be bifurcate) and two smaller laterals. The third pair usually has three branches. In those dorso-lateral processes with four branches, there are three tripinnate translucent gills located on the inner surface at the junction of each lateral papilla. In dorso-lateral processes with three branches, there are two gills. The two posterior, single, dorsal processes are simple and decrease in size towards the tail. The living adults are up to 50 mm in length.
This species can be distinguished from B. stellifer by a number of external characters. In B. hermanni the large posterior process on the rhinophore sheath is always branched, while in B. stellifer it is single. In B. stellifer the rhinophores, dorso-lateral processes and oral tentacles all have an orange-red subapical ring which is absent in B. hermanni. In B. hermanni there are three pairs of dorso-lateral processes while in B. stellifer there are five or six pairs. There are also anatomical differences in the alimentary canal and reproductive system
I've added a second page with an additional image of the above featured animal as well as the second collected individual.
As of early September, 2012 I've recorded three described species of Bornella from the waters of Okinawa's main island. These animals are: