Opisthobranch of the Week Data
Halgerda rubicunda is unknown from Okinawa's main island as of mid-August, 2002, but the close proximity of Okinawa, as well as the similar opisthofauna to that of the Kerama Islands, suggests the possibility that it is also present here on the main island of Okinawa. The above featured animal was photographed and collected in the waters from the "Tounaka Dive Site" near Zamami Island, one of a series of islands in the Kerama Islands Group, which are located 30 ~ 40 kilometers west of the Okinawa capitol, Naha. This featured animal was collected by Atsushi Ono during June of 2002 in 5m of water from underside a stone. Atsushi's photograph is used here with his kind permission. It should be noted that Atsushi considers the species to be very rare in the Kerama Islands as he has seen only the above featured individual (per. comm.).
Baba (1949) described the species based on two specimens collected from shallow waters of Sagami Bay (July, 1939 & July, 1940). The following species description is taken from Baba's publication:
Length 1-3 cm. Body elongate-elliptical, the ends equally rounded. Back firm, raised into finely reticulated ridges, with distinct knobs at the points where these ridges join. Branchial plumes 5, bi- or tripinnate. Margins of branchial cavity wavy with 5 valves. Oral tentacles digitiform. General body colour orange-red, a short longitudinal band of dull yellow medianly between the rhinophores, and there is another, but transverse, band of the same colour a short distance in front of the branchiae. No labial plates. Radula formula about 25 X 23-25.0.23-25. The outermost (3-4) laterals finely pectinated at tip, all the remaining laterals simply hamate and smooth.
I've added a scanned image (Plate XXV, figures 94 & 95) of the original drawings from Baba's description.
Mike Miller previously featured this same animal on the Slug Site and Dave Behrens, writing about Mike's featured animal, makes the following comments:
Halgerda rubicunda differs from the more typical Halgerda form, but not having strong ridges connecting tall tubercles on the dorsum. Instead the body is lower in profile, firm, reddish orange in color and has very fine, low reticulated pattern of ridges. It has distinctive patches of interesting color, one between the rhinophores and one crossing the body in front of the gills. These patches vary with mixes of white, yellow and black. Ono-san's specimen here even has some greenish pigmentation.
This species is obviously very cryptic on the sponge species it feeds upon. The body coloration closely resembles the surface of the sponge (complete with oscula) as well as the gungie encrusted areas on the coral substrate nearby. This species reminds me of several Lamellarid gastropod species, and Sclerodoris tuberculata Eliot, 1904, who also have this similar coloration resembling variability in the substrate.
There is some confusion as to the generic placement of H. rubicunda; Rudman (1978) makes the following statement in his comparison of Halgerda and Sclerodoris:
Halgerda rubicunda Baba, 1949, from Japan, is most probably correctly placed in this genus. However, the shape of the gills and rhinophores, so characteristic of the . . . species, are rather different in H. rubicunda. No information is available on the internal anatomy. Until this is studied the correct generic position of this species must remain in doubt.
As of late November, 2005, thirteen described species of Halgerda are known from Okinawa's main island and the Keramas. These thirteen species are:
Halgerda albocristata H. brunneomaculata H. carlsoni H. dalanghita H. diaphana H. elegans H. malesso H. okinawa H. onna H. rubicunda - The above featured animal H. tessellata H. wasinensis H. willeyi