Opisthobranch of the Week Data
Halgerda onna, the most recently described species of Halgerda (Fahey & Gosliner, 2001), is considered to be very rare on Okinawa and the above featured animal is the only one I've seen here and is also an image of the holotype*. The specimen was collected from a live stony coral reef, near the reef edge. Halgerda onna is named for the local village near the type locality for the species. The species has previously been seen in Guam in the 1970's: an 8mm individual and possibly an additional 6mm animal (C. Carlson, personal communication, 2001).
The following information on external morphology is from Fahey & Gosliner (2001):
The body is firm and smooth, like gelatin, but rigid. The profile is relatively low and the dorsum has a series of low, angled ridges arranged in a reticulate pattern . . . . There are small, low, pointed tubercles at the junctions of the ridges. The ridges are absent or very low in the center of the dorsum. There are small, yellow marginal tubercles along the edge of the mantle. The ground color of the dorsum and foot is white. The ridges and tubercles are the same color as the dorsum and there are short lines or dots of yellow arranged down the center of the dorsum. Irregular spots of dark brown pigment are scattered on the dorsum, especially near the mantle border. The mantle margin is outlined in yellow with a fine white edge.
The upright rhinophores are tapered towards the tip with black coloration on the anterior side of the stalk near the base, black apical coloration and a translucent white background color. There are 17-19 rhinophoral lamellae.
The six branchial leaves are moderately pinnate. The flat gill branches have black lines on the anterior and posterior sides of the branches. The tips are black. Within the finely divided pinnate portion of the gill rachis, near the apices, are numerous flattened, translucent structures that appear glandular. The anal papilla is white with a black spot near the opening.
The oral tentacles are short and digitiform. There are dark pigment spots on the ventral side of the mantle, along the sides of the body and also on the foot that are retained on the preserved animals. Along the edge of the foot is a fine white marginal line.
Phylogenetic analyses of Halgerda spp. (Fahey & Gosliner, 2001) have determined that H. onna is most closely related to H. dalanghita, united by their dark gill stripes and foot spots, elongate middle and outer lateral teeth, denticulate inner lateral teeth that are arched, and a tubular penis.
As of early May, 2013, thirteen described species of Halgerda have been featured on these pages, from Okinawa's main island and the Keramas. These thirteen species are:
*Holotype: See "Type" Dictionary for explanation.
Halgerda albocristata Halgerda batangas H. brunneomaculata H. carlsoni H. dalanghita H. diaphana H. elegans H. malesso H. okinawa H. onna - The featured animal above H. tessellata H. wasinensis H. willeyi