Opisthobranch of the Week Data
Phestilla melanobrachia is considered to be uncommon on Okinawa; I've collected seven individuals since the first was found on April of 1986, although others have been seen and in some cases, photographed.
I had searched for this attractive aeolid for several years in vain, until I quite fortuitously stumbled upon it while looking closely at one of the frequently seen wentletraps (Prosobranch: Epitonium billeeanum ) which feed on the common "cup corals" (Tubastrea spp. and Tubastrea micrantha) found here. While observing the brightly yellow-colored wentletraps, I found my first Phestilla melanobrachia. The animal had a bright orange color very similar to the coral upon which it was feeding; several white "bleached-appearing" Tubastrea "cups" (i.e., the empty calyx) were in near proximity to the slug, which it had apparently recently fed upon. P. melanobrachia feed on several species of Tubastrea; one form is usually a bright orange color (Tubastrea spp.), while another is a frequently large and branched, very dark green color (Tubastrea micrantha). I've placed a series of thumbnail images showing some of the variations of the above animals onto a single page for easier viewing.
Incidentally, the species name of the slug means "dark arms", which probably means that the original description by Bergh (1874), based on preserved specimens, which had perhaps been feeding on the darker dendrophyllids, possibly Tubastrea micrantha. Baba (1984) dealt exclusively with the yellow form of Phestilla melanobrachia collected from Seto, Kii, Japan; the animals were collected between 1-4m and were all found on colonies of Tubastrea aurea.
In as much as these animals resemble their dendrophyllid prey so closely, perhaps the best detection method useful in finding these beautiful aeolids is to inspect the occasional white "bleached-appearing" coral cups, which are a consequence of the recently eaten polyps. This method sometimes results in the discovery of one or several of these attractive animals.
Phestilla melanobrachia is one of three species found on Okinawa as of this date (May, 1999). The three species are:
Phestilla melanobrachia (the above featured animal) P. lugubris P. minor