What is Filurus dubius?

February 24, 2006
From: Gary Rosenberg

Dear Bill,
I recently came across the name Filurus dubius De Kay, 1843 and am trying to figure out how to list it in Malacolog. Filurus is treated as a synonym of Glaucus at http://www.seaslug.com/systemat.html  and in  American Seashells (Abbott 1974) it is treated as a possible synonym of Melibe (Chioraera).  The original description (DeKay, 1843 -Zoology of New York, 5: 11) says the following:

Genus Filurus.
Tentacula two. Gills in two series along the back. Vent terminal. Caudal appendage long and filiform.

Filurus dubius.
Description. Body cylindrical, enveloped in a loose transparent membrane through which the intestinal tube is apparent. Along the back are two rows of branchial? processes, six in number on each side; at their tips, furnished with five or six spiculae: these are only seen when the animal is in motion. Mouth terminal, composed of a loose festooned membrane, alternately dilating and contracting when the animal is in motion; when dilated, two small transparent tentacula are protruded. The abdomen, or upper surface, appears to be composed of numerous rings. The caudal portion becomes abruptly smaller than the body, is long, cylindrical, and tapering to a point.

Color. Abdomen silvery white; dorsal region and sides light brown; tail light greenish. The color of the body, however, appears to depend on the contained viscera.

Length of body, 0.5; of tail, 0.7 [inches].

This curious animal was taken while swimming in salt water with its body reversed. Its motion was vermicular, and it appeared to be very tenacious of life, as it lived several days in a vessel containing salt water which had not been renewed.

De Kay did not provide an illustration or state where the specimen was collected, but presumably it is from Long Island Sound, New York.

I'd appreciate any thoughts on this creatures identity.

Best wishes,
Gary Rosenberg
Department of Malacology
Academy of Natural Sciences, Philadelphia, USA

rosenberg@acnatsci.org

Rosenberg, G., 2006 (Feb 24) What is Filurus dubius?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/15932

Hi Gary,
I might as well start the day with a puzzle. I dont think this is a gastropod at all. The mention of a terminal vent [anus] immediately cancels out opisthobranchs and I presume other gastropods. If I saw this description without knowing it had been claimed as a mollusc I would have little doubt in thinking it was a polychaete. His mention of 5 or 6 spiculae which protrude from the tips of the gills when moving is to me a clear description of polychaete parapodia. Which fits with a terminal anus, seeing viscera through body wall, 'vermicular' movement, abdomen consisting of rings, etc.

One possibility is a species of Tomopteris, a planktonic genus of polychaetes some of which have a long filiform 'tail' - although they tend to have more than 6 pairs of parapodia. Another possibility is a species of Syllid worm, several of which are planktonic.

It doesn't fit Glaucus or Glaucilla which have two lobes on each side bearing cerata - no spicules and a lateral anus, and it doesn't fit any Melibe I know. Apart from shape, neither Melibe nor the related Mediterranean Tethys have been reported from the west Atlantic - they swim by lateral flexion of the body, have no spicules etc.

My suggestion would be to flick this name to the Polychaete people. I am sure it is not an opisthobranch.

Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Feb 24). Comment on What is Filurus dubius? by Gary Rosenberg. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/15932

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