Xenosiphon branchiatus (Fischer, 1894)

General description:

The external anatomy of Xenosiphon branchiatus is similar to species of the genus Sipunculus, but a distinguishing character is the “fuzzy” appearance of the middle trunk region. In the genus Sipunculus, only S. longopapilosus could be confused externally with X. branchiatus, because both have elongated papillae in the middle trunk. But the internal characteristics, such as the extra retractor muscle, attachment of the spindle muscle on the ventral side of the rectum, and the nephridial openings posterior to the anus are exclusive characteristics of X. branchiatus.

Look alikes:

Xenosiphon branchiatus may be confused with Sipunculus longipapillosus because both have elongate papillae in the middle trunk region, but the presence of the protractor muscles, the attached nephridia posterior to the anus, and the spindle muscle originating ventrally on the rectum are clear differences (Cutler, 1994).

Morphology:

(From Fisher, 1947, 1954; Cutler & Cutler, 1985 and Stephen & Edmonds, 1972)

External anatomy

Introvert much shorter than the trunk, a tenth of total length, covered by papillae (scale-like). According to Fisher (1954) the shape of the papillae is ovate, with a bluntly to broadly round tip, irregularly arranged, covering the surface of the introvert, but Cutler & Cutler (1985) described the papillae as triangular in shape.

Trunk with surface cut into a large number of small rectangles by the intersection of the longitudinal and circular muscles. In the mid-trunk region, covering 30 to 45% of the trunk, most of the skin rectangles have diagonal small canals extending from corner to corner. Off these small canals there may be one (in smallest specimens) to four (largest specimens) elongate digitate papillae (gill-like) giving a “fuzzy” appearance to this middle part of the trunk. In a live specimen (collected recently in Fort Pierce, Florida) the papillae were observed to be full of coelomic fluid, giving them a red coloration (Kawauchi, personal observation).

Tentacular crown composed of tentacles crowded into independent, elongated, longitudinal groups (16 to 18 groups) around the mouth (Fisher, 1954)

Internal anatomy

29-37 longitudinal muscles bands (LMBs), more commonly specimens have 31-34 LMBs.

Circular muscles divided in narrow bands.

A pair of nephridia originates posterior to the anus, at 15-20% of the trunk length, almost completely attached (80-90%) to the trunk wall.

The brain is bilobed, and has a dorsal tuft of digitate process.

The ventral retractor muscle (VRM) spreading over 2-4 LMBs (rarely 1-4 or 1-3), and the dorsal has the ventral edge originated from 8 or 9 (rarely 7 or 10) and spreading over 3 to 5 LMBs.

A pair of small extra retractors, the protractor muscles (PM), inserts into a pit or depression in the dorsal retractors and attaches to the posterior border of introvert.

Spindle muscle arises ventrally on rectum, attached to the rectal caecum and is not attached to the posterior end of the trunk.

Size:

Fisher’s specimens are 20 to 30 cm long (Cutler, 1994).

Distribution:

Xenosiphon branchiatus is found in shallow waters in the Pacific Ocean off Ecuador, Costa Rica, California, and Panama, as well as from the Gulf of Mexico, Florida and Puerto Rico (Fisher, 1954; Cutler & Cutler, 1985 and Cutler, 1994).

Habitat:

The specimen recently collected in Fort Pierce, Florida was found in a sand flat, in an exposed intertidal zone.

Sipuncula

Sipuncula

Xenosiphon branchiatus (Fischer, 1894)

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