Saccoglossus rhabdorhynchus, C. B. Cameron, C. Deland & T. H. Bullock, 2010

C. B. Cameron, C. Deland & T. H. Bullock, 2010, A revision of the genus Saccoglossus (Hemichordata: Enteropneusta: Harrimaniidae) with taxonomic descriptions of five new species from the Eastern Pacific, Zootaxa 2483, pp. 1-22 : 6-8

Saccoglossus rhabdorhynchus n. sp.

( Figs 1C,D ; 3A–F)

Material examined. Six specimens were dredged in 1939 from a depth of 36 meters on rocky bottom off Half Moon Bay , California, (37° 12' N, 122° 30' W). The specimens were found in the surface debris on hard shale rock. Holotype: Accession no. NMNH 1132795 . Paratype: Accession no. NMNH 1132796.

External features ( Figure 1 C, D ): The total length is about 20 to 30 mm; the length of the proboscis is 2.5 mm, and the collar is 1.5 mm, after fixation. The collar is broader than long with two posterior annular thickenings. The posterior end of the collar is broader than the anterior. The length of the branchial region is about 4 mm. Openings of the branchial sacs are externally visible. The proboscis has a shallow middorsal longitudinal groove. The genital ridges are in the form of knob-like elevations on either side of the gill pores, extending posteriorly beyond the branchial region. The color in life is bright red.

Internal features ( Figure 3 ): The nerve fiber layer of the proboscis is thickened middorsally along the shallow middorsal groove. The circular muscle fiber layer is quite conspicuous, being 4 or 5 fibers in thickness. Longitudinal muscle fibers are arranged concentrically, forming 8 or 9 rings. The proboscis cavity is narrow, extending nearly to the tip of the proboscis, completely filled anterior to the proboscis complex with irregular, granulated cells (connective tissue elements) limited all around by a layer of circular fibers. There is no dorsal septum and the ventral one extends only up to the ventral blind lumen of the stomochord ( Fig 3A ). Only the left dorsal coelomic cavity is in communication with the median proboscis canal, which opens to the exterior by a left proboscis pore. The lumen of the stomochord is continuous extending nearly to the tip. The anterior part of the stomochord is slightly bent dorsal wards and this part shows only lacunae. The ventro lateral caeca of the stomochord lumen are united with one another and wide open to the primary lumen ( Fig. 3B ). The cardiac vesicle extends ventrally over the sides of the stomochord. The glomerulus is present on the lateral sides of the stomochord and the halves are united anteriorly in front of the cardiac vesicle and the stomochord. The anterior face of the skeleton is projecting forward into a spine or obtuse projection ( Fig. 3B ). The keel of the skeleton is quite distinct and deep anteriorly ( Fig. 3C ); its cornua extend nearly to the posterior margin of the collar ( Fig. 3D ).

Both dorsal and ventral mesenteries of the collar are incomplete, being confined to the posterior third or fourth of the collar. The collar canals are short and vertically oriented, the walls being thick, with nuclei arranged in many strata ( Fig. 3E ). The perihaemal cavities extend up to the proboscis pore, separate from one another throughout. Peribuccal cavities are not distinguishable. The collar nerve cord is almost solid with only a few lacunae. It is continuous at both ends with epidermal invaginations ( Fig. 3E ). No dorsal nerve roots are present. The stomochord opens into the buccal cavity in the anterior part of the collar. There is an invagination of the trunk gut in the posterior part of the collar that fuses posteriorly with the collar gut.

The ventral part of the pharynx is about a third of the total. The ciliated epithelium of the gill septa is thick with the nuclei in many strata. The ventral longitudinal musculature is heavily formed ( Fig. 3F ). Gonads do not alternate with gill openings; only the lateral gonads are present.

Remarks. S. rhabdorhynchus is, as may be seen from the above description, similar to S. otagoensis in several characters, such as the extent of the skeletal cornua, the absence of the dorsal crest on the collar nerve cord, the absence of the ventral mesentery in the anterior part of the collar, the absence of lobed gonads, the origin of the genital ridges close to the collar and the arrangement in several strata of the nuclei in the ciliated branchial epithelium. One or more of these features are shared by certain other species of Saccoglossus , such as, S. horsti , S. shumaginensis , S. mereschkowskii , S. pusillus , S. kowalevskii , and S. ruber . While S. pusillus and S. ruber have only one character each in common with the species under consideration, S. horsti on the other hand is next only to S. otagoensis in its resemblance to the new species. Thus, they both have well developed ventral muscular ridges in the trunk; in both the genital ridges start near the collar; in both the gonads are not lobed; and finally in both the nuclei of the ciliated branchial epithelium are in several strata ( Table 1).

Although S. rhabdorhynchus more or less resembles certain other species of Saccoglossus , it exhibits some features that are shared by no other species of the genus. The complete filling up of the proboscis coelom by irregular granulated cells, the possession of a dorsal glomerulus and the complete separation of the perihaemal cavities through their entire length are characteristic of this species of Saccoglossus . The type of substratum from which this species was collected, hard shale rock, is also rather unexpected. Saxipendium coronatum lives upon rock ( Woodwick & Sensenbaugh, 1985) but this type of habitat has no parallel among the known species of Saccoglossus and is to be considered a unique feature of this species, unless in fact the debris on the shale amounts to a considerable layer of muddy sand.

Etymology. The trivial name of this species is derived from the fact that the proboscis coelom is solidly filled with granulated cells unlike that in all other known species of the genus.

Figures

FIGURE 1. Photographs of fixed specimens from the NMNH (Smithsonian). With the exception of A and F, all of the specimens have lost the anterior end of the proboscis. A, Photograph of a coiled specimen of Saccoglossus porochordus n. sp. showing the proboscis and dorsal side. B, Photograph of the ventral side of S. porochordus n. sp. C, Photograph of the dorsal side of S. rhabdorhynchus n. sp. D, Photograph of the lateral side of S. rhabdorhynchus n. sp. E, Photograph of the right side of S. shumaginensis n. sp. F, Photograph of the left side of S. shumaginensis n. sp. G, Photograph of the right side of S. sonorensis n. sp. H, Photograph of the left side of S. sonorensis n. sp. c, collar; p, proboscis; t, trunk. Scale bars = 5 mm.

FIGURE 3. Light micrographs of transverse sections of Saccoglossus rhabdorhynchus n. sp.: A, Proboscis with heartkidney complex. B, Proboscis neck showing the spine of the proboscis skeleton. C, Proboscis neck showing the proboscis vesicle. D, Anterior region of the collar. E, Posterior region of the collar. Inset: Collar canal. F, Anterior pharyngeal region of the trunk. bp, branchial pore; bs, branchial sac; bv, blood vessel; cc, collar canal; cl, collar lumen; clm, collar longitudinal muscles; cv, cardiac vesicle; dm, dorsal mesentery; g, glomerulus; gb, gill bar; nc, nerve cord; pc, proboscis coelom; np, neuropore; phd, periheamal diverticulum; pl, pharynx lumen; plm, proboscis longitudinal muscles; pv, proboscis vesicle; s, stomochord; sc, skeletal cornua; sk, skeletal keel; sl, stomochord lumen; sp, skeletal plate; ss, skeletal spine; tl, trunk lumen; tlm, trunk longitudinal muscles; vm, ventral mesentery. Scale bars A, B, C, D, E, F = 500 µm; E inset = 100 µm.