Bargmannia profunda, Pugh, 2019

Pugh, P. R., 2019, Two new species of Bargmannia (Pyrostephidae, Physonectae, Siphonophorae), Zootaxa 4686 (1), pp. 74-82 : 74-82

Bargmannia profunda sp. nov.

Material examined: The specimen from ROV Doc Ricketts Dive 26 consisted of just nine nectophores. The specimen was collected at a depth of 3323 m on 27 th May 2009 at 35°07’N, 122°46.5’W, off California, U.S.A.

Holotype: The above specimen is designated the holotype and will be donated to the National Museum of Natural History , Smithsonian Institution, Washington, D.C., U.S.A.

Diagnosis. Known only by its nectophores that measured up to 20.7 mm in length. Lower lateral ridges do not combine with meso-laterals distally. No mouth-plate. Nectosac relatively narrow and resembling a flattened cylin- der. Triangular thrust block occupying almost half the length of the nectophore.

Description. An in situ frame grab was taken of the specimen ( Figure 5 ). This shows that before collection the specimen had lost almost all of its siphosome and this could explain why no siphosomal zooids were preserved. There were no signs of any pigmentation.

Nectophores: The nine preserved nectophores varied in size from 11.8 mm in length and 6.7 mm in width to 20.7 mm and 12.3 mm, respectively. The ratio of the total length of the mature nectophore to that of the nectosac averaged 1.4. Although fairly robust, several of the preserved nectophores had lost the ectodermal lining to their nectosac, and in all others it was damaged. Although collectively it was possible to trace the arrangement of the radial canals, it was very difficult to discern the extent of the muscle-free zone on the lower side of the nectosac, and it may be larger or smaller than that represented in the illustrations.

A younger nectophore is shown in Figure 6 . At that stage the thrust block was relatively small and the ascending mantle canal extended to about half its height. All the ridges were well-defined and as with most other Bargmannia species, except B. stenotes sp. nov., the upper and lower laterals curved round to join together. The meso-lateral ridges ran from their proximal junctions with the upper and lower ridges obliquely down to end below the ostium. One characteristic feature was that the lower lateral ridges did not unite, distally, with the meso-lateral ridges, but petered out a short distance from them. At this stage the upper lateral ridges ran down the nectophore in parallel and very close to each other. Shortly above the ostium they clearly divided into inner and outer branches. The inner ones continued directly to the ostium, while the outer ones ran obliquely outwards for a short distance before petering out.

In its preserved state the nectosac was quite narrow and approximately cylindrical. The ostial opening was broad. The long ascending mantle canal gave rise to a very short pedicular canal that in turn branched off just the upper and lower radial canals on the nectosac. The lateral canals arose, slightly asymmetrically, from the upper canal close to the apex of the nectosac. They ran obliquely out to the lateral margins of the nectosac and then straight down to the ostial ring canal. As noted above, because of damage to the lining of the nectosac it was difficult to assess the exact extent of the muscle-free zone or the position of the ostial ring canal. The ostial opening was deflected slightly toward the upper side of the nectophore, but this was almost certainly a preservation artefact. There was no mouthplate.

As the nectophores matured the main change was in the size of the thrust block ( Figure 7 ). It became roughly triangular in shape coming to a rounded point proximally, without forming a digitate process. The ascending mantle canal extended to about half the length of the thrust block and typically ended in a small swelling inflected into the mesogloea. The lower lateral ridges extended to approximately the same level before petering out. Distally these ridges still did not unite with the meso-lateral ridges.At the junction between all three main ridges there was a thick- ening that extended proximally for a short distance. Shortly above the ostium the upper lateral ridges branched, the inner branch running distally to the ostium, and the outer pair obliquely outwards for a short distance.

The ostial opening was wide, accentuated by the absence of the lining to the nectosac, and tended to be inflected slightly toward the upper side. The nectosac remained cylindrical in shape, and was only slightly emarginated on its proximal side. It was difficult to gauge the extent of the muscle-free zone, but it was clear that it extended slightly onto the upper side of the nectosac, proximally. The lateral radial canals arose, slightly asymmetrically, from the upper canal at a notable distance from the insertion point of the short pedicular canal.

Remarks. The arrangement of the ridges, and particularly the fact that the lower lateral ridge did not join, distally, with the meso-lateral one suggests a similarity with that of the smaller nectophores of Bargmannia amoena . Pugh (1999) noted that the nectophores of B. amoena could be split into three size categories, which probably reflected the state of sexual maturity of the colony. Nectophores of the middle range are similar in size to those of B. profunda sp. nov., and both had a similar ratio of overall length to nectosac length. Nonetheless, there is one feature that clearly distinguishes the two species and that is the presence of a distinct mouth-plate on the nectophores of B. amoena , while for B. profunda sp. nov. it was totally absent. This alone would seem to warrant the establishment of a new species. In comparison with the specimens of B. amoena also caught in the same vicinity the nectophores of B. profunda sp. nov. were more flaccid, in their preserved state, and the lining of the nectosac had a tendency to detach itself. Finally, the photograph of the in situ specimen ( Figure 5 ) showed no sign of any pigmentation, which would be very unusual if it were B. amoena as colonies of that species have bright orange-red gastrozooids and the whole stem is of the same colour, as are the radial canals on the nectosac of the nectophores. Indeed the nectosac itself is usually suffused with a pale orange coloration.

Distribution. Known only from a single specimen collected off California, U.S. A at a depth of 3323 m. However, an in situ video frame grab of a Bargmannia colony ( Figure 8 ) seen but not collected was taken during Tiburon Dive 853, on 7 th June 2005, position 37°06.67’N, 123°57.98’W, and at the very similar depth of 3242 m ( Figure 8 ) appeared to be very similar to the type specimen of Bargmannia profunda sp nov, but as it was not collected its identity remains uncertain.

Etymology. The name is derived from the Latin profundus, meaning deep and reflecting the great depth at which the specimen was collected.

Figures

FIGURE 1. A. Very young nectophore of Bargmannia stenotes sp. nov., upper side (left) and lower side (right). Scale bar 1 mm. B. Small nectophore of Bargmannia stenotes sp. nov. Upper (left), lateral (centre) and lower (right) views. amc. ascending mantle canal; bi, bo. inner and outer branches of upper lateral ridge; lop. lateral ostial process; lrc. lateral radial canal; mfz. muscle-free zone; ns. nectosac; pedc. pedicular canal; rll., lrm., rul. lower, meso- and upper lateral ridges; tb. thrust block. Scale bar 2 mm. Proximal at top, distal at bottom.

FIGURE 5. In situ photograph of Bargmannia profunda sp. nov. Scale unknown.

FIGURE 6. Young nectophore of Bargmannia profunda sp. nov. Upper (left), lateral (centre) and lower (right) views. See Figure 1 for list of annotations. Scale bar 5 mm.

FIGURE 7. Mature nectophore of Bargmannia profunda sp. nov. Upper (left), lateral (centre) and lower (right) views. Proximal at top. See Figure 1 for list of annotations. Scale bar 5 mm.

FIGURE 8. In situ frame grab of specimen from Tiburon Dive 853.