Bargmannia stenotes, Pugh, 2019

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

Bargmannia stenotes sp. nov.

Material examined: A single specimen collected by the ROV Tiburon Dive 674 that consisted of eight nectophores, a single bract, and a few gastrozooids and stem fragments. The specimen was collected at a depth of 3342 m on 22 nd May 2004 at 35°55.15’N, 122°56.29’W. Two other specimens from the same region and similar depths may belong to this species (see below). Unfortunately, no frame grabs are available for this specimen.

Holotype: This 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. Small nectophores, up to 11.4 mm in length, with narrow nectosac tapering distally. In preserved state, the distal end of the nectophore was bent outwards so that ostium opened on upper side. Upper lateral ridges do not join with lower laterals proximally, but meso-laterals unite with lower laterals distally. Lateral radial canals arose, from upper canal, at some distance from pedicular canal. Leaf-like, thin bracts with distinctive lateral flap.

Description. Nectosome: No trace of the nectosomal stem was found with the specimen and so it can only be presumed, as is typical for other known Bargmannia species, that the nectophores were budded off on the dorsal side of the nectosome. The nomenclature for the various ridges on the nectophores has changed slightly from that used by Pugh (1999) as the terms apico- and infra-lateral have come to be considered inappropriate. Thus, as in Pugh & Baxter (2014), the apico-lateral ridges become the upper laterals, and the infra-laterals become the lower laterals. The meso-laterals remain unaltered.

Nectophores: A very young nectophore ( Figure 1A ) measured 1.45 mm in length and 1.3 mm width. It had a somewhat strange appearance as material seemed to have become attached to all the edges, making it look somewhat fuzzy. The upper, meso- and lower lateral ridges were very distinct, while the thrust block was barely developed. The ascending mantle canal was thick and very obvious. The nectosac was narrow, and was further constricted in its mid-region. There were no signs of a mouth-plate.

The other preserved nectophores were at various stages of development and measured up to 11.4 mm in length, 6.2 mm in width and 4.3 mm in depth. For a developing nectophore ( Figure 1B ), measuring 5.5 mm in length, the thrust block had considerably enlarged and on its lower side there was a deep median furrow. Proximally the ascending mantle canal inflected slightly into the mesogloea and formed a small swelling. Distally, it gave rise to a very short pedicular canal that, on reaching the nectosac, branched off only the upper and lower radial canals. The lateral radial canals arose, asymmetrically, from the upper canal at some distance from the pedicular canal. Either lateral canal could arise first. There was a large muscle-free zone on the proximal part of the lower surface of the nectosac. In the preserved state the distal end of the nectophore was bent upwards such that the ostium of the nectosac opened on the upper side of the nectophore. The nectosac itself, again in the preserved state, tapered down from its slightly emarginated proximal end and was very narrow distally.

The lower lateral ridges arose close to the proximal end of the thrust block and demarcated the lower lateral margins of the nectophore. Distally they merged with the meso-lateral ridges, on a level with the ostium, and together they continued below the ostium on either side of a small, broad mouth-plate. The upper lateral ridges did not extend down to meet them proximally, only merging with the meso-lateral ridges. They curved inwards on the upper side of the nectophore and then out lateral again. At that point they became much less prominent, and the two branches that each gave rise to were often difficult to detect. The inner branches ran to the top of the ostium and the outer pair curved round to reach its lateral margin in the region of a small lateral ostial process.

For the mature nectophores ( Figures 2 & 3 ) the thrust block had increased further in size and occupied almost half the length of the nectophore. It was broad, only tapering in its proximal third, and its proximal end was formed by a digitate process that frequently was bent upwards. The ascending mantle canal arose below the tapering section of the thrust block and continued to about half the height of the nectophore. Typically, as noted in other Bargmannia species, it was slightly inflected into the mesogloea at its proximal end, and formed a small swelling. The nectosac, in its preserved state, was narrow and tapered down from its proximal end. As with the younger nectophore, its distal end was bent upwards so that the ostium of the nectosac opened on the upper side of the preserved nectophore. There was a large muscle-free zone on the proximal part of the lower surface of the nectosac that also spread, in the central region, for a short distance onto the upper surface. In the middle of this zone, the very short pedicular canal gave rise to only the upper and lower canals. Again the lateral canals arose asymmetrically from the upper canal, at some distance from the pedicular canal. These ran obliquely to the sides of the nectosac and then ran directly to the ostial ring canal.

The arrangement of the ridges remained the same as in the younger nectophore with the upper laterals not joining with the lower ones. The lower lateral edge of the nectophore was distinctly emarginated in the region where the lower laterals ridges merged with the meso-laterals ones. The mouth-plate was now well-defined although remaining relatively small. It was either rounded or very slightly emarginated in the mid-line. The lower nerve tract could distinctly be seen running over it. The lateral ostial processes were reduced to short, narrow strips.

Siphosome:As noted above the parts of the siphosome that were preserved consisted of only a single bract; three gastrozooids and some tangled tentilla.

Bract: The single bract ( Figure 4A ) that was found with the nectophores measured 5.5 mm in length and 4 mm in maximum width. It was thin and leaf-like. Although the edges were irregular the only clear character was a small flap from the upper side of the bract that hung out laterally on the inner side. The bracteal canal did not reach either end of the bract, and arose from a distinct indentation on the inner side of the bract. Until further, more complete specimens are collected one cannot be certain that this bract actually belongs to Bargmannia stenotes sp. nov.

Gastrozooid and tentacle: The larger gastrozooid measured 3.5 mm in length ( Figure 4B ) and featured an enlarged and distinctive basigaster, from which arose the tentacle of which only remnants remained.

Tentilla: The tangled mass of tentilla ( Figure 4C ) proved difficult to study, and all that will be remarked upon was the uncoiled cnidoband, and a long terminal filament. As is usual for Bargmannia species a few stenotele nematocysts, measuring c. 60 µm in length and 48 µm in diameter were present, but the remainder of the cnidome was not investigated. However, the large cells present on the terminal filament (see Figure 4C ) were not nematocysts.

Remarks. Although this description is based on a single, very incomplete specimen there are certain characters that establish the specimen as a new species. Firstly, the single bract is very distinctive. There is, of course, the possibility that this single bract did not belong to the species being described, but evenso the morphology of the nectophores is sufficient to distinguish the specimen as a new species. Thus, for instance, the mature nectophores were much smaller than those of all the presently described species. The longest nectophore of Bargmannia stenotes sp. nov. measured 11.4 mm, while according to Pugh (1999) the maximum for B. amoena was 19 mm, although for that species there was a very large range of sizes that might indicate that there is more than one species involved. B. elongata Totton averaged 21.3 mm and B. lata nectophores ranged between 14.79 and 31.94 mm in length. Nonetheless, the extreme narrowness of the nectosac is another characteristic feature as is the extent and shape of the thrust block. This is reflected by the high ratio between the total length of the nectophore and that of the nectosac, i.e. c. 1.90, compared with the next highest, 1.67, for B. lata . The marked bend in the distal part of the preserved nectophore, resulting in the ostium opening upwards, is also very characteristic, as is the fact that the upper lateral ridges do not join the lower lateral ones. In toto these characters appear sufficient to establish the specimen as a new Bargmannia species.

Two other Bargmannia specimens were collected at similar depths, in the vicinity of Monterey Bay, that might also belong to this species, both from Doc Ricketts Dive 666 (19 th September 2014, 36°15’N, 122°09.99’W). One of these, from a depth of 3453 m, had just six nectophores in an extremely poor condition. The other, collected at a depth of 3172 m, consisted of thirteen nectophores, also in a poor condition, and two minute bracts. The nectophores were slightly larger than those of the holotype specimen of B. stenotes sp. nov., and rather than just the ostial region, in the preserved state, being bent outwards, the nectophore was distinctly bent in it mid-region, giving the ascending mantle canal a distinctly arced appearance. Although the nectophores of these two specimens closely resembled those of B. stenotes sp. nov., there was insufficient evidence to be certain that they belonged to that species.

Distribution. The type specimen was collected just north of the Davidson Seamount, California, U.S.A. at a depth of 3342 m.

Etymology. The Greek stenotes , meaning narrowness, refers to the narrowness of the nectosac in the nectophores.

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 2. Mature nectophore of Bargmannia stenotes sp. nov. Upper (left), lateral (centre) and lower (right) views. mp. mouth-plate; for other annotations see Figure 1. Scale bar 2 mm.

FIGURE 3. Photographs of one of the preserved nectophores of Bargmannia stenotes sp. nov. Upper (left, lateral (centre) and lower (right) views. Scale bar 5 mm.

FIGURE 4. Bargmannia stenotes sp. nov. A. Possible bract. Proximal end at bottom; B. Gastrozooids. C. Tentilla. bg. basigaster; cn. cnidoband; tf. terminal filament Scale bars1 mm.