Halistemma maculatum, Pugh & Baxter, 2014

Pugh, P. R. & Baxter, E. J., 2014, A review of the physonect siphonophore genera Halistemma (Family Agalmatidae) and Stephanomia (Family Stephanomiidae), Zootaxa 3897 (1), pp. 1-111 : 70-82

Halistemma maculatum sp. nov.

Halistemma rubrum Totton, 1954 (in partim) Text—fig. 16, A, D, F.

Material examined:

Dive Date Position Depth

JSLII 716 12 May 1983 c. 25°19.5'N 76°55'W? 122 m.

JSL II 983 - CGP8 26 October 1984 25°6.8'N 77°23.1'W 568 m *.

JSL II 996 - CGP4 31 October 1984 25°22.5'N 77°54.9'W 610 m *.

JSL II 1411 - DS8 6 September 1986 39°56.4'N 70°14.3'W 640 m.

JSL II 1673 - CG9 4 October 1988 26°27.9'N 78°2.9'W 828 m.

JSL II 1675 - CG1 5 October 1988 26°25.6'N 77°52.8'W 777 m.

JSL II 1679 - CG11 7 October 1988 25°50.3'N 77°14.8'W 753 m *.

JSL II 1679 - CG12 7 October 1988 25°50.3'N 77°14.8'W depth not recorded *.

JSL II 1679 -DS6 7 October 1988 25°50.3'N 77°14.8'W 720 m *.

JSL II 1681 - CG9 8 October 1988 26°25.0'N 77°52.6'W 800 m.

JSL II 1685 - CG4 10 October 1988 26°27.8'N 77°58.9'W 837 m.

JSL II 1687 -DS4 11 October 1988 26°23.0'N 78°35.6'W 715 m.

JSL I 2634 -DS3 4 November 1989 26°14.1'N 77°43.9'W 799 m.

JSL I 2646 - CG3 10 November 1989 26°3.0'N 77°32.0'W 792 m.

JSL I 2652 - CG3 15 November 1989 26°0.1'N 77°28.2'W 606 m.

JSL I 2652 - CG 4a 15 November 1989 26°0.1'N 77°28.2'W 604 m *.

* indicates specimens at the post-larval Nectalia -stage. Emboldened— holotype specimen

Holotype: The JSLII Dive 1685- CG 4 specimen has been designated the holotype and has been donated to the National Museum of Natural History , Smithsonian Institution, Washington, D.C., U.S.A.

Diagnosis: Basic Halistemma ridge pattern on nectophores, with complete vertical-laterals, joining both the upper and lower laterals at a right-angle. Lateral ridges join upper laterals. Mouth plate present and deeply divided. Three types of bract, with the Type A having multiple patches of ectodermal cells on the upper surface and, uniquely for this species, a distal bump on the inner lateral edge. Tentilla with reduced, almost vestigial, involucrum that does not completely cover the first spiral of the cnidoband, and a small acorn-shaped terminal process to the terminal filament.

Description: The description will be based largely on the type specimen from JSL II Dive 1685, unless otherwise stated. Unfortunately, this specimen was not photographed after capture, but a tank photograph of the specimen collected during JSLII Dive 1673 is shown in Figure 78 .

Pneumatophore: The pneumatophore measured 1.5 mm in length, and 0.8 mm in diameter. There was a small cluster of brownish cells at its anterior end, which suggests that that it might have had a reddish cap in life.

Nectosome: The nectophores were budded off on the dorsal side of the stem.

Nectophores: Thirty-nine detached nectophores were present in association with the type specimen, while the nectosome bore several nectophoral buds, at varying stages of development, and the attachment lamellae of the detached nectophores. Detached nectophores ranged in size from 5.5 x 4.5 mm (length x width) to 11 x 10 mm.

For the very young nectophoral buds ( Figure 79 ) the ridges stood out distinctly, as did the two lateral pairs of ectodermal cell patches, and particularly the relatively enormous lateral ostial processes that were packed with nematocysts.

For the younger nectophores ( Figure 80 ) the central thrust block was quite small and under-developed, so that the pointed axial wings extended well beyond it. As the nectophores increased in size the central thrust block became more developed and although it only reached to the same level or slightly above the now squared-off, slightly outward and downward-sloping axial wings ( Figure 81 ). In the ostial region, the distal mouth plate enlarged proportionally with the size of the nectophores, and the left and right sides overlapped extensively in the mid-line. The lateral ostial processes gradually diminished in relative size and began to lose their nematocysts, either through abrasion or having been used and subsequently discarded. These processes were then covered in small plate-like cells. There were two pairs of almost circular patches of ectodermal cells on the upper lateral sides of the nectophore that almost certainly were sites of bioluminescence. One pair lay close to the centre of the triangular facet on each side between the upper and lower lateral, and the vertical lateral ridges; and the other pair in the middle of the facets demarcated by the upper and lateral ridges.

The general arrangement of the ridges on the nectophores of Halistemma maculatum sp. nov. was similar to that of all Halistemma species as described by Totton (1954) and Pugh & Youngbluth (1988). The pair of upper lateral ridges ran across the upper surface of the nectophore from the apical margins of the axial wings obliquely toward the ostium. They closely approached each other slightly above the ostium, but then curved out laterally and petered out before reaching it.

The vertical lateral ridges arose relatively high up on the upper lateral ridges, above the apex of the nectosac, and ran obliquely down the sides of the nectophore to join with the lower lateral ridges. These lower lateral ridges ran along the entire length of the lower lateral edge of the nectophore from the outer margins of the axial wings, where they joined with the upper laterals, to below the ostium where the petered out and joined with the lateral margins of the mouth plate. There were no distinct ridges on the lower surface of the nectophore; however, folds ran down the inner apices of the axial wings to just above the apex of the nectosac.

The lateral ridges branched from the upper lateral ridges at a quarter to one third of the way along the latter's length from the apices of the axial wings. Their course was initially almost parallel to that of the vertical lateral ridges, but quite soon they curved and ran directly toward the ostium. They did not reach the latter but petered out on a level with the upper laterals, ending just over the large lateral ostial processes.

The mantle canal on the proximal or axial surface of the nectophore ran from mid-way down the central thrust block to just below the central apex of the nectosac, in the region where the nectophore had been attached to the stem. The ascending and descending branches were almost equal in length, so the pedicular canal arose at its midpoint and reached the nectosac just below the apex of the nectosac, on its lower side. There it gave rise to all four radial canals. The courses of the upper and lower canals were straight. The lateral radial canals extended out laterally and on the lower surface quite close to the top of the nectosac. Towards the lateral margins of the nectosac the lateral radial canals formed a relatively sharp hump and then formed a loop up the lateral surfaces of the nectosac. From there, they looped back down to the lower surface where they formed another loop before returning to the lateral surface of the nectosac and running down directly to the lateral margins of the ostial ring canal.

Siphosome: In each cormidium immediately anterior to the gastrozooid, which lies posterior-most, were several palpons, and anterior to them a very distinct, single stalked, female gonodendron bearing a cluster of gonophores ( Figure 87 ). Two or more palpons separated it from the male gonophores that were attached directly to the stem, and anterior to them was another cluster of palpons, at least five in number. The disposition of the bracts was could not be determined.

Bracts: Ninety-nine detached, foliaceous bracts were associated with the type specimen and these could be divided into three categories on the basis of their general morphology, which included the numbers of lateral teeth and ectodermal cell patches on the upper surface. The bracteal canal lay just within the lower surface of the bract for most of the length, only entering the mesogloea and sloping up towards the upper surface close to its distal end. It ended below a small cup-shaped oval hollow on the upper surface that, in the younger bracts at least, was filled with tightly packed elongate nematocysts. Like those on the lateral ostial processes on the nectophore, these nematocysts had generally disappeared in the fully grown bracts.

Type A —Twenty-seven bracts of this type were found in association with the type specimen and were present in enantiomorphic forms ( Figure 82 ). These distinctive bracts were elongate and fairly oval, but tapered to rounded points both proximally and distally. They ranged in length from 16.5 to 24.5 mm in length and from 8 to 11.5 mm in width at their widest part. Each bore an asymmetrical pair of lateral teeth approximately at two thirds of the length of the bract from the proximal end, and a third, reduced tooth on the outer side at about one-third the length. A very characteristic feature was a pronounced bump on the inner lateral edge towards the distal end of the bract ( Figure 82 ). A median ridge on the upper surface was present in the distal two thirds of the bract. The mesogloea was markedly thickened in the proximal third of the bract, but rapidly decreased in thickness on a level with the proximal end of the median ridge, where the lower surface of the bract became concave. For the remainder of the length of the bract the mesogloea was slightly thickened on the outer side, while on the inner side it was very thin, often resulting in that side folding inwards. The bracteal canal originated on the inner side of the bract, very close to the proximal end. In mature bracts ( Figure 82 , left) it was thickened in the proximal third of the bract. At approximately the same level as the proximal end of the median ridge, and at the level of the reduced inner tooth, the canal decreased noticeably in diameter, but remained in contact with the lower wall of the bract. Thus as the thickness of the mesogloea and the bract itself decreased the canal followed the contour of the lower surface and curved upwards. In younger bracts ( Figure 82 , right), however, the canal remained thickened for most of the length of the bract, tapering only towards the distal end. The bracteal canal remained in contact with the lower wall throughout most of its length, but close to its distal end it penetrated into the mesogloea and ran obliquely toward the upper surface, where it ended below a small oval processes, on the upper side of the bract close to its distal tip. It is presumed that a cluster of small nematocysts once occupied this area, but on most bracts they been detached. Between 3 and 7 oval ectodermal patches of differing size were observed, distributed randomly (not paired) on the upper surface on either side of the mid-line, although due to their tendency of being abraded, their number varied somewhat.

Type B —This was the most numerous type with forty-five bracts in enantiomorphic forms being present with the holotype ( Figure 83 ). They were quite elongate, asymmetrical and widened at the distal end, measuring from 9 to 20.5 mm in length and from 5.5 to 12 mm in width. There were four lateral teeth that were asymmetrical and misaligned. The inner proximal one was longer and more pointed than the others, while the inner distal one was often reduced to a slight bump. There was only one ectodermal cell patch on the outer half of the upper surface of the distal part of each bract. A distinct conical, pointed process formed the proximal end of the bract. A medial ridge stretched from approximately the mid-length of the bract to almost the distal end on its upper surface. The bracteal canal originated at approximately one sixth of the length of the bract, from its proximal end, and on the inner lateral surface. It narrowed on a level just below the proximal end of the median ridge, but only penetrated into the mesogloea close to the distal tip, where it ran diagonally up to end below a small cupulate structure on the upper side of the bract. In the proximal region of the bract the mesogloea on the outer half of the bract was thickened. This thickening continued, in the central region of the bracts, around the mid-line, and thinning towards the lateral edges forming a thin and flimsy convexo-concave surface on the under side. The bracteal canal ran down the middle of this thickening and so there was no marked change in its course as found in the type A bracts.

Type C —These bract, twenty-seven in number, were somewhat similar to the previous type but less asymmetrical and with a pair of ectodermal patches on the upper surface, in the distal half ( Figure 84 ). They were also the most variable in shape, which appears to be age related, with the smaller ones being more rounded, while the larger older ones were elongate. They ranged in size from 5.5 to 22 mm in length and from 3.5 to 10 mm in width. There were two pairs of slightly misaligned lateral teeth, with the inner proximal tooth being somewhat smaller than the others and there was a small infolding at the level of the outer proximal one. A median ridge, on the upper surface, extended from just below the distal end for just under half of the length of the bract. Proximal to the origin of the bracteal canal, on the inner side of the bract, the mesogloea was relatively thick and formed a cylindrical process. Distal to this the mesogloea was thicker in the outer half, but also tapered down distally, so that the bracteal canal formed a smooth upward curve as it ran distally. In the larger bracts the thickened portion extended to approximately the same level as the proximal inner tooth, while in the younger ones it remained thickened for most of its length, only tapering when it entered the mesogloea before ending below the small cupulate process on the upper surface.

Gastrozooid and tentacle: The gastrozooids ( Figure 85 ) were variable in form and measured up to c. 5.5 mm in length. There was a distinct basigaster.

Tentilla: The mature tentilla are shown in Figure 86 . At the distal end of the highly contractile pedicel was a very small, almost vestigial involucrum, which hardly covered any of the cnidoband. The cnidoband usually consisted of six spiral coils, which were usually closely wound but occasionally somewhat looser. Two types of nematocysts were present; stenoteles and, probably, anisorhizas. The stenoteles, whose capsules measured 68 x 25 µm, where few in number and were found only on the outer edges of the proximal one or two spirals of the cnidoband. The other banana-shaped nematocysts were variable in size ranging from 39 to 69 µm in length and 6.5 to 9 µm in diameter. Large numbers of platelets, 8–9 µm in diameter, were visible on the cnidoband. The long terminal filament bore two types of nematocysts; probably acrophores, c. 18.5 x 9 µm, and desmonemes, c. 22 x 9 µm, as discussed above. The terminal filament ended in a process that was acorn-shaped, with a cupulate proximal part. This process was considerably smaller than that found on the tentilla of Halistemma cupulifera , and measured c. 180µm in length and 100 µm in diameter, with the cupulate proximal part occupying slightly under half the length. This part contained small nematocysts, c. 14 x 9 µm, while the distal part was covered in platelets, about 16 µm in diameter.

Palpon: The thin-walled, narrow palpons were featureless apart from a darker papilliform distal end ( Figures 85 , 87 ). They measured up to 6.5 mm in length. They possessed a long narrow palpacle, but no nematocysts were found on it or on the palpon itself.

Gonophore: The single male gonodendron was attached immediately posterior to a gastrozooid and the female one, with its thickened stalk, immediately followed it. The mature male gonophores were elongate structures ( Figure 88 ) measuring up to 3 mm in length and 0.9 mm in diameter. The female gonophores ( Figure 88 ) were almost spherical, with a short triangular pedicel and a widely open mouth. They measured up to 1.2 mm in length and 1 mm in diameter. No nematocysts were found on the gonophores of either sex.

Nectalia-stage: Five of the specimens of Halistemma maculatum sp. nov. were at the post-larval Nectalia -stage of development (see Material examined), of which three were collected during a single dive. One of these, with a nectophore and a C-type bract still attached is shown in Figure 89 . The nectosome bore up to eighteen nectophores that did not differ from those of adult specimens. The short siphosome, in live, bore a corona of long larval bracts ( Figures 90 , 91 ) that, in their preserved state measured up to 32.5 mm in length. They bore two pairs of distinct lateral teeth, and a series of irregularly sized and positioned ectodermal patches was present on the upper side of the bract for much of its length ( Figure 90 ). However, abrasion often made these patches difficult to see. There was a median longitudinal ridge on the upper side of these bracts that ran from their distal end to the approximate level of the proximal pair of lateral teeth. However, in the proximal half of the bract this ridge was only weakly defined. The mesogloea was thickest at the proximal and distal ends of the bract, and in the central part of the bract the lateral sides delimited a distinct concave cavity. The distal end of the bract was triangular in cross section. The bracteal canal originated close to the proximal end of the bract and followed the median line on the lower side of the bract; thereby firstly curving slightly upwards and then downwards. It entered the mesogloea close to the distal tip of the bract, curling upwards and then straight toward the distal point ending below a small cupulate process. Only in this final section did the canal show and signs of narrowing. The cupulate process was relatively large in the younger bracts, and was filled with nematocysts; but when the latter were lost or used up the cavity decreased in size. Adult bracts were often found, in small numbers, with these Nectalia -stage specimens, and were either of the A- or, more commonly, the C-type.

The short siphosomal stem typically bore two or three gastrozooids, including the protozooid, and a number of palpons, usually about five per gastrozooid. Larval type tentilla branched from the tentacle of the protozooid, while the other gastrozooids had tentacles with adult type tentilla. The larval tentilla ( Figure 92 , A) somewhat resembled those that D. Carré (1971, Plate II, fig. 1) illustrated for Halistemma rubrum , but there were certain notable differences. The tentilla were borne on long contractile pedicels and consisted of a sac, enclosing the cnidoband and a double elastic band, with a domed terminal process, with a short terminal filament projecting out from its distal summit.

The terminal process and filament bore no nematocysts, and the former often was annulated. Immediately proximal to the terminal process a dark band formed the beginning of the cnidoband, which stretched down one side of the enclosing sac for about two-thirds of its length. Two types of nematocysts were found. A row of about 25 stenoteles was present on either side of the cnidoband, but only on its distal two-thirds, that is the part closer to the pedicel. These stenoteles measured c. 65 µm in length and 27.5 µm in diameter. The remainder of the cnidoband was filled with banana-shaped nematocysts, probably anisorhizas, which measured c. 40 µm in length and 6.5 µm in diameter.

In some, but not all, of the Nectalia -stage specimens of Halistemma maculatum sp. nov. the palpons ( Figure 92 , B) showed certain differences from the ones found on the adult specimens. The distal proboscis region was often more pronounced, with a dark subterminal annular structure. In addition and usually, but not always, at the proximal end of the palpon there were included darker structures that were made up of variously sized oil globules. On occasion this these structures projected out from the side of the palpon.

Remarks: As noted in the remarks on Halistemma cupulifera, Totton (1954) found two forms of nectophores, which he called "e-type" and "f-type", and which he ascribed to Halistemma rubrum . We believe that the "e-type" nectophores belong to H. cupulifera , and that the "f-type" ones ( Figure 93 ) belong to this new species, H. maculatum sp. nov., although it is difficult to be certain of this.

Nevertheless, the nectophores of Halistemma cupulifera and H. maculatum sp. nov. are of a similar size. For both a mouth plate is present and the vertical lateral ridges are complete. The main difference between them is that in H. cupulifera the axial wings are squarely truncated and the thrust block extends to the same level, whereas in H. foliacea the axial wings are obliquely truncated and the broader thrust block does not extend to their level. However, such differences would probably not be apparent on specimens that were collected and probably mutilated by nets, such as Totton's specimens ( Figure 93 ).

The form of the different types of bract for each of these two species is very different and should clearly distinguish them, as might the arrangement of the gonodendra. However, the clear distinguishing feature is the structure of the tentilla. The terminal filament of the tentillum of Halistemma cupulifera ends in a very large cupulate process, while the terminal process on the filament of H. maculatum sp. nov. is very small (cf. Figures 34 and 86 ).

Distribution: All the specimens were collected in the region of the Bahamas, within an area only slight over a degree square. As well as the specimens examined, two others, from the same region, are known. These are:

JSLI Dive 2631-DS6, 2 November 1989, 26°22.5'N 78°38.3'W, depth 777m

JSLI Dive 2634-DS7, 4 November1989, 26°14.1'N 77°43.9'W, depth 806 m.

Apart from the specimens from JSLII Dive 716 and 1679-CG12—the former's depth of collection being rather dubious and the latter's unrecorded; all the specimens were collected with the moderately narrow depth range of 568 to 837 m. The mean depth for all the specimens was 727 ± 91.45 m, with the five " Nectalia " stage specimens tending to be among the shallower occurring ones (mean depth 651± 80.5 m).

Totton's (1954) "f-type" nectophores of Halistemma rubrum came from Discovery St. 1586 at 2°39.4'N 50°46.4'E, in the Indian Ocean off Somalia and possibly also from Mahabiss St. 145 at 4°59'S 73°16'E, central Indian Ocean to the south of the Maldives .

Etymology: The species is named for the ectodermal patches or maculae found on the nectophores and bracts.

Figures

FIGURE 34. Halistemma cupulifera. Four tentilla from the JSLII Dive 1450 specimen. cn: cnidoband; i: involucrum; tf: terminal filament; tp: terminal process. Scale bar 1 mm.

FIGURE 78. Halistemma maculatum sp. nov. Tank photograph of specimen collected during JSLII Dive 1673. Length not recorded.

FIGURE 79. Halistemma maculatum sp. nov. Very young nectophore from JSLII Dive 1679-CG12 specimen. Scale bar 1 mm.

FIGURE 80. Halistemma maculatum sp. nov. Upper (left) and lower (right) views of young nectophore from type specimen. Scale bar 5 mm.

FIGURE 81. Halistemma maculatum sp. nov. Upper (top left), lower (top right) and lateral (bottom) views of a mature nectophore from type specimen. Scale bars 5mm.

FIGURE 82. Halistemma maculatum sp. nov. Upper views of Type A bracts. Proximal at top, distal at bottom. Scale bar 5 mm.

FIGURE 83. Halistemma maculatum sp. nov. Upper views of Type B bracts. Proximal at top, distal at bottom. Scale bar 5 mm.

FIGURE 84. Halistemma maculatum sp. nov. Upper views of Type C bracts. Proximal at top, distal at bottom. Scale bar 5 mm.

FIGURE 85. Halistemma maculatum sp. nov. Piece of siphosomal stem from holotype, showing a gastrozooid and tentacle, and several palpons with palpacles. Scale bar 5 mm.

FIGURE 86. Halistemma maculatum sp. nov. Mature tentilla from type specimen. Proximal at top, distal at bottom. Scale bar 1 mm.

FIGURE 87. Halistemma maculatum sp. nov. Piece of stem from JSLII Dive 2646-CG3 specimen. fg: female gonophores; mg: male gonophore; p: palpon. Scale bar 5 mm.

FIGURE 88. Halistemma maculatum sp. nov. Male (top) and female (bottom) gonophores from JSLII Dive 2646-CG3 specimen. Proximal at top, distal at bottom. Scale bar 1 mm.

FIGURE 89. Nectalia stage of Halistemma maculatum sp. nov. from JSL II Dive 1679. Scale bar 5 mm.

FIGURE 90. Halistemma maculatum sp. nov. Upper views of Nectalia-stage bracts of JSLII Dive 996-SS4 specimen. Proximal to the left. Scale bar 5 mm.

FIGURE 91. Upper view of Type A (top left) and various views of Nectalia-stage larval bracts (all others) of Halistemma maculatum sp. nov. from JSLII 1679-CG12 specimen. Proximal at top, distal at bottom. Scale bars 2 mm.

FIGURE 92. Halistemma maculatum sp. nov. A. Larval tentilla, and B. palpons from JSLII 1679-D6 Nectalia-stage specimen. Scale bars 1 mm.

FIGURE 93. "F-type" nectophores, probably of Halistemma maculatum sp. nov., from Totton (1954, Text—fig. 16, C, D, F) Scale bar 2 mm.