Sphaeronectes tiburonae, Pugh, 2009

P. R. Pugh, 2009, A review of the family Sphaeronectidae (Class Hydrozoa, Order Siphonophora), with the description of three new species, Zootaxa 2147, pp. 1-48 : 35-38

Sphaeronectes tiburonae sp. nov.

Diagnosis: Large hemispherical nectophore, somewhat truncated on its proximal-basal side, 8 mm in height. Nectosac restricted to lower two-thirds of nectophore. Hydroecium small and tubular, up to one-third height of nectosac. Minute globular somatocyst, 0.25 mm in diameter. Lateral radial canals extending up to close to apex of nectosac, but then curving and running directly to ostium.

Material examined: A single specimen collected during Tiburon Dive 981 (11 th May 2006; 36°36.13'N, 122°22.57'W: depth of collection 1896 m). The specimen was initially fixed in 5% formalin, and later transferred to Steedman's preserving fluid. It is designated the type and is deposited at the Smithsonian's National Museum of Natural History ( USNM1124196 ) .

Description: The specimen consisted of a single nectophore, and a short part of the anterior region of the siphosome, which was tightly contracted into the hydroecium. The posterior part of the siphosome was broken off during collection. Photographs of the somewhat stressed specimen, taken shortly after recovery of the ROV, are shown in Figure 28.

Nectophore: The fragile nectophore ( Figures 28 & 29) was hemispherical in shape with a rounded, convex apex, rather Sphaeronectes haddocki , although it was obliquely truncated on its proximal side between the top of the external opening of the hydroecium and the base of the nectophore. In life it measured 8 mm in height and 8 mm across, in the proximal-distal direction, and 7.8 mm in the lateral direction. There was a slight decrease in the diameter of the nectophore above the ostium, but nowhere near as marked as in the two species described above. Nonetheless the ostial opening was quite narrow, measuring 4.5 mm in diameter. Typically it was equipped with a narrow velum.

The animal, when being photographed in the laboratory on board ship, was clearly stressed, probably as a result of temperature shock, and its nectosac underwent a number of violent contortions, such that it was difficult to establish its true relaxed condition. Some of the photographs showed the nectosac to be conical, with a pointed apex, but the configuration shown in Figure 28 (bottom) appeared to closest to the relaxed state. Based on this, the height of the nectosac was c. 5.5 mm, and thus it stretched to approximately twothirds the height of the nectophore itself. After preservation the nectophore shrank considerably in size and became greatly distorted, with the only obvious features being the hydroecium packed with the siphosomal stem, and the minute somatocyst, while the nectosac was very shallow and difficult to distinguish, with no signs of any radial canals.

The hydroecium was small and tubular, with only a relatively small opening onto the proximal side of the nectophore, and extended up to about one third the height of the nectosac. Typically the siphosomal stem was attached to the nectophore in the upper distal corner of the hydroecium, and at that point arose the somatocyst, which was without an obvious pedicle. The somatocyst was globular in shape and minute in size, measuring only 0.25 mm in diameter. In relative terms, with regard to the size of the nectophore, the somatocyst of Sphaeronectes tiburonae is by far the smallest of any Sphaeronectes species. No pigmentation was observed in the somatocyst, but it was seen to contain numerous oil droplets of varying sizes. As with S. haddocki , there was no obvious pedicular canal connecting the somatocyst to the nectosac. The four radial canals arose together, with the upper canal running over the apex of the nectosac and then down to the ostial ring canal, and the lower canal running directly down to it. The lateral radial canals looped upwards to close to the apex of the nectosac, but then curved down and ran directly to the ostial ring canal, without forming the additional loop that was found on the nectophores of the species described above. On the proximal side of the nectosac the angle between the upper canal and each of the lateral canals was c. 30°.

Siphosome: As noted above only the very anterior most part of the siphosomal stem remained with the specimen, and this was greatly contracted into the hydroecium. The cormidia were at a very early stage of development and no useful observations could be made concerning them.

Distribution: The species is known from a single specimen collected in the vicinity of Monterey Bay, California, at a depth 1896 m. There are several in situ frame grabs of probable Sphaeronectes specimens in the VARS database at MBARI that were taken between depths of 1385 and 2764 m. This depth range is very different from that known for the other two species described herein and so the records may refer to S. tiburonae , although one cannot discount the possibility that yet another Sphaeronectes species might be present at deeper depths.

Remarks: Although the nectophore of Sphaeronectes tiburonae is only slightly smaller that those of S. christiansonae and S. haddocki , the former can easily be distinguished on the basis of its minute somatocyst, its less curved lateral radial canals, and the small tubular hydroecium.

Etymology: Sphaeronectes tiburonae is named for the ROV Tiburon whose pilots so skilfully collected most of the specimens of the species described herein, and a great number of other siphonophore species beside. Unfortunately the ROV is now decommissioned, and it remains to be seen whether its replacement can be manoeuvred as skilfully.