Description - Add description
Mitral ring square, with circular gate and two divergent simple spines on each corner. Basal ring twice as broad, square, with four convex sides and one simple spine on each corner. Four columellae straight. All twelve rods with three smooth edges.
Dimensions: Breadth of the mitral ring 0.02, of the basal ring 0.05.
Habitat: Central Pacific, Station 266, depth 2750 fathoms.
|Benson, 1966, p. 312-315; pl. 22, figs. 3-6:|
Pseudocubus obeliscus Haeckel
Pseudocubus obeliscus Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 1010, Pl. 94, fig. 11.
Plectophora triacantha Popofsky, 1908, Deutsche Südpolar-Exped., vol. 10, pp. 262-263, Pl. 29, fig. 1, Pl. 30, fig. 1.
Obeliscus pseudocuboides Popofsky, 1913, Deutsche Südpolar-Exped., vol. 14, pp. 280-281, Pl. 29, figs. 4, 5.
Test consisting of a truncate-pyramidal cephalis composed of a lower, three-bladed to cylindrical, collar ring, elliptical to subtrapezoidal in outline, joined by three to five but generally four straight, upward-divergent, three-bladed columellae to an upper larger, three-bladed ring, elliptical to polygonal in outline but not coplanar, often with convex upward bars between the columellae. One of the columellae corresponds to the apical bar of the incomplete sagittal ring and extends from the median bar; each of two columellae originate from the junction of the collar ring and the primary lateral bars; the fourth columella is generally ventral, representing the ventral side of the incomplete sagittal ring. The median bar lies below the plane of the collar ring and is joined to it by the thin, cylindrical, primary lateral bars and the apical bar; the vertical bar or spine is absent; a free, thin, conical, vertically oriented axial spine arises from the ventral end of the median bar; the dorsal spine is free, thin, conical, relatively short. The primary lateral bars extend laterally from the collar ring as three-bladed spines with distal branches. In many specimens an outer ring-like structure surrounds the inner truncate-pyramidal cephalis. This is developed from 3-5 or more three-bladed spines (including the primary lateral spines) which originate from both the collar, ring and upper ring and extend radially outward but horizontally; at their distal ends, three-bladed bars are developed horizontally between the spines of each ring as well as vertically between the spines of both rings, together forming an outer box-like structure surrounding the cephalis on all sides. In many specimens the outer network of bars is less regular than indicated, but the inner cephalis distinguishes the species. Three-bladed to conical spines generally extend from and are collinear with the bars or columellae comprising the cephalis and the surrounding network. Numerous accessory spines, three-bladed to conical, arise from the surface of the rings and bars.
Measurements; based on 30 specimens from stations 106, 115, and 133: diameter of collar ring (dorsal-ventral) 25-34 µm, of upper ring (dorsal-ventral) 37-59 µm; maximum height of pyramidal cephalis 27-50 µm; maximum diameter of outer box-like network 74-117 µm; length of axial spine l0-20 µm, of dorsal spine 5-15 µm.
Remarks. Popofsky's illustration of Plectophora triacantha most closely resembles the Gulf species because it shows the continuation of spines laterally beyond the rings, presumably representing a rudimentary outer lattice surrounding the cephalis. Obeliscus pseudocuboides Popofsky as in the species mentioned above has a collar ring with the basal tripodium. Although Popofsky misinterpreted some of the basal spines, his illustration does show a basal tripodium similar to that of the Gulf species. The orientation of his drawing, however, makes interpretation difficult. In both species Popofsky based their definition on only one or two specimens. Most specimens from the Gulf conform to P. triacantha but a few were observed that are similar to O. pseudocuboides.
The reasons for naming this species Peudocubus obeliscus Haeckel are twofold: First, it is the earliest named, but Haeckel's illustration of it does not show the basal tripodium. Several specimens from the Gulf were observed with this structure broken away and, therefore, are very similar to Haeckel‘s diagrammatic illustration. Secondly, Haeckel placed Pseudocubus rightfully within the Superfamily Acanthodesmiacea, whereas Popofsky placed the two species mentioned above in the Superfamily Plagoniacea. The presence of the collar ring, basal tripodium, and a portion of the sagittal ring (apical and median bars, at least) as well as an upper ring is grounds for the inclusion of this species within the former superfamily.
All three species mentioned above were originally interpreted as having the smaller ring above the larger ring. As mentioned under the remarks of Pseudocubus this interpretation is wrong; the smaller collar ring lies below the larger ring.
Distribution. This species is cosmopolitan in the Gulf but has a much higher frequency in the northern half of the Gulf, especially at stations located within or near the diatomite facies. Its distribution in the northern half of the Gulf is controlled by upwelling that accounts for the diatom blooms. Its distribution in the southern half of the Gulf shows no response to upwelling in this region. It is absent at stations 27, 90, 130, 203, and 214. Its highest frequency (6.2%) is at station 133 where it is common and is the second most abundant species. It is common at stations 136, 191, 192, and 208. With the exception of 208 these stations are all located within the diatomite facies. Station 208 is adjacent to a diatom-rich area. The second highest frequency (6.0%) of this species in the Gulf is at this station where it is the fifth most abundant species. At all other Gulf stations it is rare but with higher frequency in the northern half of the Gulf than in the southern half. Its absence or near absence at marginal stations indicates its preference for an offshore or more nearly oceanic habitat.
Haeckel (1887, p. 1010) reported this species from the central Pacific at “Challenger” station 266. Popofsky
(1908, p. 263) reported Plectophora triacantha from 0-385 meters water depth near the coast of East Antarctica. Obeliscus pseudocuboides was reported by Popofsky (1913, p. 281) from the western tropical part of the Indian Ocean. The Gulf species, therefore, is apparently cosmopolitan at all latitudes.