Ikenoue, Dumitrica and Bjørklund, 2016
Description - Add description
|Joergensenium arcticum Ikenoue, Dumitrica and Bjørklund n. sp|
Text-Fig. 2.2; Plates 1-6.
Synonyms: 2003 Actinomma leptoderma leptoderma Itaki et al. p. 1517; Plate 1, Fig. 12
Holotype: Plate 2, Fig. 7, Station NAP, NAP11t shallow #25, Stored at the National Museum of Nature and Science, Tokyo with registration number MPC-04118, England Finder coordinates E17/4.
Paratype 1: Plate 1, Fig. 1, Station 32, 100-250 m, Stored at the National Museum of Nature and Science, Tokyo with registration number MPC-04119, England Finder coordinates T30/3.
Paratype 2: Plate 1, Fig. 2, Station 32, 100-250 m, Stored at the National Museum of Nature and Science, Tokyo with registration number MPC-04120, England Finder coordinates J10/3.
Paratype 3: Plate 1, Fig. 3, Station 32, 100-250 m, Stored at the National Museum of Nature and Science, Tokyo with registration number MPC-04121, England Finder coordinates K45/4.
Paratype 4: Plate 3, Fig. 6, Station 56, 250-500 m, Stored at the National Museum of Nature and Science, Tokyo with registration number MPC-04122, England Finder coordinates F11/2.
Description: Shell spherical consisting of one simple or double cortical shell and one double medullary shell consisting of microsphere and second medullary shell. Microsphere looks like a pentagonal prism the corners of which bear one ray. One lateral edge is straight, the other four are curved and bear also a ray in their middle, so that the total number of rays originated in the microsphere is 14. All microspheric rays stop in the wall of the second medullary shell (partly seen on Plate 2, Figs. 1c, 2a, 3c; Plate 6, Figs. 2b–c) and are interconnected by arches that form in its wall with slightly marked constrictions and slightly marked cupolas. Second medullary shell thin-walled with rounded pores of irregular size and arrangement. Their cupolas bear one to two three-bladed radial bars prolonged outside the cortical shell into about 10-15 relatively long pointed spines. Cortical shell is spherical and may be single (Plate 2, Figs. 4-8; Plate 5, Fig. 1; Plate 6, Fig. 1) or double (Plate 3, Figs. 1-7; Plate 4, Figs. 1-2; Plate 5, Figs. 2-3).When single, it is thin walled and has rounded pores of various shapes, sizes and arrangement (Plate 2, Fig. 7a; Plate 5, Fig. 1).When double, the external one is close to the internal shell, spongy-like, fibrillar, formed of thin threads uniting very irregularly the ends of numerous by-spines arising from the wall of the inner cortical shell (Plate 3, Figs. 3, 5-6; Plate 5, Figs. 2-3).
Material: Numerous specimens, juvenile and adult forms with all of the in-between growth stages.
Dimensions: (averaged based on 20 specimens; Table 4): Diameter of microsphere 20 µm (18-24 µm, of second medullary shell 42 µm (39-47 µm, of cortical shell 84 µm (75-94 µm), of second cortical shell 108 µm (96-124 µm).
Etymology: From its occurrence in the Arctic Ocean.
Range and occurrence: This species is living mainly in the upper water layers (100-250 m depth) and increase in number during early stage of sea-ice formation (see Chapter 4.2 below). Its first occurrence is not known as most cores with Tertiary as well as Quaternary sediments in the Arctic Ocean usually are barren for siliceous microfossils.
Remarks: This species was identified as Joergensenium sp. A in Ikenoue et al. (2015), Plate 4, Figs 1-3).Whether this species is being recruited from the Pacific Ocean via the Bering Sea is not confirmed by the plankton data from the south-western part of the shallow Chukchi Sea, presented in this paper. However, the samples studied are essentially barren of any radiolarian fauna, only three species were observed and none of them was J. arcticum.
As mentioned under the genus description, structurally, the microsphere consists of an initial spicule formed of a median bar (MB), two apical rays (A), and four basal rays (B). Basal rays are united by a system of arches that form together with MB a pentagonal prism, and each of its ten corners bears a ray. One lateral edge of this prism (MB) is simple, the other four are curved and in their middle they bear an additional radial bar (a), so that the total number of rays extending fromthemicrosphere is 14 (Text-Fig. 2.2). Their distal ends stop in the wall of the outer medullary shell where they are united by arches forming slightly marked constrictions that repeat, by their position, the edges of the macrospheric pentagonal prism. Morphologically, by having 3-4 concentric shells and many radial spines the species J. arcticum n. sp. could be considered close to the series of boreal species Actinomma boreale (Cleve), Actinomma leptodermum (Jørgensen), Drymyomma elegans Jørgensen belonging to Actinommidae of Spumellaria (see Bjørklund, 1974a, 1974b, 1976; Cortese and Bjørklund, 1997). These actinommids have a round microsphere with rather small and regular pores. A major difference between J. arcticum and the actinommids is the complicated structure of the microsphere in J. arcticum that we have mentioned under the species description.
|Ikenoue et al. 2016||