N5

Antarctissa strelkovi Petrushevskaya

Antarctissa strelkovi Petrushevskaya, 1967, p. 89, fig. 51, III-IV

DESCRIPTION "This species has a structure typical for the genus. The first segment is separated from the second by a slight constriction. The ratio of "cephalis" width to "thorax" width is 1: 1.5 - 2. Pores on both segments are rounded, randomly distributed; their sizes vary greatly. The shell walls are comparatively thin. On the surface there are thorns and even long secondary spines. These spines arise at the first segment, extend laterally along the sides of the second segment and project downward at its lower edge (in sediment specimens, the spines are usually broken off in the vicinity of the base). The elements of the inner skeleton are much thinner than in A. denticulata and their outward extensions are more distinct...differs from [A. denticulata] in the presence of long secondary spines on its surface, general form of the shell and thinner, transparent wall" (from Petrushevskaya, 1967; translation courtesy W.R. Riedel).

DIMENSIONS "Length of the first segment (externally) 45 microns - 55 microns, its width 60 microns - 65 microns, length of the second segment 70 microns - 90 microns, width 70 microns - 110 microns, overall length of the shell is up to 150 microns." (from Petrushevskaya, 1967; translation courtesy W.R. Riedel).

REMARKS

1. "When counting, many specimens are found which are difficult to assign to [A. denticulata or A. strelkovi]. A. denticulata is characterized by the smooth outline of the shell due to filling of the constriction between the cephalis and the thorax. This constriction is well defined in typical specimens of A. strelkovi.

"Several varieties of A. strelkovi can be distinguished but it seems that there are specimens with characteristics which are intermediate between each one of these varieties and one of the varieties of A. denticulata. (from Lozano, 1974). See Lozano (1974) for further discussion of specific gradations.

Plate 18, figures 2a,b

N6

RECENT DISTRIBUTION

1. Hays, 1965, Fig. 3 (Helothus histricosa);

"...generally restricted to sediments south of the Polar Front except in the Argentine basin where it occurs in some abundance, often associated with warm-water species...H. histricosa occurs in zone phi but is a much less important component of this assemblage than it is of the typical Antarctic assemblage found in the overlying diatomite facies.

"H. histricosa is probably the most abundant and ubiquitous radiolaraian species in the Antarctic, and if this fauna were to be characterized by any one species, H. histricosa would be a good choice. Its dominance in the fauna is frequently replaced near the Polar Front by Spongoplegma antarcticum Haeckel.

"With the exception of several samples, particularly those near the ice in the Ross Sea where Peromelissa denticulata is the dominant species, no other Antarctic species seriously threatens the dominance of Helotholus histricosa."

2. Lozano, 1974, Fig. IV-18; "It has a similar pattern of distribution as A. denticulata but some important differences can be noted.

"It is more abundant south of the APF varying betewen 12.2 and 36.8 percent (RC13-263) except in core V15-133) where it accounts for only 6.8 percent of the total radiolaria. It reaches values of over 3 percent close to the Subtropical Convergence being generally more abundant at the same latitudes in the Indian than in the Atlantic Ocean. As A. denticulata it is more abundant in the sediments of the Argentine, Crozet, and Aguhlas basins, but unlike A. denticulata it is more abundant in the Crozet basin than in the Argentine basin north of 40 degrees S and to the east of 30 degrees W it is practically absent in the Atlantic sector but it is found in a few cores in the Indian Ocean. It is also absent in the three cores east of the Crozet basin. The northward transport by bottom currents in the Argentine, Aguhlas and Crozet basins is as evident as for A. denticulata.

"It is never found in proportions over one percent outside of the Argentine and Crozet basins beneath August surface temperatures over 12 degrees C. Outside the Argentine basin, in the Atlantic Ocean it is practically absent under waters warmer than 10 degrees C. Under Antarctic waters it seems to increase in relative abundance from west to east reaching higher values between 15 degrees W and 20 degrees E and apparently decreasing again toward the east but the number of samples is not enough to show a clear pattern.

N7

"As A. denticulata and A. strelkovi appear to intergrade we combined them in the factor analysis (Fig. IV-19). All the samples south of the APF and west of 20 degrees E have values of over 30 percent reaching a maximum of 51.9 percent in core RC13-263. Only one of the five samples east of 20 degrees E has a value of over 30 percent. Values of 30 percent are not found north of 2 degrees north of the average position of the APF which is within the limits of seasonal fluctuation for the APF (Gordon, 1971).

"The distribution of A. denticulata plus A. strelkovi seems to have a more uniform pattern. It seems that one takes the place of the other as if they were varieties of the same species or at least species with a common ancestor (see section on taxonomic notes). The addition of the two species only emphasizes the features described for each of them."

3. Morley, 1977, counted together with A. denticulata and its varieties; "This group loads highest in factor 2 (polar). At present this group occurs most frequently in the polar regions. Although its maximum concentration is reduced at 18,000 YBP, the 20% countour is positioned at approximately the same latitude as today."