S25

Actinomma antarcticum (Haeckel)

Spongoplegma antarcticum Haeckel, 1887, p. 90

Cladococcus aquaticus Popofsky, 1908, p. 214, pl. 23, figs. 3, 4

Actinomma antarcticum (Haeckel), Nigrini, 1967, p. 26, pl. 2, figs. 1a-d

Diploplegma (?) aquatica (Popofsky), Petrushevskaya, 1967, p. 18, fig. 9, I-III, fig. 10, I-IV

DESCRIPTION

"Cortical shell large, spherical, with a rough surface. Shell thickness varies from a single lattice plate with subcircular to subangular pores of different sizes to a thick spongy meshwork; intervening bars heavy.

"Numerous radial beams join the cortical shell to a superficially completely spongy medullary meshwork. However, by breaking open this meshwork a central subspherical shell having 2.5-3 subcircular pores on a half-equator can be seen.

"According to Hays (1965) the medullary meshwork is not always present, but from his extensive examination of Antarctic material he has concluded that forms with and without a central meshwork are conspecific." (from Nigrini, 1967).

"Cortical shell large; spherical, spongy surface rough, varying greatly in thickness from a single mesh to a spongy network equaling one-quarter of the radius. In spite of spongy texture, pores usually well-defined, rounded, irregular in size and shape, ranging in diameter from 1 to 10 times the width of bars, 15 - 30 across diameter. Bars vary in thickness but tend to be rather heavy, about 8-15 wide, bearing very short thorns on each node. Some specimens show the development of a second and sometimes a third cortical shell. Texture of these inner shells similar to the outer cortical shell, but generally the bars are thinner.

"Cortical shell or shells connected to a loose, subspherical, spongy medullary meshwork without a central cavity by numerous radial rodlike beams (as many as 40) branching distally as they join spongy cortical shell.

" The medullary meshwork is not present in all specimens but the association of forms with and without this central meshwork in the same sample suggests that both are the same species. Also numerous specimens have been observed without the medullary shell but with rodlike spines projecting radially from the inner surface of the cortical shell. Many of these spines show no evidence of breakage, which suggests that the medullary meshwork may develop later in the life cycle of the animal than the cortical shell." (from Hays, 1967, p. 165).

Plate 3, figures 1a,b; 2a,b; 3

S26

A. antarcticum " may have one, two, or three cortical shells. The most common form in our area is that with two cortical shells. These cortical shells may be very close to each other giving the appearance of a single thick shell, or the distance between the two shells may be as much as one-third of the radius of the outer shell. A few specimens were observed in which the medullary mesh is replaced by spongy tissue, in others it is poorly developed but generally it is clearly defined. Nigrini described the presence of a medullary capsule with a central cavity. This was not observed in the southern specimens but is present in specimens under subantarctic waters.

"Sometimes the medullary mesh is found without showing signs of being broken. Specimens with bars and the beginning of the cortical shell were also observed suggesting that growth of the shell proceeds from the central part to the cortical shell. Forms like the last two mentioned above may constitute a significant part of the total radiolarian fauna (2.6 percent in cores RC12-291 and RC15-96).

"North of about 45 degrees S, forms which fit better Nigrini's description of A. medianum become predominant. Generally they have a single cortical shell but sometimes they have a delicate extracortical shell, as Nigrini referred to it. In A. antarcticum the two cortical shells are equally well developed. According to Nigrini (1967), in A. medianum, the radial beams which extend from the outer surface of the medullary meshwork to the cortical shell, sometimes extend externally. These spines are better developed in specimens from cores RC11-117 to RC11-120 east of the Crozet basin. Specimens as small as 140 microns in diameter were observed. It is suggested here that the forms described by Nigrini as A. antarcticum and A. medianum are morphological variants of the same species. Moore (1974) believes the same is true for A. medianum and A. arcadophorum. A more detailed study is necessary to reach a more definitive conclusion." (from Lozano, 1974).

DIMENSIONS "Diameter of cortical shell 225-375 microns, of medullary shell 100-130 microns" (from Hays, 1965).

REMARKS

1. For a more complete synonymy see Nigrini, 1967. The fact that this species can have up to three cortical shells may require some taxonomic revision. Petrushevskaya (1975, p. 571) places this species in the genus Rhizosphaera Haeckel emend. Holland and Enjumet, but she does not specify the placement of its two close relatives, A. medianum and A. arcadophorum. Therefore, all three species are left herein in the genus Actinomma until a more complete taxonomic revision can be made.

S27

RECENT DISTRIBUTION

1. Hays, 1965, fig. 4; " Spongoplegma antarcticum is one of the most abundant species in the Antarctic fauna and in several samples constitutes over 50% of the species counted. Although the sampling in this is not sufficient to be certain, there seems to be a tendency for this species to reach its greatest relative abundance in the vicinity of the Polar Front. Farther to the south Helotholus histricosa is apt to be the dominant species and to the north the Antarctic fauna is replaced by a warmer-water fauna. This situation is best illustrated in the series of cores V-16-57 through V-16-66 south and east of Africa. Exceptions to this pattern are also present, e.g., V-14-58, which lies well south of the Front and contains a relative abundance of S. antarcticum of over 50%.

"The stratigraphic range of this species is not known but in the Antarctic it occurs in zone phi; however, it may extend back as far as the Miocene if Plegmosphaera churchi and/or Rhodosphaera nipponica are synonymous with S. antarcticum."

2. Lozano, 1974, fig IV-15; "This species accounts for 1 to 6 percent of the Radiolarian in samples south of the average position of the APF. It reaches relatively high values along the APF and immediately to the north (13.5 percent in RC8-44). In the Atlantic it is found only south of 40 degrees S except for the samples in the northwestern part of the Argentine Basin. It is generally present under subantarctic waters in the Indian Ocean sector. It reaches maximum abundance in core V14-64A (18.9 percent) under southern subantarctic waters. In the Indian Ocean it is found in some samples north of the subtropical convergence but invariably they can be interpreted as being transported by bottom currents or as intermediate forms which could be classified as Actinoma medianum, according to Nigrini (1967). The transported specimens can be distinguished by the presence of two equally developed cortical shells.

"Except for cores RC14-12A (1.7 percent) and RC11-104 (2.2 percent) with typical Antarctic forms, it is not found in proportions over one percent under August surface temperatures over 8 degrees C. Maximum August temperature under which it is found is 14 degrees C in the Indian Ocean (RC8-36) and over 15 degrees C in the Argentine basin (V18-155)."