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Actinomma antarcticum (Haeckel, 1887)

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Cortical shell with loose spongy framework and rough surface, four to six times as broad as the enclosed simple medullary shell. Pores of the latter regular circular, twice as broad as the bars. From its surface arise numerous (forty to fifty or more) short radial beams which become forked and compose, by communication of lateral branches, the spongy cortical shell.

Dimensions.--Diameter of the spongy cortical shell 0.2 to 0.3, of the latticed medullary shell 0.05 to 0.06
Haeckel 1887
Benson, 1966, p. 134-136; pl. 3, figs. 2-3:

Diploplegma banzare Riedel

Diploplegma banzare Riedel, 1958, B.A.N.Z.A.R.E. Repts. ser. B, vol. 6, pt. 10, pp. 223-224, Pl. 1, figs. 3, 4.

Test consisting of two, irregular, spherical, medullary shells and one or two spherical cortical shells without radial main spines. Inner medullary shell spherical to subspherical with irregular outline and large irregular pores, 3-5 on half the circumference; joined to the outer medullary shell by numerous (20-30 or more) cylindrical, thin to heavy beams which arise from its surface. Outer medullary shell spherical to subspherical with irregular outline, with large irregular pores similar to those of the inner medullary shell, 7-10 on half the circumference, joined to the cortical shell by numerous (50-100 or more) radial beams arising from its surface; beams range from thin, delicate, and cylindrical to heavy and three-bladed, and they generally extend as short, heavy thorns on the surface of the cortical shell. Cortical shell spherical, with rough surface and generally thick wall, with unequal, irregular pores of all shapes and sizes, slightly smaller than those of the medullary shells, 10-20 on half the circumference. Several specimens with a double cortical shell [=A. antarcticum] with a narrow space between, together giving the outer shell a spongy appearance; outer cortical shell similar to inner one. In some specimens the inner medullary shells are indistinguishable, together giving the appearance of a solid spherical mass of loose spongy meshwork.

Measurements; based on 30 specimens from stations 27, 34, 60, and 194: diameter of outer cortical shell (3 specimens) 221-264 Ám. of inner cortical shell 172-239 Ám, of outer medullary shell or inner solid spongy sphere 98-129 Ám, of inner medullary shell 43-62 Ám.

Remarks. The major difference between the Gulf specimens of this species and the Antarctic forms described by Riedel (1958, p. 223) is the pretence of two internal, irregular, concentric spheres in the former instead of an inner solid sphere of loose spongy meshwork. However, in numerous unbroken specimens and in those with double or thick cortical shells the inner spheres appear as a solid spongy mass. Many specimens from the Gulf have irregular, closely spaced, double cortical shells which are spongy in appearance, not unlike the Antarctic forms described by Riedel.

Distribution. This species has a much greater occurrence in the northern Gulf than in the central or southern parts. It is absent at stations 64, 81, 115, 130, 136, 192, 203, 208, and 214 and rare at all others except stations 194 and 206 where it is common (9.2% and 8.0% respectively). At station 194 it is the second most abundant species. Its high frequency at these northern Gulf stations may reflect its tolerance of higher salinity and temperature and the greater range in these properties in the surface waters. At stations 184 and 90 it is rare but has a greater frequency (1.4%) than at most of the other stations. Its general rarity or absence throughout the rest of the Gulf can be explained by its dilution by other species which are less tolerant of salinity and temperature changes.
According to Riedel (1958, p. 224), a form similar to the Antarctic members of this species is widespread in the tropical parts of the Pacific and Indian Oceans and in the northern Pacific. The typical form has a more delicate cortical shell which is rarely spongy. It is this form which is probably the one present in the Gulf, although numerous specimens from the Gulf have a double cortical shell which appears spongy. This species or species-group therefore is apparently cosmopolitan.


Benson 1966


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