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Stylatractus pluto (Haeckel, 1887)

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Benson, 1966, p. 184-186; pl. 7, figs. 14-17:

Xiphatractus pluto(Haeckel)

Amphisphaera pluto Haeckel, 1887, Challenger Rept , Zool , vol. 18, p 144, P1 17, figs 7, 8.
Xiphatractus stahli Dreyer?, 1889, Jenaische Zeitschr. Naturwiss., vol 23, pp 129-130, Pl. 6, fig. 17.
Stylatractus neptunus Haeckel?, Riedel, 1958, B.A.N.Z.A.R.E. Repts., ser. B, vol 6, pt 10, p 226, P1. 1, fig 9.

Test consisting of three ellipsoidal shells with two unequal polar spines. Basic structure of test similar to that of Xiphatractus cronos. Major difference between this species and Xiphatractus cronos is in the details of the cortical shell and polar spines. Cortical shell with very thick wall (10-12 µm), ellipsoidal in all specimens, with large, subequal, subcircular, hexagonally arranged pores, 7-8 on half the minor circumference; openings of all pores filled with 2-4 or more subcircular to irregular, subequal pores, in many specimens not completely developed, giving the primary pores a “scalloped” appearance. Polar spines absent in a few specimens, represented by short, triangular thorns in others, in most specimens robust, long, conical, in some specimens grooved at their bases, three-bladed in a few, always unequal. Inner shells generally difficult to observe but not unlike those of the preceding species.

Measurements; based on 30 specimens from stations 27, 34, and 46: major diameter of outer shell 108-199 µm, of second shell 63-86 µm, of first shell 27-43 µm; minor diameter of outer shell 92-160 µm, of second shell 53-80 µm, of first shell 22-31 µm; length of greater polar spine (when present) 5-117 µm, of lesser polar spine (when present) 2-98 µm; breadth at base of spines 10-34 µm.

Remarks. This species agrees best with Riedel's description and illustration (1958, p. 226, P1. 1, fig. 9) of a species he described from the Antarctic but identified tentatively with Stylatractus neptunus Haeckel (1887, p. 328, P1. 17, fig. 6). The pores of the cortical shell of the Gulf specimens are unlike those of S. neptunus Haeckel, but agree with those of Amphisphaera pluto Haeckel (1887, p. 144, P1. 17, figs. 7, 8) in that the larger pores are more or less “scalloped.” In fully developed specimens from the Gulf the large pores are filled with 2-4 or more secondary pores, and if these pores are incompletely developed, the large pores appear as in A. pluto, i.e., “scalloped.” Xiphatractus stahli Dreyer (1889, p. 129, P1. 6, fig. 17) agrees with the Gulf specimens in all details, particularly in the details of the polar spines, but Dreyer did not mention the presence of large, equal, cortical shell pores but does state that the small circular pores of this shell appear to be arranged in groups of 2-5. This could be explained if they represent secondary pores filling the spaces of the larger primary pores. The dimension of all three species mentioned above are in agreement with those of the Gulf species.

Distribution. The distribution of this species in the Gulf is similar to that of Xiphatractus cronos although it is less frequent. It occurs rarely only at stations 27, 34, 46, 56, 60, 71, 81, and 95. Thus it is a more nearly oceanic species not tolerant of waters with higher salinity and temperature. It does not appear to be controlled by upwelling.
Riedel (1958, p. 226) states that specimens closely resembling the Antarctic forms of this species have been found in the northern Pacific and in the tropical parts of the Pacific and Indian Oceans. Haeckel (1887, p. 144) reported Amphisphaera pluto from the central Pacific. Dreyer (1889, p. 130) reported Xiphatractus stahli from the same region. It appears that the Gulf species has a cosmopolitan distribution.

Benson 1966











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