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Hexastylus triaxonius Haeckel, 1887

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Shell thin walled, with smooth surface. Pores irregular, polygonal, three to six times as broad as the bars; four to six on the radius. Six spines triangular pyramidal, as long as the diameter of the shell, as broad at the base as one pore.

Dimensions.--Diameter of the shell 0.05, pores 0.004 to 0.008, bars 0.0015; lenght of the spines 0.04, basal breadth 0.006.
Haeckel 1887
Benson, 1966, p. 139-141; pl. 3, figs, 6-7:

Hexastylus triaxonius Haeckel

Hexastylus triaxonius Haeckel, 1887, Challenger Rept., Zool., vol.18, P. 175, Pl. 21, fig. 2.
Hexastylus dictyotus Haeckel. 1887, Challenger Rept., Zool., vol. 18, p. 176, Pl. 21, figs. 8, 9.

Single spherical shell with six heavy, three-bladed, nearly equal, mutually perpendicular spines ranging in length from less than half of to nearly the same as the diameter of the shell. In most specimens the radial spines continue inward as short, thin, conical centripetal spines. Surface of shell smooth to spiny with short, thin, conical spines arising from along the intervening bars as well as from their nodes. Pores subcircular to subelliptical, subequal. subregularly arranged, 7-9 on half the circumference, without polygonal frames. Shell wall of variable thickness but generally thin (4-5 Ám) the number and length of by-spines generally increase with increasing wall thickness; few specimens have a completely smooth shell surface. A few specimens were observed with five spines, others with seven (one spine bifurcated proximally) and one with eight (two spines each on opposite poles of the shell in addition to the four remaining normal spines). A few specimens have six spines of which only two to four are mutually perpendicular. One specimen was observed with numerous tangential by-spines and partially infilled pores. A thin, barely visible, suboctahedral inner shell was observed in one specimen.

Measurements; based on thirty specimens from stations 71 and 81: diameter of shell 69-92 Ám; length of main spines 27-74 Ám, of byspines 0-12 Ám.

Remarks. Hexastylus triaxonius Haeckel and H. dictyotus Haeckel are within the size range of and have similar proportions of spine length to shell diameter as the species from the Gulf. The former has a smooth surface, the latter a spiny surface; both have pores similar to the Gulf specimens. Since the former has page preference over the latter, its name was retained for this species.

Distribution. This species is rare but occurs generally throughout the Gulf. It is absent at the marginal stations 64, 130, 203, and 214 and is common only at station 184. It occurs in slightly greater numbers in the central and northern parts of the Gulf, which may indicate its tolerance of slightly higher salinity and temperature but which also may be explained by northward decrease in the total number of species. In the section across the Gulf between stations 91 and 99 it has a greater frequency at stations 91, 95, and 99 than at 92 and 93. This may be explained by its response to upwelling in the coastal areas at opposite ends of this section.
Apparently H. triaxonius is a tropical to subtopical oceanic species. It was reported from about latitude 30░ N in the central Pacific at "Challenger" station 256. H. dictyotus was recorded from "Challenger" station 272 in the central Pacific at about 4░ S latitude. It has not been reported from high latitudes.
Benson 1966


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