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Eucyrtidium hexastichum
(Haeckel, 1887)
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Description - Add description
Benson, 1966 (partim.), p. 506-508; pl. 34, figs. 15-16, (not 13-14):
Lithostrobus hexastichus Haeckel
Lithostrobus hexastichus Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 1470, Pl. 80, fig. 15.
Stichopilium annulatum Popofsky, 1913, Deutsche Südpolar-Exped., vol. 14, pp. 403-404, Pl. 37, figs. 2, 3.
Basic structure of cephalis including four collar pores, an indistinct ventral tubule (text-fig. 26), a three-bladed apical horn, and the shape of test and number and relative size of joints similar to Eusyringium siphonostoma but with equal pores arranged hexagonally, in transverse, not longitudinal, rows and terminal joint without a constricted, tubular mouth. Dorsal and primary lateral thoracic ribs raised or wing-like; extend within the wall of the proximal portion of the first abdominal joint in a few specimens. Test generally smaller than that of E. siphonostoma, and joints separated by constrictions in several specimens. Generally 6-7 joints but ranging from 5-9.
Measurements; based on 30 specimens from stations 27, 34, 60, and 92: maximum length of test 96-221 µm, maximum breadth 62-101 µm; length of cephalis 15-18 µm, of thorax 16-32 µm, of abdominal joints 12-37 µm; breadth of cephalis 15-23 µm, of thorax 33-49 µm; length of apical horn 6-25 µm, of vertical spine 5-7 µm, of dorsal and primary lateral spines (when present) 2-l2 µm.
Remarks. Two general forms of this species are present in the Gulf. Both are identical in all details except for pore size. One has small pores ranging in size from l-4 µm and arranged in 8-12 transverse rows per joint; the other has larger pores ranging from 2-8 µm and arranged in 3-5 rows per joint. The former as well as tests with pores of intermediate size are rare in the Gulf; therefore, whether or not the two forms should be separated either as distinct species or subspecies could not be determined.
Both Lithostrobus hexastichus Haeckel and Stichopilium annulatum Popofsky have equal pores arranged hexagonally in transverse rows. The former apparently lacks the three primary thoracic ribs but they are present in the latter. The ribs are inconspicuous in many of the specimens from the Gulf.
Distribution. The distribution of this species in the Gulf is similar to that of Eusyringium siphonostoma, but its frequency is much less. It is absent at stations 90, 130, 203, and 214. It is rare at all stations except 192 where it is common (2.6%). It is very rare (less than 1.0%) at stations 27, 46, 64, 71, 81, 91, 93, 95, 99, 115, 194, and 206 and rare to nearly common at the other stations. Its frequency in the northern half of the Gulf is slightly greater than that in the southern half. This may be due to its response to upwelling in the diatomite region of the Gulf, but its presence at stations 184, 194, and 206 (nearly common at 184) where diatom percentage is low in the sediments may be the result of its tolerance of waters with slightly higher than average temperature and salinity. Because of its rare occurrence at most of the northern Gulf stations the relative importance of these two factors cannot be assessed. Its common occurrence at station 192, which is located within the diatomite facies, indicates that it does respond to upwelling. Its relative frequency in the southern half of the Gulf is too low and shows no significant fluctuations to suggest its response to upwelling in this region.
This species is apparently confined to the tropical regions of all oceans. Lithostrobus hexastichus Haeckel was reported from "Challenger" stations 270-274 in the central tropical Pacific, and Stichopilium annulatum Popofsky from the tropical parts of the western Indian and the Atlantic Oceans.
| | Benson 1966 | Benson, 1983, p. 503; pl. 9, figs. 9-11:
Eucyrtidium(?) hexastichum (Haeckel) group
Remarks. Members of this Eucyrtidium-like group are similar in shape to E. hexagonatum and posses the indistinct vertical tubule. They differ by having pores aligned transversely, not longitudinally. Because these forms have a lateral cephalic vertical tube and pores arranged in transverse rows, they would qualify for inclusion in the subfamily Artostrobiinae Riedel of the family Artostrobiidae Riedel emend. Foreman (Nigrini, 1977, p. 243), except that they have only four instead of six collar pores.
In the Gulf of California two general forms of this group are present. One has larger pores arranged in three to five transverse rows per abdominal segment (Benson, 1966, pl. 34, figs. 13-14; this chapter, pl. 9, fig. 10). The other has smaller pores arranged in eight to twelve rows per segment (Benson, 1966, pl. 34, figs. 15-16; this chapter, pl. 9, figs. 9, 11). The latter, as well as tests with pores of intermediate size, are less abundant in the Gulf sediments than are the former.
| | Benson 1983 |
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