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Theopilium tricostatum Haeckel, 1887

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Benson, 1966, p. 444-447; pl. 30, figs. 1-2:

Theopilium tricostatum Haeckel

Theopilium tricostatum Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 1322, Pl. 70, fig. 6; Popofsky, 1913, Deutsche Südpolar- Exped., vol. 14, pp. 375-376, Pl. 37, fig. 6.

Test smooth, consisting of a small subhemispherical cephalis, a campanulate thorax, and a relatively flat, broad, brim-like abdomen. Cephalis smooth, with small circular pores as broad as the intervening bars, with a dorsal sagittal constriction occupied by the apical bar which extends nearly vertically as an eccentric, relatively thin, conical to three-bladed apical spine, in a few tests classifiable as a horn; vertical spine extends from the vertical bar and is generally thin, conical, of variable length. Thorax divided from cephalis by a slight stricture occupied by the collar ring; campanulate, with hexagonally arranged pores in 7-12 (generally 8) transverse rows; pores small, subcircular in the two or three proximal rows, increasing gradually in size and becoming hexagonal in shape distally, pores of each row approximately equal in size, terminal row of pores generally smaller. A nearly flat or widely inflated, truncate-conical, latticed, abdominal brim, slightly convex outward, separated from the thorax by a distinct angular contour change occupied by an inner septal ring; pores of brim slightly smaller but similar to thoracic pores, arranged hexagonally in transverse rows (5-8 or more). Four collar pores present; a pair of pores corresponding to jugular pores at top of dorsal face of thorax on either side of the dorsal thoracic rib; secondary lateral bars present as ribs occupying the dorsal portion of the collar stricture; they do not extend as free spines; primary lateral and dorsal bars extend as thoracic ribs for a distance of 2-6 rows of thoracic pores and in many specimens terminate in thin, conical spines.

Measurements; based on 30 specimens from stations 27, 34, 91, 133, 136, 184, and 191: length of test (4 specimens) 68-98 µm, of cephalis 12-20 µm, of apical spine 6-31 µm, of vertical spine 6-25 µm, of primary lateral and dorsal spines 4-18 µm; breadth of cephalis 15-20 µm, of thorax 80-l38 µm, of abdominal brim 139-289 µm.

Remarks. This species differs from Coracalyptra cervus (Ehrenberg) from the Gulf in the much broader and flatter abdominal brim, a more constant number (generally 8) of transverse rows of thoracic pores, more regular pores of both thorax and brim, the lack of a forked apical horn, the presence of thinner apical and vertical spines, the lack of lateral cephalic spines, a smaller length and greater breadth, the presence of dorsal and primary lateral thoracic ribs, and in general a more delicate test. These two species appear to be closely related and should be placed within a single genus. The extension from the collar ring of the dorsal and primary lateral bars as either free spines or thoracic ribs is subject to intraspecific variation. In T. tricostatum the presence of thoracic ribs is constant, whereas in C. cervus, either free spines from the collar ring or thoracic ribs are present in different individuals.

Distribution. This species is cosmopolitan in the Gulf, being absent only at stations 90, 130, 194, 203, and 21.4. It is common at stations 81 (2.6%), 106 (3.2%) where it is the fifth most abundant species, and 206 (2.1%) in the northern shelf region of the Gulf near a diatomite facies. It is nearly common but rare at stations 91, 93, and 115, and rare at all others. Except at station 81 its average frequency in the southern half of the Gulf is less than that in the northern half, a fact which indicates that it does respond to upwelling in the latter region. Its high frequency at station 208 may be explained in part by this as well as the smaller number of species present at this station. Its absence or very rare occurrence at marginal stations and at stations in the northern shelf region removed from diatom-rich sediments indicates its preference for more nearly oceanic, offshore waters. It does not respond to upwelling in the southern half of the Gulf, although its high frequency at Station 81 may be explained by this but more likely by its being a common member of the oceanic waters overlying this station.
Haeckel (1887, p. 1322) reported this species from the surface of the central Pacific at "Challenger" stations 260-274. Popofsky (1913, p. 376) reported this species from the western tropical part of the Indian Ocean and from the southern Atlantic off the west coast of South Africa at water depths of 400 meters and 200 meters, respectively. It appears that this species is widespread in Recent tropical seas.
Benson 1966











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