Staurotheca densa, Pena Cantero & Vervoort, 2003

Cantero, A. L. Peña & Vervoort, W., 2003, Species of Staurotheca Allman, 1888 (Cnidaria: Hydrozoa: Sertulariidae) from US Antarctic expeditions, with the description of three new species, Journal of Natural History 37 (22), pp. 2653-2722 : 2678-2680

Staurotheca densa sp. nov.

(figure 6; table 2)

Selaginopsis sp. 5 Peña Cantero, 1991: 100–102, pls 14, 48, 65 figure c; Peña Cantero and García Carrascosa, 1994: 122, figure 4p–q; 1995: 48, figures 19, 20, 63c.

Material examined. 12 / 1002, three fragments up to 20 mm long, with a single male gonotheca ( USNM 1003147 ) ; 12 / 1003, one extremely fragmented stem (largest fragment ca 50 mm long) ( USNM 1003148 ; RMNH-Coel. 30258) ; 27 / 1877, a mass of branches ca 40 mm in diameter, with female gonothecae (holotype, USNM 1003149 ; paratypes RMNH-Coel. 30259 and MNCN 2.03 /260) ; 32 / 2120, nine fragments up to 10 mm long ( USNM 1003150 ) ; 6 / 418, two fragments up to 12 mm long ( USNM 1003151 ) ; 691 / 32, one fragment ca 15 mm long ( USNM 1003152 ) ; 731 / 1944, a mass of branches ca 70 mm in diameter ( USNM 1003153 ; RMNH-Coel. 30260; MNCN 2.03 /261) .

Description. Colonies compact (up to 70 mm in diameter), mono- or polysiphonic, without distinct main stem. Branching subdichotomous and in one plane; branching pattern, however, hidden by presence of anastomoses, giving the colony an appearance of a dense net. Branches with perisarc constrictions, delimiting internodes of variable length and of approximately same diameter.

Hydrothecae present along whole length of colony and arranged in decussate verticils, typically with three to four hydrothecae per verticil, forming six to eight longitudinal rows (figure 6A, B). Verticils of hydrothecae closely packed; distal part of hydrothecae in a verticil sometimes overlapping basal part of hydrothecae in verticil above.

Hydrothecae (figure 6A–E) immersed into branches for less than half their volume. Adcauline hydrothecal wall free for one-fifth to one-third of its length; free part straight or slightly concave. Abcauline wall nearly straight or distinctly concave. Hydrothecal aperture almost circular and either perpendicular to longitudinal axis of branch or slightly tilted downwards. Rim of hydrothecal aperture uneven, provided with a slight abcauline elevation. Hydrothecal rim with a few renovations. Opercular apparatus occasionally present, composed of a single disk-shaped flap. Diaphragm mushroom-shaped, with two abcauline protuberances projecting into hydrothecal lumen.

Male and female gonothecae present, inserted directly under hydrothecae. Female gonotheca urn-shaped (figure 6F), provided with a wide neck bearing a large, circular aperture. Male gonotheca fusiform (figure 6G), distally narrowing into a coneshaped neck bearing a small, circular aperture. External wall with a series of transverse striae over distal third.

Cnidome composed of microbasic mastigophores in two size groups: a larger (20.3–21.7×4.9–5.3 Mm) and a smaller group (7.7–9.1×2.1–2.5 Mm).

Remarks. Staurotheca densa sp. nov. shares the presence of anastomoses of the branches with S. antarctica , S. compressa , S. cornuta , S. dichotoma , S. frigida , S. glomulosa , S. nonscripta and S. vanhoeffeni (cf. table 6). Nevertheless, they all

Hydrothecae

Length of abcauline wall 650–870 Length of free adcauline wall 120–230 Length of adnate adcauline wall 630–650 Length of adcauline wall 700–770 Diameter at rim 280–300 Maximum diameter 380–400 Diameter at diaphragm ca 330 Gonothecae

Length of male gonotheca ca 1380 Maximum diameter of male gonotheca ca 800 Diameter of aperture of male gonotheca ca 170 Length of female gonotheca ca 1900 Maximum diameter of female gonotheca ca 1400 Diameter of aperture of female gonotheca ca 650 Length of distal neck of female gonotheca ca 500

have significant differences from S. densa sp. nov. Thus, in S. antarctica , S. compressa , S. cornuta and S. frigida the female gonotheca is globular, rests on a digitiform process and the external wall is ornamented by a series of digitiform projections. In all these species, moreover, the hydrothecae are much smaller than in S. densa sp. nov. and are usually distributed in decussate pairs (sometimes also in verticils of three hydrothecae); in S. cornuta the hydrothecae are always arranged in decussate verticils of three hydrothecae and the rim is provided with a strongly marked abcauline elevation. In S. glomulosa and S. nonscripta the hydrothecae are always arranged in decussate pairs and the hydrothecal rim is even. In S. dichotoma the hydrothecae are usually arranged in decussate pairs and sometimes in verticils of three hydrothecae and, moreover, the hydrothecal aperture is even. Finally, in S. vanhoeffeni the hydrothecal rim is even and the hydrothecae are almost completely sunken into the branches.

Ecology and distribution. Staurotheca densa sp. nov. is a shelf species. Peña Cantero and García Carrascosa (1994, 1995 as Selaginopsis sp. 5 ) recorded it from depths of 198–215 m, whereas we found it from 73 to 570 m. Peña Cantero and García Carrascosa (1994, 1995) reported fertile material collected in January; in our material the colonies with gonothecae were found in January and March. Staurotheca densa sp. nov. may be used as a substratum by other hydroids ( Lafoea sp. , Oswaldella sp. , Symplectoscyphus sp. ). It has been found on bottoms of pebbles.

Staurotheca densa sp. nov. appears to have a circum-Antarctic distribution. Our material comes from both the Ross Sea region, where it was found east of Pennell Bank and off Cape Hallett (Victoria Land), and from the region of the Antarctic Peninsula. In this area, S. densa sp. nov. has been collected off Elephant Island, north-east of Joinville Island, north of d’Urville Island, off Anvers Island (Palmer Archipelago) and off Deception Island (South Shetland Islands). Peña Cantero and García Carrascosa (1994, 1995) reported it from off King George Island, also in the South Shetland Islands region.

Etymology. The specific name densa refers to the dense appearance of the colony, owing to the frequent anastomoses and branching.