Oswaldella frigida, Peña Cantero & Vervoort, 2004

Peña Cantero, A. L. & Vervoort, W., 2004, Species of Oswaldella Stechow, 1919 (Cnidaria: Hydrozoa: Kirchenpaueriidae) from US Antarctic expeditions, with the description of three new species, Journal of Natural History 38, pp. 805-861 : 830-832

Oswaldella frigida sp. nov.

( figure 8; table 2)

Material examined. 691/26, one stem ca 75 mm high (holotype, USNM 1003325 ) ; 721/1063, three stem fragments up to 50 mm high ( USNM 1003326 ; RMNH-Coel. 30212; MNCN 2.03 / 233) .

Description. Colonies consisting of monosiphonic and unbranched stems up to 75 mm high. Hydrocaulus provided with apophyses alternately arranged in one plane and forming two longitudinal series. Cauline apophyses directed upwards, forming an angle of ca 45 ° with long axis of stem. Stem divided into internodes with usually one or two apophyses per internode although up to four apophyses have been observed. Cauline apophyses with three ( figure 8C), occasionally four ( figure 8B) nematophores: two axillary nematophores, each one emerging through a simple perisarc hole, and one more emerging through a ‘mamelon’. Occasionally an extra nematophore is present, emerging through a second ‘mamelon’. Cauline apophyses supporting hydrocladia with asymmetrical branching ( figure 8A). First hydocladial internode bifurcated, giving rise to two unequally developed prongs, this process being repeated at the next internodes; ramifications all in one plane. Cauline apophyses and hydrocladia separated by a distinct node ( figure 8A, B).

Hydrocladia homomerously divided into hydrothecate internodes, each provided with one hydrotheca and two nematophores ( figure 8 D–K): one mesial superior nematophore, emerging through a simple perisarc hole situated behind free adcauline hydrothecal wall, and one mesial inferior nematophore emerging through a hole

Hydrothecae

Length of abcauline wall 80–120

Length of free part of adcauline wall 20–30

Diameter at rim 130–150

Internode

Length 680–730

Diameter under hydrotheca 150–170

situated on a strong elevation of the internode and deprived of any kind of nematotheca. Top of distal hydrocladial internodes truncated.

Hydrotheca shallow, placed on proximal half of internode or in the middle ( figure 8 D–K). Hydrothecal aperture frontally depressed; rim uneven due to the presence of two strongly developed lateral lobes. Hydrotheca provided with a conspicuous free portion of its adcauline wall.

Gonothecae absent.

Remarks. Oswaldella frigida sp. nov. is allied to O. bifurca in general appearance, but differs in many respects (cf. table 1). In O. bifurca there is usually a single nematophore on the cauline apophyses emerging through a simple axillary perisarc hole (occasionally two), in O. frigida sp. nov. there are typically four nematophores on the cauline apophyses, two axillary nematophores emerging each from a perisarc hole and two more emerging each from a ‘mamelon’ (occasionally only a single ‘mamelon’). Also, though in both species the hydrotheca is frontally depressed, in O. frigida sp. nov. that frontal depression is much better developed; in fact, in many hydrothecae of O. bifurca the rim is even. Moreover, whereas in O. bifurca the hydrotheca is adnate over the entire adcauline wall, that adcauline hydrothecal wall usually has a distinct free portion in O. frigida sp. nov. In O. bifurca , the top of the distal hydrocladial internodes is pointed, whereas in O. frigida sp. nov. that part is truncated. In O. bifurca the hydrocladial arrangement is usually symmetrical, but in O. frigida sp. nov. that disposition is asymmetrical. In O. bifurca the first hydrocladial internode is distinctly bifurcated, giving rise to two secondary prongs of equal development, while in O. frigida sp. nov. that first internode gives rise to two unequally developed prongs that could be seen as the first-order hydrocladium giving rise to several secondary hydrocladia. In O. bifurca the cauline apophyses form an angle of ca 70 ° with the long axis of the stem, whereas in O. frigida sp. nov. that angle is ca 45 °. In O. frigida sp. nov. there is a distinct node separating cauline apophyses and hydrocladia; such a node is absent in O. bifurca , only in the youngest stems is it possible to find an indistinctly indicated node. In O. bifurca the stem is slightly geniculate, due to the arrangement and structure of the cauline apophyses, whereas in O. frigida sp. nov. the stem is straight. Finally, in O. bifurca the hydrocladia proximally follow the angle of ca 70 ° of the cauline apophyses, curving strongly upwards distally, whereas in O. frigida sp. nov. the hydrocladia completely follow the angle of ca 45 ° of the cauline apophyses.

Ecology and distribution. Oswaldella frigida sp. nov. has been collected from depths of 44 to 124 m off Low Island and off Nelson Island, in the South Shetland Islands .

Etymology. The specific name frigida refers to the frigid conditions under which this species lives.