Oswaldella medeae, 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 : 839-842

Oswaldella medeae sp. nov.

( figure 12; table 3)

Material examined. 12/1003, four stem fragments up to 20 mm long (holotype, USNM 1003339 ; paratypes, RMNH-Coel. 30217 and MNCN 2.03 / 237); 32/2018, one distally broken stem ca 9 mm high ( USNM 1003340 ) ; 32/2021, two stem fragments up to 7 mm long, each with a single hydrocladium ( USNM 1003341 ) .

Hydrothecae

Length of abcauline wall 120–200

Length of free part of adcauline wall 30–50

Diameter at rim 180–230

Nematothecae

Length 20–30

Internode

Length 750–1150

Diameter under hydrotheca 180–230

Description. Colonies apparently with monosiphonic and unbranched stems, but present material fragmentary, largest fragment up to 20 mm long. Hydrocaulus with apophyses alternately arranged in one plane, forming two longitudinal series ( figure 12A). Cauline apophyses directed upwards, at an angle of ca 45 ° with long axis of stem and provided with three ( figure 12B, C) or four ( figure 12D) nematophores: two axillary nematophores, each emerging through a simple perisarc hole, and one or two nematophores emerging through a strongly developed ‘mamelon’, situated laterally on upper part of apophysis. Hydrocaulus divided into internodes with one or two apophyses per internode ( figure 12A). Cauline apophyses supporting hydrocladia ( figure 12 A–D) from which they are separated by distinct node; secondary hydrocladia present ( figure 12B).

Hydrocladia homomerously segmented; hydrothecate internodes with one hydrotheca and two nematophores ( figure 12 C–K): one mesial superior emerging through a simple perisarc hole just behind free adcauline hydrothecal wall and another mesial inferior nematophore, emerging through a perisarc hole on a strong elevation of the internode and provided with a small, scale-shaped nematotheca. Top of distal hydrocladial internodes truncated.

Hydrotheca shallow, situated in middle of hydrocladial internode or on its distal half ( figure 12). Hydrothecal aperture circular and distinctly tilted downwards. Adcauline wall of hydrotheca with considerable free part. Abcauline wall straight, angle with long axis of internode ca 45 °.

Gonothecae absent.

Remarks. Oswaldella medeae sp. nov. is well characterized by the shape of the hydrotheca, with a strong downward tilt, and by the presence of one or two ‘mamelons’ on the cauline apophyses, besides two axillary nematophores (cf. table 1).

Oswaldella medeae sp. nov. is allied to O. grandis by the presence of two axillary nematophores and two ‘mamelons’, as well as by the general shape of the hydrotheca which is also shallow (cf. table 1). They are clearly different as in O. grandis the hydrotheca is almost fully adnate to the hydrocladial internode, the abcauline wall is very steep and the hydrothecal aperture is perpendicular to the long axis of the internode. They also differ in the general structure of the colony, since O. grandis is a much more robust species, with stems reaching 500 mm height. On the other hand, the cauline apophyses are relatively shorter and in close contact with the stem. Also, the hydrocladia are much more branched, having several third-order hydrocladia. Finally, in O. grandis the mesial inferior nematotheca is much more reduced.

By the occasional presence of one ‘mamelon’ besides the two axillary nematophores O. medeae sp. nov. is allied to O. antarctica , O. crassa , O. garciacarrascosai , O. obscura and O. vervoorti (cf. table 1). However, O. medeae sp. nov. clearly differs from those species. Oswaldella antarctica , O. crassa , O. garciacarrascosai and O. obscura have elongate hydrothecae, whereas in O. medeae sp. nov. these are shallow. In this character O. medeae sp. nov. is also close to O. vervoorti , though in this species the hydrotheca is larger and always situated on the distal half of internode, the hydrothecal aperture is approximately perpendicular to the long axis of the internode, and the mesial inferior nematotheca is larger. In O. vervoorti hydrocladia up to fourth-order are present, whereas in O. medeae sp. nov. only secondary hydrocladia have been observed.

The presence of a shallow hydrotheca with a strong downward tilt of the rim brings O. medeae sp. nov. near to O. herwigi (cf. table 1), but in this species, together with one ‘mamelon’, there is a single axillary nematophore on the cauline apophyses. Moreover, the asymmetrical structure of the hydrocladia is completely different and hydroids of that species have alternately arranged hydrothecate and athecate internodes.

Oswaldella encarnae also has hydrothecae in which the rim is slightly tilted downwards (cf. table 1), but in this species the hydrothecae are distinctly larger, the hydrocladia are unbranched and there are only axillary nematophores on the cauline apophyses, with ‘mamelons’ being absent.

Ecology and distribution. Oswaldella medeae sp. nov. appears to have a circum- Antarctic distribution. It was collected at depths between 210 and 503 m at Pennell Bank (Victoria Land), in the Ross Sea region, and at the north-east of Joinville Island (Antarctic Peninsula).

Etymology. The specific name medeae is a dedication to Medea Peña Sancho, daughter of the first author.