Oswaldella laertesi, Peña, Alvaro L., 2007

Peña, Alvaro L., 2007, Breaking molds: Oswaldella laertesi, sp. nov., a unique Antarctic species of Oswaldella Stechow, 1919 (Cnidaria: Hydrozoa: Kirchenpaueriidae), Zootaxa 1612, pp. 63-68 : 64-67

Oswaldella laertesi , sp. nov.

( Figures 1–2 , Table 1)

Material examined. Stn 97 ID 2655, one stem fragment, c. 14 mm long (Paratype, NIWA 29736); Stn 178 ID 7960, one stem, c. 100 mm high, with gonothecae (Holotype, NIWA 29735).

Description. Stem c. 100 mm high, unbranched and polysiphonic in first 20 mm. Hydrocaulus provided with apophyses regularly disposed in three longitudinal rows, usually in a spiral arrangement, not in verticils. Cauline apophyses directed upwards, forming an angle of c. 45º with long axis of stem. Stem irregularly divided into internodes, usually with three apophyses. Cauline apophyses with six nematophores: two nematophores each emerging through a “mamelon” situated laterally on upper part of apophysis, and four axillary nematophores, each emerging through a simple perisarc hole. Cauline apophyses supporting hydrocladia, with distinct node between the two.

First hydrocladial internode bifurcated, giving rise to two secondary hydrocladia. These forming up to two third-order hydrocladia. Hydrocladia homomerously divided into hydrothecate internodes. Forked hydrocladial internodes with a hydrotheca between each prong and three nematophores: one mesial inferior (absent in first hydrocladial internode), with a much reduced nematotheca, and two mesial superior nematophores emerging through a simple perisarc hole behind free part of adcauline hydrothecal wall, one on each prong. Unforked hydrocladial internodes with one hydrotheca and two nematophores: 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 situated in middle of hydrocladial internode or on its distal half. Hydrotheca shallow, usually diameter at aperture larger than abcauline length. However, abcauline length slightly increasing along hydrocladia, for instance length could be 176 µm in first unforked hydrocladial internode and 256 µm in 15th. Hydrothecal aperture circular, perpendicular to long axis of internode. Adcauline wall with tiny but distinct free portion. Abcauline wall straight, but directed outwards. Rim even; sometimes with tiny adcauline elevation.

Presumed male gonothecae inserted at hydrothecal base on elevation of internode, fusiform, with a subterminal, oval aperture.

Cnidome composed of microbasic mastigophores of two size classes ( Table 1).

Remarks. Oswaldella laertesi , sp. nov., is unique amongst species of the genus because of the peculiar arrangement of the cauline apophyses and, consequently, of the hydrocladia (cf. Table 2). In fact, arrangement of apophyses in three longitudinal series is unique even amongst members of the family Kirchenpaueriidae , since this family is characterized by the alternate arrangement of apophyses forming two longitudinal series (cf. Millard 1975; Bouillon 1985; Bouillon et al. 2004). The presence of more than two longitudinal rows of cauline apophyses, and consequently of hydrocladia, is found also in mature colonies of Nemertesia , a genus of the related family Plumulariidae (cf. Millard 1975; Bouillon 1985). However, in that genus the structure is different, since hydrocladia are arranged in alternating verticils, with the hydrocladia of one verticil normally alternating with those above and below forming a double number of longitudinal rows. By contrast, the cauline apophyses in O. laertesi are approximately spirally arranged, forming three regular longitudinal rows, instead of in alternating verticils.

With its low hydrotheca, Oswaldella laertesi , sp. nov., is close to several other species of the genus (cf. Table 2), namely O. bifurca , O. blanconae , O. encarnae , O. frigida , O. gracilis , O. grandis , O. herwigi , O. medeae , O. niobae , O. tottoni , and O. vervoorti . However, it clearly differs from all of them. In several species the cauline apophyses either are deprived of “mamelons” (i.e. O. bifurca , O. encarnae , O. gracilis and O. tottoni ) or have only one (i.e. O. blanconae , O. herwigi , O. niobae and O. vervoorti ), whereas in O. laertesi there are two mamelons. Consequently, it is closer to O. frigida , O. grandis , and O. medeae . Oswaldella frigida , as well as O. bifurca , differs from O. laertesi in hydrothecal shape, having the hydrothecal aperture frontally depressed. Moreover, the hydrocladial structure is completely different with only secondary hydrocladia present and the hydrotheca is in the middle or on the basal half of the internode. Oswaldella medeae differs from O. laertesi by the presence of only secondary hydrocladia, and by the strong downward tilt of the hydrotheca.

The greatest similarities are found with O. grandis , with which it also shares the hydrocladial branching, the general shape of the hydrotheca, the position of the hydrotheca on the internode, the development of the mesial inferior nematotheca, and the large size of the gonothecae. There are, nevertheless, important differences. In O. grandis the colonies are much larger (up to 500 mm high), only two axillary nematophores are present in the cauline apophyses, the hydrocladial internodes are larger, and the length of the hydrothecal abcauline wall is shorter (up to 228 µm). Moreover, Oswaldella grandis is endemic to West Antarctica, being known only from the South Shetland Islands area and the Weddell Sea (cf. Peña Cantero & Ramil 2006).

Differences from the remaining species of the genus are even larger (see Table 2).

Ecology and distribution. Oswaldella laertesi , sp. nov., is known only from off Cape Adare, in the Ross Sea area, where it was collected at depths between 343 and 736 m. The fertile colony was collected in February.

Etymology. The specific name laertesi is a dedication to Laertes Peña Sancho, son of the author.

Hydrotheca

Length abcauline wall 136–256 Length free part of adcauline wall 8–24 Diameter at rim (frontal view) 208–240 Hydrocladial internode

Length 760–1016 Diameter under hydrotheca 200–272 Gonotheca

Length 2200–2300 Maximal diameter 340–480 Nematocysts

Larger size group 13–15 x 4–4.5 Smaller size group 6– 7 x 2

Figures

FIGURE 1. Oswaldella laertesi, sp. nov. (A) Hydrocladial branching and arrangement of hydrothecae and gonothecae; (B) cauline apophysis with “ mamelons ” and first forked hydrocladial internode; (C) forked hydrocladial internode showing branching, mesial inferior nematophore and superior nematophores; (D – H) unforked hydrocladial internodes with hydrotheca, mesial superior nematophore and mesial inferior nematotheca (all drawings from the Holotype). Scale bar: 1 mm (A), 250 µm (B – H).

FIGURE 2. Oswaldella laertesi, sp. nov. (A) Hydrocladial branching; (B) gonotheca; (C) hydrotheca in lateral view; (D) hydrotheca in frontal view; (E) mesial inferior nematotheca (see arrow); (F) mesial superior nematophore (see arrow); (G) cauline apophyses showing “ mamelons ” (see arrows); (H) cauline apophysis showing axillary nematophores (see arrows) (A – F from Holoype; G – H from Paratype). Scale bars: 200 µm (A – B), 100 µm (C – H).