Schizotricha auroraaustralis

Ecology and distribution. The material of Schizotricha auroraaustralis sp. nov. was collected at depths between 423 and 433 m, with gonothecae in January. It is tentatively considered endemic to East Antarctica.

Description. Stem polysiphonic, unbranched, composed of a main aXial tube and several accompanying ones, whose number decreases distally; most distal part of stem only with main tube. Main aXial tube on one side of stem, not covered by accessory tubes for most of its length. Main tube divided into homomerous hydrothecate internodes, bearing hydrothecae and nematothecae. Secondary tubes only provided with nematothecae. Cauline internodes with a short apophysis without nematothecae (Fig. 4 B). Cauline apophyses alternately arranged, forming an angle slightly smaller than 90 ° at basal part of stem, but opening distally, being almost in one plane at distal part. Cauline internodes (Fig. 4 B) with a hydrotheca at aXil between apophysis and internode and three nematothecae: two flanking hydrothecal aperture and one infrathecal nematotheca (occasionally two). Cauline apophyses giving rise to hydrocladia, forming an acute angle with stem. A basal ahydrothecate internode, provided with one nematotheca, present between cauline apophysis and first hydrothecate hydrocladial internode (Fig. 4 A – B). Only occasionally basal ahydrothecate internode absent (in this case, cauline apophyses might be slightly longer and have a nematotheca). Hydrocladia homomerously divided into internodes (Fig. 4 A), but with a short ahydrothecate internode following hydrocladial apophyses of forked internodes (Figs 4 A, 5 A), typically provided with one nematotheca (occasionally two). Hydrocladia branched, up to fifth order (Fig. 4 A). Forked hydrocladial internodes following ahydrothecate internodes. Hydrocladial apophyses forming an acute angle with internode, and provided with one or no nematothecae (Fig. 4 A, C). Length of hydrocladial internodes roughly constant along hydrocladia. Forked hydrocladial internodes (Figs 4 C, 5 A) with an aXillary hydrotheca, two nematothecae flanking hydrothecal aperture, one infrathecal nematotheca (occasionally two), and one nematotheca on apophysis in third and / or fourth internodes (occasionally on the second too) (Fig. 4 A). Unforked hydrothecate hydrocladial internodes (Figs 4 D – G, 5 B – C) with one hydrotheca in the middle and three nematothecae: two flanking hydrothecal aperture and one infrathecal nematotheca. Hydrotheca elongate (Figs 4 D – G, 5 B – D), length increasing along hydrocladia. Adcauline wall completely adnate to internode. Abcauline wall straight. Hydrothecal aperture circular; rim even. Gonothecae inserted on small apophyses between hydrotheca and infrathecal nematotheca of hydrocladial internodes. Gonothecae elongate pear-shaped (Fig. 4 H), provided with a distal, slightly oblique, circular aperture, and a basal chamber delimited by a circular diaphragm and provided with two nematothecae. Measurements (in µm). Hydrotheca: length, 220 – 350 (1 st and 16 th unforked hydrocladial internodes, respectively), diameter at aperture, 190 – 210. Hydrocladial internodes: length, 640 – 800 (1 st and 16 th unforked hydrocladial internodes, respectively), diameter under hydrotheca, 280 – 180 (1 st and 16 th unforked hydrocladial internodes, respectively). Cauline internodes: length, 620 – 720, diameter under apophysis, 260. Gonotheca: height, 1000 – 1100, maXimum diameter, 400 – 500, diameter at aperture, 220 – 240.

Remarks. Schizotricha auroraaustralis sp. nov. is recognizable from other Antarctic / sub-Antarctic species of the genus by the constant presence of a single infrathecal nematotheca on the unforked hydrocladial internodes, the forked hydrocladial internodes and the cauline internodes (only occasionally a second nematotheca has been noticed in the last two). In addition, it is also characterized by the presence of one nematotheca on the ahydrothecate internodes, one or none on the hydrocladial apophyses, and none on the cauline apophyses, although when the basal ahydrothecate internode is absent the cauline apophysis may be longer and provided with a nematotheca. From the known Antarctic / sub-Antarctic species of the genus, Schizotricha auroraaustralis sp. nov. is allied with Schizotricha discovery Soto Àngel & Peña Cantero, 2015, Schizotricha multifurcata Allman, 1883, Schizotricha nana Peña Cantero, Svoboda & Vervoort, 1996, Schizotricha southgeorgiae Peña Cantero & Vervoort, 2004 a, Schizotricha trinematotheca Peña Cantero & Vervoort, 2005 and Schizotricha unifurcata Allman, 1883 by the presence of ahydrothecate internodes, but differs from all of them by several important features. Schizotricha nana has the closest nematotheca sets, but it is easily distinguishable by having branched stems, hydrocladia up to second order and, particularly, low hydrothecae. Schizotricha multifurcata and S. unifurcata differ, amongst other things, by having branched stems, two nematothecae on the ahydrothecate internodes and a much higher number of infrathecal nematothecae on the cauline and forked hydrocladial internodes. Schizotricha discovery, S. southgeorgiae and S. trinematothecae all have unbranched stems. However, S. trinematotheca differs in having two to four nematothecae on the ahydrothecate internodes and a much higher number of nematothecae everywhere, even having suprathecal nematothecae. Schizotricha discovery differs in having one or two nematothecae on the ahydrothecate internode, one or two infrathecal nematothecae on the hydrocladial internodes and, particularly, in having four to five infrathecal nematothecae on the cauline internodes. Finally, S. southgeorgiae differs in having two infrathecal nematothecae on the forked hydrocladial internodes, two to five infrathecal nematothecae on the cauline internodes and the complete absence of nematothecae on the hydrocladial apophyses.

Etymology. The specific name auroraaustralis honors to the research vessel Aurora Australis, and its crew, with which the material studied was collected.

Material examined. 63 EV 314, a colony 250 mm high, with gonothecae (Holotype, IK – 2009 – 0483).