Corhiza megatheca

Description Relatively flaccid, upright colonies, reaching as much as 7 cm in height, arising from rhizoid stolon securing the whole into sediment. Stem fascicled for most of its length, becoming monosiphonic distally; accessory tubes tortuous, anastomosing, devoid of nematothecae; main tube almost straight, unsegmented, athecate, giving rise at more or less regular intervals to short hydrocladial apophyses springing in all directions around stem; occasionally two apophyses are given off at the same level; apophyses ending in straight nodes distally. Cladia up to 1.1 cm long, heteromerously segmented by alternating straight and oblique nodes; first segment relatively short, delimited by straight nodes at both ends, with distally-placed nematotheca at its upper side; remainder of hydrocladium with regular structure, composed of alternating ahydro- and hydrothecate internodes. Ahydrothecate internodes comparatively longer than their hydrothecate counterparts, with proximal node transverse and distal node oblique; first internode exceedingly long, with 4 – 6 frontal nematothecae in row, subsequent ones shorter and provided with 2 frontal nematothecae, length of internodes decreasing distally, so that only 1 nematotheca is to be found on them. Hydrothecate internodes with proximal node oblique and distal node transverse, with one hydrotheca and its three associated nematothecae: a mesial one, not reaching the hydrothecal base, and pair of relatively short laterals, borne on moderately-long apophyses, together reaching half-length the adnate wall of hydrotheca. Hydrothecae very deep, tubular, somehow flared at margin, rim circular, entire. Gonothecae borne on cladia, given off singly from lateral side of hydrothecate internode, between hydrothecal basis and mesial nematotheca; pedicel of two short, quadrangular, athecate segments; gonotheca large, piriform, thin-walled, tapering basally and provided there with 3 nematothecae; aperture apical, ovoid, provided with watch-glass-shaped operculum. All nematothecae bithalamic, conical, upper chamber shorter than basal one, with rim scooped on both ab-and adaxial sides, distinctly more on the latter.

Figs 11 B, 13 H – J; Table 5

Remarks There is little doubt that the present materials belong to the same species as that described by Ansín Agís et al. (2009) under the basionym Antennella megatheca. The morphology of their trophosomes, as well as their measurements (see Table 5), agree well. It is likely that these authors had only tips of colonies, not exceeding 13 mm in length, that were provided with a limited number of cladia. This prompted them to consider the distal, monosiphonic parts of the main stems as representing potential stolons. No fertile specimens were reported so far, but the colony from sample MNHN-IK- 2015 - 600 bears gonothecae. Corhiza megatheca is unique among its congeners in that its cladia, instead of being given off from the component tubes of the stem, arise from a single, main tube, while all other counterparts have a strictly accessory function. This particular condition raises the question of the generic limitation in Corhiza Millard, 1962. At least morphologically speaking, the genus appears to be polyphyletic, with several species not meeting the original scope defined by Millard (1962: 275), viz, “ fascicled stem composed of a number of interwoven and intercommunicating tubes of equal diameter and importance. Hydrocladia arising from the component tubes in a completely irregular fashion, not rebranching ”. Indeed, C. sociabilis Millard, 1980 is a branched species with a pinnate appearance, in which three types of component tubes have been identified: 1) one to several axial tubes, each branched or unbranched; tubes bearing alternate cladia; 2) companion tubes, provided with hydrothecae, that accompany each branch; 3) peripheral tubes carrying nematothecae only (Millard 1980). Another species, formerly known as C. valdiviae (Stechow, 1923) (Millard 1975) is shown to belong to the new genus, Thamnopteros Galea gen. nov. (see below). For the time being, it is preferred to leave provisionally the present species in Corhiza, until additional data at both specific and generic levels become available.

Distribution Known only from off New Caledonia (Ansín Agís et al. 2009; present study).

Material examined PACIFIC OCEAN • 1 sterile colony, ca 7 cm high; off New Caledonia, stn DW 4725; 22 ° 41 ′ S, 167 ° 05 ′ E; 240 – 256 m; 20 Aug. 2016; KANACONO leg.; MNHN-IK- 2015 - 595 • 1 ca 5 cm high, fertile colony; off New Caledonia, stn DW 4711; 22 ° 47 ′ S, 167 ° 24 ′ E; 335 – 338 m; 18 Aug. 2016; KANACONO leg.; MNHN-IK- 2015 - 600.