Macrorhynchia spiralis Galea 2020, sp. nov.

Macrorhynchia spiralis sp. nov.

urn:lsid:zoobank.org:act: DA1E42B9-C682-4639-B3B1-4A9A7E2C7143

Figs 16 E–G, 19; Table 3

Diagnosis

Species of Macrorhynchia with a distinctly spirally-twisted stem, giving rise at each bend to a side branch that branches soon again, forming a bifid ramification. Stem and side branches strongly fascicled. Internodes short, with a proximal frontal nematotheca, a cladial apophysis above with its simple, conical nematotheca and a fronto-axillar nematotheca; cauline nematothecae saccate, with deeply scooped adaxial walls. Cladia alternate, fairly close to one another; cormidia moderately long, hydrotheca cupshaped, rather deep, aperture slightly sloping frontally, rim with 9 triangular cusps with rounded tips, of which the median, abaxial one is the most prominent; a slightly sigmoid, adaxial, intrathecal septum given off softly obliquely, reaching almost to abaxial wall; mesial nematotheca with short, spoutlike aperture, not reaching middle level of hydrotheca; lateral nematothecae barrel-shaped, with rim scooped out adaxially. Pseudophylactocarps, composed of chains of atrophied hydrothecae, borne on proximal parts of branches. Phylactocarp borne on a modified cladium, after 1–2 normal cormidia; first (occasionally second) segment, an atrophied hydrotheca, with its mesial nematotheca displaced more distally, and a short, lateral projection (on one side) bearing a couple of nematothecae similar to the laterals; gonotheca lenticular, flimsy, on top of atrophied hydrotheca, between the lateral nematothecae; gonophore, a medusoid.

Etymology

From the Latin ‘ spīra ’, meaning ‘spiral’, with reference to the distinctive shape of the stem.

Material examined

Holotype

PACIFIC OCEAN • upper part of 1 colony, ca 6.5 cm high, with female gonophores; off New Caledonia, stn DW4954; 24°12ʹ S, 159°41ʹ E; 300 m; 5 Sep. 2017; KANADEEP leg.; MNHN-IK-2015-519.

Paratypes

PACIFIC OCEAN • upper part of 1 colony, ca 8 cm high, with gonophores; off New Caledonia, stn DW5007; 22°12ʹ S, 159°02ʹ E; 290–750 m; 19 Sep. 2017; KANADEEP leg.; MNHN-IK-2015-520 • 1 entire sterile colony, ca 10.5 cm high; off New Caledonia, stn DW5008; 22°12ʹ S, 159°02ʹ E; 300–750 m; 19 Sep. 2017; KANADEEP leg.; MNHN-IK-2015-522.

Description

Colonies erect, sympodially built, with fascicled stems and side branches, grading to monosiphonic distally. Stem straight basally for a varied length, then bends at irregular intervals adopting a spiral arrangement. At each bend, a side branch arises as a rectilinear prolongation of the preceding portion of the stem, and usually ramifies once a short distance from its origin, both branches bearing hydrocladia for most of their length; proximally, a linear succession of saccate nematothecae with large apical foramina. Division into internodes indistinct wherever the perisarc is thick, but becomes evident towards the distal parts of the branches; internodes relatively short (400–420 µm long, 245–250 µm wide at node), with a large, proximal nematotheca, a cladial apophysis above, a fronto-axillar nematotheca, and a simple, conical nematotheca (with small, rounded, apical aperture) on the basal part of apophysis; nematothecae saccate, adnate for most of their length, having apically a large aperture whose rim is lowered adaxially. Cladia alternate, up to 14 mm long, composed of short cormidia (475–490 µm long, 95–100 µm wide at node), each accommodating a hydrotheca and its three nematothecae: one mesial and a pair of laterals. Hydrotheca cup-shaped, rather deep (390–400 µm), abaxial wall 215–240 µm long, straight for most of its length, gently expanding towards aperture; the latter 205–215 µm wide, slightly sloping frontally; rim with 9 rounded cusps, the uneven, abaxial one being the most prominent, and slightly inwardlycurved, the 4 pairs on the edges being comparatively broader and of varied development; an internal septum, of a slightly sigmoid appearance, is given off softly obliquely from the lower part of the adaxial thecal wall, penetrating deep into the lumen, almost reaching the abaxial wall; mesial nematotheca fused to the abaxial wall of hydrotheca for most of its length, leaving distally a short (45–60 µm long), spout-shaped aperture, not reaching the middle of the hydrotheca; lateral nematothecae broadly barrelshaped, 110–120 µm long and ca 50 µm wide at aperture, rim scooped out on adaxial side. Fertile colonies produce two distinct structures: one not associated to the gonosome, the other being part of it. The former are pseudophylactocarps, composed of a chain of modified hydrothecae devoid of lumina, but retaining their complement of nematothecae (the latter are all tubular, with a rounded distal aperture and an ovoid, proximal, adaxial one); they are abundantly borne on small, alternate apophyses arising regularly and slightly laterally to the nematothecae of the proximal (acladiate) portion of branches, and encircle their long axis; no gonothecae have been observed associated with the pseudophylactocarps. The gonosome proper is borne on the distal parts of distally-modified cladia that retain proximally 1 (occasionally up to 4) normal cormidia, followed by 1–2 elongated internodes representing atrophied hydrothecae (devoid of lumina) that retain their normal complement of nematothecae, in addition to which a pair of nematothecae (similar to the laterals) is mounted on a conspicuous lateral, subterminal projection. There is some variation in the structure of the proximal part of the fertile cladium: the proximalmost hydrotheca (when only one is present) may be suppressed and replaced by one or two short internodes, the proximalmost bearing one nematotheca, the second three (one mesial and a pair of laterals). Gonothecae lenticular, 1150–1380 µm long and 750–795 µm wide, with flimsy perisarc, inserted between the pair of lateral nematothecae of the distalmost, atrophied hydrotheca; the gonophore is a medusoid, composed of a central spadix encircled by a single layer of large oocytes (the holotype is a female colony), and bearing apically a belt of refringent corpuscles.

Nominal species Brief description, with emphasis on the distinctive features, and geographical distribution

Remarks

Hydrothecae with similar shape as those of M. spiralis sp. nov., i.e., cup-shaped, with intrathecal adaxial but not abaxial ridge, are found in a number of congeners. Their distinguishing features with respect to the new species are emphasized in Table 3.

As noted above, this species produces a medusoid gonophore. At least three other nominal species reproduce in a similar way, namely M. filamentosa (De Lamarck, 1816), M. philippina Kirchenpauer, 1872 and M. racemifera (Allman, 1883) (Galea 2018), suggesting that this mode of dispersal could be the rule within the genus, as it probably is in the plumulariid genus Dentitheca Stechow, 1920 (Galea et al. 2012).

Distribution

Known only from off New Caledonia (present study).