Dynamena opposita Galea & Schuchert 2019, sp. nov.
Creators: name: Galea, Horia R.
name: Schuchert, Peter
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Animalia
Cnidaria
Hydrozoa
Leptothecata
Sertulariidae
Dynamena
Dynamena opposita

description: Dynamena opposita Galea, sp. nov.
urn:lsid:zoobank.org:act: 375154A7-28FA-4897-A522-362AA994DDA1
Figs 10A, 11; Table 6
Diagnosis
Stems simple, monosiphonic, bearing pinnately-arranged hydrocladia in opposite pairs. Division into internodes irregular; each internode with a pair of indistinct apophyses supporting cladia and their associated axillary hydrothecae, as well as 0–3 pairs of hydrothecae below. Cladia divided into more or less regular internodes comprising 0–3 pairs of opposite hydrothecae. Hydrothecae tubular, deeply immersed into internode, not contiguous, aperture facing outwards, provided with two lateral, triangular cusps. Gonothecae unknown.
Etymology
From the Latin ‘ oppŏsĭtŭs, -a, -um ’, meaning ‘opposite’, to illustrate the relative position of the two rows of stem apophyses supporting the cladia.
Material examined
Holotype PACIFIC OCEAN • a 3.8 cm high colony without gonothecae, detached from substrate, with mostly only the perisarc left; off New Caledonia, stn DW4735; 22°40′ S, 167°40′ E; 233– 195 m; 22 Aug. 2016; KANACONO leg.; MNHN-IK-2015-470.
Description
Colony erect, 3.8 cm high, stiff when out of liquid, arising from creeping, branching hydrorhiza; stem monosiphonic, unbranched, composed of a short (3.5 mm long), athecate, basal part above the origin from stolon, and a much longer, distal thecate and cladiate part; the latter divided into irregular internodes by means of transverse constrictions of the perisarc; each internode long, composed distally of a pair of axillar hydrothecae above the insertion of two opposite cladia, as well as 0–3 pairs of hydrothecae below; hydrothecae opposite and distant from each one within a pair. No distinct stem apophyses supporting the cladia; successive pairs of cladia distant of 1.5–4 mm, depending of the number of intervening pairs of hydrothecae; cladia up to 1.3 cm long and comprising up to 16 pairs of successive hydrothecae; divided by transverse nodes, occasionally indistinct, into rather irregular internodes, generally comprising a single pair of hydrothecae distally, although 2–3 of these may occur elsewhere; first, proximal most internode comparatively longer than subsequent ones. Hydrothecae in strictly opposite pairs, not contiguous, though much closer to one another compared with their cauline counterparts; almost tubular, gently curving outwards, deeply immersed into the corresponding internode, leaving 1/6-1/7 of their adaxial wall free; the latter consequently short and distinctly concave; adnate adaxial wall concave; abaxial wall slightly curved outwards, especially distally; margin thickened, provided with two conspicuous, lateral, triangular cusps with rounded tips; opercular apparatus lost. Hydranths badly-preserved, presence of an abaxial gastric diverticulum could not be ascertained. Gonothecae absent. Perisarc of the colony thick and brown.
Remarks
According to Galea & Ferry (2015: 230–231, table 2), only two nominal species of Dynamena described so far form pinnate colonies with opposite cladia, namely D. anceps (Fraser, 1938) and D. japonica Stechow, 1920.
Through the courtesy of D. Geiger of the Santa Barbara Museum of Natural History (SBMNH), CA, USA, one of us (H.R.G.) was able to see several macrophotographs of the lecto- and one of the paralectotypes of D. anceps, demonstrating that it is a much slenderer species, with hydrothecae free from their corresponding internodes for a much longer portion than in D. opposite sp. nov.
Unlike the new species, D. japonica is a large hydroid (colonies reaching as much as 20 cm in height), with thick and brown perisarc (Hirohito 1995), with cladia separated by 2–8 pairs of hydrothecae, the latter being closely approximated on both stems and cladia (Hirohito 1995; Stechow 1913, as Thuiaria articulata (Pallas, 1766); Park & Rho 1986, as T. articulate). In addition, its hydrothecae are bigger, having notably a wider rim (Hirohito 1995).
Distribution
Only known from off New Caledonia (present study).
description: Published as part of Galea, Horia R. & Schuchert, Peter, 2019, Some thecate hydroids (Cnidaria: Hydrozoa) from off New Caledonia collected during KANACONO and KANADEEP expeditions of the French Tropical Deep-Sea Benthos Program, pp. 1-70 in European Journal of Taxonomy 562 on pages 28-30, DOI: 10.5852/ejt.2019.562, http://zenodo.org/record/3474305