Actinopluma mirifica

Description Colony erect, ca 14.5 cm high, rigid, coplanar, arising from rhizoid stolon anchoring it in marine sediment; stem with 2 unbranched side branches, all coplanar and fascicled; accessory tubes with longitudinal row of nematothecae. Monosiphonic parts of stem and branches undivided, but equivalents of internodes with cladial apophysis and 3 nematothecae above, on same side as apophysis; apophyses alternate along stem, two rows coplanar; apophysis with quite distant, conspicuous conical mamelon on upper side, with rounded aperture on summit, and proximal, bithalamic nematotheca; no delimitation between apophysis and cladium; two accessory cladia are given off at acute angles from the lateral sides of apophysis, insertion sites below the mamelon; all three cladia unbranched, pointing out- and upwards, pair of accessory cladia forming comparatively more acute angle with stem than with their main counterpart. Main cladium relatively thick, up to 7 mm long, irregularly divided into hydrothecate internodes by means of transverse nodes; each internode with 1 – 4 hydrothecae and a number of nematothecae in row between them; accessory cladia, up to 3.5 mm long, comparatively thinner than their main counterpart, comprising generally a succession of nematothecate internodes (nodes transverse) of varied length, each bearing 1 – 8 alternate nematothecae, slightly displaced laterally with respect to longitudinal axis of internode; more distal parts of longest accessory cladia with 1, occasionally 2, intervening hydrothecate internodes. Hydrothecae shallow, fully adnate adaxially to their corresponding internodes, abaxial wall with thickened perisarc; conspicuous perisarc plug at junction between adaxial wall with basis; belt of desmocytes close to basis; no nematothecae directly associated to hydrotheca. Hydranths missing, only remains of coenosarc present. Gonothecae absent.

urn: lsid: zoobank. org: act: 3 C 5853 F 3 - E 9 EB- 421 B-B 725 - A 5 B 5 C 3 EDAA 9 B Figs 3 – 5; Table 1

Diagnosis Colony erect, rigid, coplanar, with sparingly branched stem; stem and branches fascicled. Accessory tubes with a row of nematothecae along their length. Main tube of the stem undivided, but equivalents of internodes with indistinct cladial apophysis and a row of 3 bithalamic nematothecae above; apophyses alternate along the stem, the two rows coplanar; each with a conspicuous conical mamelon with broad, rounded aperture, and a bithalamic nematotheca a short distance below to it; no delimitation between cladium and apophysis; cladia irregularly divided into hydrothecate internodes by transverse nodes; each internode with 1 – 4 hydrothecae and a number of nematothecae in a row between them; each cladium accompanied by 2 comparatively slender, accessory cladia given off from below each side of the basal mamelon; accessory cladia generally composed of successive nematothecate internodes of varied length (nodes transverse); occasionally internodes with 1 – 2 comparatively smaller hydrothecae intervene distally. Gonothecae unknown.

Remarks As noted above, the segmentation of the cladia is very irregular. When only one hydrotheca is to be found on an internode, there is a number of nematothecae below and above it, respectively: 2 – 1, 2 – 3, 3 – 1, 3 – 2, 4 – 1, 4 – 2, 6 – 1. When two hydrothecae are confined to an internode, the number of nematothecae below the proximal hydrotheca, between the two hydrothecae, and above the second hydrotheca, could be as follows: 2 – 5 – 2, 3 – 4 – 1, 3 – 5 – 1, 3 – 5 – 2, 4 – 4 – 1, 4 – 5 – 1. In the case of internodes bearing three hydrothecae, the number of nematothecae below the first hydrotheca, between the first and second, the second and third hydrotheca, and above the third hydrotheca, could be: 3 – 5 – 5 – 1 and 3 – 5 – 5 – 2. When four hydrothecae are confined to an internode, the number of nematothecae below the first hydrotheca, between the first and second, second and third, third and fourth, and above the fourth hydrotheca, is generally 3 – 5 – 5 – 5 – 1. In the colony from MNHN-IK- 2015 - 613, an additional accessory hydrocladium is given off from one side of the first cladial hydrotheca.

Distribution Only known from its type locality, Coral Sea off New Caledonia (present study).

Etymology From Latin ‘ mīrĭfĭcus, - a, - um ’, meaning ‘ marvelous’, to qualify the existence of such a peculiar, uncommon hydroid species.

Material examined Holotype PACIFIC OCEAN • 1 ca 14.5 cm high, sterile colony; off New Caledonia, stn CP 4956; 23 ° 13 ′ S, 159 ° 35 ′ E; 295 – 295 m; 6 Sep. 2017; KANADEEP leg.; MNHN-IK- 2015 - 612. Paratype PACIFIC OCEAN • 1 ca 11 cm high, sterile colony; off New Caledonia, stn DW 4953; 24 ° 10 ′ S, 159 ° 41 ′ E; 320 – 270 m; 5 Sep. 2017; KANADEEP leg.; MNHN-IK- 2015 - 613.