Plumularioid hydroids (Cnidaria: Hydrozoa) from off New Caledonia collected during KANACONO and KANADEEP expeditions of the French Tropical Deep-Sea Benthos Program

This study reports on 25 species of hydroids occurring in the collections gathered during KANACONO and KANADEEP expeditions carried out in the SE of New Caledonia in 2016, and off the western coast of the island in 2017, respectively. Of these, 19 have not been dealt with in earlier reports on these collections. Two new genera and four new species are described, viz, Actinopluma mirifica Galea gen. et sp. nov., provisionally assigned to the family Kirchenpaueriidae Stechow, 1921, Schizoplumularia helicoidalis sp. nov., belonging to the Plumulariidae McCrady, 1859, and Corhiza patula Galea sp. nov. and Thamnopteros uniserius Galea gen. et sp. nov., both placed in the family Halopterididae Millard, 1962. The gonotheca and the medusoid gonophore of Plumularia contraria Ansín Agís et al., 2014 are described for the first time, allowing a genus transfer to Dentitheca Stechow, 1919, as D. contraria comb. nov. Plumularia conjuncta Billard, 1913, known earlier from a minute portion of colony, is redescribed based on a complete, though infertile, specimen. Similarly, complete specimens corresponding to the hydroid previously referred to as Antennella megatheca Ansín Agís et al., 2009 are documented, allowing a provisional reallocation to Corhiza Millard, 1962 and a description of its so far unknown gonothecae. Fertile material assignable to the poorly-known Monostaechas fisheri Nutting, 1905 allows the recognition of this hydroid as a valid species, distinct from M. quadridens (McCrady, 1859). Most taxa are illustrated to validate the reliability of their identifications. Finally, phylogenetic reconstructions of the families Aglaopheniidae, Plumulariidae, and Halopterididae, based on the 16S rRNA, allowed a first genetic characterization of some of the species dealt with in this work.


Diagnosis
Hydroids with upright, rigid, coplanar colonies with sparingly-branched stem; stem and branches fascicled. Main tube of the stem undivided, but equivalents of internodes with a lateral apophysis and a number of nematothecae above; apophyses alternate, coplanar, each with a mamelon, continued directly into a main hydrocladium; two accessory cladia, projecting outward and slightly upward, are given off laterally from below both sides of the mamelon; main hydrocladium divided into internodes with hydroand nematothecae; accessory cladia shorter and thinner than main counterpart, generally composed of nematothecae-bearing internodes, occasionally with 1-2 intervening hydrothecate internodes distally. Hydrothecae cup-shaped, without associated nematothecae. Gonothecae unknown.

Etymology
From Ancient Greek 'ἀκτίς', meaning 'ray', and Latin 'plūma', meaning 'plume', to describe the radiate appearance of this new hydroid genus. The gender is feminine.

Remarks
Based on morphological grounds alone, the absence of lateral nematothecae flanking the hydrotheca place this genus in the family Kirchenpaueriidae Stechow, 1921. This allocation should be regarded as provisional, pending the collection of additional material suitable for molecular studies, expected to clarify unambiguously its systematic position.
In having hydrocladia provided with two accessory appendages, the colonies of this so far monotypic genus somehow resemble macroscopically Oswaldella Stechow, 1919, but in the latter, the hydrocladia branch dichotomously at the level of the proximalmost internode, the original cladium not extending beyond this internode (Peña Cantero et al. 1997). Galea gen. et sp. nov. urn:lsid:zoobank.org:act:3C5853F3-E9EB-421B-B725-A5B5C3EDAA9B 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.

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.    Galea gen. et sp. nov., main cladium with couple of accessory counterparts (A); proximal portion of another main cladium (B), accessory cladia not shown; middle portion of a cladium with aberrant branching (C); accessory cladium with nematothecate internodes and intervening hydrothecate internode (D); all from holotype MNHN-IK-2015-612. Scale bar: 300 µm.
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).

Remarks
For a comprehensive account on the trophosome of this species, refer to Ansín Agís et al. (2014).
Gonothecae were not documented in the original account, but the material MNHN-IK-2015-585 is a fertile colony. Its gonothecae are borne on the side branches in the axil formed by the cladial apophyses with their corresponding internodes; they are broadly piriform, with a truncate distal end that bears a wide, circular aperture closed by a filmy perisarc, possibly becoming a deciduous operculum that is lost upon the release of the gonophores. The latter are undoubtedly medusoids, as suggested by the presence of an apical belt of small, spherical, refringent corpuscles. Two or three large oocytes, occupying the whole subumbrellar cavity, are clearly discernible inside each gonophore (Fig. 6B).
The elongated hydrothecae, whose lateral walls produce two arched lobes flanking their apertures, the characteristic shape of the gonotheca, and the production of medusoid gonophores, suggest that this species is better assignable to the genus Dentitheca Stechow, 1919. For the same reasons as those listed above, additionally supported by molecular evidence (Moura et al. 2018), Plumularia elongata Billard, 1913 is equally assigned to Dentitheca, as D. elongata comb. nov. As noted by Ansín Agís et al. (2014), this species shows certain similarities with D. contraria comb. nov., notably the shape of its hydrothecae. However, Billard's species forms planar colonies, while those of D. contraria comb. nov. have spirally-arranged cladia-bearing branches; additionally, the cladial internodes of the latter continue for a longer distance above the hydrotheca, and are provided there with an additional nematotheca, otherwise absent in D. elongata (compare Fig. 6A and Fig. 6C).

Remarks
A recent, comprehensive account is given by Ansín Agís et al. (2014). Despite their opinion, this species is here reassigned to Dentitheca on the account of the emarginated hydrothecal rim on the adaxial side, creating two latero-posterior, triangular lobes, and the morphology of its gonothecae, as noted above under D. contraria comb. nov.
The Philippines record by Nutting (1927, as Plumularia dendritica), very likely belongs to the present species. Despite Nutting' statement that his material agrees "very exactly with the original descriptions and figures, and a comparison with the type specimens in the museum of the State University of Iowa confirms this view", it is believed that this author was mistaken by some obvious similarities between the hydrothecae of these two species. Indeed, upon a close examination, it appears that the hydrothecal rim of D. dendritica has a pair of distinct, lateral cusps before the adaxial emargination, while the adaxial wall of D. habereri produces comparatively less marked cusps (compare Galea (2010: fig. 7d-e) with Fig. 6D herein). It is very unlikely that a common species of the tropical western Atlantic occurs in the tropical western Pacific as well, and no subsequent records of D. dendritica have been reported from this area.

Description
Colony ca 30 cm high, detached from its substrate immediately above origin from hydrorhiza; flaccid, delicate, composed of lightly fascicled, slender, exceedingly elongate stem, composed of a few auxiliary tubes surrounding main tube that gives rise regularly to spirally-arranged, monosiphonic, up to 2 cm long hydrocladia-bearing branches, 0.35-0.55 mm distant from one another; main stem neither divided into internodes nor with internal partitions of perisarc, with row of frontal nematothecae, and short, inconspicuous apophyses supporting lateral branches. The latter arch graciously outwards and nearly horizontally; division into internodes inconspicuous, though incomplete transverse septa delimit segments composed of 1-2 apophyses for hydrocladia; nematotheca a certain distance above each apophysis on opposite side of internode; each internode with dense, annular, internal perisarcal ridges; apophyses alternate, shifted on to lower side of branches, resulting in cladia facing downwards and forming acute angle between two rows; mamelon and pair of nematothecae on each side of axil. Cladia sinusoid, up to 2.5 mm long, heteromerously divided into internodes by means of transverse nodes; proximally, 1-2 short internodes, first one athecate, second always bearing nematotheca; remainder of cladium composed of sequence of up to 3 hydrothecate internodes alternating with short, ahydrothecate internodes provided with proximally-placed nematotheca on their upper side; all internodes with two incomplete internal ridges near both ends; hydrothecate internode about thrice length of their ahydrothecate counterparts, with centrally-placed, deep, tubular, wholly adnate hydrotheca and its three associated nematothecae: one mesial, far below hydrothecal base, borne on small protuberance, and pair of laterals, not situated at same level, borne on indistinct apophyses; hydrothecal rim entire, circular in apical view, slightly scooped when seen in lateral view, aperture tilted abaxially; all nematothecae bithalamic, conical, with high basal chamber and relatively shallow upper chamber, whose rim is slightly lowered on adaxial side. Gonothecae unknown.

Remarks
Considered as a variety of Plumularia insignis Allman, 1883 by Billard (1913), it was raised to species by Ansín Agís et al. (2016) upon the reexamination of Allman's hydroid. Plumularia conjucta was exclusively known earlier from its holo-(a 5 mm long fragment) and schizoholotype (a 1.5 mm long fragment).

Distribution
Nearly cosmopolitan (Schuchert 2013), except for Arctic and Antarctic waters. According to Schuchert (2014), the taxon could be regarded as either a multitude of cryptic, geographically delimited lineages, or as a species with an extensive population subdivision.

Remarks
For a description of this species, refer to Ansín Agís et al. (2016).

Distribution
Scattered records from off New Caledonia (Ansín Agís et al. 2016;present study

Diagnosis
Schizoplumularia with slender, flaccid, geniculate stem, lightly fascicled proximally, then composed of only two adjacent tubes for most of its length; tubes with scattered nematothecae along their length; at each geniculation, one tube, while diverging as a cladia-bearing branch, gives rise to an auxiliary  tube running upwards along the second tube, a situation that is reversed throughout the stem; branches undivided, but each equivalent of internode with a latero-distal cladial apophysis (the latter with a mamelon and 2 axillar nematothecae) and 1-2 nematothecae on side opposite the apophysis; apophyses shifted on to the upper side of the branch; cladia heteromerously segmented into short, ahydrothecate internodes with 1 nematotheca alternating with comparatively longer hydrothecate internodes bearing a small, cup-shaped hydrotheca and its 3 associated nematothecae.

Etymology
From Greek 'ἑλικοειδής', meaning 'in the form of a helix', to describe the arrangement of the cladiabearing branches along the stem.

Description
A 15.5 cm high colony of very delicate and flaccid appearance, detached from its hydrorhiza; stem slender, composed proximally of bundle of few tubes running parallel to each other and communicating, at intervals, through common holes in the perisarc; nematothecae scattered along their length; more distally, remainder of stem composed of only two contiguous tubes forming alternately-placed geniculations at intervals of 3.5-5.5 mm; at each geniculation, one of tubes diverges from stem at acute angle and transforms itself into cladia-bearing branch; it also gives rise simultaneously, at level of axil thus formed with its henceforth single counterpart, to another tube running upward along that counterpart, so as to ensure the obligatory presence of two adjacent tubes composing stem; at next geniculation, there is a reversal of roles played by couple of tubes: that newly-added at previous geniculation continues unaffected (skips one geniculation), while its counterpart detaches distally from stem as cladia-bearing branch; stem tubes with two parallel rows of alternately-placed nematothecae; between successive geniculations, stem acquires slight torsion, so that its cladia-bearing branches are arranged in helicoidal manner along it. Stem, branches and cladia very slender; perisarc straw colored. Cladia-bearing branches up to 2 cm long, unsegmented, but each equivalent of internode with distallyplaced cladial apophysis and 1-2 nematothecae on side opposite to apophysis (up to 6 nematothecae in two closely-set whorls found on proximalmost 'internode'); apophyses alternate, not coplanar, but forming wide angle between two rows; axil with conical mamelon provided with rounded aperture on summit, and two nematothecae, one on each side. Cladia up to 2.5 mm long, heteromerously segmented by means of transverse nodes into alternating ahydrothecate and hydrothecate internodes; proximalmost internode ahydrothecate, slightly shorter than its subsequent counterparts, with proximal nematotheca on its upper side; ahydrothecate internodes with generally two (occasionally up to four) internal, incomplete perisarc ridges near both ends, and proximal nematotheca placed frontally; hydrothecate internodes, up to 6 per cladium, comparatively longer than their ahydrothecate counterparts, with almost centrally-placed hydrotheca and its three associated nematothecae: a mesial one and pair of laterals; up to eight internal incomplete perisarcal ridges per hydrothecate internode. Nematothecae of colony all alike: trumpet-shaped, bithalamic, lower chamber tall, upper chamber shallow, wall of the latter lowered on adaxial side. Gonothecae in axils of cladia-bearing branches; immature in material at hand; broadly piriform, tapering gradually below into an indistinct, laterally curved pedicel; distal end truncate, not fully formed.

Remarks
The present material is, with little doubt, conspecific with part of that originally assigned to S. elegans by Ansín Agís et al. (2016), as for instance the colony from SMIB 4, stn DW53 that displays a "spirallybuilt" appearance (see their fig. 6a) and "larger hydrothecae" (see their fig. 8c-d). Although these authors acknowledged that "the remaining characters are similar to the other examined colonies" of S. elegans, they refrained from separating it specifically.
However, when compared to all available specimens of S. elegans occurring in the present collections, the colony from sample MNHN-IK-2015-610 is comparatively more slender and decidedly lax, and, most importantly, displays a spiral arrangement of the hydrocladia-bearing branches around the stem; the latter is lightly fascicled for the proximal 2.5 cm of its length, while the remainder (13 cm high) is composed of only two contiguous tubes. In S. elegans, the stems are fairly fascicled for nearly their whole length, with only the very tips of the colonies being monosiphonic, and their cladia-bearing branches of both rows are strictly coplanar (compare Fig. 9A and Fig. 9B).
The material MNHN-IK-2015-596, besides displaying shorter stem internodes (and consequently more closely-set cladia-bearing branches) compared to the holotype, has similar cladia ( Fig. 10D-E). Due to the scarcity of the available specimens of S. helicoidalis sp. nov., its intraspecific variability could not be assessed properly. In order to avoid any possible identification error, these specimens are not selected as a paratype, despite one of them being provided with (immature) gonothecae.

Description
Colony upright, ca 10 cm high, arising from elongated (2-3 cm high) mass of interlacing hydrorhizal fibers anchoring it in sediment, giving rise from its upper part to bundle of individual hydrocladia, all pointing upwards. Cladia up to 6 cm high, composed proximally of up to 1.5 cm long ahydrothecate part with two closely-set, parallel rows of frontal nematothecae, placed mostly alternately, occasionally in opposite pairs; ahydrothecate part ending in deeply-incised, oblique node, separated from remainder of cladium through prosegment, the latter also ending distally in oblique node; prosegment with hydrotheca and its five associated nematothecae (a mesial one and two pairs of laterals), as well as four additional nematothecae imperceptibly displaced laterally, left and right, along longitudinal axis of internode. Remainder of hydrocladium undivided, but equivalents of internodes with hydrotheca, its four associated nematothecae (two pairs of laterals), as well as row of 2-4 nematothecae above (commonly 3, but proximal 'internodes' with 4 of these, and distal 'internodes' with only 2). Hydrotheca cup-shaped, fully adnate, abaxial wall straight for most of its length, slightly curved inwardly at aperture; rim smooth, aperture circular, facing upwards. All nematothecae bithalamic and movable; mesial nematotheca indistinguishable morphologically from its counterparts situated proximally in row, all with slightly scooped rim of upper chamber; lateral nematothecae in pairs, first pair borne on tips of moderately-developed apophyses, second pair seated near bases of same apophyses; first pair greatly surpassing hydrothecal rim, with tall basal chamber, upper chamber shallow, conical, with rim entire; second pair comparatively shorter, wall of upper chamber scooped on adaxial side. Gonothecae absent.

Remarks
There is little doubt that the present material is conspecific with that described and illustrated earlier by Ansín Agís et al. (2009). Both display a single row of nematothecae between successive hydrothecae (except for the proximalmost internodes), and this represents a deviation from the current concept of A. varians (Billard, 1911), in which the nematothecae are arranged in two parallel rows, even though their distribution in pairs is not always met with (Billard 1913;present study). Additionally, a comparison of the material at hand with true specimens of A. varians from Indonesia (sample HRG-1584) reveals a series of discrete differences, such as the shape of the hydrotheca (compare Fig. 12B-C and 12D-E, respectively) and the size of the anterior pair of lateral nematothecae (compare Fig. 13B and 13C). Their morphometrical differences are summarized in Table 4. For these reasons, the New Caledonian material is provisionally assigned to the hydroid of Billard (1911), pending a more comprehensive morphological study based on a relevant number of colonies, as well as genetic data. Billard (1913) distinguished his A. varians from A. balei (Billard, 1911) on the following accounts: 1) the proximal part of the stems always bears a prosegment in the former, but never in the latter; its occurrence was documented by a number of authors (e.g., Billard 1913: fig. 4a;Hirohito 1995: fig. 79d;   Fig. 11. A. Antennella aff. varians (Billard, 1911) (Billard, 1911) vs true A. varians and A. balei (Billard, 1911), in µm. * Di Camillo, pers. comm.
In spite of the opinion of Schuchert (1997), A. balei is here kept distinct from A.varians. Indeed, the examination of several samples from three distant localities in Indonesia reveals a uniformity of morphological characters: 1) although deeply-incised, strongly oblique nodes occur here and there along the stems, delimiting internodes with numerous hydrothecae, a prosegment is lacking in all specimens; 2) the proximal part of each stem (± 1 cm long) bears a varied number of hydrothecae until the first strongly oblique node occurs; in some specimens, this portion could be segmented by slightly oblique nodes into internode bearing irregularly 1, 2, 3, etc. hydrothecae; these nodes, although distinct, are neither that deeply-incised, nor that strongly oblique as the scarce nodes that occasionally divide the caulus above; 3) there is no single mesial nematotheca, even in the internodes immediately above a deeply-incised, strongly-oblique node; 4) there are generally 2 pairs (occasionally 3 proximally) of nematothecae in two parallel rows above each hydrotheca, the members of each pair being always situated at the same level; 5) a deeply-incised, strongly-oblique node generally passes through two consecutive pairs of stem nematothecae, one being confined to the internode below, the other to the internode above; 6) all nematothecae have a hypertrophied perisarc; the first pair of laterals is borne on long, horizontal apophyses fused to the lateral walls of the hydrotheca; the second pair is sessile, and the nematothecae have a size comparable to their anterior counterparts; the wall of the upper chamber of the anterior nematothecae is sinusoid on the adaxial side.
Antennella biarmata Nutting, 1927 is very likely a junior synonym of A. balei, judging from the shape and size of the apophysis supporting the lateral nematothecae, and additional characters mentioned in the original account.
The material assigned by Rees & Vervoort (1987) to A. varians could not be allocated with certainty to either A. varians or A. balei, due to an unusual segmentation of the stems and some characters displayed by the nematothecae.

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 aband adaxial sides, distinctly more on the latter.

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.  Table 6 Diagnosis Stem composed of many loosely-aggregated tubes arising from root-like hydrorhiza; individual cladia arising distally, in all directions, from some of the component tubes; proximal parts of cladia with a row of frontal, bithalamic nematothecae; distal node oblique, followed by a regular succession of alternating hydrothecate and ahydrothecate internodes delimited by a heteromerous segmentation; ahydrothecate internodes short, with a frontal nematotheca proximally; hydrothecate internodes equally short, with a very deep, cylindrical hydrotheca with strongly flared margin, and its 3 associated nematothecae: one mesial, and a pair of lateral; mesial nematotheca with wall of upper chamber lowered adaxially; lateral nematothecae borne on short apophyses, upper chamber globular, with deeply emarginated wall on adaxial side. Gonothecae unknown.

Etymology
From Latin 'pātŭlus, -a, -um', meaning 'provided with a wide aperture', to illustrate the shape of its hydrothecae.

Description
Colony upright, ca 5 cm high, arising from dense, root-like hydrorhiza, composed of numerous branching, anastomosing tubes that converge, then loosely aggregate above substrate to form ca 3 cm high, fascicled stem; all tubes have equivalent roles; division into internodes absent; perisarc thick, smooth to slightly wrinkled. Some of component tubes of stem modified distally to become cladia; these detach from stem at acute angle, pointing upwards, in all directions around it. Cladia up to 2 cm long, composed of free, ahydrothecate proximal part with smooth perisarc, occasionally with either signs of breakage followed by subsequent regeneration, or divided by transverse nodes into succession of internodes of varied length, distally bearing row of nematothecae; last internode with oblique node distally; remainder of cladium heteromerously segmented into hydrothecate and ahydrothecate internodes by means of alternating transverse and oblique nodes; most often, transverse node only slightly indicated, especially dorsally; oblique nodes generally well-marked, though here and there they may be totally absent. Hydrothecate internodes short, with proximally oblique and distally transverse nodes, a hydrotheca and its three associated nematothecae: one mesial and pair of laterals; hydrotheca thin-walled, deep, tubular, half adnate, flaring distinctly at aperture; mesial nematotheca not reaching hydrothecal base, upper chamber with rim lowered on adaxial side; lateral nematothecae borne on short apophyses, whole structure far from reaching hydrothecal margin; upper chamber globular, with rim distinctly scooped on adaxial side. Ahydrothecate internodes with proximal node transverse and distal node oblique, of nearly same length as their hydrothecate counterparts; proximally frontal, conical nematotheca with rim of upper chamber lowered on adaxial side. Gonothecae absent.

Remarks
Corhiza patula Galea sp. nov. is unique among its congeners through the distinctive shape and size of its hydrothecae. Only the hydrothecae of C. megatheca, if this species proves congeneric, approach their morphology, but they are slightly smaller and not distinctly flared at rim. Unlike in the new species, it has been shown above that the cladia of C. megatheca are given off exclusively from a single tube of the fascicled stem. The third species occurring in the study area, C. pauciarmata Ansín Agís et al., 2009 (see below), has comparatively shorter hydrothecae (compare Fig. 14A and 14E).

Distribution
Known only from off New Caledonia (present study).

Remarks
Although the ahydrothecate internodes of cladia are comparatively shorter, no other differences could be noted with the specimens described and illustrated by Ansín Agís et al. (2009).

Distribution
Scattered records from off New Caledonia (Ansín Agís et al. 2009;present study

Description
Stems upright and quite rigid, simple, monosiphonic, ca 8 cm high, arising from reticulate hydrorhiza; composed of long (5-6 cm) proximal part devoid of hydrothecae and hydrocladia, and comparatively shorter (2-3 cm) distal part bearing hydrothecae and hydrocladia. Proximal part with few closely-set transverse nodes immediately above origin from stolon, as well as few, irregular, more distant ones above; nematothecae in two closely-set, longitudinal rows, only in distal part of this portion of stem; nematothecae alternate along most length of nematothecate part of stem, distalmost ones being gradually arranged in subopposite to opposite pairs. Upper part of stem starting with two consecutive prosegments delimited at both ends by deeply-incised, very oblique nodes, each comprising a proximally-placed, quite damaged hydrotheca, 11 nematothecae, and cladial apophysis arising only from side of second hydrotheca; nematothecae: one mesial, two pairs of laterals, as well as 3 opposite pairs above hydrotheca, in two closely-set, longitudinal rows. Remainder of stem slightly geniculate, with faintly-marked to almost imperceptible segmentation through slightly oblique nodes; each internode with hydrotheca proximally, short cladial apophysis laterally, and at least 9 nematothecae: one mesial, two pairs of laterals and 2 pairs above hydrotheca; presence of axillar nematotheca could not be checked properly, for cladial apophyses being shifted on to anterior part of stem, forming acute angle between two rows. Cladia up to 5 mm long, bearing up to 11 hydrothecate internodes; proximally short, almost indistinct, athecate internode, with proximal node straight and distal node oblique; nodes on remainder of cladium generally indistinct but, when present, transverse, immediately above each hydrotheca; each internode with hydrotheca, flanked by double pair of nematothecae, as well as 3-4 additional nematothecae above; anterior pair of laterals borne on conspicuous apophyses, thecae projecting above hydrothecal rim; posterior pair sessile, at base of apophyses supporting anterior pair; proximalmost nematotheca among those situated between consecutive hydrothecae always situated behind adaxial wall of proximal hydrotheca, either decidedly in axil or certain distance above. All nematothecae conical, bithalamic, wall of upper chamber slightly lowered on adaxial side. Hydrotheca tubular, rather deep, ⅔ rd adnate, abaxial wall straight or nearly so, aperture circular, forming an angle of ca 40° with internode. Single gonotheca arising laterally from below basis of cladial hydrotheca; urn-shaped, slightly flattened, tapering abruptly basally, distally truncate, and there provided with ovoid, deciduous, filmy flap; two exceedingly long nematothecae inserted at above ⅓ rd of its length.

Remarks
The present material agrees generally well with the type (Vervoort 1966;Schuchert 1997) and specimens examined by Vervoort & Watson (2003) and Ansín Agís et al. (2009), although it shows a number of differences: 1) it bears 2 opposite pairs of nematothecae above the cauline hydrothecae, while the occurrence of only two nematothecae in a row has been documented previously; 2) there are 3-4 nematothecae, instead of only two as documented earlier, between two successive cladial hydrothecae, of which the proximalmost can be either decidedly axillar or situated a short distance above the axil; 3) the gonotheca is urn-shaped and not piriform, as described by Vervoort & Watson (2003), and bears 2 exceedingly long nematothecae.
I can see little reason to separate specifically the present material from that described earlier, as the species is still poorly-known, and the intraspecific variation insufficiently documented. The gonotheca in my material could belong to the opposite sex with respect to that described earlier by Vervoort & Watson (2003); as to the length of the nematothecae it carries, this could likely not be used as a valid taxonomic criterion at this stage, given that, in this hydroid, at least the lateral nematothecae display a "very variable" length (Vervoort & Watson 2003).

Remarks
For synonymy and a description, refer to Schuchert (1997).
In the present material, the lateral nematothecae associated to the cauline hydrothecae on the side opposite to the cladial apophyses are often absent (Fig. 14O); in only rare instances, a pair is formed (Fig. 14P). The cladia display a regular structure: they begin with a short, quadrangular, athecate segment, followed by an ahydrothecate internode bearing a superior nematotheca and ending distally in an oblique node; afterwards, the segmentation is heteromerous, with up to 8 hydrothecate internode alternating with ahydrothecate counterparts (Fig. 14N); the latter are short, delimited proximally by a transverse node and distally by an oblique node, and bear a frontal nematotheca; the hydrothecate internodes are slightly longer, are delimited proximally by an oblique node and distally by a transverse one, and bear distally a hydrotheca and its 4 associated nematothecae: a mesial, a pair of laterals, and a scale-shaped axillar one. There is an important sexual dimorphism of the gonothecae: the female, given off from below the cauline hydrothecae, are large, ovoid and bear proximally 3 nematothecae (Fig. 15D); the male gonothecae, borne on cladia, are comparatively minute, and bear a single basal gonotheca (Fig. 15E).

Description
Colonies upright, up to 2 cm high, coplanar, rigid, with thick, brown perisarc, arising from creeping, branching, slightly flattened hydrorhiza. Stem monosiphonic; proximal part straight, irregularly divided into few internodes of varied length by means of transverse nodes; distalmost nodes with two parallel, closely-set rows of frontal nematothecae; remainder of stem divided into regular internodes by transverse nodes; each internode moderately long, straight, bearing one or two lateral apophyses and 4-6 frontal nematothecae in two closely-set rows; apophyses subterminal on internodes; when two of these are present on same internode, either opposite or subopposite; two types of apophyses: first type very short, ending in straight node and bearing secondary stem, and second type comparatively longer, ending in oblique node and bearing cladium; first regular thecate stem internode with either pair of dissimilar apophyses, one bearing secondary stem, the other a cladium, or with pair of apophyses supporting two secondary stem; in the latter case, following internode may bear two dissimilar apophyses; remainder of stem internodes with single apophyses supporting cladia only, distal part of stem assuming scorpioid shape, typical of genus. Secondary stems, like main stem, divided into regular internodes, each bearing nematothecae and upwardly-directed apophysis supporting cladium; consequently, all their cladia point upwards with respect to colony, and their hydrothecae all facing towards main stem. All cladia borne directly on main stem have their hydrothecae facing downwards in proximal part of colony and, as stem curves away in scorpioid manner, hydrothecal apertures of more distal cladia increasingly facing outwards or even upwards, depending of length of distal part of stem and number of cladia it bears. Cladia up to 8 mm long, heteromerously divided into hydrothecate and ahydrothecate internodes; up to 10 hydrothecate internodes per cladium, relatively short, each with proximal node oblique and distal node transverse, bearing hydrotheca in middle and its complement of nematothecae: one mesial, a pair of laterals, as well as axillar one on one side of hydrotheca; hydrotheca deep, tubular, half adnate, free adaxial wall and abaxial wall nearly straight, aperture circular, rim smooth; all nematothecae movable and bithalamic; laterals conical, borne on conspicuous apophyses, whole set reaching hydrothecal rim; upper chamber of lateral nematothecae lowered on adaxial side, and there with sinusoid margin. Cladial ahydrothecate internodes of varied length, with proximal node straight and distal node oblique, each bearing frontal nematotheca proximally. Stems either mono-or dioecious; gonothecae sexually dimorphic, borne exclusively on cladia by means of 2-segmented pedicel, given off laterally between base of hydrotheca and its mesial nematotheca; female gonothecae large, piriform, slightly flattened laterally, distally truncate and there with broad, thickened, ovoid aperture, proximally with 2-3 nematothecae; male gonothecae, comparatively smaller, ovoid, circular in transverse section, distally with small, rounded aperture, proximally with a nematotheca.

Remarks
The illustrations and measurements provided by Ansín Agís et al. (2009) leave little doubt that their material is conspecific with that dealt with herein. Although they identified it as M. quadridens (McCrady, 1859), it could be demonstrated that they are specifically distinct. Indeed, Caribbean specimens (Galea 2010), corresponding strictly to earlier descriptions of McCrady's species, are more delicate in appearance and build exclusively scorpioid sympodia, while the present material is composed of comparatively stronger colonies with distinct, upright, straight main stems (Fig. 17J).
Unlike the present material from New Caledonia, the cladial ahydrothecate internodes of the type material of M. fisheri possess two nematothecae instead of only one, although no other differences could be noted between them. It is thus assumed that the present material is, with little doubt, conspecific with M. fisheri. Supporting this hypothesis, Watson (2011, as M. quadridens) noted the presence of 1-2 nematothecae in her specimens, with distinct stems that could probably be assignable as well to Nutting's (1905) hydroid. Their color in life was "dark purplish grey", thus different from the paleyellow colonies of M. quadridens from the Caribbean collected by Galea (2010). Although Watson argued that her Australian material was indistinguishable from a colony of M. quadridens from Sapelo Island, GA, USA, no information, notably on the condition of the stem, was provided to support her statement.
Only male gonothecae, bearing a single basal nematotheca, have been documented in M. fisheri (Schuchert 1997). The present material also possesses female gonothecae, of a much larger size compared to those of M. quadridens, and bearing 2-3 basal nematothecae, further supporting the specific separation between these species. Similar gonothecae were reported upon by Watson (2011: fig. 9e).
Specimens in material MNHN-IK-2015-605 show a more complex branching pattern of the main stem, with an increasing number of ramifications per site, and the formation of up to third order stems. Some examples of branching are illustrated in Fig. 18. This appears to be a variety of the nominal species, distinguished by its proportionally smaller and shallower hydrothecae and longest (trumpet-shaped) lateral nematothecae, the other morphological characters, including the shape and size of gonothecae of both sexes, being similar to those of the nominal species. Fig. 18. Monostaechas fisheri Nutting, 1905, morphotype with shallower hydrothecae and longer lateral nematothecae: patterns of branching. Slender lines represent cladia, thicker lines show primary, secondary or tertiary stems; dots correspond to the hydrothecae.
A series of records attributed to M. fisheri are, most probably, based on misidentifications. Indeed, the Cape Verde material studied by Bedot (1921b) is said to exhibit all characters of M. fisheri var. simplex Billard, 1913, which is now recognized as the simplest mode of colony growth met with in McCrady's hydroid (e.g., Schuchert 1997).
Additionally, Vannucci's (1949Vannucci's ( , 1950 material reportedly comprised both simple and ramified stems, although their mode of branching was not described in detail. On the account of their geographical occurrence and the size of their female gonothecae with respect to the hydrothecae (Vannucci 1950: pl. 1 fig. 6), it is believed that she actually had specimens of M. quadridens. Migotto (1996) agrees that " Vannucci's description (1949) of M. fisheri […] clearly conforms with the current conception of M. quadridens".
The colony illustrated by Park (2010: fig. 75a, as M. quadridens) shows a peculiar branching pattern, approaching the condition met with in the present species, and both could be conspecific. However, her description fits the classical concept of M. quadridens, as well as the size of the illustrated gonothecae (Park 2010: fig. 75b, d) do, but, since many colonies were available, it is conceivable that some belonged to one species, while some others to the second one.

Diagnosis
Colonies tall, branched or unbranched; stems and, when present, side branches stiff, thick and strongly fascicled, giving rise regularly or irregularly to hydrocladia-bearing branchlets from individual deeper tubes; branchlets monosiphonic, Halopteris-like, with basal prosegment, followed by a regular succession of modules (separated or not by nodes), each carrying a hydrotheca, its associated nematothecae, and a cladial apophysis laterally; cladia alternate, except for 1 st and/or 2 nd cormidium of each branchlet, where they can be opposite; cladia with short, proximal, athecate internode, followed by a succession of modules (either divided or undivided by nodes) composed of a hydrotheca and its associated nematothecae. Lateral nematothecae of varied length, occasionally exceedingly long, members of a pair usually of different lengths. Gonothecae provided with several basal, bithalamic nematothecae.

Etymology
From Ancient Greek 'θάμνος', meaning 'bush', and 'πτερόν', meaning 'wing' or 'feather', to illustrate the appearance of the colonies of hydroids assignable to the present genus. The genus is masculine.

Remarks
The species under discussion herein (see below) shows obvious similarities with a hydroid described by Stechow (1923c) under the basionym Heteroplon valdiviae. The latter forms large, ramified, fascicled colonies, giving rise irregularly, from the deeper tubes, to monosiphonic, Halopteris-like branchlets facing in all directions around the stem and/or its branches. The axes of branchlets are provided proximally with a prosegment, and occasionally with as many as four (Millard 1962); the prosegment bears a hydrotheca, its complement of nematothecae, and a pair of opposite cladial apophyses; the remainder of axis comprises a succession of hydrothecate internodes, each bearing a cladial apophysis lateral to a hydrotheca (proximalmost hydrotheca occasionally has a pair of opposite cladial apophyses); the apophyses are arranged alternately along the axis. With the exception of a short, proximal, athecate internode, followed by a nematothecate internode, the remainder of cladium is composed of a succession of thecate internodes separated by oblique nodes. Each hydrotheca, on both axes and cladia, is surrounded by 5 nematothecae: a mesial (well below its basis), a pair of laterals (of varied length and borne on short, conical apophyses), and a pair of axillar (Millard 1975). Stechow's (1923) species was sometimes assigned to Halopteris (e.g., Millard 1957Millard , 1962 and sometimes to Corhiza (Millard 1975;Schuchert 1997). Millard (1975) argued that a genus allocation to Corhiza was more opportune, for the "appearance of the colony is very similar to C. scotiae, differing from it in the fact that the diverging tubes of the stem form sub-branches instead of hydrocladia". The diagnosis of Corhiza was modified accordingly, so as to include species with "branches […] bearing cauline hydrothecae and pinnately arranged hydrocladia", as well (Millard 1975: 334).
However, Millard's (1975) opinion that unites into a single genus species whose fascicled stems and, if present, branches give rise to either Antennella-like hydrocladia or Halopteris-like branchlets, is not shared here. If this principle is applied to the genera Antennella and Halopteris, they must also be regarded as congeneric, although increasing evidence demonstrates that both are polyphyletic (Moura et al. 2018).

Description
Colony erect, originally ca 17.2 cm high, now broken into two, almost equal parts, of rigid appearance, with brown perisarc, arising from rhizoid stolon. Stem unbranched, 2.5 mm thick at base, strongly fascicled, displaying superficially a number of tortuous tubes that seem to wrap an internal beam of straight counterparts, as shown by a couple of small, less organized areas on surface that reveal a possible sheath-like general structure. Tubes on stem surface provided with two longitudinal, laterally-set rows of opposite nematothecae along their length. Upper half of stem gives rise unilaterally, at regular intervals, to monosiphonic, Halopteris-like branches arising from internal beam of component tubes; before each of these tubes becomes free from the fascicled stem, the distal ends of their fused parts become apparent at the surface of the stem for a varied length, displaying on their exposed side two closely-set rows of alternate nematothecae; a short distance (no longer than 1 cm) after diverging from the stem, each tube acquires suddenly a well-organized structure: a basal prosegment is followed by a regular succession of hydrothecae (with their associated nematothecae) and lateral apophyses supporting the cladia. Except for the prosegment, that is delimited at both ends by deeply-incised, very oblique nodes, the remainder of tube is unsegmented. The prosegment bears a hydrotheca devoid of cladial apophyses, its 5 associated nematothecae (1 mesial, far below the base, 1 pair of laterals and 1 pair of axillar), as well as up to 4 nematothecae in two rows above the hydrotheca. Remainder of tube with modules composed of a hydrotheca and its 5 associated nematothecae (as above), an apophysis lateral to it (except for proximalmost hydrotheca that has a pair), as well as 0-1 nematothecae above the hydrotheca. Hydrothecae shifted alternately left and right; truncated cones in shape (broader at base than at aperture), half free from the 'internode'; abaxial wall straight, free adaxial wall slightly concave, both with quite thick perisarc; mesial nematotheca movable, with wall of upper chamber lowered on adaxial side; lateral nematothecae borne on fairly-developed apophyses fused distally to the hydrothecal wall, as illustrated by a peg-like projection of perisarc, running frontally for a short distance over the hydrothecal wall; lateral nematothecae of varied length, generally short on proximalmost cormidia, and exceedingly long distally; basal chamber very tall, upper chamber shallow, with circular, entire rim; axillar nematothecae bithalamic, wall of upper chamber slightly lowered on adaxial side. Cladial apophyses short, athecate, shifted laterally so that the two rows of hydrocladia they carry form an acute angle between them. Hydrocladia up to 4 mm long, unsegmented, except for a short, proximal, quadrangular segment; cladia comprising up to 10 modules, each with one hydrotheca and its 5 associated nematothecae (as above); there are no additional nematothecae between 2 successive hydrothecae; the lateral nematothecae are generally exceedingly tall and often of unequal length. Gonothecae, likely female, borne on cladia, each inserted laterally between the base of a hydrotheca and its mesial nematotheca through a short apophysis; pedicel of one short, quadrangular piece; gonotheca piriform, with 2-3 proximal, quite long nematothecae, distally truncate, with distinctly thickened, circular rim, apparently without lid.

Remarks
The new species differs from Thamnopteros valdiviae (Stechow, 1923) in having: 1) all Halopterislike branchlets given off unilaterally from the stem, instead of pointing in all directions around it; 2) undivided axes and cladia; 3) cladia missing an intervening nematothecate internode between the proximalmost, quadrangular segment and the remainder of cladium; 4) hydrothecae with an even rim, not sinuated laterally; 5) bithalamic instead of scale-shaped axillar nematothecae.

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

Genetic results
The newly sequenced specimens were used to build updated phylogenetic hypotheses for the families Aglaopheniidae, Plumulariidae, and Halopterididae, based on the 16S rRNA region (Figs 21-23). Overall, all families showed a pervasive polyphyly of genera, as previously demonstrated by other studies using the same DNA region (e.g., Galea et al. 2018;Moura et al. 2018), but non-monophyly was also commonly observed using multi-locus approaches (Maronna et al. 2016). Therefore, a thorough integrative morpho-molecular revision is needed to better define the generic and specific boundaries in these plumularioid families.
In the Aglaopheniidae tree (Fig. 21), the newly produced sequences represent the first genetic data for the analysed species, except for Gymnangium expansum (Jäderholm, 1903), which clusters with a conspecific sequence. Sequences of the recently-described Cladocarpus pennatus Galea, 2020 cluster in a monophyletic group, divergent from all other Cladocarpus sequences, and the same happens for   Aglaophenia digitulus Vervoort & Watson, 2003, highlighting the peculiar nature of these species and their possible belonging to other genera. Finally, Cladocarpus keiensis Schuchert, 2003 clusters with C. unilateralis Schuchert, 2005 andC. bocki Jäderholm, 1919, and all together form a fully-supported monophyletic group with 3 species of Streptocaulus, suggesting that the scope of these genera is in need of a revision.
Regarding the Plumulariidae (Fig. 22), the newly-produced sequence of Dentitheca habereri (Stechow, 1909) clusters with a conspecific sequence from Japan, and this species is closely-related to D. contraria comb. nov. (confirming the assumption based on morphological grounds alone, see above), even though with low nodal support, and with D. dendritica (Nutting, 1900). Sequences of Schizoplumularia are sister to the group composed of the species of Dentitheca mentioned above, and the two species, Schizoplumularia elegans Ansín Agís et al., 2016 and S. helicoidalis sp. nov. do not form a monophyletic group in our phylogenetic hypothesis. Sequences of Schizoplumularia spp. and D. contraria comb. nov. were also produced for the first time with this work.

Fig. 23.
Phylogenetic hypothesis for the Halopterididae Millard, 1962. The species discussed in this study is highlighted in yellow, and the newly-produced sequence is in bold. Numbers at nodes represent BPP and BS, respectively, and are shown only when both BPP ≥ 0.9 and BS ≥ 75.

Discussion
The present study, dealing with the plumularioid hydroids gathered by KANACONO and KANADEEP expeditions, supplements earlier reports on the hydroid fauna from off New Caledonia (Ansín Agís et al. 2009Agís et al. , 2014Agís et al. , 2016, and adds a number of new records. Among these, two new genera and four new species are described, viz, Actinopluma mirifica Galea gen. et sp. nov Stechow, 1919 andCorhiza Millard, 1962, respectively. Monostaechas fisheri Nutting, 1905, a species of so far questionable validity, is rediscovered and redescribed, and its distinguishing characters from M. quadridens (McCrady, 1859), a species to the synonymy of which it has often been assigned, are underlined, to support their specific separation.