Synthecium hians Millard, 1957,

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, European Journal of Taxonomy 562, pp. 1-70: 20-23

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Synthecium hians Millard, 1957


Synthecium hians Millard, 1957 

Figs 6View Fig C–F, 8A–F; Table 4

Synthecium hians Millard, 1957: 204  , fig. 9A–C.

Synthecium hians – Millard 1964: 25  ; 1975: 238, fig. 77C–D; 1980: 144, fig. 5A–B.

Material examined

PACIFIC OCEAN • several colonies, some fertile, with stems up to 13.5 cm high, with strongly fascicled bases; off New Caledonia, stn DW4726; 22°40′ S, 167°03′ E; 240– 181 m; 20 Aug. 2016; KANACONO leg.; a fully fertile colony was used for DNA extraction, DNA 1400; voucher MHNG-INVE- 120860; MNHN-IK-2015-486 • three colonies without gonothecae, with strongly fascicled bases: a profuse one, 8× 7 cm, a less profuse one, with only four stems, 6 × 8 cm, and one with only a single intact stem and many stumps, 7× 5 cm; off New Caledonia, stn DW4718; 22°47′ S, 167°09′ E; 350 m; 19 Aug. 2016; KANACONO leg.; MNHN-IK-2015-494GoogleMaps  many stems without gonothecae, up to 11 cm high, most of them independent, a few ones aggregated basally through their intertwined hydrorhizae; off New Caledonia, stn DW4784; 22°51′ S, 167°44′ E; 310–322 m; 29 Aug. 2016; KANACONO leg.; one stem was used for DNA extraction, DNA 1398; voucher MHNG-INVE- 120858; MNHN-IK-2015-484GoogleMaps  many colonies with strongly fascicled bases, up to 11 cm high, some bearing gonothecae; off New Caledonia, stn DW4724; 22°40′ S, 167°07′ E; 260– 255 m; 20 Aug. 2016; KANACONO leg.; a fragment of stem from one colony was used for DNA extraction, DNA 1397; voucher MHNG-INVE- 120857; MNHN- IK-2015-483GoogleMaps  several colonies with fascicled stems and fragments resulting from their breakage, largest 18 cm high, all without gonothecae; off New Caledonia, stn DW4735; 22°40′ S, 167°40′ E; 233– 195 m; 22 Aug. 2016; KANACONO leg.; MNHN-IK-2015-501GoogleMaps  .


Colonies erect, bushy, up to 18 cm high, arising from root-like hydrorhiza composed of tortuous, branching and anastomosing fibers firmly adhering to the substrate; usually, profuse colonies with strongly fascicled base, up to 1 cm wide, of varied length, although smaller colonies composed of many independent stems exist. Stems diverging at varied angles from their common base; composed of a smooth, basal athecate and ahydrocladiate part of varied length, and a much longer, distal part bearing both hydrothecae and hydrocladia; division into internodes not always distinct but, when present (nodes transverse), each internode composed of a pair of opposite hydrothecae proximally, followed by a pair of opposite apophyses supporting cladia and their associated axillary hydrothecae, as well as a second pair of opposite hydrothecae above; often, the hydrothecae are damaged, leaving scars in the stem, and not regenerating subsequently. Stem apophyses short, delimited from corresponding cladia by transverse constrictions of the perisarc, obvious mainly on upper parts of the colonies. Cladia given off laterally and slightly upwards in opposite, coplanar pairs; cladia distant of 3–4 mm, up to 1.8 cm long and bearing up to 18 thecate ‘internodes’; division by nodes indistinct, occasionally a transverse constriction of the perisarc intervenes between successive pairs of hydrothecae; the first ‘internode’ bears an unpaired hydrotheca facing downwards, while the 2–4 subsequent internodes display subalternate pairs of hydrothecae, changing gradually to strictly opposite distally. Often, the cladia show signs of breakage and subsequent regeneration at level of the proximal most internode; in this case, the first internode, similarly to the subsequent ones, bears invariably a pair of strictly opposite hydrothecae. Hydrothecae deep, cylindrical, curved outwards, adnate to the corresponding internode for most of their length; free adaxial wall short, slightly concave, perisarc thin, gently flaring distally; adnate adaxial wall distinctly curved, perisarc moderately thick, ending downwards into perisarc plug; abaxial wall more or less straight, distal ⅔ with distinctly thickened perisarc, often forming an internal swelling in middle, distal end slightly everted; aperture circular, rim relatively scooped in lateral view; up to three closely-set renovation could be note occasionally. Gonothecae, generally occurring in pairs, replace the (strongly) damaged stem hydrothecae in the lower halves of the colonies, arising from a hole in their adnate adaxial wall; urn-shaped, laterally flattened and thus bilaterally symmetrical; in a frontal view of the colonies, the gonothecae show their narrowest side; flattened sides are provided with 6–7 broad, convexly-arched ridges, the distalmost being the most prominent, grading proximally to the less conspicuous; viewed from their narrowest sides, the gonothecal ridges adopt a concave shape. Male and female gonothecae (sexes not always discernible), similar in shape. Female gonothecae with ca ten large, ovoid eggs.


In rare instances, there can be no additional pair of hydrothecae between two successive pairs of cladia other than the axillar hydrothecae associated to the lower pair of cladia. In other cases, besides the axillary hydrothecae, only one pair (instead of two) of hydrothecae above separate two successive pairs of cladia. In one instance, a gonotheca replaced one cladium of a pair.

There is little doubt that the present material belongs to the species of Millard (1957), especially giving the following features: 1) the presence of a thick, tangled hydrorhiza, “with diameter of stolons equal to that of stem” ( Millard 1957); as noted by Millard (1975), in “rich colonies the hydrorhizal tubes may rise up from the surface in a tangled bundle simulating a fascicled stem, the individual tubes anastomosing with one another and with the bases of the stems”; 2) the distinctive segmentation of the stem; 3) the presence of an unpaired proximal hydrotheca, followed by a few subopposite pairs; 4) the shape and size of hydrothecae, especially their narrow base, “widening strongly to margin, adnate for most of its length, only very slightly bent outwards […]. Margin smooth, everted […]. Diaphragm oblique, with outer edge reaching sometimes as far as half-way up the abcauline wall. Perisarc thin, sometimes thickened below margin on abcauline wall” ( Millard 1957: 204).

The gonothecae, were described subsequently by Millard (1980) and, in her material, they arose directly from the hydrorhiza or “from with the ends of short tubes which are probably damaged stems or hydrorhizal fibers” ( Millard 1980: 144).

A closely related congener (or, possibly, a mix of two species) to both S. hians  and S. rectangulatum  sp. nov. (see below) is the one likely erroneously assigned by Vervoort & Watson (2003) to S. protectum Jäderholm, 1903  . Indeed, these authors mention a basally fascicled colony with stems up to 12 cm high (NZOI Stn I85), and the gonotheca described and illustrated in their fig. 60F shows obvious similarities to those of the two species discussed herein. However, since there is no formal description provided by these authors, their material could not be assigned with certainty to one of the species dealt with herein. In addition, it should be stressed that S. protectum  has comparatively smaller hydrothecae, and its gonothecae are radically different ( Galea 2007: 77, fig. 18E–J, table 31, as S. robustum Nutting, 1904  ).


South Africa ( Millard 1957) and New Caledonia (present study).

Table 4. Measurements of Synthecium spp., in µm.

  Synthecium hians Millard, 1957  Synthecium rectangulatum Galea  , sp. nov.
  MNHN-IK-2015-486 Millard (1967, 1980 *) MNHN-IK-2015-485
- internode length ca 2500 3800–4000
- diameter at node ca 205 170–230
- length of 1st thecate segment 1200–1410 1900–2305
- distance between consecutive pairs of hydrothecae 780–975 510–740 1715–1950
Hydrotheca (cladial)
- free adaxial wall 30–70 30–50 315–400
- adnate adaxial wall 520–585 300–370 635–685
- maximum width 285–310 500–520
- diameter at rim 290–325 230–270 420–435
- length ca 2665 2100–3000 * ca 2525
- width (wide side) ca 1850 2000–2400 * ca 1560
- width (narrow side) ca 1605 ca 1445














Synthecium hians Millard, 1957

Galea, Horia R. & Schuchert, Peter 2019

Synthecium hians –

Millard N. A. H. 1975: 238
Millard N. A. H. 1964: 25

Synthecium hians

Millard N. A. H. 1957: 204