Rhysia halecii ( Hickson & Gravely, 1907 )

Peña Cantero, Álvaro L., 2015, Review of some little-known benthic hydroids (Cnidaria, Hydrozoa) from the Southern Ocean, Zootaxa 3972 (3), pp. 369-392 : 387-389

Rhysia halecii ( Hickson & Gravely, 1907)

( Figs 9 A–D, 10A–C)

Stylactis halecii Hickson & Gravely, 1907: 8 pl. 1 figs 5–6, pl. 4 fig. 33; Kramp, 1932: 7; Iwasa, 1934: 262, figs 15–16; Boero & Bouillon, 1993: 259; Peña Cantero, 2004: 768; Bouillon et al., 2006: 157, fig. 88E, F.

Halerella halecii — Stechow, 1925: 401; 1962: 417.

Stylactaria halecii — Namikawa, 1991: 810.

Not Stylactis halecii — Hirohito, 1988: 128, 131–132, fig. 48a, b.

Material examined. Discovery Antarctic Expedition 1901-04: Type (NHM 1907.8.20.4), 28.II.1902, McMurdo Bay (Ross Sea), 20 fms, colony growing on Hydrodendron arboreum , with gonozooids.

Description. Stolonal colony growing on stems of Hydrodendron arboreum ( Fig. 9 A–D). ‘Encrusting hydrorhiza consisting of a dense mass of branching and anastomosing tubes (fig. 6). Each tube is covered with its own very thin sheath of perisarc (fig. 33) and there is no common sheath of coenosarc covering the hydrorhiza as a whole, … In the central parts of the hydrorhiza the tubes are closely packed, anastomose freely, and are disposed in several layers. At the periphery however the tubes are reduced to a single layer, become more dispersed, and anastomose slightly’ ( Hickson & Gravely 1907: 9).

Colony polymorphic, with gastrozooids, gonozooids and dactylozooids ( Fig. 9 A–D).

Gastrozooids ( Fig. 9 C, 10A), c. 1050 µm high and c. 250 µm in maximum diameter, with conical hypostome and a distal crown of eight to nine filiform tentacles [six to ten according to Hickson & Gravely (1907)]. Nematocysts concentrated on their tips.

Dactylozooids short (c. 200 µm high and 80 µm in maximum diameter), finger-shaped, distally swollen and with a high concentration of large nematocysts ( Fig. 10 C). Dactylozooids covered by a perisarc sheath, except for distal pad.

Gonozooids shorter than gastrozooids (c. 850 µm high and c. 370 µm in maximum diameter), with short, rounded hypostome, without tentacles or just with a few tentacle buds, without gonophores (gametes developing into body wall) ( Figs 9 A–B, D, 10B).

‘The body of the blastostyle is usually considerably dilated and has a superficial resemblance to a simple ovoid sporosac’. ‘The medusoid structure of the gonosome is completely reduced in the male, … The sperm cells (fig. 33, sp.) are found in a dense cluster between the ectoderm and endoderm even in the youngest blastostyles we have examined (0.1 mm in length), and neither in these nor in the older blastostyles can we find any true medusoid structures.’ ( Hickson & Gravely 1907: 9)

Cnidome consisting of three types of nematocysts: desmonemes, isorhizas and microbasic euryteles (the last type represented by three size categories). All types on gastrozooid body. However, the largest type, largest microbasic eurytele, concentrated on hypostome of gastrozooids, hypostome and tentacle buds of gonozooids, and dactylozooids; also present on the spadix. Isorhizas and normal microbasic euryteles concentrated on tentacle tips of gastrozooids. Coma-shaped microbasic euryteles also present on gonozooids, but scarce.

Measurements (in µm). Cnidome: Type I (isorhizas) [range 20.0–22.0 x 7.0–8.5, mean 21.2±0.5 x 7.8±0.5 (n=10); ratio, range 2.5–3.0, mean 2.7±0.1 (n=10)], Type II (microbasic euryteles) [range 11.5–12.5 x 6.0–8.0, mean 12.3±0.3 x 7.2±0.6 (n=10); ratio, range 1.5–2.0, mean 1.7±0.1 (n=10)], Type III (microbasic euryteles) [range 27.0–30.5 x 11.0–14.5, mean 28.3±1.0 x 12.8±1.1 (n=10); ratio, range 2.1–2.6, mean 2.2±0.2 (n=10); shaft c. 25 long], Type IV (coma-shaped microbasic euryteles) [range 12.5–13.5 x 5.5–6.0, mean 12.9±0.3 x 5.6±0.2 (n=10); ratio, range 2.2–2.4, mean 2.3±0.1 (n=10)], desmonemes [range 5.0–6.0 x 3.5–4.0].

Remarks. Although Hirohito (1988) considered the genus Rhysia (family Rhysiidae ) synonymous with Stylactis, Brinckmann-Voss et al. (1993) retained both the family and the genus as valid [This is also the currently accepted view (e.g. Bouillon et al. 2006; Schuchert 2014)], arguing that the structure of the gonozooid, with gonophores reduced to mere gonads, is very different from that of representatives of the family Hydractiniidae .

Brinckmann-Voss et al. (1993) correctly pointed out that the relationships of R. halecii with the other species of the genus were difficult to determine because female gonozooids and nematocysts were not described in the original description. Here, I have tried to complete the description of this species and, consequently, clarify its relationships with the other species of the genus by studying its cnidome.

Brinckmann-Voss et al. (1993) already indicated that Rhysia fletcheri could be distinguished from Rhysia autumnalis and R. halecii by morphological characters (see Table 2 in Brinckmann-Voss et al. 1993). In fact, R. fletcheri has gastrozooids with nematocyst clusters around the hypostome and there are no dactylozooids [according to Schuchert (pers. com.), it can have tentaculozooids]. On the other hand, they indicated that R. autumnalis and R. halecii were largely distinguished by meristic characters, that both species were widely separated zoogeographically, and that they occur under quite dissimilar environmental conditions.

Brinckmann-Voss et al. (1993) also studied the cnidome of R. autumnalis and R. fletcheri , finding desmonemes and microbasic euryteles of two size classes and indicating that the size and distribution of large and small microbasic euryteles [20.2 x 9.6 µm and 9.6 x 4.8 µm, respectively] on the hypostome and tentacles are similar in both species: large microbasic euryteles concentrated on the hypostome and much more numerous in gastrozooids than in gonozooids; small microbasic euryteles and desmonemes on the tentacles, abundant on those of gastrozooids but scarce on gonozooid tentacles, being confined to the tips.

Schuchert (2008), however, indicated the presence of desmonemes (4– 5 x 3 µm) and three size classes of microbasic euryteles (29–31 x 11 µm, 16 x 5–6 µm and 8 x 4–5 µm) in the type material of R. autumnalis ; medium euryteles on the hypostome and smallest ones on tentacles.

Rhysia halecii differs clearly from both species in the cnidome. As I have shown above, its cnidome consists of desmonemes, three categories of microbasic euryteles, and also isorhizas, which have never been described in the other species.

Hirohito (1988) assigned to R. halecii material from Sagami Bay ( Japan) that is morphologically very similar to Hickson & Gravely’s species, although the size of gastrozooids, gonozooids and dactylozooids is two-fold. In addition, the gonozooids seem to have well-developed tentacles, when present, whereas in R. halecii the tentacles, if present, are short or simple tentacle buds. In any case, Hirohito’s material clearly differs from R. halecii in the size of the largest nematocysts (microbasic euryteles): up to 15 x 5 µm in Hirohito’s material and up to 30.5 x 14.5 µm in R. halecii . There are also important geographical reasons to consider both species different. As indicated above, Hirohito’s material comes from Japan, whereas R. halecii comes from High Antarctica . Hirohito’s material might correspond to a different, new species of Rhysia since it also differs from the other two species of the genus.

Ecology and distribution. Rhysia halecii is only known from McMurdo Bay, in the Ross Sea, where it was collected at 36 m, in February ( Hickson & Gravely 1907).

Figures

FIGURE 9. Rhysia halecii (Hickson & Gravely, 1907): A, overview of part of a colony; B, several gonozooids; C, several gastrozooids; D, detail of part of the colony showing high density of gonozooids. Scale bar: 2 mm (A), 1 mm (C – D).

FIGURE 10. Rhysia halecii (Hickson & Gravely, 1907): A, gastrozooid; B, gonozooid; C, dactylozooid. Scale bar: 250 µm (A, B), 125 µm (C).