Zyzzyzus robustus Petersen 1990

Zyzzyzus robustus (Petersen, 1990)

Table 4

Zyzzyzus robustus Petersen, 1990: 183, figs. 32–33.— Schuchert, 2001: 42 (footnote).

Material examined. Preserved: Greenland: off Akuliaruseq (60°09’N. 44°10’W) (140 m), 12 Jul 1970, polyps on steep rocky slope (type with sponge fibers in the hydrorhiza); Cape Farewell Expedition st. 93; southwest Qeqertat (59°52.8`N, 43°31.5`W) (225 m) (no date), 10 polyps without gonophores, on encrusting bryozoans, ZMUC (without collection number).

Description. Hydroids 7.3–29.0 mm in height, covered by thick, heavy perisarc, growing on rock, sponge, and encrusting bryozoans. Hydrorhiza swollen, with 1–3 large, strong, club-shaped processes, covered by stiff perisarc. Hydranth large, oblong; hydranth body 1.4–3.7 mm high, 0.7–3.5 mm in diameter at base. Oral tentacles filiform, ca. 40 in number, arranged in five whorls, adnate to hypostome around mouth, tentacles laterally flattened in cross section; most proximal whorl arising at right angles to hypostome, most distal whorl reaching no more than one-third of hypostomial length, 0.8– 0.14 mm long, 0.06–0.21 mm in diameter. Aboral tentacles filiform, 28–30 in number, tentacular bases contiguous to each other at base of hydranth body and in one whorl, tentacles laterally flattened in cross section, 0.8–3.8 mm long, 0.08–0.30 mm in diameter. Fertile hydranths with 21 blastostyles, ca. 1 mm in length, arranged in one imperfect or irregular whorl above aboral tentacles, each blastostyle with 8–14 spherical cryptomedusoid gonophores. Hydrocaulus trunk-like, bent from median to distal parts, cylindrical and covered by heavy, stiff perisarc at basal part, distal part of hydrocaulus narrowing, marked by end of stiff perisarc, surrounded by thinner perisarc. Transition between hydranth base and hydrocaulus smooth, without marked groove at region of perisarc secretion.

Cnidome. Large stenoteles 10.2–13.5 x 8.2–12.2 Μm (11.2 ± 0.7 x 9.1 ± 0.8, n = 55); basitrichous isorhizae 5.3–6.8 x 1.8–2.5 Μm (6.2 ± 0.6 x 2.2 ± 0.3, n = 11) (aboral tentacles).

Additional data. “Hydrocaulus filled with parenchymatic endoderm, 20–30 peripheral longitudinal canals of equal diameter. […] Blastostyles consisting of stout shaft distally branching into seven to eight very short, stout branches, each bearing a gonophore” (Petersen 1990: 183–184).

Colour. Hydranth and hydrocaulus yellowish-brown in alcohol-preserved material.

Type. One polyp ca. 36.0 mm high (50 mm after Petersen 1990: 184). Hydrorhiza 6.5–6.8 mm in length, 2.3–2.8 mm in diameter. Hydrocaulus 25.8 mm in total length. Hydranth 3.7 mm in length, ca. 3.5 mm in diameter at base, with 40 filiform oral tentacles 0.9–1.4 mm in length, 0.12–0.21 mm in diameter; and 30 filiform aboral tentacles 2.9–3.8 mm in length, 0.18–0.35 mm in diameter. Fertile hydranth with 21 blastostyles, 1 mm in length, with several spherical cryptomedusoid incompletely developed. Hydrocaulus 5.8 mm in length with one primary polyp 7.3 mm in length; hydranth body 1.4–1.5 mm in length, 0.7–0.9 mm in diameter, ZMUC (without collection number).

Additional material. One polyp from southwest Qeqertat (59°52.8`N, 43°31.5`W) (225 m) (no date), without gonophores, hydranth ca. 2.3 mm in length, ca. 2.6 mm in diameter at base, with detached oral and aboral tentacles. Hydrocaulus with two newly settled small polyps, 11 mm and 3 mm in height, respectively, ZMUC (without collection number).

x ± SD (in Μm) Range (in Μm) n

Hydrorhiza

Number of processes 2 ± 1 1–3 1 Length - - - Diameter 1967 ± 678 992–2815 12 Hydrocaulus

Length 11222 ± 9787 5857–25875 4 Proximal diameter 1771 ± 954 603–3376 27 Median diameter 1537 ± 1051 492–3089 26 Distal diameter 948 ± 542 457–2367 30 Hydranth

Total length 2254 ± 954 1455–3744 8 Length from aboral tentacle bases to hypostomial tip 1431 ± 495 926–2163 8 Diameter at base 1845 ± 1073 687–3506 12 Oral tentacles

Number 40 - 1 Length 586 ± 400 883–1451 13 Diameter 137 ± 53 61–214 40 Aboral tentacles

Number 29 ± 1 28–30 2 Length 1754 ± 873 813–3817 12 Diameter 191 ± 78 77–352 60 Gonosome

Total number - - - Gonophore

Number 11 ± 2 8–14 2 Length - - - Diameter 190 ± 38 149–270 19 Blastostyles

Number 21 - 1 Length 1047 1047 1 Diameter - - -

Remarks. We corroborate the main morphological characters described by Petersen (1990). The diagnostic characters of Z. robustus are: absence of a groove at region of secretion of perisarc, long and monopodial blastostyles, and distance between adjacent aboral tentacles inferior to diameter of tentacular base.

Zyzzyzus robustus is morphologically unique compared with other species of the genus, but Petersen (1990: 184) remarked “the general morphology of Z. robustus nevertheless agrees so closely with that of the other species that it would seem unreasonable to erect a separate genus for it”. Morphologically it resembles species of Bouillonia Petersen, 1990, including its larger dimensions and its bathyal habit (cf. Petersen 1990; Schuchert 2001, Svoboda et al. 2006). Additional phylogenetic studies are necessary to establish the taxonomic affinities of the species.

Petersen (1990: 184) considered young specimens of Z. robustus to be similar to Z. warreni, especially the thinner perisarc, noting that “as the hydroid grows to a size about 10-15 mm […] becomes thicker and harder, preventing further expansion of the tubers and the lower part of the hydrocaulus; growth lengthwise is still possible”. In fact, the specimens of Z. robustus reach a larger size than other species of Zyzzyzus.

A synopsis of differences among species of Zyzzyzus is given in Table 7.

Distribution. Northwestern Atlantic: off Akuliaruseq (60°09`N, 44°10`W), southwest Qeqertat (59°52.8`N, 43°31.5`W) (Petersen 1990).

Ecology. Substratum. The largest polyp in type material has two meshes of sponge fibres on its hydrorhiza, indicating that it was embedded in sponges, but dislodged from them during or after the collecting process. Remaining materials are attached to rocks and encrusting bryozoans. One newly-settled polyp from type material has a colony of the hydroid Clytia sp. growing along the hydrocaulus extension, arising from small hydrorhizal pedicels and with badly preserved hydrothecae and gonothecae.

Depth. The uniqueness of Z. robustus is also reflected in its ecology. It is the only non-littoral species, having been collected at depths from 140 to 225 m off Greenland. Other specimens tentatively assigned to the genus Zyzzyzus from deep-water (110–400 m) are those described by Hirohito (1988: 26). He identified them as “ Zyzzyzus (?) sp.” but noted: “as well-developed gonophores can not be observed, I feel some hisitation (sic) in referring it to the genus Zyzzyzus ”.