Eudendrium merulum Watson, 1985

Calder, Dale R. & Faucci, Anuschka, 2021, Shallow water hydroids (Cnidaria, Hydrozoa) from the 2002 NOWRAMP cruise to the Northwestern Hawaiian Islands, Zootaxa 5085 (1), pp. 1-73 : 10-11

Eudendrium merulum Watson, 1985

Figs 2c–e , 3

Eudendrium merulum Watson, 1985: 200 , figs 53–58.

Type locality. Australia: Victoria, Bass Strait, 0.5 km S of Clonmel Island, 38°45’S, 146°43’E, from wreck of streamer Blackbird , 6 m ( Watson 1985) .

Voucher material. Nihoa, 06.x.2002, three branched colonies or colony fragments, to 8 mm high, one with female gonophores, coll. A. Faucci, ROMIZ B5405. — Kure Atoll, on algae and calcareous rubble, 25.ix.2002, two mostly stolonal colonies, to 5 mm high, without gonophores, coll. A. Faucci, ROMIZ B5476.

Cnidome. Microbasic euryteles, small ( Fig. 3a ), (n=10): 6.4–7.0 µm long × 2.4–2.7 µm wide (undischarged) Microbasic euryteles, large ( Fig. 3b, c ), (n=6): 19.0–20.8 µm long × 10.0–11.3 µm wide (undischarged)

Remarks. Approximately 80 species are currently recognized worldwide in the genus Eudendrium Ehrenberg, 1834 . Hydroids of the group are easily distinguished in having monopodial growth, perisarc reaching to a groove at the base of relatively large, naked, urn-shaped hydranths, a typically flared or knob-shaped hypostome, and solid, filiform tentacles arranged more or less in a single whorl. Their gonophores are also distinctive, arising in a whorl around bases of the hydranths. Those of the male usually comprise a linear series of oval to nearly round chambers, while those of the female initially possess a spadix curving over a single ovum.

While hydroids of Eudendrium are distinctive, identification of species assigned to the genus can be particularly troublesome. Within the last few decades, the nematocyst complement and morphology of the gonophores, particularly those of the female, have been emphasized as characters of particular value in the morphological distinction of species.

In terms of the cnidome, all known species of Eudendrium have small microbasic euryteles. Most, but not all, of the others also have additional “complementary nematocysts”. While these often comprise nematocysts of an entirely different category, they may simply be microbasic euryteles of a noticeably larger size. The locations of the complementary nematocysts, and the shapes that aggregations of them sometimes create, are also of taxonomic value.

Complementary nematocysts in hydroids examined here consisted solely of large microbasic euryteles ( Fig. 3a–c ). On hydranths, these were scattered around the base above the perisarc groove and around the rim of the hypostome. In this overall character, they align with some 20 of the known species of the genus ( Calder 2017: 34–36), and they conform with a so-called “ Eudendrium ramosum ( Linnaeus, 1758) group”.

Colonies from the present collection were small (up to 8 mm high) and stolonal to alternately branched. One of the specimens nevertheless had female gonophores, indicative of a species having small colonies. Developing female gonophores, with unbranched spadices, were observed on a nearly atrophied hydranth. Fully mature gonophores were present on a blastostyle, lacking tentacles and with expanded and wrinkled pedicel perisarc ( Fig. 2e ). The perisarc of the hydrocaulus and branches was smooth, except for a few annulations basally and with occasional irregular ones elsewhere. Although hydranths were mostly in poor condition, the best ones bore a single whorl of about 20–23 tentacles.

The characters of these hydroids, together with the presence of large euryteles as complementary nematocysts, most closely resemble E. merulum Watson, 1985 . While the trophosomes also resemble those of E. generale von Lendenfeld, 1885 , female gonophores occur on atrophied rather than fully developed hydranths, as in that species ( Watson 1985). In addition, complementary euryteles tend to be larger and their shafts are spinier in material examined here. Also similar in morphology are hydroids of E. pusillum var. amoyicum Hargitt, 1927 [= E. generale amoyicum ], and E. kirkpatricki Watson, 1985 . Colonies of the first of these species appear to be more profusely branched ( Ling 1938; Xu et al. 2014a), while complementary nematocysts of the second are larger and possess a longer, less spiny shaft ( Watson 1985). Hydroids from Fiji having a similar colony form were identified as Eudendrium sp. by Gibbons & Ryland (1989). Finally, stolonal colonies of this hydroid resemble E. breve Fraser, 1938 a , but the complementary microbasic euryteles were much larger than even the largest of those reported in that species (13.0–14.6 × 5.8–6.8 µm) ( Calder et al. 2021). While specimens from this collection have been assigned here to E. merulum, DNA analyses are needed to sort out the taxonomy of this and related species ( Schuchert 2008).

Hydroids of E. merulum may be restricted to the Pacific Ocean. Schuchert (2008) found that hydroids identified as this species from the eastern Atlantic Ocean, Mediterranean Sea, and Black Sea all represented distinct genetic lineages, and that they likely differ from the Australian population.

Reported Distribution. Hawaiian archipelago. First record.

Elsewhere. Australia ( Watson 1985); Yemen (Marques et al. 2000). As noted above, records of the species from the eastern Atlantic and adjacent seas are likely erroneous ( Schuchert 2008).

Order Leptothecata Cornelius, 1992

Figures

FIGURE 2. Pennariidae, Oceaniidae, Eudendriidae, and Phialellidae. a, Pennaria disticha Goldfuss, 1820, hydranth with a medusa bud and part of pedicel, Pearl & Hermes Atoll, ROMIZ B5404. Scale equals 0.2 mm. b, Turritopsis sp., hydrocaulus with two hydranths, Midway Atoll, ROMIZ B5490. Scale equals 0.2 mm. c, Eudendrium merulum Watson, 1985, hydranth and part of pedicel, Nihoa, ROMIZ B5405. Scale equals 0.1 mm. d, Eudendrium merulum Watson, 1985, hydrocaulus and proximal ends of three branches, Nihoa, ROMIZ B5405. Scale equals 0.2 mm. e, Eudendrium merulum Watson, 1985, female gonophores, Nihoa Island, ROMIZ B5405. Scale equals 0.1 mm. f. Phialellidae (undetermined), three hydrothecae, Gardner Pinnacles, ROMIZ B5406. Scales equal 0.05 mm.

FIGURE 3. Eudendriidae. Eudendrium merulum Watson, 1985, nematocysts, material from Nihoa, ROMIZ B5405. a, microbasic eurytele, small, undischarged. b, microbasic eurytele, large, undischarged. c, microbasic eurytele, large, discharged.