Macrorhynchia fallax Galea & Maggioni 2024, sp. nov.
Creators: name: Galea, Horia R.
name: Maggioni, Davide
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Cnidaria
Hydrozoa
Leptothecata
Aglaopheniidae
Macrorhynchia
Macrorhynchia fallax

description: Macrorhynchia fallax, sp. nov.
urn:lsid:zoobank.org:act: AD646F86-77CA-43F0-8F5F-4046057ED127
Figs 21, 22, 34
? Macrorhynchia philippina — Schuchert, 2003: 221, fig. 67 [non Macrorhynchia (Aglaophenia) philippina Kirchenpauer, 1872: 45, pl. 1 fig. 26, pl. 2 fig. 26, pl. 7 fig. 26].
Material examined. Holotype: MSNMCoe373, Indonesia, Bali, Amed, dive site known as “Ghost Bay”, -8.332965°, 115.643044, 15–20 m, 18 Apr 2023, sterile colony composed of several branched stems, tallest 7 cm high, GenBank: OR872020 (18S), OR872040 (28S), OR866288 (COI).— Paratype: MSNMCoe374, same collecting data as for the holotype, 10–15 m, 04 Oct 2022, sterile colony composed of several branched and unbranched stems, tallest reaching 10.5 cm high, GenBank: OR872019 (18S), OR872039 (28S), OR866287 (COI). Additional material: HRG-1783, same data as for the holotype, 15 m, 25 Apr 2023, a 4 cm high, sterile colony. —HRG-1784: same data as for the holotype, 10–20 m, 07 Oct 2022, 2 sterile colonies ca. 11 cm high. —HRG-1785: same data as for the holotype, 10–15 m, 04 Oct 2022, a 10.5 cm high, sterile colony.
Description. Colonies large (up to 11 cm high in the field), of bushy appearance, with fascicled stems and branches, usually fixed in sediment by means of a dense, ramified, stolonal fiber network attached not only to the proximal tip of the caulus, but also laterally to a whole proximal portion of it, ensuring an effective anchoring among sand grains and small pebbles. Branching occurring at irregular places, with a tendency to alternate, to ensure a good balance of the colonies in an environment dominated by currents; up to 3 rd order branches, given off laterally and slightly in front of the stem or higher order branches. Main tubes of stem and branches reinforced dorsally by bundle of undivided auxiliary tubes (not provided with nematothecae), but communicating with one another and with their main counterparts through pores in the perisarc; main tubes of side branches not arising from the main tube of the stem, but from different cauline auxiliary tubes; all main tubes in a colony regularly divided into moderately-long (460–590 µm), rather slender internodes, delimited by 130–175 µm wide, transverse nodes; each internode provided with a 110–135 µm long, lateral, subterminal, slightly anteriorly-shifted apophysis supporting a cladium, and two nematothecae, one towards each end; apophyses biseriate, alternate, relatively short, with a basal, conical mamelon (45–50 µm high, 15–20 µm wide at aperture) and an oblique node distally; nematothecae sac-shaped, elongate (120–140 µm long, 60–75 µm wide), monothalamic, adnate for more than half their length to the internode behind, aperture either small (10–15 µm wide), circular (mounted atop a conical projection provided with a second, axillar aperture behind) or gutter-shaped (the apical aperture having fused with the axillar one); proximal nematotheca a short distance above the proximal node; distal nematotheca adjacent to the cladial apophysis. Proximal parts of the main tubes of branches of varied length, undivided, and provided with a succession of frontally-placed, widely-spaced nematothecae in a row, ending distally in oblique hinge joint, the latter allowing the much longer, distal, cladiate part to move freely in the current. Cladia borne on both stem and branches, distant of ca. 1 mm from one another (on the same side), long (up to 8 mm), given off at an angle of 50–55° with the main tubes, composed of a regular succession of up to 18 cormidia delimited by slightly oblique, 85–90 µm wide nodes (in lateral view); each cormidium relatively short (400–440 µm), accommodating a hydrotheca and its associated nematothecae; hydrothecae adnate for most of their length (leaving distally but a reduced, 46–60 µm long portion free from the internode), elongate, concave along their longitudinal axis; adnate adaxial wall 305–315 µm long, thin, concave, in proximal most hydrothecae projecting basally an incomplete annular ridge into the internode behind; mesial nematotheca adnate for about 2/3 of its length (distal 80–100 µm are free), leaving only a short portion of the abaxial hydrothecal wall free, the latter being fully occupied by a prominent, triangular septum projecting halfway into the thecal lumen; hydrothecal aperture 165–175 µm wide, tilted abaxially, margin provided laterally with a pair of board, very low, triangular cusps, in addition to the central, abaxial, linguiform cusp formed by the upper tip of the intrathecal ridge; mesial nematotheca arching adaxially, length not surpassing the level of the hydrothecal rim, provided distally with a small (ca. 25 µm wide), rounded aperture, and basally (in the axil formed with the abaxial hydrothecal wall) with an additional, rounded foramen, nematotheca separated from the hydrothecal lumen by a thin, concave, perisarcal septum; lateral nematothecae 125–150 µm long, horn-shaped, adnate to the hydrothecal wall for most of their length, apically provided with a small (ca. 15 µm wide), circular aperture, and basally with a large, ovoid foramen on their adaxial side. Hydranths bearing a whorl of 14–16 filiform, unicoronate tentacles surrounding a small, dome-shape hypostome. Gonosome absent.
Color in life (Fig. 21): stems and branches with a greenish to pale brown tinge, cladia white.
Remarks. The new species resembles M. philippina Kirchenpauer, 1872, but it is readily distinguished in situ through the absence of a striking irisation of its cladia, and the rather greenish to light brown tinge of its stems and branches (instead of being dark brown to black). Macroscopically, the examination of several colonies revealed that it has longer (6.5–8 mm) and more widely-spaced cladia (10–12 per side/cm), compared to specimens of M. philippina from various localities (Bali, Martinique, Madeira, Madagascar and United Arab Emirates), in which their length is 3.5–5 mm and their density of 13–17 per side/cm. Cladia not reaching the length met with in the new species were also reported in Kirchenpauer’s hydroid by Migotto (1996, up to 4.8 mm), Calder (1997, up to 5 mm), Pictet (1893, up to 6 mm), Watson (2000, up to 6 mm). Genetically, M. fallax is closely-related to M. philippina and M. filamentosa (Lamarck, 1816), being nevertheless divergent from them (Fig. 34).
Microscopically, the hydrothecae of M. fallax sp. nov. are comparatively deeper and slenderer, their abaxial wall is longer (and, implicitly, the intrathecal septum is broader), and the mesial nematothecae less adjacent to their corresponding hydrothecae; additionally, the internodes of the main tubes are longer than in M. philippina, and the nematothecae they bear are comparatively more elongate than the relatively compact and broader ones in Kirchenpauer’s hydroid. Additionally, unlike in M. philippina, internal perisarcal ridges are absent from the cladial internodes of the new species, and this is also true for the oldest parts of the colonies.
Some literature records assigned to M. philippina may have been based on specimens belonging to the present species instead, as demonstrated by their long cladia and more elongate and slenderer hydrothecae compared to Kirchenpauer’s hydroid, e.g. Schuchert (2003), whose colonies had ca. 10 cladia per side/cm (deduced from his fig. 67A) and up to 18 cormidia/per cladium.
Macrorhynchia ambigua Watson, 2000, a closer congener from northern Australia, builds more denselypinnate colonies and its hydrothecae come closer in shape to those of M. philippina (Watson 2000: fig. 55B and D, respectively).
In our opinion, the absence of the gonosome in the specimens examined does not represent an impediment in describing them as belonging to a new species. Indeed, the phylactocarps of Macrorhynchia have a rather uniform structure, and emphasis is primarily placed on the trophosome, especially the hydrothecal features.
Etymology. From the Latin fallax, -acis, meaning misleading, to emphasize its deceptive resemblance to its closest congener, M. philippina.
Distribution. Known with certainty only from Bali.
description: Published as part of Galea, Horia R. & Maggioni, Davide, 2024, On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species, pp. 1-57 in Zootaxa 5428 (1) on pages 29-31, DOI: 10.11646/zootaxa.5428.1.1, http://zenodo.org/record/10842930