Halecium turbinariae, Galea & Maggioni, 2024

Galea, Horia R. & Maggioni, Davide, 2024, On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species, Zootaxa 5428 (1), pp. 1-57 : 11-15

Halecium turbinariae , sp. nov.

urn:lsid:zoobank.org:act:415C365F-8FD6-4393-A2A5-846E1783028D

Figs 9‒11 , 29

Halecium sp. — Gravier-Bonnet & Bourmaud, 2005: 65, fig. 2.

Material examined. Holotype: MSNMCoe361, Indonesia, Bali, Pemuteran , vicinity of the pontoon, -8.142954°, 114.660906°, 01 Apr 2023, female colony growing on floating, ripped Turbinaria sp. , GenBank: OR872056 (16S), OR866286 (COI) . Paratypes: MSNMCoe362, same collecting data as for the holotype, female colony.— MSNMCoe363, same collecting data as for the holotype, male colony.—MSNMCoe364, same collecting data as for the holotype, male colony. Additional material: MSNMCoe365, same collecting data as for the holotype, sterile colony.—MSNMCoe366, same collecting data as for the holotype, sterile colony.

Description. Colonies, often profuse, are formed on both the thalli and blade stalks of Turbinaria spp. , never occurring on the distally-expanded, externally-exposed part of the blades, thus finding shelter from predation and/or direct mechanical injury ( Fig. 9A, B ); arising from dense, branching and anastomosing hydrorhizal web creeping on the surface of its algal substrate ( Fig. 9C, D ); stems ( Fig. 10A‒H ) exclusively monosiphonic, borne on minute apophyses of the hydrorhiza from which they are delimited by a deeply-incised, transverse constriction; a distinct basal bulge on first internode immediately above constriction; usually short (175‒375 µm), slightly inverted-conical cauli, ending distally into a hydrotheca, are formed, although small, sympodially-grown stems, composed of up to six hydrothecate internodes arising irregularly in more than one plane, co-occur seldomly; branches, given off singly ( Fig. 10D‒H ) or in opposite pairs ( Fig. 10C ), are borne on short (60‒70 µm), upwardly-curved apophyses arising from below a hydrotheca, and are provided basally with a bulge, similarly to that of the first internode. Hydrothecae ( Fig. 10I ) shallow (25‒30 µm), walls slightly flared (140‒155 µm wide at base and 155‒175 µm at aperture), rim not everted; renovations occasional ( Fig. 10C‒F ), up to two observed; a band of ovoid desmocytes above a transversely-set, thin diaphragm ( Fig. 10I ); hydranths ( Fig. 10A ) composed proximally of a tubular column capable of great extension, and of a distally-swollen, digestive region comprising the hypostome on top, surrounded by a whorl of 17‒21 filiform, unicoronate tentacles; endoderm of polyps densely filled with zooxanthellae ( Fig. 9E, F, J ), conferring them in life a distinctive, brown color. Colonies dioecious; gonothecae arising laterally, either singly ( Fig. 10K, L ) or in pairs ( Fig. 10J ), from below the primary or secondary hydrothecae; sexually dimorphic; male ( Fig. 10J ) elongate-ovoid (615‒725 µm long, 245‒270 µm wide), thin- and smooth-walled, enclosing an unbranched, tubular, central blastostyle surrounded by a homogenous mass of sperm cells; female gonothecae ( Fig. 10K, L ) sac-shaped (865‒1050 µm long, 355‒430 µm wide), laterally-flattened, with an “anterior”, almost straight outline, and a “dorsal”, arched wall, ending distally in an almost conical projection; commonly 2‒3 (occasionally only one), large (240‒270 µm wide) spherical oocytes are formed; spadix trifid, with two parallel-running, “anterior” branches, ending distally into two functional hydranths ( Fig. 11A ), and an unpaired, “dorsal” branch, curving over the oocytes ( Fig. 10K, L ); aperture of female gonotheca provided with a twin hydrotheca ( Fig. 10M, O ), their respective lumina, deeply immersed into the gonothecal wall, being separated anteriorly by a distinct, longitudinal, perisarcal septum ( Figs 10M , 11C ), and posteriorly by a thick perisarc sheet ( Fig. 10O ) projecting into the gonotheca for a certain distance, forming an “anterior” compartment for the hydranths, separated from the “dorsal” one restricted to the oocytes ( Figs 10N , 11B ). Zooxanthellae also present in the gonophores, except for the gametes proper. Long, slender, distally bifid propagules, given off laterally from below some hydrothecae, have been observed infrequently (not illustrated).

Cnidome ( Figs 9G‒I , 10P ): large, ovoid, microbasic euryteles [(8.1‒9.3) × (3.6‒4.1) µm, a single capsule seen discharged], banana-shaped microbasic mastigophores [(6.0‒6.5) × (1.5‒1.7) µm, none seen discharged], and sausage-shaped microbasic heteronemes [(5.7‒6.4) × (1.4‒1.5) µm, none seen discharged].

Color in life: brown.

Remarks. 16S rRNA data supports the divergence of Halecium turbinariae sp. nov. from all other sequenced congeners ( Fig. 29 ).

The new species is primarily distinguished from its congeners through its substrate preference and the occurrence of zooxanthellae in its coenosarc. The hydroid from the SW Indian Ocean whose frustulation was documented by Gravier-Bonnet & Bourmaud (2005, as Halecium sp. ) is evidently the same. Despite the presence of female gonothecae in their material ( Gravier-Bonnet & Bourmaud 2005: 65), the authors refrained from assigning it a specific name, stating that “more taxonomic work has to be done to identify it with confidence” ( Gravier-Bonnet & Bourmaud 2005: 68). The fertile material from Bali allows us to provide, herein, a comprehensive description and to assign it conclusively to a new species.

Only a few congeners share with H. turbinariae sp. nov. the presence of zooxanthellae in their coenosarc, namely H. nanum Alder, 1859 ( Calder 1991), H. lankesterii ( Bourne, 1890) ( Peña Cantero & García Carrascosa 2002) and H. xanthellatum Galea, 2013 (original account). Its mode of branching and the structure of its trophosome have much in common with those of both H. nanum and H. xanthellatum ; however, unlike in the new species, the female gonothecae of H. nanum have transversely-corrugated hydranth tubes ( Congdon 1907: figs 21‒22, as H. marki ; Fraser 1912: fig. 29C; Calder 1991: fig. 12C), while those of H. xanthellatum are dorsally-ringed and their aperture surpasses the top of the theca ( Galea 2013: fig. 5G). On the other hand, the female gonotheca of H. lankesterii has a broad, dome-shaped top and its aperture is situated much lower than in H. turbinariae ( Peña Cantero & García Carrascosa 2002: fig. 13C).

The female gonotheca of the new species is very similar to that of: 1) H. labiatum Billard, 1933 [original account: fig. 8L‒M; Galea & Ferry (2015: fig. 3J‒K); Calder et al. (2019: fig. 4B)], but in that species the internodes are long and geniculate ( Billard 1933: fig. 8K); 2) H. praeparvum Calder, 2017 , but here it produces as much as 4‒6 oocytes, and the species can also be distinguished through its strongly flaring hydrothecae, and an occurrence in cold waters (Bay of Fundy, Canada).

Etymology. Named after its occurrence on thalli of Turbinaria algae ( Fucales : Sargassaceae ).

Distribution. Glorioso Islands ( French Southern and Antarctic Lands) ( Gravier-Bonnet & Bourmaud 2005, as Halecium sp. ), Bali ( Indonesia) (present study).

Incertae sedis 3

Figures

FIGURE 9. Halecium turbinariae, sp. nov. A–D. Colonies on Turbinaria sp. (formalin-fixed material, note discoloration of the polyps over time). E. Hydranth, showing endoderm filled with zooxanthellae. F. Hydrant column, more enlarged. G–I. Cnidome. J. Zooxanthella, showing large nucleus and cytoplasm filled with chloroplasts. Scale bars: G–J = 10 µm; F = 100 µm; E = 500 µm.

FIGURE 10. Halecium turbinariae, sp. nov. A–H. Many stems, two of which (A, B) are illustrated with their polyps. I. Detail of two neighboring cauli. J. Male gonothecae with gonophores. K–O. Female gonothecae, two of which (K, L) showing oocytes; an empty gonotheca is seen in both frontal (M) and lateral (N) views; aperture seen from above (O). P. Cnidome. Scale bars: P = 10 µm; I–O = 200 µm; A–H = 500 µm.

FIGURE 11. Female gonotheca of Halecium turbinariae, sp. nov. A, B. Ripe gonotheca, showing branched blastostyle ending in pair of feeding polyps (A), and oocytes behind (B). C–E. Empty gonotheca seen frontally (C), laterally (D) and apically (E). Scale bar: 200 µm.

FIGURE 29. Phylogenetic hypothesis of the genus Halecium based on the 16S rRNA region. Numbers at nodes represent BPP and MLBS, respectively.