Nemalecium caeruleus, 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 : 15-18

Nemalecium caeruleus , sp. nov.

urn:lsid:zoobank.org:act:08E68E3D-DF5F-43F0-BF29-15EBE40D3DAB

Figs 12 , 13A‒H & M‒U , 14 , 30

? Nemalecium lighti — Gravier-Bonnet & Mioche, 1996: 169, 172–197, fig. 2C, F.— Gravier-Bonnet & Migotto, 1996 (pro parte): 208–213, fig. 2 [non Nemalecium lighti ( Hargitt, 1924) ].

? Nemalecium sp. — Di Camillo et al., 2008: 1594.

? Nemalecium sp. 1 — Gravier-Bonnet & Bourmaud, 2012: 107.

Material examined. Holotype: MSNMCoe367, Indonesia, Bali, Amed, Lipah Bay , -8.350152°, 115.683861°, 2‒4 m, 26 Jan 2020, GenBank: OR872061–OR872064 (16S), OR872010 (18S) .— Paratype: MSNMCoe368, same collecting data as for the holotype, GenBank: OR872057–OR872060 (16S), OR872009 (18S).— Additional material: MSNMCoe369, Nemalecium sp. , same collecting data as for the holotype, GenBank: OR872065– OR872068 (16S), OR872011 (18S).

Description. Colonies living in anfractuosities of scleractinian corals ( Fig. 12A ), where they form a continuous belt along the lateral walls (but not the bottom of the cavities), partly embedded among sand grains, detritus,filamentous algae and diatoms ( Fig. 12B ); stems erect, monosiphonic, either unbranched ( Fig. 13M ) or sympodially-branched ( Fig. 13B‒H, N‒R ), up to 5 mm high, borne on short apophyses of a creeping, branching hydrorhiza firmly adhering to the substrate; composed of fairly long hydrophores, often with 1‒3 intervening, proximal athecate internodes with slight bulges at both ends ( Fig. 13B‒H, M‒R ), nodes straight; a hydrotheca on distal part of the hydrophore; one ( Fig. 13C‒F, H, N, O ) or a pair ( Fig. 13B, G, P‒R ) of 75‒125 µm long apophyses are given off laterally from below the hydrothecal base, each bearing a hydrophore of varied length, ending distally into a hydrotheca; an intervening athecate internode may be occasionally present between the apophysis and the hydrophore ( Fig. 13D, N, P, Q ); subsequent branching pattern similar throughout the colony, with up to 5 th order branches formed, giving rise to cymose colonies (e.g. Fig. 13F, G, P, Q ), composed of up to a dozen polyps; hydrophores 155‒425 µm long, intervening athecate internodes 230‒380 µm long, diameter at nodes 70‒90 µm. Hydrothecae ( Fig. 13S‒U ) moderately deep (55‒65 µm high), with circular, 170‒200 µm wide apertures, walls slightly diverging distally from a 125‒140 µm wide, transversely-set, thin diaphragm (topped by a belt of large, ovoid desmocytes), rim smooth, sometimes inwardly-curved; rare renovations could be noted, with the secondary hydrothecae borne on fairy long hydrophores ( Fig. 13F, O ). Hydranths ( Fig. 13A ) highly extensible, up to 4 mm long, composed proximally of an exceedingly long, slender, tubular, non-digestive portion (containing large, vacuolated cells), and distally of a short, globular, digestive part (filled with gland cells) bearing apically a conical hypostome, encircled by a whorl of 22‒25 filiform, amphicoronate tentacles, among which occur 1‒2 scorpioid nematodactyls; tentacle bases with adjacent clusters of gland cells, appearing visually as refringent patches; when a couple of nematodactyls is present, they are set at an angle of 80‒120° between each other with respect to the perimeter of the hypostome; non-digestive and digestive parts of the hydranth separated by a narrow constriction, allowing the head to move freely in all directions. Gonothecae not seen.

Cnidome ( Fig. 14 ): pseudostenoteles [(23.8‒28.9) × (10.0‒11.0) µm], banana-shaped microbasic mastigophores [(5.8‒6.2) × (1.5‒1.7) µm], large, ovoid microbasic heteronemes [(7.6‒7.9) × (2.7‒3.1) µm], and small, ovoid microbasic heteronemes [(5.2‒5.5) × (2.4‒2.6) µm].

Color in life ( Fig. 12 ): the hydranth column is, for most of its length, translucent, except for its lumen and distalmost part that are white; the apical, digestive region is translucent, the tentacles have a distinctive fluorescent bluish tinge, while the hypostome and nematodactyls are milky white.

Remarks. The new species is immediately distinguished from its two congeners [viz. N. lighti ( Hargitt, 1924) and N. gracile Galea et al., 2012 ] both ecologically and morphologically. Unlike them, it lives exclusively in coral anfractuosities, forming minute, sympodial stems, able to find shelter in such small cavities. Additionally, it forms a well-supported monophyletic clade, well separated from specimens of N. lighti from multiple localities ( Fig. 30 ). However, genetic divergence exists also between N. caeruleus sp. nov. and other morphologically indistinguishable specimens identified as Nemalecium sp. in this study and in Boissin et al. (2018).

The present material matches the typical ecology and distinctive “white blue opalescent coloration of the top of the hydranths”, documented by Gravier-Bonnet & Mioche (1996: 169) in specimens from Reunion Island. Gravier-Bonnet & Migotto (2000, as N. lighti ) discussed its reproduction, although based on mixed specimens of this species with Brazilian counterparts of N. lighti . The authors appear not to have undertaken a morphological comparison of the colonies from the two localities 4, although a thorough description of N. lighti from Brazil was published earlier ( Migotto 1996: 36, fig. 7H–I). Differences in the “shape and size of the corpuscles” of the medusoid were noted, and it was concluded that “we are not dealing with one widespread or nearly circumtropical species, but rather with two or more sibling species” ( Gravier-Bonnet & Migotto 2000: 213). Moreover, there could be additional, though unstated, differences between their material and specimens of N. lighti described by Bouillon (1986) and Calder (1991) 5, and the authors decided not to name the species “until thoroughly morphological comparisons are not [sic] undertaken” ( Gravier-Bonnet & Migotto 2000: 208). The present material from Bali allowed us to characterize the hydroid morphologically and genetically, and to assign to it a specific name.

Fertile colonies were found during the warm season (from November to February) of the southern hemisphere ( Gravier-Bonnet & Mioche 1996: 174), and they were either mono- or dioecious ( Gravier-Bonnet & Migotto 2000: 208). The gonothecae are reportedly borne on both the hydrorhiza and stem internodes, and are broadly conical (with smooth to slightly undulated walls), tapering basally, distally being wide and truncate 6. The gonophore is a medusoid; the males are ovoid, while the females are almost rounded; the origin of the manubrium is slightly eccentrically-placed, and it does not open distally into a mouth, being encircled by a compact mass of gametes occupying the whole subumbrellar cavity; there are no canal system and sense organs, except for a belt of refringent corpuscles around the aperture, the latter being closed by a velum. The gonophores are released at dawn, or when the colonies are left in the dark. Unfortunately, Gravier-Bonnet & Migotto (2000) did not state whether the illustrations provided in their fig. 2 represent French or Brazilian specimens, or both.

In colonies from Reunion Island, the hydroid coexisted with unidentified species of Rhizogeton and Sarsia “either at the same time or alternately” ( Gravier-Bonnet & Mioche 1996), although only a few sterile polyps of a corynid were noted to occur among the colonies from Bali. No propagulae were observed in situ so far, although they were produced in vitro ( Gravier-Bonnet & Migotto 2000).

Etymology. From the Latin caerŭlĕus , -a, -um, meaning (sky) blue, to describe its color in life.

Distribution. Based on ecological preferences and visual appearance, the hydroid is scientifically documented from Reunion Island ( Gravier-Bonnet & Mioche 1996; Gravier-Bonnet & Migotto 2000; both as N. lighti ), Maldives ( Gravier-Bonnet & Bourmaud 2012, as Nemalecium sp. 1 ) and Indonesia ( Di Camillo et al. 2008, as Nemalecium sp. ; present study). However, at present, only the material from Bali can be unambiguously assigned to N. caeruleus and other distributional records should be confirmed through molecular analyses. Additionally, photographic evidence from iNaturalist.org shows similar specimens (mainly identified as Nemalecium sp. ) from India, Christmas Islands, Taiwan, southern Japan, New Caledonia, Fiji and Hawaii, whose specific identities need to be assessed.

Figures

FIGURE 12. Nemalecium caeruleus, sp. nov., in situ. A. Whole colony in coral crevice.B, C. Colony details, showing individual polyps and their colors in life. Photo in A taken by J-M. Bertot.

FIGURE 13. Nemalecium caeruleus, sp. nov. A. Distal portion of a stem with two hydranths. B–H, M–R. Various stems, to show mode of branching, and counterparts of Nemalecium sp. (I–L). S–U. Detailed view of the mode of branching and hydrothecae. From specimens MSNMCoe367 (B–G), additional unregistered colonies (A, M–U), and MSNMCoe369 (I–L). Scale bars: S–U = 200 µm; A–R = 500 µm.

FIGURE 14. Cnidome of Nemalecium caeruleus, sp. nov. A. Pseudostenotele. B. Banana-shaped microbasic mastigophore. C. Large, ovoid microbasic heteroneme. D. Small, ovoid microbasic heteroneme. Scale bar: 10 µm.

FIGURE 30. Phylogenetic hypothesis of the genus Nemalecium based on the 16S rRNA region. Numbers at nodes represent BPP and MLBS, respectively.