Macrobiotus hoianicus Stec, 2026, sp. nov.
Authors: Stec, Daniel
Type: Taxonomic treatment
Published: 2026-05-18
Keywords: Biodiversity, Taxonomy, Animalia, Tardigrada, Eutardigrada, Parachela, Macrobiotidae, Macrobiotus, Macrobiotus hoianicus

<p><b><i>Macrobiotus hoianicus</i> sp. nov.</b></p><p>urn:lsid:zoobank.org:act: 4E0E4B5B-7E3B-43D2-82AE-C02244AAE4DF</p><p>Figs 22–29; Tables 8–9</p><p><b>Etymology</b></p><p>The species name <i>hoianicus</i> refers to Hội An, Vietnam, where the species was collected. The adjective is formed in reference to the type locality.</p><p><b>Material examined</b></p><p>77 animals, 48 eggs mounted on microscope slides in Hoyer’s medium, 6 animals and 5 eggs examined under SEM and two animals processed for DNA sequencing.</p><p><b>Type material</b></p><p><b>Holotype</b></p><p>VIETNAM • Quảng Nam Province, Hội An, vicinity of Fukian Assembly Hall; 15°52′39″ N, 108°19′50″ E; 11 m a.s.l.; Aug. 2018; D. Stec and K. Miler leg.; moss collected from tree bark in urban environment; ISEA PAS, slide VN.004.13.</p><p><b>Paratypes</b></p><p>VIETNAM • 76 specs; same data as for holotype; ISEA PAS, slides VN.004.01 to VN.004.06, VN.004.13, VN.004.14 • 48 eggs; same data as for holotype; ISEA PAS, slides VN.004.07 to VN.004.12.</p><p><b>Description</b></p><p><b>Animals</b></p><p>Body transparent in juveniles and whitish in adults; after fixation in Hoyer’s medium, the body becomes transparent (Fig. 22). Eyes present in alive and Hoyer-fixed specimens. Cuticular pores are scattered randomly over the entire body surface (Figs 22–23), being more sparsely distributed on the ventral side. Pores are roundish to oval and similar in size (0.6–0.1 μm in diameter). Granulation is present over the entire body cuticle, being denser in the dorso-caudal region; however, it is clearly visible only under SEM (Fig. 23). Under PCM, body granulation is discernible only in the dorso-caudal part of the body (Fig. 22). In addition to the more or less evenly distributed body granulation, two dense patches of granulation are present on the left and right sides of the caudal body portion, between the main body trunk and the terminal body segment (Figs 22–23). Distinct granulation patches are also present on the external surfaces of legs I–III (Figs 24–25), whereas on the internal surfaces granulation occurs only on legs II–III (Figs 24–25). Dense and distinct granulation is also present on the lateral and dorsal surfaces of leg IV (Figs 24–25). On the internal surface of legs I–III, a pulvinus-shaped cuticular bulge is centrally located, and a faint cuticular fold occurs in the distal portion of the legs just above the claws (Figs 24– 25). Both structures are visible only when the legs are fully extended and properly oriented on the slide.</p><p>Claws small and slender, of the <i>hufelandi</i> type (Fig. 26). Primary branches bear distinct accessory points, a common tract, and a well-developed stalk connecting the claw to the lunula (Fig. 26). Lunulae on legs I–IV are smooth (Fig. 26). A divided cuticular bar and double muscle attachments occur above claws I–III but are very poorly visible (Figs 24–26).</p><p>Mouth antero-ventral. Bucco-pharyngeal apparatus of the <i>Macrobiotus</i> type (Fig. 27), with a ventral lamina and ten small peribuccal lamellae followed by six buccal sensory lobes. Under PCM, the OCA is of the <i>maculatus</i> type, with the first and second bands of teeth not visible (Fig. 27); however, all three bands are clearly discernible under SEM (Fig. 27). The first band of teeth consists of numerous extremely small cones arranged in a single row, located anteriorly in the oral cavity on the bases of the peribuccal lamellae and immediately posterior to them (Fig. 27). The second band of teeth is situated between the ring fold and the third band and is composed of several rows of small, sparsely distributed cones, larger than those of the first band (Fig. 27). The third band of teeth is located in the posterior portion of the oral cavity, between the second band and the opening of the buccal tube (Fig. 27); it is discontinuous and divided into dorsal and ventral portions. Under PCM, the dorsal and ventral teeth appear as a single transverse ridge, each with two dark thickenings (Fig. 27). Under SEM, the dorsal and ventral portions are clearly distinguishable: the dorsal portion forms a continuous ridge bearing two peaks (teeth), whereas the ventral portion is composed of two larger lateral teeth and one smaller median tooth (Fig. 27). The pharyngeal bulb is spherical, with triangular apophyses, two rod-shaped macroplacoids, and a small triangular microplacoid (Fig. 27). The macroplacoid length sequence is 2<1, with the first and second macroplacoids constricted centrally and subterminally, respectively, the latter constriction being extremely delicate (Fig. 27). Measurements and morphometric statistics of the animals are provided in Table 8.</p><p><b>Eggs</b></p><p>Eggs laid freely, white, spherical to slightly ovoid (Figs 28–29). The surface between egg processes is of the <i>hufelandi</i> type, i.e. covered with a reticulum (Figs 28–29). The reticular mesh is composed of distinct pores (0.4–0.6 μm in diameter), the diameter of which is usually similar to the width of the nodes and bars forming the mesh, giving the reticulum a robust appearance (Figs 28–29). Interbasal meshes are usually larger than peribasal meshes; however, the peribasal meshes do not form distinct rings around the bases of the egg processes (Figs 28–29). Under PCM, dark thickenings can be distinguished around the bases of the egg processes, which under SEM correspond to the portions linking the reticulum with the walls of the egg processes (Figs 28–29). Egg processes are of inverted goblet shape, with slightly concave trunks and concave terminal discs (Figs 28–29). Terminal discs are round and strongly serrated to moderately dentate (Figs 28–29). Each terminal disc has a distinct concave central area, which may contain scattered granulation. Granulation is also consistently present on the serrated margins or teeth of the terminal discs (Figs 28–29). Measurements and morphometric statistics of the eggs are provided in Table 9.</p><p><b>Reproduction</b></p><p>The type population of <i>M. hoianicus</i> sp. nov. is dioecious. Both males with testes filled with sperm and females with ovaries containing oocytes were observed in specimens freshly mounted in Hoyer’s medium.</p><p><b>Differential diagnosis</b></p><p>By having (i) patches of dense cuticular granulation on the body in areas other than the legs, (ii) the first band of teeth in the OCA not visible in light microscopy, (iii) eggs with processes of inverted goblet shape bearing terminal discs lacking flexible filaments, and (iv) an egg surface between processes covered by a reticulum, the new species is most similar to only one species of <i>Macrobiotus</i>: <i>Macrobiotus kamilae</i> Coughlan & Stec, 2019, known so far only from India (Coughlan & Stec 2019). However, the new species differs from <i>M. kamilae</i> in several diagnostic characters: (i) the presence of only a single patch of granulation located on the dorso-lateral cuticle of the caudal body region (three distinct granulation patches present on the dorso-lateral cuticle in <i>M. kamilae</i>); (ii) the absence of granulation on the internal surface of leg I (granulation present in <i>M. kamilae</i>); (iii) the presence of smooth lunulae on the hind legs (lunulae dentate in <i>M. kamilae</i>); (iv) a narrower buccal tube (external and internal widths 1.8–3.8 μm and 0.9–2.0 μm in the new species vs 4.1–6.0 μm and 2.4–4.3 μm in <i>M. kamilae</i>); (v) a shorter second macroplacoid (1.7–4.6 μm in the new species vs 4.8–8.0 μm in <i>M. kamilae</i>); and (vi) a shorter placoid row (7.9–16.8 μm in the new species vs 17.1–26.7 μm in <i>M. kamilae</i>).</p><p><i>Phylogeny</i></p><p>Both Bayesian inference and maximum likelihood analyses produced trees with highly similar topologies and well-supported clades (Fig. 30), reflecting congruent relationships within Macrobiotidae clade I (sensu Stec <i>et al.</i> 2021a). All four new species described in this study were recovered within the genus <i>Macrobiotus</i> and were consistently placed within <i>Macrobiotus</i> clade C (sensu Stec <i>et al.</i> 2021a). Within clade C, <i>M. hoianicus</i> sp. nov. is recovered as sister to all remaining taxa in the clade. <i>Macrobiotus witalinskii</i> sp. nov. forms a clade with <i>Macrobiotus porifini</i> Kuzdrowska, Mioduchowska, Gawlak, Bartylak, A. Kepel, M. Kepel & Kaczmarek, 2021 and <i>M. kristenseni</i>, with <i>M. kristenseni</i> being its closest relative. <i>Macrobiotus surmaczi</i> sp. nov. clusters with <i>Macrobiotus noongaris</i> Coughlan & Stec, 2019 and <i>Macrobiotus scoticus</i> Stec, Morek, Gąsiorek, Blagden & Michalczyk, 2017, with <i>M. scoticus</i> recovered as its sister species. <i>Macrobiotus dalaticus</i> sp. nov. forms a clade with <i>Macrobiotus paulinae</i> Stec, Smolak, Kaczmarek & Michalczyk, 2015 and <i>M. kamilae</i>, with <i>M. kamilae</i> recovered as its sister species. In all analyses, each of the newly described species formed a well-supported, distinct monophyletic lineage, corroborating their delineation as separate species-level taxa (Fig. 30).</p>