Thelastoma variabile Phillips, McAllister, Turner & Bernard, 2025

Phillips, Gary, Mcallister, Chris T., Turner, Jackson H., Moulton, Jaydeep Kolape John K. & Bernard, Ernest C., 2025, Review of Thelastoma Leidy, 1849 (Nematoda: Oxyuridomorpha: Thelastomatidae) and description of T. variabile sp. nov. from the North American indigenous millipede Narceus americanus (Spirobolida: Spirobolidae), Zootaxa 5665 (4), pp. 451-508 : 484-491

publication ID	https://doi.org/10.11646/zootaxa.5665.4.1
publication LSID	lsid:zoobank.org:pub:B2FB0C47-1B54-48B0-9DCD-7554AC1FC59C
persistent identifier	https://treatment.plazi.org/id/0F3B87AE-4F56-2A0F-FF7E-8BB7F319FE6B
treatment provided by	Plazi
scientific name	Thelastoma variabile Phillips, McAllister, Turner & Bernard
status	sp. nov.

Treatment

Thelastoma variabile Phillips, McAllister, Turner & Bernard , sp. nov.

( Figs. 1‒5 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 , 8 View FIGURE 8 )

Type locality and habitat. USA: Oklahoma, McCurtain County, Hochatown (34°10′17.0286″N, 94°45′05.7414″W), around bases of oaks ( Quercus spp. ) and pines ( Pinus spp. ), in hindgut and midgut of 75 of 82 Narceus americanus (Palisot de Beauvois, 1817) ( Shelley et al. 2006) .

Type designation and deposition. Female holotype identification number T- 808t; male paratype identification number T-8094p; female paratype numbers identified as T-8086p through T-8093p. United States Department of Agriculture ( USDA), Agricultural Research Service ( ARS), Mycology and Nematology Genetic Diversity and Biology Laboratory , Building 010A, Room 111, 118, BARC-West, 10300 Baltimore Avenue , Beltsville , Maryland 20705.

GenBank accession number. PP694142.

Etymology. The new species is named in recognition of the great variation in tail length among the collected specimens.

Adult female (n = 59). Robust nematodes, length 2,830 ‒6,063 µm, width at midbody 193‒406 µm ( Figs. 2A View FIGURE 2 , 8A View FIGURE 8 ); measurements and morphometrics given in Table 4. Cuticle strongly annulated with annules of varying widths until reaching the anus ( Figs. 1A View FIGURE 1 , 2C, E–G View FIGURE 2 , 5A, C, E View FIGURE 5 ). First annule much longer than the succeeding 10 cephalic annules, often as much as 2‒3× the length of each following neck annule ( Figs. 1A View FIGURE 1 , 2C View FIGURE 2 , 5A, C View FIGURE 5 ). Annules posterior to the cephalic annule rounded then becoming flattened and angled in profile distally ( Figs. 2C, E–G View FIGURE 2 ). Head elevated, eight labial papillae surrounding a raised, triradiate buccal region; stoma capped by three lips, forming the exterior opening of the stoma ( Figs. 5A–D View FIGURE 5 ). Each lip with two projections, lips fitting together to form circular oral opening ( Figs. 5A–D View FIGURE 5 ). Stoma relatively shallow, subquadrate, approximately 14 µm long by 12 µm wide ( Fig. 1A View FIGURE 1 ). Stoma basally with three robust, multi-cusped teeth ( Figs. 1A View FIGURE 1 , 2C, D View FIGURE 2 ); pinnate structures absent from stoma lumen. Glandlike somatic extensions of anterior and posterior arcade syncytia present but weakly developed ( Fig. 1B View FIGURE 1 ). Minute slit-like amphids near base of labial papillae ( Fig. 5D View FIGURE 5 ). Lateral alae and lateral field absent. Esophageal corpus long, cylindrical, posteriorly constricting into an isthmus, transitioning into the basal bulb ( Figs. 2A View FIGURE 2 , 3A View FIGURE 3 , 8 View FIGURE 8 ). Basal bulb pyriform with grinding valve, terminal cardia shallow with three small cells ( Fig. 8B View FIGURE 8 ). Anterior portion of intestine dilated ( Figs. 2A View FIGURE 2 , 8 View FIGURE 8 ). Nerve ring at about 39% of total esophagus length ( Figs. 3A View FIGURE 3 , 8B View FIGURE 8 ). Excretory pore located near base of basal bulb or slightly anterior or posterior (ratio z, range = 0.97‒1.18) ( Fig. 3A View FIGURE 3 ). Intestine gradually narrowing posteriorly. Annules at level of anterior intestine larger than in cephalic region, approximately 14 µm long. Four coelomocytes: one coelomocyte located near anterior flexure of gonad, second between basal bulb and junction of gonads, third at or slightly anterior to vulva, fourth about one body length posterior to vulva. Gonads paired, amphidelphic, doubly reflexed, posterior ovary looping around intestine just posterior to intestinal dilation ( Figs. 2A View FIGURE 2 , 8B View FIGURE 8 ), anterior ovary looping well anterior to anus. One posterior spermatheca ( Fig. 2A View FIGURE 2 ). Vulva located near midbody, ratio V 40.1‒53.4% of total body length, slightly protruding but without distinct anterior flap ( Fig. 2A View FIGURE 2 , 5E View FIGURE 5 ; vagina directed anteriorly ( Figs. 2A View FIGURE 2 , 3D View FIGURE 3 , 8A View FIGURE 8 ). Annules posterior to vulva gradually lengthening to about 20‒24 µm, terminating just anterior to or at anus ( Fig. 5E View FIGURE 5 ). Eggs oval, smooth, one polar body ( Fig. 2A View FIGURE 2 , 3B, C View FIGURE 3 , 8A View FIGURE 8 ). Phasmids pore-like, approximately 1‒2 µm in diameter and located posterior to base of tail (at about 25‒35% of tail length) ( Figs. 4A, B, E–G View FIGURE 4 , 5F View FIGURE 5 ). Tail length and profile highly variable, from long filiform to dagger-like, tip pointed; tail occasionally very short, conoid, rounded ( Figs. 2A View FIGURE 2 , 4 View FIGURE 4 , 5F View FIGURE 5 , 8A View FIGURE 8 ).

Adult male. Male length about 20‒38% that of females. Length 1,197 µm, width 87 µm ( Fig. 2B View FIGURE 2 ). Cuticle weakly annulated, first annule about twice as long as succeeding individual annules ( Fig. 1C View FIGURE 1 , 3E View FIGURE 3 ). Lateral alae absent, lateral field a slightly raised ridge. Labial papillae not observed, head not elevated as in female. Amphids not observed. Stoma shallow, funnel-shaped, with ventral tooth and weakly formed dorsal tooth ( Fig. 1C View FIGURE 1 ). Nerve ring at about 41% of total esophagus length ( Fig. 3E View FIGURE 3 ). Basal bulb pyriform, valved ( Fig. 2B View FIGURE 2 ). Anterior intestine not dilated as in female, consistent width in anterior portion of body, then tapering posteriorly to cloaca ( Fig. 2B View FIGURE 2 ). Excretory pore at approximately 26% of body length, well posterior to basal bulb (z = 0.57), inconspicuous. Four coelomocytes: the first at the anterior flexure of the testis, second at about the middle of the testis, third and fourth at junction of testis and vas deferens ( Fig. 2B View FIGURE 2 ). Gonad monorchic, reflexed. One spicule, capitulum slightly enlarged, distally tapering to a point ( Figs. 1D, E View FIGURE 1 ). Gubernaculum absent or possibly represented by a tiny structure located near midpoint of spicule ( Fig. 1E View FIGURE 1 ). Genital cone conspicuous. Papillae consisting of one ventro-sublateral precloacal pair about two body widths anterior to cloaca, one ventral precloacal pair, one ventromedial papilla, one sublateral pair at level of cloaca and one pair at about 28% of the tail length located at bend in tail ( Figs. 1D, E View FIGURE 1 ). Tail tapering, with a 40° dorsal bend at the paired papillae, then tapering distally to a fine point ( Figs. 1D View FIGURE 1 , 2B View FIGURE 2 ).

Diagnosis. Females large, cephalic annule dome-like, conspicuous; stomal aperture round, formed from 3 inner lips each with 2 triangular projections. Stoma with 3 multi-cusped teeth. Pinnate stomal projections absent. Secretory-excretory pore about level with base of esophagus (ratio z). Nerve ring at about 40% of the esophagus length (ratio d). Vulva position usually less than 50% of body length (ratio V), about 60% of the body length excluding tail (ratio V’). Vulval flap absent. Tails filiform to dagger-like to short-conoid, longest tails 4× length of shortest tails. Phasmid apertures minute, circular, usually at about 25‒30% of tail regardless of tail length. Male with tapering stoma, large ventral tooth, lateral field slightly raised, not expanded into alae. Posterior region with four pairs of papillae: one sublateral pair well anterior to cloacal cone, one pair each anterior and above cloacal orifice and one pair on tail; single midventral papilla posterior to cloacal orifice. Spicule present, tapering, tip pointed. Tail bent dorsally.

Remarks. Of the North American Thelastoma spp. in diplopod hosts, T. variabile sp. nov. most resembles the type species of the genus, T. attenuatum . Similarities include the overall shape of the nematode; oval eggs with similar dimensions; the posterior intestine looping just posterior to the dilation of the intestine in both species; ratio V (47% for T. attenuatum , 46.4% for T. variabile sp. nov.; and the same host species, although the type localities (Pennsylvania, Oklahoma) are more than 2,000 km apart.

Differences between T. variabile and T. attenuatum are primarily in their morphometrics. Measurements in the original description of T. attenuatum by Leidy (1849, 1853) were made according to the old British system using lines and inches. It is ambiguous how long a line was, but according to Klein (1974), 1 line is roughly equivalent to 2,120 µm; thus, the equivalent range in body length of T. attenuatum was 2,540 ‒3,180 µm, compared to 2,830 ‒6,052 µm in T. variabile sp. nov. For measurements of Virginia specimens Kloss (1965) gave lengths of 4,038 ‒4,697 µm, and Lee (1974) listed a range of 2,000 ‒3,600 µm. Basir (1956) illustrated the excretory pore far anterior to the basal bulb (z = 1.35), but in T. variabile sp. nov., it is primarily located around the base of the basal bulb (mean z = 1.04).

Kloss (1965) apparently received specimens of Spirobolus marginatus Say 1821 (= Narceus annularis / americanus complex) from the millipede specialist Dr. Richard L. Hoffman. The specimens were collected from Buffalo Mountain, Floyd County, Willis, Virginia, approximately 700 km from where Leidy presumably collected the original specimens of T. attenuatum in 1848 or 1849. Measurements listed by Kloss (1965) did not include ratios, so they were estimated based on her drawings. The nerve ring ratio d for T. attenuatum is 32% compared to a range of 35‒47% in T. variabile sp. nov.; and the excretory pore ratio z is more anterior in T. attenuatum sp. nov. (z = 1.25) than that of T. variabile sp. nov. (z = 0.97‒1.18).

Leidy (1849) only described T. attenuatum females. Kloss (1965) reported the discovery of a single male found in the intestine of S. marginatus . In describing the male, she indicated that the excretory pore was located at the base of the basal bulb in T. attenuatum , whereas in T. variabile sp. nov. it is much more posterior to the basal bulb; lateral alae were described for T. attenuatum but are absent in T. variabile sp. nov.; the tail was about 26% of the body length opposed to 13% in T. variabile sp. nov., indicating a much shorter tail overall; the spicule length in T. attenuatum was 39 µm vs. 30 µm for T. variabile sp. nov.; and the tail shape of T. attenuatum was relatively straight and filiform as opposed to bent dorsally for T. variabile sp. nov.

Of the other North American Thelastoma spp. , female T. variabile sp. nov. are much longer (2,830 ‒6,063 µm) than T. collare (1,400 ‒2,700 µm). The anterior annules of T. collare in profile have sharply angled posterior edges, whereas in T. variabile sp. nov. they are rounded; and the female tail of T. collare is relatively consistent in length and shape as opposed to the wide variation of the conoid, dagger-like and long, filiform tails in T. variabile sp. nov. The male T. variabile sp. nov. lacks lateral alae but they are present in T. collare . Female T. krausi are much shorter than T. variabile sp. nov. (1,820 ‒2,880 µm vs. 2,830 ‒6,063 µm), possess a hexagonal oral opening (circular in T. variabile sp. nov.) and have a slit-like phasmid aperture (round in T. variabile sp. nov.). The papilla pattern of T. variabile sp. nov. consists of four pairs of papillae (one sublateral pair, well anterior; one sublateral pair, precloacal; one pair post-cloacal; one medial papilla; and one pair of papillae located at tail bend) which differs from the descriptions for T. krausi (three pairs of papillae and one medial papilla) and T. collare (four pairs of papillae and no medial papilla). However, the more anterior pre-cloacal pair seen in T. variabile sp. nov. may have been overlooked in these species.

Thelastoma spicatum View in CoL was described from S. marginatus View in CoL (assumed to be Narceus sp. as outlined by Upton et al. (1983), the same host as T. variabile sp. nov.). Cobb (1929) used an antiquated formula ( Cobb 1890) to tabulate the morphometric measurements of T. spicatum View in CoL . Analyzing this formula, it appears that T. spicatum View in CoL has an overall body length of approximately 2,800 µm, far shorter than T. variabile sp. nov., and a relatively longer esophagus (b = 5.6) compared to T. variabile sp. nov. (mean b = 7.5).

Comparing T. variabile sp. nov. with T. labiatum View in CoL (= T. myolabiatum Christie, 1938 View in CoL ) by means of measurements is tenuous at best, primarily because T. labiatum View in CoL was measured in inches and Leidy (1853) did not illustrate the entire nematode, but only the most anterior portion of the head; therefore, a comparison was done with Christie’s specimens. Thelastoma myolabiatum View in CoL is a much smaller nematode than T. variabile sp. nov. (1,600 µm vs. 2,830 ‒ 6,063 µm) and its head appears more constricted ( Leidy 1850, 1853), which conforms to an illustration made by Cobb (1929). In comparison with T. variabile sp. nov., all measurements and ratios were smaller for T. myolabiatum View in CoL ; the only notable morphometric similarity was the V value, 46% for T. myolabiatum View in CoL and mean of 46.4% for T. variabile sp. nov.

The only known female of the Cuban species T. bivalvulum has a much shorter body (1,924 µm) than does T. variabile sp. nov. It was also described and illustrated as having prominent elongated valves extending from the posterior corpus into the basal bulb. This feature is not present in T. variabile sp. nov. The new species can be separated from Thelastoma spp. on other continents and in other hosts by reference to Tables 2 and 3. These tables are available from the authors as Excel spreadsheets to facilitate sorting and comparison.