Rhynchonema nordestinum, Mota & Neres & Esteves, 2025

Rhynchonema nordestinum sp. nov.

( Figures 6–7 View FIGURE 6 View FIGURE 7 , Table 2)

Material studied. Three males in glycerin slides.

Type material. Holotype male ( MOUFPE 0040 View Materials ), other paratypes: two males (NM LMZOO-UFPE 537–538 ).

Type locality. Holotype male and paratypes: Potiguar Basin , located along the coast of the state of Rio Grande do Norte and the extreme west of the state of Ceará, in the Northeast region of Brazil ( 5°01'34.8"S; 36°20'11.2"W). Collected in 2009. 8 m depth. In sediments from rhodolith beds. GoogleMaps

Etymology. The specific epithet ‘ nordestinum’ is given to the species due to the location of the Potiguar Basin, where the holotype was collected.

Description. (Males). Body cylindrical 749–950 μm long. Maximum body diameter corresponding to 6–7 times the head diameter. Cuticle strongly annulated throughout the length of the body, presenting vacuoles with no distribution pattern, exhibiting a circular/rough appearance (which are best observed anterior to the amphideal fovea). The cuticular annules closest to the amphideal fovea region are wider than the others. Cephalic sensilla arrangement not observed, probably lost during sample processing. Somatic setae distributed in a disorderly manner throughout the body. Oval amphideal fovea, occupying 63–75% of corresponding body diameter, the equivalent of the width of five body annules, located 38–45 μm from the anterior end. It presents 22–23 cuticular annules anterior to the amphideal fovea. Buccal cavity elongated extending to half of the amphideal fovea, total length equivalent to 22% of the total length of the pharynx. Nerve ring and secretory-excretory pore not observed (the same condition was observed in all analyzed specimens). Pharynx cylindrical (205–234 μm), without terminal bulb. Cardia partially surrounded by the intestine. Reproductive system with two outstretched testes, located to the left of the intestine. Spicules symmetrical, 20–28 μm long (0.9–1.3 times the cloacal body diameter), robust and slightly curved. Gubernaculum occupying 54–64% of spicules’ length with well-developed dorsal apophysis. Precloacal supplements absent. Tail conical, about 3.5–3.9 times the cloacal body diameter (the annules in this region are smaller compared to the rest of the body). Three caudal glands.

Diagnosis. Rhynchonema nordestinum sp. nov. is characterized by the presence of a cuticle annulated with vacuoles and somatic setae short, spread throughout the body without a distribution pattern. Oval amphideal fovea, occupying 63–75% of corresponding body diameter, equivalent to five cuticular annules in length. 22–23 annules anterior to the amphideal fovea. Buccal cavity elongated, extending to half of the amphideal fovea. Spicules symmetrical, robust and slightly curved. Gubernaculum with dorsal apophysis. Two testes in the anterior and left region of the intestine. Tail conical about 3.5–3.9 times anal body diameter.

Differential diagnosis. In this section, only males of the species were used for comparisons.

Rhynchonema nordestinum sp. nov. differs from other species by the morphology of the amphideal fovea, spicules, and gubernaculum along with its apophysis. The species that most resemble Rhynchonema nordestinum sp. nov. are Rhynchonema chiloense and Rhynchonema megamphidum , similar in terms of the morphology of the amphideal fovea (oval, occupying more than 60% of the corresponding body diameter, with a length equivalent to 4–6 cuticular annules), symmetrical, curved spicules with a more robust appearance, as well as the presence of a gubernaculum with an apophysis. However, the species differ in the number of cuticular annules anterior to the amphideal fovea: R. chiloense has 46 annules, R. megamphidum has 29–34 annules, and the new species has 22–23 annules. The location of the amphideal fovea in relation to of the pharyngostome is also divergent. In R. chiloense , the structure is located near the end of the pharyngostome (final portion situated in the anterior third of the amphideal fovea); in R. megamphidum , the amphideal fovea is entirely located posterior to the pharyngostome; whereas in R. nordestinum sp. nov., it is located in the terminal part of the pharyngostome (final portion situated in half the amphideal fovea). The species have a gubernaculum with an apophysis, but they differ in the morphology of this structure. R. chiloense presents a gubernaculum with a caudal apophysis, R. megamphidum presents a short dorsal apophysis and a distal part with sharp tips oriented toward the anterior part of the body ( vs. R. nordestinum sp. nov. gubernaculum with a dorsal apophysis). Furthermore, the presence of vacuoles was not mentioned in any species, as observed in the described species.

The presence of an oval amphideal fovea, as well as symmetrical spicules and the presence of a gubernaculum with apophysis, is observed in other species of the genus: Rhynchonema amakusanum , Rhynchonema falciferum and Rhynchonema sieverti . However, the morphology of spicules and gubernaculum are divergent: in R. amakusanum and R. sieverti , spicules are thin and curved, with globular capitulum. In R. amakusanum the gubernaculum presents dorso-caudal apophysis, while in R. sieverti it has a small dorsal apophysis. In R. falciferum the spicules are without curvature, almost straight, and the gubernaculum presents asymmetrical apophysis ( vs. R. nordestinum sp. nov., spicules are robust and slightly curved and the gubernaculum presents dorsal apophysis).