Rhynchonema parvum, Mota & Neres & Esteves, 2025

Rhynchonema parvum sp. nov.

( Figures 8–9 View FIGURE 8 View FIGURE 9 , Table 2)

Material studied. Three males in glycerin slides.

Type material. Holotype male ( MOUFPE 0041 View Materials ), other paratypes: two males (NM LMZOO 539–540 ).

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°04'10.1"S 36°22'31.1"W). Collected in 2013. 4 m depth. In sediments from rhodolith beds. GoogleMaps

Etymology. The specific epithet ‘ parvum ’ is derived from the Latin meaning “small” given that the species has the smallest amphideal fovea within the genus.

Description. (Males). Body cylindrical 696–766 μm long, thinner in the cephalic portion. Maximum body diameter corresponding to 5–7 times the head diameter. Cuticle annulated. Body annules are characterized as being very thin and narrow, with the exception of those located in the region of the body along the pharynx. The cuticle contains vacuoles, which are more clearly visualized at the level of the pharynx. Only six labial setae could be observed (very thin). Somatic setae along the entire body. Amphideal fovea circular and small, occupying 16–28% of corresponding body diameter (equivalent to three body annules), with a prominence of the corpus gelatum. Cuticular annules anterior to the amphideal fovea are thin and small, and it is not possible to count them (the same condition was observed in all analyzed specimens). Buccal cavity elongated and irregular with total length equivalent to 40–43% of the total length of the pharynx, extending to the end of the amphideal fovea. Nerve ring and secretory-excretory pore not observed (the same condition was observed in all analyzed specimens). Pharynx cylindrical (161–166 μm) slightly expanded at the posterior region, sometimes looking like a terminal bulb. Cardia oval surrounded by cylindrical intestine. Reproductive system with outstretched testes to the left of the intestine. Spicules symmetrical, 33–40 μm long (1.5–1.7 times the cloacal body diameter), very thin and curved; also elongated (cephalized proximal extremities). Gubernaculum occupying 46–50% of spicules’ length. Poorly developed dorsal apophysis. Precloacal supplements absent. Tail conical, about 3.6–4.6 times the cloacal body diameter. Three caudal glands.

Diagnosis. Rhynchonema parvum sp. nov. is characterized by a cuticle that is annulated with very thin annules and by the presence of somatic setae throughout the body. Amphideal fovea circular and small, occupying equivalent to three cuticular annules in length, 16–28% of corresponding body diameter. Buccal cavity irregular, extending posteriorly to the amphideal fovea. Spicules curved and elongated, with cephalized proximal extremities. Gubernaculum with dorsal apophysis. Two testes. Tail conical, corresponding to 3.6–4.6 cloacal body diameter.

Differential diagnosis. In this section, only males of the species were used for comparisons. Furthermore, some measurements and proportions missing from the original descriptions were obtained from the available images.

Rhynchonema parvum sp. nov. is differentiated from other species of the genus due to the morphology and size of the amphideal fovea, position of the amphideal fovea in relation to the pharyngostome and morphology of the spicules. The species that most resemble Rhynchonema parvum sp. nov. are Rhynchonema ambianorum and Rhynchonema lyngei , morphologically similar concerning the amphideal fovea (circular, occupying less than 40% of the corresponding body diameter) and symmetrical spicules with proportional sizes, R. ambianorum 23 μm, R. lyngei 32 μm e R. parvum sp. nov. 33–40 μm. However, neither species have vacuoles, as seen in the new species and the position of the amphideal fovea is different. In R. ambianorum and R. lyngei the amphideal fovea is located posterior to the pharyngostome, whereas in the new species, it is located anterior to the pharyngostome. Furthermore, the species present morphological differences in the spicules and gubernaculum. In R. ambianorum the spicules are arched, with claw-like distal parts, and the gubernaculum is barely visible and devoided of apophysis. R. lyngei presents spicules with uniformly rounded arched capitulum and gubernaculum with angular dorsal apophysis ( vs. R. parvum sp. nov. presents spicules thin, curved and elongated and gubernaculum with poorly developed dorsal apophysis).