Strophomenia boricua Cobo & Strong, 2025

Strophomenia boricua Cobo & Strong sp. nov.

Figs 4 View Figure 4 , 5 View Figure 5

Type material.

Holotype: USNM 1691556 About USNM , West Puerto Rico, Caribbean Sea, off Cabo Rojo, Mona Passage ; 18.034167°N, 67.407167°W; 389 m depth. Serial sections (28 slides 5 µm), fragment of specimen (mid-body region) in 95 % ethanol, DNA sequences (mtCOI, 18 S rRNA, 28 S rRNA). GoogleMaps

Diagnosis.

Animal slender, elongate (18 × 1–1.8 mm), sienna to orange in color in life. Cuticle thick, hollow acicular sclerites dominate. Epidermal papillae pedunculate. Radula lacking; radular sac vestigial. Ventrolateral foregut glands of type B, fusing dorsally in the foregut. Tubes running only on the right side of the body in their anterior region. Five bundled seminal receptacles on each side. Dorsoterminal sensory organ well developed. Spawning ducts paired. Respiratory folds, copulatory stylets, and abdominal spicules lacking.

Description.

External aspect and sclerites. Animal slender, elongate ( 18 mm long, 1–1.8 mm wide) with rounded ends (Fig. 4 A, B View Figure 4 ). In life, the body is brown to orange (Fig. 4 C View Figure 4 ); it becomes dark brown when preserved in ethanol (Fig. 4 A, B View Figure 4 ). Sclerites do not protrude externally, and their arrangement is not visible without magnification. With three types of hollow acicular sclerites located in the dorsal and mid body region (Fig. 4 D, E View Figure 4 ): 1) the main type (140–200 μm long, 15–20 μm wide) is strongly curved, with a characteristic distal end bearing three ridges; 2) the second most abundant type (140–160 μm long, 15–20 μm wide) has also ridged distal ends but emerges straight from the body surface; 3) less common are slightly curved hollow acicular sclerites with a pointed distal end and a rounded base (120–200 μm long, 15–30 μm wide). Along the pedal groove, the arrangement of sclerites is dense and distinctive (Fig. 4 D, F View Figure 4 ). Acicular sclerites like those on the dorsal surface are found, also bearing ridged distal ends, but with a flatter tip and curvature oriented in the opposite direction (120–140 μm long, 12–15 μm wide). In the most ventral portion, these diminish in number and are replaced by numerous knife-shaped scales, characteristic of the pedal groove (40–60 μm long, 10–18 μm wide). The pedal groove and atrio-buccal cavity (Fig. 4 A View Figure 4 ) are externally distinguishable.

Internal anatomy. Cuticle thick with sclerites in several layers (Fig. 5 View Figure 5 ). The thickness of the cuticle is constant through the body, but thinner ventrally (up to 200 μm dorsal, 50–80 μm ventral) and is traversed by abundant pedunculate epidermal papillae (Fig. 5 I View Figure 5 ). Pedal groove contains a single, almost rectangular, pedal fold (25 × 12.5 μm) (Fig. 5 H View Figure 5 ). The atrium (250 μm long, 200–210 μm wide, 90–100 μm high) opens ventrally, with small papillae anteriorly and two main sensory structures ventrally. Cerebral ganglia almost rectangular in cross-section (115 μm long, 100 μm wide, 90 μm high). Pedal pit densely ciliated, triangular in shape (110 μm long, 60 μm wide, 30 μm high). The mouth opens at the posterior end of the atrium and continues into a robust, muscular foregut, almost circular in cross section (525 μm long, 80–120 μm wide, 60–100 μm high). A dorsal caecum extends to the mid-anterior region of the body. The foregut merges directly with the midgut caecum in the radular region, without forming a distinct esophagus. Ventrolateral foregut glands are of type B ( García-Álvarez and Salvini-Plawen 2007) (Fig. 5 G View Figure 5 ) and connect dorsally to form a single duct in the foregut (Fig. 5 A View Figure 5 ). The foregut glands continue first as two independent tubes only on the right side of the midgut (Fig. 5 B View Figure 5 ) and posteriorly they are positioned on both sides of the midgut and are folded: in some sections, each side shows a main tube and one or two smaller ventral ones (Fig. 5 C View Figure 5 ). A rudimentary radular sac is present after the fusion of the foregut and the dorsal caecum, but no radula is developed (Fig. 5 C View Figure 5 ). Midgut with serial constrictions. Spawning ducts are paired along most of their length (450 μm length, 80–100 μm width, 50–60 μm heigh), open ventrally into the mantle cavity (120 μm length, 20–30 μm width, 20–40 μm heigh). Mantle cavity small, ciliated (520 μm length, 40–50 μm width, 120–140 μm heigh) (Fig. 5 D, E View Figure 5 ). Bundles of five seminal receptacles on each side, located in the connection of the pericardioducts with the anterior portion of the spawning ducts (Fig. 5 F View Figure 5 ). A single, well-developed dorsoterminal sensory organ is present (Fig. 5 D, E View Figure 5 ). Respiratory folds, copulatory stylets, and abdominal spicules are absent.

Etymology.

‘Boricua’ is a term that refers to a native of Puerto Rico or someone of Puerto Rican descent. It originates from ‘ Borikén ’ (also spelled ‘ Borinquen ’), the Taíno name for the island of Puerto Rico, and is widely used by Puerto Ricans to refer to themselves. The use of boricua aims to honor the cultural identity of Puerto Rico and highlights the geographic origin of the species. Used as a noun in apposition.

Remarks.

This species is assigned to genus Strophomenia ( Strophomeniidae ) based on the absence of a radula, and the presence of type B ventrolateral foregut glands and of bundled seminal receptacles ( García-Álvarez and Salvini-Plawen 2007). It is distinguished from its congeners in the combination of reproductive features (fewer seminal receptacles) and arrangement of foregut glands (Table 2 View Table 2 ). Molecularly, it is recovered as sister to Anamenia gorgonophila (Kowalevsky, 1880) (Fig. 6 View Figure 6 ). Strophomenia boricua sp. nov. is distinguished from all described species of the genus (Table 2 View Table 2 ) by a unique combination of internal features, including a reduced number of seminal receptacles (five per side vs. 8–23 in other species) and a rudimentary radular sac (previously only reported in S. debilis ; Nierstrasz 1902). It further differs from S. debilis in the asymmetrical arrangement of the ventrolateral foregut glands and the nearly rectangular shape of the pedal fold (triangular in all other known species). The discharge of the paired spawning ducts as a single short tube in the mantle cavity, shared only with S. scandens ( Heath 1905) , also supports its distinction. The genus Strophomenia is well supported based on the current morphological diagnostic characters: seminal receptacles, absence of a radula, distinctive pharyngeal gland structures (type B and commonly located on just one side of the body after the connection with the foregut), a well-developed dorso-terminal sensory organ, and paired genital openings (except for S. scandens , although the single opening is very short; Heath 1911). A more detailed analysis of the sclerites may also reveal that the carinated distal ends observed in S. boricua sp. nov. as well as the arrangement of the sclerites in the pedal region (observed for other Strophomenia species; Heath 1911) are also diagnostic of the genus. However, distinguishing species within Strophomenia remains challenging. Definitive diagnostic characters are limited, and the distinction is based on a combination of characters that must be studied in detail (Table 2 View Table 2 ). Ecological traits, particularly coral-host associations, may offer valuable clues for species delimitation within the genus. This new solenogaster species is associated with the coral host Villogorgia nigrescens . Two specimens were observed on a colony, ~ 30 cm in height.