Solenofilomorpha crezeei sp. nov. (Figs. 4–6)

Type Material. Holotype: AMNH _ IZC 00386163, one set of 1.5- µ m-thick serial sagittal sections of epoxy-embedded specimens stained with toluidine blue, collected on February 28, 1999. Paratypes: AMNH _ IZC 00386164, one set of 2- µ m-thick serial sagittal sections of epoxy-embedded specimens stained with toluidine blue; and AMNH _ IZC 00386165, epoxy-embedded whole mount, both collected in February 2005 from Bakeman Beach, Cape Rosier, Maine, U.S.A. (44 ◦ 18’39”N, 68 ◦ 48’11”W).

Type Locality. Wadsworth Cove, Castine, Maine, U.S.A. (44 ◦ 24’13”N, 68 ◦ 48’33”W). Fine-grained shallow subtidal sediment.

Other Material Examined. Numerous living specimens in squeeze preparations; 9 sets of 2- µ m-thick serial sections of epoxy-embedded specimens; 8 whole mounts for fluorescence imaging of musculature.

Etymology. Species named for Michael Crezée, in gratitude for his friendship during the writing of his seminal monograph on the Solenofilomorphidae and in celebration of the 50th aniversary of that monograph.

Synonyms. Solenofilomorpha “crezeei ”: Hooge (2001), Todt (2009); Solenofilomorpha sp.2: Todt & Tyler (2006), Jondelius et al. (2011), Atherton & Jondelius (2021), Atherton & Jondelius (2022), Nilsson (2011), Kånneby & Jondelius (2013).

Description. Mature specimens ~ 1.4 mm long and 75−220 µ m wide (Figs. 4A, 5A–D) but up to 2.2 mm in length when fully stretched. Body cylindrical, with anterior and posterior ends rounded. Anterior quarter of body slightly narrower than rest; posterior quarter of body with crenulated margins and usually held slightly curled as the animal glides over substrate (Fig. 5B).

Epidermis completely ciliated; without prominent mucoid or rhabdoid glands.

Body translucent; almost colorless in reflected light and light brown in transmitted light. Eggs and gut contents dark brown to olive, often with large transparent vacuoles in digestive tissue.

Body-wall musculature a simple grid of outer circular and inner longitudinal muscle fibers as described by Hooge (2001).

Frontal glands well developed, reaching beyond the posterior end of the pharynx (to U32). Strand-like mucoid parenchymal glands throughout the body stain a more intensive, darker shade of pink than frontal glands.

Mouth opening slit-like on the ventral surface in anterior half of body (U24, Figs. 4A, 5E). Mouth without strong sphincter muscles, opens to long (U24–U32), tubular, muscular pharynx composed of a ciliated, insunk, glandular epithelium subtended by longitudinal and deeper (abluminal) circular muscle fibers as described by Todt & Tyler (2006) and Todt (2009). A well-developed, prominent, funnel-like sphincter delimits the proximal end of the pharynx.

Testis unpaired, compact; extending from a level posterior to the pharynx to the male copulatory organ (U32– U58; Figs. 4A, B).

Ovary unpaired, dorsal; extending from frontal glands posteriorly to bursa (U37–U63; Fig. 4).

Common gonopore on ventral surface shortly behind middle of body (U63); opens to a long ciliated, tubular antrum (Fig. 4A, B, 5A, 6A, B). Anteriorly, the antrum opens directly to the male copulatory organ (at U60) conspicuous in squeeze preparations of live animals where the seminal vesicle forms a dark cap on the proximal end of the penis (Fig. 5A).

Copulatory organ comprises a muscular penis capped by a squat sclerotized cone (Figs. 5F, 6B) and a seminal vesicle filled with sperm (Figs. 4A, 5F, 6). The proximal end of the antrum is richly glandular, its long-necked glands filling much of the volume around the copulatory organ. Seminal vesicle, penis, and the proximal glandular part of the antrum are surrounded by a thick common muscular sheath, mostly of thin, somewhat disorganized circular fibers (Fig. 6). The seminal vesicle itself is spherical, 50 µ m in diameter, with a thick muscular wall composed of strong longitudinal fibers extending from the antrum and with thinner, less-well-organized circular fibers (Figs. 4B, 5F, 6A, B). The penis is slightly longer than broad (~ 25 µ m × ~ 20 µ m) and consists of stacked crescent-shaped muscle-bound chambers; the sclerotized cone capping its distal end stems from actin-reinforced cell processes originating from globular cells surrounding it just outside the muscular sheath where that meets the antrum (Figs. 4B, 6). The penis cone is only weakly refractile, not as prominent in brightfield or even phase-contrast microscopy as sclerotized bursal nozzles of other acoels, for example, and it appears simpler, less well-organized, than those nozzles.

The seminal bursa (at U63–U69), which connects to the antrum on its posterior side, appears syncytial, without a clear lumen, and contains loosely packed allosperm (Figs. 4B, 5F).

Remarks. In its habitus—that is, elongate, cylindrical body with rounded ends and narrow rostrum, crenulated posterior, and elongate pharynx— Solenofilomorpha crezeei sp. nov. looks like other species of Solenofilomorpha. More specifically, its elongate mouth, tubular pharynx with glandular epithelium but lacking pharyngeal glands and having a narrow opening to the digestive syncytium, and its seminal vesicle without invaginations, all fall in line with the diagnosis of the genus.

The genus Solenofilomorpha presently contains five species: S. funilis Crezée, 1975, from North Carolina, U.S.A.; S. guaymensis, Crezée, 1975, from Sonora, Mexico; S. longissima Dörjes, 1968, from Helgoland Island, Germany; S. justinei Nilsson, Wallberg, & Jondelius, 2011, from New Caledonia; and S. pellucida Kånneby & Jondelius, 2013, from Las Cruces, Chile.

S. crezeei sp. nov. is distinctly different from all of these species in having a much more muscular and thick-walled seminal vesicle and muscular sheath around the seminal vesicle and penis. It could also stand distinct from the other species in having a sclerotized penis cone, but we suspect other species have not been sufficiently studied in this respect (using actin-binding probes); the sharp, densely staining granular plug at mid-antrum in S. funilis (what Crezée [1975] depicts as termini of antrum glands), for instance, could well be a sclerotized cone, and the dense mass filling the proximal half of the antrum in S. guaymensis may also be. So, too, could the eosinophilic glands at the proximal end of the antrum in S. justinei, but this species is described as lacking a penis. While Dörjes (1971) described a muscular penis in S. longissima, Crezée (1975) interpreted this structure in Dörjes’s sections of this species (the sole ones in existence) as non-muscular and probably in a state of resorption.

Both S. funilis and S. guaymensis have long genital atria that lead to the male and female copulatory organs, but in neither case are these ciliated as they are in S. crezeei. The antrum of S. justinei is long and ciliated, but this species has no penis or bursa, and its copulatory organ is quite simple.

A bursa comparable to that of S. crezeei sp. nov. is present in S. funilis and S. guayensis but absent from the other three species. In the three species with a bursa, it connects to the antrum rather proximally, just distal to what could be called a penis. That of S. funilis is more like that of S. crezeei sp. nov. in having a well-defined wall.