Myopea callaeum, Hooge & Todt & Tyler, 2025

Hooge, Matthew D., Todt, Christiane & Tyler, Seth, 2025, New solenofilomorphids (Solenofilomorphidae: Acoela) from the northwest Atlantic, Zootaxa 5686 (1), pp. 121-131 : 122-125

publication ID	https://doi.org/10.11646/zootaxa.5686.1.5
publication LSID	lsid:zoobank.org:pub:3CE268AE-7ED7-47FE-A04C-415472200DA8
DOI	https://doi.org/10.5281/zenodo.16987000
persistent identifier	https://treatment.plazi.org/id/B178CF60-FFDF-6A4E-FF14-FB34FC0FFD66
treatment provided by	Plazi
scientific name	Myopea callaeum
status	sp. nov.

Treatment

Myopea callaeum sp. nov. ( Figs. 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 )

Type Material. Holotype: AMNH _ IZC 00386160 About AMNH , one set of 1.5- µ m-thick serial sagittal sections of epoxy-embedded specimens stained with toluidine blue, collected on June 14, 1998 . Paratypes: AMNH _ IZC 00386161 About AMNH , one set of 1.5- µ m-thick serial sagittal sections of epoxy-embedded specimens stained with toluidine blue; and AMNH _ IZC 00386162 About AMNH , epoxy-embedded whole mount, both collected in September 2004 .

Type Locality. Crow Neck , Cobscook Bay, Maine, U.S.A. (44 ◦ 52’35”N, 67 ◦ 07’36”W). Medium to coarsegrained sand, often with detritus, from the upper subtidal (almost intertidal) GoogleMaps .

Other Material Examined. Numerous living specimens in squeeze preparations; 5 sets of 2- or 1.5- µ m-thick serial sections of epoxy-embedded specimens; 6 whole mounts for fluorescence imaging of musculature.

Etymology. The species name is the Latin transliteration of the Greek ( Kallaion ) for a rooster’s comb, and refers to the red color and crenulated body margins of the species.

Synonyms. Myopea “callaeum ”: Hooge (2001), Hooge et al. (2002), Todt (2009); Myopea sp. : Todt & Tyler (2006), Jondelius et al. (2011), Atherton & Jondelius (2021), Atherton & Jondelius (2022), Nilsson (2011), Kånneby & Jondelius (2013).

Description. Mature specimens up to 1500 µ m long and ~ 260 µ m wide ( Fig. 1A View FIGURE 1 .) Anterior and posterior ends rounded and lateral body margins of posterior half crenulated. Body color mostly red in both transmitted and reflected light (from pigment dissolved in tissue fluids). With lateral chordoid tissue ( Fig. 1A View FIGURE 1 ), which is especially prominent in starved animals.

Epidermis completely ciliated, composed of insunk mucoid epithelial cells that stain metachromatically with toluidine blue. Without prominent rhabdoid or mucoid glands.

Frontal organ opens as small pit at frontal pore, its glands moderately developed, reaching posteriorly approximately one-quarter the length of the body, to shortly behind pharynx (to U26; Figs. 1A View FIGURE 1 , 2A View FIGURE 2 , 3A View FIGURE 3 ).

Body-wall musculature a simple grid of outer circular and inner longitudinal muscle fibers ( Fig. 2B View FIGURE 2 ) as described by Hooge (2001).

Mouth opening oval and positioned subterminally at U10–U20, thus close to anterior tip of the body ( Fig. 2 View FIGURE 2 , 3A View FIGURE 3 ). Frontal mouth area (a shallow groove in the epidermis) extends anteriad from the mouth to anterior tip of the body. Pharynx extending ~ 300 µ m posteriorly from mouth, with ciliated epithelium and musculature like that of body wall—i.e., with longitudinal muscles immediately under the epimyum (that part of epidermis superficial to musculature; Crezée 1975) and circular muscle fibers deep (abluminal) to them as described by Todt & Tyler (2006) and Todt (2009). Strong sphincter fibers around the mouth, but no prominent posterior sphincter at the juncture between pharynx and digestive syncytium ( Fig. 2B View FIGURE 2 ). Clusters of long-necked unicellular pharyngeal glands bearing acidophilous secretory granules lie lateral to the pharynx, their cell bodies situated along the posterior end of the pharynx and their gland necks opening into its distal quarter to half. At its juncture with the digestive syncytium, the pharynx is wrapped in a conspicuous thick layer of cells containing numerous lipid droplets.

Testis unpaired, dorsal, compact; positioned posterior to pharynx and anterior to male copulatory organ (U20– U30; Fig. 1A View FIGURE 1 , 3A View FIGURE 3 ). Tracts of sperm reach to the anterior end of the seminal vesicle without forming a false seminal vesicle.

Ovary unpaired, dorsal; germinal area behind copulatory organ and reaching posteriorly to U75–U80 ( Figs. 1A, C View FIGURE 1 ; 3A View FIGURE 3 ).

Common gonopore on ventral surface in anterior half of body (U44). It opens to a short ciliated common antrum which connects it to an elongate (100 µ m by 58 µ m), thick-walled (8 µ m), muscular seminal vesicle ( Fig. 1C View FIGURE 1 , 3 View FIGURE 3 ). The penis, at the distal, ventroposteriorly directed end of the seminal vesicle is muscular and fully invaginated into the seminal vesicle. Its center is lined by globose secretory cells with finely granular contents that stain orthochromatically with toluidine blue and that appear prominently in squeeze preparations of live specimens ( Fig. 1B View FIGURE 1 ).

At the posterior edge of the gonopore and antrum is an indistinct seminal bursa composed of diffuse, fibrous tissue ( Fig. 1C View FIGURE 1 , 3 View FIGURE 3 ). No sperm were seen within seminal bursa.

Remarks. In having a muscular penis invaginated into the seminal vesicle and a relatively short and cupshaped pharynx with a wreath of glands emptying into its distal portion, Myopea callaeum sp. nov. fits clearly into the genus Myopea . It stands distinct from the other two described species of Myopea — M. crassula Crezée, 1975 , and M. latafaucium Crezée, 1975 , both from North Carolina, U.S.A. —in its red coloration, its prominently glandular (and less muscular) penis, and its elongate, thick-walled seminal vesicle. It is more similar to M. crassula in possessing a diffuse seminal bursa; a bursa is not present in M. latafaucium . The quite short ciliated antrum in M. callaeum sp. nov. stands in contrast to the much longer ciliated antrum of M. crassula and more resembles the short ciliated portion of the otherwise longer antrum of M. latafaucium . M. callaeum sp. nov. is more like M. crassula in having its mature oocytes extending no farther posteriorly than the first two-thirds to three-quarters of the body length, while they reach to the posterior tip of the body in M. latafaucium .