Arctonula arctica (Sars, 1851)

Serova, K. M., Belikova, E. V., Kotenko, O. N., Vishnyakov, A. E., Bogdanov, E. A., Zaitseva, O. V., Shunatova, N. N. & Ostrovsky, A. N., 2022, Reduction, rearrangement, fusion, and hypertrophy: evolution of the muscular system in polymorphic zooids of cheilostome Bryozoa, Organisms Diversity & Evolution (New York, N. Y.) 22 (4), pp. 925-964 : 936-939

Arctonula arctica ( Figs. 1B , 8 , 17C )

Avicularium

Cystid and mandible Two small adventitious avicularia are situated proximo-laterally near the autozooidal orifice ( Figs. 1B , 8A ). The avicularian cystid is oval in a frontal plane. It is associated with the lateral wall of autozooid and deeply immersed in the autozooid, but does not reach its basal wall. The slightly widening proximal edge of sclerotinized, semicircular mandible rests on the transverse skeletal pivotal crossbar, dividing the frontal side of the avicularium into two parts: mandibular and postmandibular; the latter occupied by the frontal membrane ( Figs. 8A, B, D , 17C ). The mandible is hollow, consisting of three walls and enclosed cavity connected with the body cavity of the avicularium via oval opening in the lower wall ( Fig. 8B ). An external (upper) and internal (lower) walls merge at the free mandible edge, forming a thickened peripheral sclerite with slightly curved and tapering margin ( Fig. 8B, D ). The most part of the lower mandibular wall is internal ( Fig. 8B ), with the upper vestibular wall positioned beneath. This wall is distally attached to the sclerite, then extending proximally towards the mandible base and continuing to the palate (lower vestibular wall). Proximally, the mandibular walls are connected by the narrow vertical wall ( Fig. 8B, D ).

elements of avcularium (lateral view) (note longitudinal muscles of tentacle sheath in C). Large opening in the lower mandibular wall is visible in B and C. Abbreviations: dm, diaphragm, fm, frontal membrane, g, ganglion of vestigial polypide, mab, abductor, mad, adductor, md, mandible, p, palate, rm, retractor muscles, t, tendon of adductor, ts, tentacle sheath, vp, vestigial polypide

Polypide and muscular system The vestigial polypide is an oval “cell mass” situated in a distal part of avicularian cystid in front of the adductor muscles ( Figs. 8B–D , 17C ). Its upper part is represented by a rudimentary lophophore surrounded by a tentacle sheath, whereas its lower part consists of a round ganglion. The tentacle sheath proceeds distally to a small invagination in the center of the palate ( Fig. 8B, D ) that is a lower wall of the modified vestibulum.

Diaphragmatic muscles (3–4 annular muscle fibers forming a loose, narrow “cone” and sometimes interconnected by the “spiral” fibers) are situated in the uppermost narrow part of the tentacle sheath. The wall of the latter also bears 7–8 thin longitudinal muscle fibers. The upper ends of these muscles intersect with the annular muscles in the lower half of the diaphragm ( Fig. 8C ). Distal ends of the retractors composed of two to three muscle fibers insert to the polypide between the ganglion and the tentacle sheath; their proximal ends attach to the lower part of the cystid wall in the distal part of the avicularium ( Fig. 8B–D ). Some confocal images showed that they are striated.

tentacle sheath). C, inset Muscular elements of avicularium (frontolateral view) (note longitudinal muscles of tentacle sheath in C and inset). In C dotted line indicates borders of avicularian mandible. Abbreviations: dm, diaphragm, g, ganglion, mab, abductor, mad, adductor, md, mandible, rm, retractor muscles, sc, sclerite of mandible, ts, tentacle sheath, vp, vestigial polypide

Two pairs of symmetrical adductors and abductors occupy most of the avicularian cystid volume. Smaller smooth abductors are comprised of five short and relatively thick muscle bundles each. A single bundle consists of several muscle cells ( Fig. 8B, E ). Two zones of their attachment to the frontal membrane are short, narrow, and parallel to each other. The lower ends of the abductors are attached to the proximal and lateral cystid walls of the avicularium, behind the adductor attachment zones and slightly overlapping with them.

Larger striated adductors insert to the upper vestibular wall behind the sclerite by a pair of tendons ( Figs. 8B–E , 17C ). The distal ends of both tendons run just beyond than the distal margin of the central opening, slightly widening at the sites of attachment. Because of their large size, the proximal attachment zones of adductors occupy most of the cystid walls in the lower third of the avicularium ( Fig. 8C, E ).

Figures

Fig. 1 Partial view of living colonies of the studied species (stereomicroscope). A Terminoflustra membranaceotruncata (erect lobate); B Arctonula arctica (encrusting); C Rhamphostomella ovata (encrust-

Fig. 8 Morphology and anatomy of adventitious avicularia in Arctonula arctica. A Stereomicroscope; B, D histological sectioning, light microscopy; C, E CLSM, phalloidin staining. A Part of living colony showing autozooids and paired lateral avicularia (some shown by arrows). B, D Sagittal sections of avicularium with closed mandible (mandibular sclerite and small invagination of palate are visible on both images, pivotal crossbar shown by arrow). C, E Muscular

Fig. 17 Schemes of cheilostome autozooid (A), interzooidal avicularium of Terminoflustra membranaceotruncata (B), and adventitious sessile avicularia of Arctonula arctica (C) and Smittoidea propinqua (D). All with uplifted operculum/mandible. Thin muscle fibers radiating from diaphragm shown in B. Longitudinal muscles are visible in tentacle sheath of vestigial polypide in B–D. Abbreviations: db, duplicature bands, dm, diaphragm, f, funiculus, fm, frontal membrane, lw, lower mandibular wall, mab, abductor, mad, adductor, md,