Small, but smart: Fine structure of an avicularium in Dendrobeania fruticosa (Bryozoa: Cheilostomata)

Abstract

Bryozoans are small benthic suspension‐feeding colonial animals. Among this phylum, there are representatives showing a lesser or greater degree of polymorphism, and the most common type of polymorphic zooids is the avicularium. Here we present a detailed description of the bird's‐head shaped avicularium in Dendrobeania fruticosa. The body cavity of the avicularium demonstrates an acoelomate condition: along the cystid walls, there is neither the layer of extracellular matrix toward the epidermis, nor coelomic lining. However, a layer of extracellular matrix and epithelialized cells lie under the epidermis of the tentacle sheath. Probably, such construction helps the tentacle sheath to acquire some rigidity—it is the only region of the body wall without an ectocyst. We did not find typical funicular strands in the avicularium, but there is a delicate mesh composed of stellate cells with thin and long projections, which sometimes isolate the spaces filled with a heterogeneous matrix. The proximal ends of the adductors, abductors, and polypide retractors are attached to the body wall via typical epidermal tendon cells, which possess numerous bundles of tonofilaments. The distal ends of the abductors and adductors attach to the frontal membrane or upper vestibular membrane, respectively. The inner organic layer of the ectocyst in these regions forms large protrusions, from which numerous thin outgrowths branch off. We suggest them to be a functional analogue of apodemes and apodemal filaments in arthropods. “Apodemal” tendon cells have long and thin projections that line the outgrowths of the ectocyst and surround the distal ends of the muscle cells. At these sites, “apodemal” tendon cells possess numerous tonofilaments. The vestigial polypide includes the tentacle sheath, rudimentary lophophore, cerebral ganglion, and polypide retractors. The sensory part of 5HT‐positive cells of the frontal membrane is dendrite‐shaped and embedded in the inner organic layer of the ectocyst.

Colonial system of integration and communication pores in a polymorphic bryozoan Dendrobeania fruticosa (Bryozoa: Cheilostomata)

“…Indirect confirmation of this assumption is provided by differences in the cellular composition of the CSI in autozooids and heteromorphs of D. fruticosa . Thus, the CSI of avicularia is composed only of stellate cells (Shunatova et al, 2022), while the CSI of kenozooids includes only elongated cells (Shunatova et al, 2021). Based on differences in the sizes of avicularia and kenozooids (about 300 μm and 2–4 cm in length, respectively), we again come to the suggestion that the strands of the CSI composed of elongated cells provide transport of nutrients over a larger distance, whereas the stellate cells distribute them locally.…”

“…Exclusively the stellate cells contact cells of the rosette complexes at both sides of the pore plate (Figure 18f). The avicularian CSI is composed of stellate cells only, and its structure has been recently described by us (Shunatova et al, 2022).…”

“…Indirect confirmation of this assumption is provided by differences in the cellular composition of the CSI in autozooids and heteromorphs of D. fruticosa. Thus, the CSI of avicularia is composed only of stellate cells (Shunatova et al, 2022), while the CSI of kenozooids includes only elongated cells (Shunatova et al, 2021).…”

“…As for the “funicular system” in heteromorphic zooids, it has been recently described for a single species, Dendrobeania fruticosa (Shunatova et al, 2021, 2022). The authors demonstrated differences in the cellular composition of the “funicular system” in avicularia and kenozooids.…”

To be a transit link: Similarity in the structure of colonial system of integration and communication pores in autozooids and avicularia of Terminoflustra membranaceotruncata (Bryozoa: Cheilostomata)

“…The large spherical electron‐dense inclusions of storage cells in this species are homogeneous and similar to those reported in autozooids of the cheilostome Celleporella hyalina (Nekliudova et al, 2019). However, electron‐dense inclusions of storage cells have a striated appearance in gonozooids and autozooids of cyclostomes Crisia eburnea and Crisiella producta (Nekliudova et al, 2021), as well as in the autozooids, avicularia, and kenozooids of the cheilostome D. fruticosa (Shunatova et al, 2021, 2022, 2023). The inclusions in storage cells of Bugulina flabellata have a different appearance (see Plate V(f) in Dyrynda & King, 1983) compared to other published data.…”