Morphology of ctenostome bryozoans: 1.Arachnidium fibrosum

Abstract

The morphology of ctenostome bryozoans remains little investigated with only few species having been subject to more detailed studies. From all the seven main different superfamilies, only few representatives have been studied. The superfamily Arachnidioidea has particularly been neglected concerning detailed morphological and histological details. So far, not a single analysis specifically studied a representative of the family Arachnidiidae. Arachnidium-like forms have, however, often been regarded as potential cheilostome ancestors, the most successful group of bryozoans to date. The lack of any morphological data on this family called for a detailed investigation of one of its representatives. Hence, we analysed the general morphology and histology of Arachnidium fibrosum. Most striking morphological features previously unrecognized are a cardiac constrictor, previously almost unknown in the family, a single pair of apertural muscles consisting of proximal parieto-diaphragmatic and distal parieto-vestibular muscles, six pairs of duplicature bands, a lophophoral anus and retractor muscles attaching to the foregut. Although comparative data are limited, there seem to be two distinct different clades of arachnidiid ctenostomes that are characterized by their aperture and details of gut morphology. Further analysis of additional arachnidioidean species are required to confirm this.

Morphology of ctenostome bryozoans: 2. Haywardozoon pacificum, with implications of the phylogenetic position of the genus

“…The gut in Haywardozoon pacificum has an elongated cardia, short to vestigial caecum, vestibular anus (Schwaha, 2020b), and lacks a cardiac constrictor found in many ctenostomes (Schwaha, 2020c; Schwaha & Wanninger, 2018). These characteristics are identical to the morphology found in alcyonidioideans (Schwaha & De Blauwe, submitted; d'Hondt, 1983; Schwaha, 2020a; Schwaha & Wanninger, 2018). In contrast, species of the genus Hislopia have a mid‐sized cardia, extremely large caecum (see also Hirose & Mawatari, 2011; Schwaha et al, 2011), a mid‐tentacle sheath anus (Schwaha, 2020b) and a prominent cardiac, muscular proventriculus (Schwaha et al, 2011; Table 1).…”

“…Soft‐body morphological features are still little studied for inferring evolutionary relationships, but have a high potential for revealing phylogenetic relationships (see also Schwaha, 2020a, 2020b, 2020c). The present study is the second in a series just started by Schwaha and De Blauwe (submitted) dealing with precise soft‐body morphological analyses of ctenostome bryozoans. Herein, we analyze the newly described deep‐sea species Haywardozoon pacificum Grischenko, Gordon, & Melnik, 2018.…”

Morphology of ctenostome bryozoans: 3.Elzerina,Flustrellidra,Bockiella

“…Another character typical for this superfamily is the highly cuticularized vestibular wall, which has also been described for other members of the clade ( Alcyonidium , Matricon, 1963; Haywardozoon , Schwaha, Grischenko, et al, 2020; Pherusella , Decker et al, in press). In most other ctenostome superfamilies, the cuticle of the vestibular wall is thin (see, e.g., Braem, 1890, 1951; Schwaha et al, 2019; Schwaha & De Blauwe, 2020). Apertural muscles are present as parieto‐diaphragmatic muscles, which insert only at the proximal end of vestibular wall.…”

“…Specimens were photographed and documented with a Nikon SMZ25 (Nikon, Tokyo, Japan) stereomicroscope equipped with a Ds‐Ri2 camera, or a Hirox RH‐2000 (Hirox, Tokyo Japan). For embedding, sectioning and 3D‐reconstruction, samples were processed as previously described previously (e.g., Decker et al, 2020; Schwaha, Grischenko, et al, 2020; Schwaha & De Blauwe, 2020). In brief, samples were dehydrated with acidified dimethoxypropane, embedded into epoxy resin and afterwards serially sectioned at 1 μm thickness at a Leica UC6 ultramicrotome (Leica Microsystems, Wetzlar Germany).…”

“…A series of morphological studies of ctenostome bryozoans was started to analyze the details of soft‐tissues by sectioning and 3D reconstruction techniques. (Schwaha, Grischenko, & Melnik, 2020; Schwaha & De Blauwe, 2020). This contribution is the third in the series providing new morphological data on three different genera of the Alcyonidioidea.…”