Immergentia, Silén, 1946

Johnson, Mildred J., Saadi, Ahmed J., Kuklinski, Piotr, Smith, Abigail M., López-Gappa, Juan & Schwaha, Thomas, 2024, Digging into boring bryozoans: new characters and new species of Immergentiidae, Organisms Diversity & Evolution 24 (2), pp. 217-256 : 222

Genus Immergentia, Silén, 1946

Emended diagnosis: Non-pedunculate boring bryozoans, living in shells of molluscs. Colony typically enantiomorph, with alternating left-right symmetries of zooids. Autozooids vase shaped, slightly asymmetrical, with a tapered or rounded basal tip slightly bent in direction to shell surface. Zooids vertically or obliquely oriented in shell. Primary cystid appendages (pcy) from pointed both ends of zooidal aperture. Secondary cystid appendages (scy) in mid-region of zooids. Additional cystid anastomoses present at various locations of autozooid. Polypide with 8–10 tentacles, gut with distinct cardiac constrictor, and bulbous caecum. Lowto mid-lophophoral anus. Intercalary kenozooids connecting primary or secondary cystid appendages. Sac zooids bulb- or vase-shaped and typically 1/3 to 2/3 the size of an autozooid. Single embryos brooded, developing in degenerated zooid.

Remarks: Silén (1946) described the aperture as regularly quadrangular with a thickened cuticle at distal end. Though borehole apertures are typically spindle-shaped, resembling a bulged ‘S’ shape, they can also appear as oval or circular. Cystid appendages refer to thin extensions of the zooid itself, referred to as connecting prolongations in the original diagnosis of the genus (Silén, 1946) and subsequently stolo-like threads (Silén, 1947).

Amendments to the genus now include the presence of two types of kenozooids found in a colony. First, intercalary kenozooids that connect primary or secondary cystid appendages. The second type of kenozooids is sac zooids separated from a connecting stolon or cystid appendage by a septum.

Though Silén (1947) first reported brooding in immergentiids, this was not included in the initial diagnosis of the genus. Soule (1950) then added this to his generic description. The development of an embryo within the tentacle sheath of a degenerated zooid is confirmed here.

Based on samples examined and literature, a minimum of eight and a maximum of ten tentacles have been reported. Additionally, analysis of the soft-body morphology revealed the presence of a cardiac constrictor in the gut not reported before. The anus is typically located in the low- to mid-lophophoral position of the polypide.