General description of bryozoans

Bryozoa were used to be united together with Phoronida and Brachiopoda into the Tentaculata group (Hyman, 1959). Now investigators stated that Lophophorata are united group. Nowadays all three taxa are grouped into Lophotrochozoa group (Protostomia) (Nielsen, 2012; see review in Ostrovsky, 2013) or into Lophodeuterostomia bringing them closer to Deuterostomia. Some researchers place Lophophorata at the base of Proto-/Deuterostomia branching.

Bryozoans are colonial organisms, formed by modules – zooids, which size usually do not exceed 1 mm. Bryozoans are filtrators, mainly feeding with plankton. The majority of bryozoan colonies are sessile, only about dozen species are exception. Usually Bryozoans are divided into 3 classes: Gymnolaemata, Stenolaemata and Phylactolaemata. Species from two first classes mostly live in marine waters, Phylactolaemates inhabits only in fresh water.

All bryozoan species are hermaphrodites. After the cross fertilization, larvae covered with chaetae and able to swim are formed. All freshwater bryozoan colonies are also able to form special hibernating gemmae with a thick chitinous cover – statoblasts. One colony could form over 100 statoblasts.

Freshwater bryozoans are widespread and could be found both in streams or stagnant waters. All species, excluding Cristatella mucedo, are sessile animals, usually living on a different substrata – stones, wood, channel walls, water plants. Among freshwater bryozoan colonies two major forms could be described – plumatellid and lophopodid form (after the genera Plumatellida and Lophopodida). Plumatellid form is an arborescent colonies, tightly attached to the substrata, formed by pipe-shaped zooids. Zoooids cover – ectocyst – is chitinous. Depending on a bryozoan species, polypides could be placed all along the colony branch or could be concentrated on their end. Lophopodid colonies could form different colonies – from elongated to globed. Their ectocyst is gelatinous, polypides usually are situated close to each other. All zooids are monomorphic – i.e. all zooids in a colony are organized similarly. Freshwater bryozoan zooids are huger than of marine species, usually > 1mm long. Every zooid has a highly specialized feeding apparatus – lophophore, consisting of intruding crown of tentacles, covered by chaetae and extruding part of the body wall – tentacle sheath. Mainly bryozoans fed on different phytoplankton – diatoms, desmidians and chlorophytes.

Zooid body is divided into two parts – polypide (intruding tentacle crown with the gut and its muscles) and cystide (where all the internal organs of zooid are placed). All freshwater bryozoan species (excluding Frederecillidae) has a horse-shoe shaped lophophore. On the anal side of the lophophore two large branches are placed – these are lophophore arms with two rows of tentacles on each. The number of tentacles could vary from 20-30 up to 80-100 depending on the species. Osculum, surrounded by tentacles and covered with a tiny fold (epistome), which is indicative for Phylactolaemates, leads to U-shaped gut. Anus is situated under the tentacle crown. From the gut loop to the body wall goes a funiculus. The retractor muscles of the polypide are attached to the base of the lophophore, they are going basally along the gut and are attached to the cystid wall beneath.

Bryozoan nervous system started to be researched from the beginning of 19th century. As a result of the pioneer investigation, the nervous system of a bryozoan species from Lophopus genera was described. It was stated that it consists of two paired ganglions, connected by a commissure. It was shown that these ganglions are situated above the pharynx in a special cavity, and also some huge nerves, innervating lophophore, tentacles, body wall and some other parts of the zooid were described. The majority of knowledge about bryozoan nervous system were obtained at the beginning of 20th century (Gerwerzhagen, 1913; Marcus, 1926). A lot of new data concerning bryozoan nervous system appeared in the scientific literature. It was highly promoted by the appearance of new immunohistochemical and histological techniques in nervous system investigations, and also by the appearance of CLSM. Recently, using these methods, larvae nervous system was investigated in details (Gruhl, 2009, 2010), and also a zooid nervous system was described for several marine and freshwater species (Schwaha et al., 2011; Shwaha, Wanninger, 2012).

Fig. 1. General scheme of a freshwater bryozoan zooid nervous system: a – anus, bwn – body wall nerves, cnr – circumoral nervous ring, dn – distal nerves, fn – frontal nerves, g – cerebral ganglion, lh – lophophore horns, m – mouth opening, mrn – main radial nerve. Image by K.V. Shunkina

After these investigations, a general scheme of zooid nervous system could be described as following. Central nervous system of every zooid consists of a small oval hollow ganglion, situated on the back wall of the pharynx at the lophophore base (Fig. 1). The major part of neuropile is situated on the anal side of the ganglion and a little bit displaced to its basal part. A pair of thick nerves goes from the anal side of the ganglion to the lophophore arms – these are so called lophophore “horns”. From the oral side of the ganglion the other pair of nerves are forming a circumoral nervous ring. Laterally from the both sides of the cerebral ganglion few radial nerves are innervating a group of tentacles, which are situated right on each side of the ganglion. Basally three pairs of nerves are lying in the septum between meso- and metacoel, they are descending to the oral side of the zooid and form a nervous net in the introvert body wall. The other pair of nerves, which is slightly smaller than previous, is going to the intestine, its derivations form a nervous plexus of the intestine. Some thin nerves from the basal radial nerves are also forming a part of nervous plexus.

Lophophore tentacles are innervated by radial nerves, branching from the lophophore horns and peroral nerve ring. Distally every radial nerve divides into 4 nerves, which are innervating two adjacent tentacles. In every tentacle a part of these nerve is lying laterally and form fronto-lateral tentacle nerves. The other part curves and goes to the abfrontal (outer) side of the tentacle, forming an abfrontal tentacle nerve. A part of these nerves are also placed laterally thus forming fronto-lateral and abfronto-lateral nerves. All along the radial nerve smaller proximal nerves are ascending to the adjacent tentacles. They curve a little to the frontal (inner) side of the tentacle thus forming frontal tentacle nerve.

More detailed information concerning freshwater bryozoan nervous system and its comparative characteristics for different species could be found in articles of Shunkina K.V. and coauthors (Shunkina et al., 2013 [in Russian], 2014f, 2014b, 2015).