Autozooids of the cheilostome bryozoan Aquiloniella scabra contain rod-like bacteria in the funicular bodies – the complex swellings of the funicular strands. Each funicular body contains symbionts in the central cavity surrounded by a large, synthetically active internal “sheath-cell” (bacteriocyte) and a group of the flat external cells. The tightly interdigitating lobes of these cells form a capsule well-isolated from the body cavity. Slit-like spaces between bacteria are filled with electron-dense matrix and cytoplasmic processes of various sizes and shapes (often branching) produced by the “sheath-cell”. The cell ultrastructure and complex construction of the funicular bodies as well as multiplication of the bacteria in them suggest metabolic exchange between host and symbiont, involving the nourishment of bacteria. We suggest that the bacteria, in turn, influence the bryozoan mesothelial tissue to form the funicular bodies as capsules for bacterial incubation. We present ultrastructural data, discuss possible variants in the development of the funicular bodies in Bryozoa, and propose the possible role of bacteria in the life of their bryozoan host.

Bacteriophage communities associated with humans and vertebrate animals have been extensively studied, but the data on phages living in invertebrates remain scarce. In fact, they have never been reported for most animal phyla. Our ultrastructural study showed for the first time a variety of virus-like particles (VLPs) and supposed virus-related structures inside symbiotic bacteria in two marine species from the phylum Bryozoa, the cheilostomes Bugula neritina and Paralicornia sinuosa. We also documented the effect of VLPs on bacterial hosts: we explain different bacterial ‘ultrastructural types’ detected in bryozoan tissues as stages in the gradual destruction of prokaryotic cells caused by viral multiplication during the lytic cycle. We speculate that viruses destroying bacteria regulate symbiont numbers in the bryozoan hosts, a phenomenon known in some insects. We develop two hypotheses explaining exo- and endogenous circulation of the viruses during the life-cycle of B. neritina. Finally, we compare unusual ‘sea-urchin’-like structures found in the collapsed bacteria in P. sinuosa with so-called metamorphosis associated contractile structures (MACs) formed in the cells of the marine bacterium Pseudoalteromonas luteoviolacea which are known to trigger larval metamorphosis in a polychaete worm.

23Bacteriophage communities associated with humans and vertebrate animals have been 24 extensively studied, but the data on phages living in invertebrates remain scarce. In 25 fact, they have never been reported for most animal phyla. Our ultrastructural study 26 2 showed for the first time a variety of virus-like particles (VLPs) and supposed virus-27 related structures inside symbiotic bacteria in two marine species from the phylum 28 Bryozoa, the cheilostomes Bugula neritina and Paralicornia sinuosa. We also 29 documented the effect of VLPs on bacterial hosts: we explain different bacterial 30 'ultrastructural types' detected in bryozoan tissues as stages in the gradual destruction 31 of prokaryotic cells caused by viral multiplication during the lytic cycle. We speculate 32 that viruses destroying bacteria regulate symbiont numbers in the bryozoan hosts, a 33 phenomenon known in some insects. We develop two hypotheses explaining exo-and 34 endogenous circulation of the viruses during the life-cycle of B. neritina. Finally, we 35 compare unusual 'sea-urchin'-like structures found in the collapsed bacteria in P. 36 sinuosa with so-called metamorphosis associated complexes (MACs) known to trigger 37 larval metamorphosis in a polychaete worm. 38 39 Importance 40 Complex symbiotic systems, including metazoan hosts, their bacterial symbionts and 41 bacteriophages are widely studied using vertebrate models whereas much less is 42 known about invertebrates. Our ultrastructural research revealed replication of the 43 viruses and/or activation of virus related elements in the bacterial symbionts inhabiting 44 tissues of the marine colonial invertebrates (phylum Bryozoa). The virus activity in the 45 bacterial cells that are believed to be transmitted exclusively vertically is of a special 46 importance. In addition, in the bacterial symbionts of one of the bryozoan hosts we 47 observed the massive replication of the structures seemingly related to the 48 Metamorphosis associated complexes (MAC). To our knowledge, MACs were never 49 reported in the animal prokaryotic symbionts. Our findings indicate that Bryozoa may be 50 new suitable model to study the role of bacteriophages and phage-related structures in 51 the complex symbiotic systems hosted by marine invertebrates.52 3In the marine realm, the large variety of viruses are found across diverse taxa, 78 including protists and various invertebrates such as sponges, cnidarians, flatworms, 79 polychaetes, mollusks, crustaceans and echinoderms (reviewed in Johnson