Andrew Ostrovsky is Professor of the Department of Invertebrate Zoology at the St Petersburg University. Together with his colleagues from the University of Vienna, he has analyzed which evolutionary innovations helped the bryozoans to survive on Earth for 490 million years. This is longer than many other animal groups. The many secrets of these tiny aquatic colonial invertebrates include: embryo incubation; ‘friendly relationships’ with bacteria; ‘mobile’ colonies; and protective colonial ‘organs’.
The research findings have been published in the Biological Reviews journal.
One of the biggest challenges in evolutionary biology is understanding of the causes of the evolutionary success and taxonomic diversity in living organisms. Why did some species that had existed on our planet for a long time become extinct; while others, sometimes very ancient, survive? If such ‘longevity’ is a measure of evolutionary success, then what enables it? The researchers from the St Petersburg University and the University of Vienna were Professor Andrew Ostrovsky, Dr Thomas Schwaha, and Professor Andreas Wanninger. They studied bryozoans to find answers to these questions. Bryozoa is a phylum of aquatic organisms that arose at the very beginning of the Ordovician period (490 million years ago), or, possibly, earlier.
Some bryozoan colonies look like small corals, although these grouos are non-related. These filter feeders have survived three so-called evolutionary radiations, that are periods of rapid increase in taxonomic diversity. So far, more than 24,000 fossilised forms and more than 6,000 living bryozoan species have been described. In aquatic habitats, predominantly marine, bryozoans may be found on all types of bottom substrates: mobile and immobile, hard and soft, organic and inorganic. Some species have adapted to live at depths of 8,200 metres; most bryozoans, however, inhabit much shallower waters. Having colonised freshwater habitats, Bryozoa became an example of an extremely successful group. In taxonomic diversity and abundance in ancient and modern bottom assemblages, they concede only to sponges and cnidarians. Interestingly, these three groups of colonial invertebrates together provide shelter, food and brooding grounds for a large number of other aquatic animals, thus performing the function of plants in terrestrial ecosystems.
‘The evolution of Bryozoa can be described using a slogan “If you can’t escape, defend yourself!”’ said Andrew Ostrovsky. ‘I will mention only a few of their key evolutionary novelties. The first to mention is the acquisition of a protective calcareous exoskeleton that evolved in Bryozoa independently at least two or, possibly, three times. The second were protective frontal shields of various morphologies. The third evolutionary innovation is matrotrophic embryonic brooding in the form of incubation of embryos in protective brood chambers, supported by extraembryonic nourishment. The fourth evolutionary change is the defensive symbiosis with bacteria: their toxic metabolites enable bryozoan larvae to survive, making them inedible for the attacking fish larvae.’
Additionally, the list of the evolutionary innovations includes the appearance of various protective colonial ‘organs’. These help colonies to remove sediment from the surfaces and to defend themselves from micropredators. Some bryozoan species do not grow on solid substrates, but form their colonies on non-stable silty or sandy bottom. Others inhabit shells of some molluscs, having learned to drill holes in them. Another indicator of their evolutionary success must be the diversity of reproductive patterns. These include the transition from long-living feeding larvae to short-living non-feeding ones. The latter show much higher morphological variability. Non-feeding larvae are always incubated and, together with incubation chambers, are believed to have evolved multiple times from a feeding larva. Besides, it must be mentioned that placentation in bryozoans evolved independently multiple times. This is also true for the appearance of various methods of asexual development based on regeneration. The latter include one of the rarest phenomena – polyembryony. This is when a fertilised egg gives rise to the primary embryo that buds more than a hundred larvae.
The study has been supported by grants from St Petersburg University, the Russian Foundation for Basic Research (Grant No 16-04-00243-a), the Russian Science Foundation (Grant No 18‐14‐00086) and the Austrian Science Fund (projects numbers: P 32088‐B, P22696‐B17, P27933‐B29).
It should be emphasised that during their very long evolution, bryozoans ‘invented’ many different forms of colonies. In fact, there is no such a variety in any other group of colonial animals. Moreover, the evolutionary ‘experiments’ of bryozoans with their colony construction resulted in the evolution of unique motile colonies that can ‘walk’ or crawl along the bottom.
‘The mentioned diversity is only part of the innovations that Bryozoa acquired during their evolution. These evolutionary novelties enabled bryozoans to survive during the major global disasters. Effectively competing with larger neighbours-epibionts and resisting predators, they have remained an extremely successful, diverse and environmentally important component of most benthic ecosystems of our planet,’ stressed Andrew Ostrovsky.