Bryozoa, also known as moss animals, are a diverse group of aquatic invertebrates that live in colonies of tiny individuals called zooids. They are found in freshwater, brackish and marine habitats all over the world, and have a fossil record dating back to the Ordovician period. Bryozoa are fascinating creatures that have a unique feeding structure called a lophophore, a ring of ciliated tentacles that filter food particles from the water. They also have a variety of colony forms and specialized zooids that perform different functions such as reproduction, defense and attachment. In this article, we will explore some of the most interesting aspects of bryozoa biology and ecology.
What are Bryozoa?
Bryozoa are members of the phylum Bryozoa (or Ectoprocta), which contains over 5,000 living species and 15,000 extinct ones . They are coelomate animals that have a body wall (cystid) that secretes an exoskeleton (cuticle) of organic or mineralized material. The exoskeleton can be soft or hard, transparent or pigmented, smooth or ornamented. Inside the cystid, there is a retractable organ system (polypide) that contains the mouth, gut, nervous system and reproductive organs. The mouth is surrounded by a lophophore, which is used for feeding and respiration. The lophophore can have different shapes and sizes depending on the species, but it always consists of a crown of tentacles that bear cilia. The cilia create water currents that bring food particles to the mouth, where they are ingested and digested in the U-shaped gut. The anus is located outside the lophophore, near the base of the zooid.
Bryozoa are almost exclusively colonial animals, meaning that they live in groups of genetically identical or related zooids that share a common exoskeleton and are connected by tissue or fluid. The only exception is Monobryozoon ambulans, a solitary species that lives in Antarctic waters. Colonies can vary in size, shape and complexity depending on the species and environmental conditions. Some colonies are encrusting, forming thin sheets or patches on hard substrates. Others are erect or prostrate, forming branches, fans, trees or discs. Some colonies are gelatinous or membranous, lacking a rigid exoskeleton. Some colonies can even move slowly by using spiny defensive zooids as legs.
Bryozoa have three main classes: Phylactolaemata, Stenolaemata and Gymnolaemata. Phylactolaemata contains about 50 freshwater species that have soft exoskeletons and lophophores with horseshoe-shaped tentacles. Stenolaemata contains about 900 marine species that have mineralized exoskeletons (usually calcareous) and lophophores with cylindrical tentacles. Gymnolaemata contains about 4,000 marine species (and a few brackish ones) that have organic or mineralized exoskeletons (usually chitinous) and lophophores with flattened tentacles. Gymnolaemata includes two orders: Ctenostomata and Cheilostomata. Ctenostomata have soft exoskeletons and simple colony forms. Cheilostomata have hard exoskeletons (usually calcareous) and complex colony forms with various specialized zooids. Cheilostomata is the most diverse and abundant order of bryozoans in modern seas.
How do Bryozoa reproduce?
Bryozoa can reproduce both asexually and sexually. Asexual reproduction occurs by budding, which is the formation of new zooids from existing ones. Budding can occur at different rates and patterns depending on the species and environmental conditions. Some species can bud continuously throughout their lives, while others can bud seasonally or sporadically. Some species can bud only from certain parts of the colony, while others can bud from any part.