Phylactolaemata
Encyclopedia
Phylactolaemata is a class
of the phylum
Bryozoa
whose members live only in freshwater
environments. Like all bryozoans, they filter feed
by means of an extensible "crown" of ciliated tentacles called a lophophore
. They live in colonies, each of which consists of clone
s of the founding member. Unlike those of some marine bryozoans, phylactolaemate colonies consist of only one type of zooid
, the feeding forms known as autozooids. These are supported by an unmineralized "exoskeleton
" made of gelatin
ous material or protein
, secreted by the zooids.
The class contains only one extant order
, Plumatellida.
Phylactolaemate colonies reproduce sexually
, and each member is a simultaneous hermaphrodite
that functions as both male and female. They also reproduce asexually by a method that is unique among bryozoans and enables a colony's lineage to survive the variable and uncertain conditions of freshwater environments. Throughout summer and autumn they produce disc-shaped statoblasts, masses of cells that function as "survival pods" rather like the gemmules of sponges. Statoblasts form on the funiculus (cord) connected to the parent's gut, which nourishes them. As they grow, statoblasts develop protective bivalve-like
shells made of chitin
. When they mature, some types stick to the parent colony, some fall to the bottom, some contain air spaces that enable them to float, and some remain in the parent's cystid (outer casing) to re-build the colony if it dies. Statoblasts can remain dormant for considerable periods, and while dormant can survive harsh conditions such as freezing and desiccation. They can be transported across long distances by animals, floating vegetation, currents and winds. When conditions improve, the valves of the shell separate and the cells inside develop into a zooid that tries to form a new colony. A study estimated that one group of colonies in a patch 1 square metres (10.8 sq ft) produced 800,000 statoblasts.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of the phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Bryozoa
Bryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
whose members live only in freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
environments. Like all bryozoans, they filter feed
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
by means of an extensible "crown" of ciliated tentacles called a lophophore
Lophophore
The lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
. They live in colonies, each of which consists of clone
Cloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...
s of the founding member. Unlike those of some marine bryozoans, phylactolaemate colonies consist of only one type of zooid
Zooid
A zooid or zoöid is a single animal that is part of a colonial animal. The zooids can either be directly connected by tissue or share a common exoskeleton...
, the feeding forms known as autozooids. These are supported by an unmineralized "exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
" made of gelatin
Gelatin
Gelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...
ous material or protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
, secreted by the zooids.
The class contains only one extant order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, Plumatellida.
Phylactolaemate colonies reproduce sexually
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
, and each member is a simultaneous hermaphrodite
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
that functions as both male and female. They also reproduce asexually by a method that is unique among bryozoans and enables a colony's lineage to survive the variable and uncertain conditions of freshwater environments. Throughout summer and autumn they produce disc-shaped statoblasts, masses of cells that function as "survival pods" rather like the gemmules of sponges. Statoblasts form on the funiculus (cord) connected to the parent's gut, which nourishes them. As they grow, statoblasts develop protective bivalve-like
Bivalve shell
A bivalve shell is part of the body, the exoskeleton or shell, of a bivalve mollusk. In life, the shell of this class of mollusks is composed of two parts, two valves which are hinged together...
shells made of chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
. When they mature, some types stick to the parent colony, some fall to the bottom, some contain air spaces that enable them to float, and some remain in the parent's cystid (outer casing) to re-build the colony if it dies. Statoblasts can remain dormant for considerable periods, and while dormant can survive harsh conditions such as freezing and desiccation. They can be transported across long distances by animals, floating vegetation, currents and winds. When conditions improve, the valves of the shell separate and the cells inside develop into a zooid that tries to form a new colony. A study estimated that one group of colonies in a patch 1 square metres (10.8 sq ft) produced 800,000 statoblasts.