What Is a Bryozoan? Colonies of Tiny Animals Mistaken for Plants and Corals
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Lacy crusts and branching fronds on shells, rocks and seaweed are colonies of microscopic animals, each in its own box, connected and cooperating. They are abundant as fossils, common in living seas, and almost entirely unknown to people who are not specialists.
The animal and the colony
Each individual is under a millimetre across and lives in a small chamber it secretes, usually of calcium carbonate though some groups build flexible chambers instead. It feeds by extending a crown of ciliated tentacles that drives a current of water past itself and captures suspended particles, and it withdraws that crown rapidly into the chamber when disturbed. A colony arises by budding from a single settled larva, so all members are genetically identical, and the colony grows as an encrusting sheet, a branching structure, a fan or a free-living disc depending on the group. Individuals within a colony are connected by tissue and share resources. Some members are specialised for tasks other than feeding, including defence, cleaning the colony surface and reproduction, which makes the colony closer to a single organism with differentiated parts than to a collection of animals living adjacently.
Why they matter to geology
The group has a fossil record stretching back nearly half a billion years and is useful in several ways:
- •Abundance, since some limestone units are built substantially from their skeletons
- •Reef building, since they contributed to reef frameworks in several periods, sometimes as primary builders
- •Stratigraphy, since some groups evolved rapidly and serve to date and correlate rocks
- •Palaeoenvironment, since colony shape responds strongly to water energy, depth and sediment, so growth form indicates conditions
- •Growth banding, since colonies record seasonal change and can supply temperature information
- •Preservation quality, since the carbonate skeleton survives well and retains fine detail that allows species identification
The identification problem
They are confused with other things constantly and the confusions are instructive. Encrusting colonies on shells and stones look like lace or like a fine mesh, and are frequently taken for coral, which is a different group entirely with a different feeding structure and a different relationship between the individual and the skeleton. Branching colonies look like seaweed and several species have common names describing them as such, with one flexible form washing up in quantity and being known as hornwrack. Freshwater species form gelatinous masses that are routinely reported as unidentified blobs and cause alarm in reservoirs and water systems. The group also resembles the unrelated entoprocts, which share a superficial similarity and differ fundamentally in anatomy, and that resemblance caused decades of classification argument. Their small size means most people who have handled them, on beaches or on the undersides of rocks, did not know what they were looking at.
Living with them
The group has practical significance beyond the fossil record. Encrusting species are a major component of marine biofouling, settling on hulls, pipes, buoys and aquaculture equipment, where they add weight, increase drag and require removal, which makes them a persistent cost to shipping and to coastal infrastructure. Freshwater species block intake pipes at water treatment plants and power stations, and the gelatinous colonies are large enough to obstruct screens. Several species have spread well outside their original ranges in ballast water and on hulls, and one introduced form has caused substantial damage to kelp beds by encrusting the fronds so heavily that they become brittle and break. On the other side, their sensitivity to water conditions makes them useful indicators of pollution and of habitat quality, and their skeletons preserve chemical records used to reconstruct recent environmental change in coastal waters.
How colonies compete
Encrusting species live on surfaces that are always in short supply, which makes competition for space the dominant pressure and produces behaviour that is unexpectedly aggressive for animals of this kind. Colonies growing towards each other meet along a boundary and one may overgrow the other, and the outcomes are consistent enough that competitive hierarchies between species have been mapped, with the results showing intransitive relationships in which one species beats another that beats a third that beats the first. That pattern matters to ecology generally, since it allows many species to coexist rather than one winning outright. Some colonies raise their growing edges, develop spines or deploy specialised defensive members against neighbours and against predators. Chemical defences are also present, and the compounds produced by some species have attracted pharmaceutical interest, with one anticancer compound originating in a marine bryozoan and requiring synthetic production because the natural yield was impossibly small.
The takeaway
Each individual is under a millimetre in its own secreted box, feeding with a crown of ciliated tentacles, and a colony grows by budding from one larva so its members are clones. Some members are specialised for defence and cleaning, which makes the colony closer to one organism than to a group. They are mistaken for coral, seaweed and unidentified blobs, and they compete for space by overgrowing each other.