(Text by kind permission of Dr Cris Little)
(SEM Images by kind permission ofThe Natural History Museum Picture Library)
A whole phylum of marine animals often overlooked by UK divers are the bryozoans. Bryozoans are actually very abundant and together with hydroids make up a substantial proportion of the brown “turf” found covering rocks below the kelp line at many diving locations. In fact they are often mistaken for algae or hydroids.
Perhaps this is because bryozoan colonies are typically small. Most are less than 5cm high. However, as a group they are actually very important in marine ecosystems, filtering a substantial quantity of seawater and forming living habitats and/or food for numerous other animals.
Bryozoans also have an economic importance as fouling organisms on pipelines and ship hulls. One species called Bugula neritina is the source of a potential anti-cancer drug called “bryostatin” currently undergoing clinical trials.
The name bryozoa comes from the Greek bryo meaning “moss” and zoan meaning “animal”. Bryozoans are actually highly complex colonial animals constructed of individual zooids that are all connected by living tissue. The zooids are protected within a cup or box shaped exoskeleton of calcified chitinous tissue. The degree of calcification governs how rigid or flexible is the overall structure.
Bryozoans are eaten by a variety of grazing animals, such as sea urchins and various molluscs. The latter includes nudibranchs, some of which feed only on a particular species of bryozoan. For example, the yellow and white nudibranch Polycera quadrilineata only feeds on a common bryozoan called the “common sea mat” Membranipora membranacea which encrusts kelp fronds. Towards the end of the summer you can see massive orgies of these pretty nudibranchs busy mating and laying spirals of white eggs next to their sea mat prey.
Whilst many bryozoans are inconspicuous, one species that many divers would recognise is the cauliflower-like ross coral, Pentapora foliacea, mainly because of its size - some colonies grow up to one metre in diameter. The coral part is a misnomer. In years gone by ross coral was presumably confused with true corals because it was hard to the touch, or perhaps the term coral had a wider definition in the past.
Being very brittle ross corals are easily damaged. A slow rate of growth means that once damaged a ross coral will take many years to re-grow. Like true corals on coral reefs, damage by divers or boat anchors can be devastating.
Ross coral is not the only bryozoan that has a misleading common name. The bryozoan Flustra foliacea is called “hornwrack” i.e. hard seaweed, perhaps from the days before the differences between marine plants and colonial, plant-like animals were fully appreciated. Don't laugh too hard, there are still people out there who think a whale is a large fish! Hornwrack has a free-standing branching structure with flat rounded lobes.
Different bryozoan species have skeletons calcified to varying extents, so some bryozoan colonies like ross coral form hard structures while others are more jelly-like in texture.
Many of the less calcified bryozoans, like the common sea mat, live attached to seaweed in the intertidal and shallow subtidal areas. The more calcified ones tend to live in sheltered environments or deeper water, especially the more delicate species.
Amongst the kelp roots and below the kelp line you can find fine tufts and spirals of bryozoans such as Bugula turbinata, Bugula plumosa and Cellaria fistulosa. These are less calcified than ross coral and move gently with current and wave surge. Indeed, they are particularly common in areas of high current flow. With a leafy appearance and brown coloration it is easy to see how these sorts of bryozoans can be mistaken for algae. But look carefully and you will see the tentaculate zooids that confirm their animal nature.
If you look closely at bryozoan colonies you will see that they often have beautiful exoskeletal architecture. Some of these exoskeleton features are illustrated here using pictures from a low power scanning electron microscope (SEM), but any diver can also see them whilst underwater using a x15 hand lens.
Many of the 5000 or so bryozoan species exhibit polymorphism. In plain English this means that the within a colony there are several types of zooids, each with a different function reflected in its different shape.
The basic zooid is the feeding zooid, called the autozooid. This has a feeding structure called the lophophore that is covered in ciliated tentacles and looks somewhat like extended barnacle arms. The lophophore is protruded into the seawater, and beating of the cilia creates a feeding cureent which draws plankton towards the mouth. When danger threatens the lophophore can be pulled rapidly back into the safety of the zooidal skeleton by a muscle which is said to be the fastest contracting muscle in the animal kingdom.
Each autozooid has a mouth, a U-shaped gut and anus that discharges outside the ring of feeding tentacles. Hence bryozoans are also known as Ectoprocta. In some bryozoan groups the lophophore is further protected when not extended by a small trap-door structure called the operculum which seals the opening of the zooid.
The lophophore can't be seen on these SEM pictures as they have been prepared with bleach for imaging, but the box-like skeletons of the autozooids stand out well and some of the pictures also show the operculum.
Other polymorphic zooids found in bryozoans include: gonozooids, large, non-feeding zooids which brood the larvae before their release; avicularia, defensive zooids in which the operculum is modified into a jaw-like structure; and vibracula, zooids with an operculum modified into a bristle-shaped structure seemingly used for brushing sediment and fouling organisms off the surface of the colony. Even the spines present in bryozoans such as Flustra foliacea may be polymorphic zooids.
You can see all of these zooid types in the SEM pictures. But don't stop there, take a cheap plastic hand magnifier diving and you can see these intricate structures at most UK dive sites. It will add a whole new dimension to your diving. So little is known about the ecology and distribution of bryozoans that many opportunities exist for furthering our understanding of this fascinating group of animals.