High-density Housing (for Bryozoans)

This beautifully geometric piece of natural architecture is produced by the animal commonly called the sea-mat and scientifically known as a bryozoan (which means 'moss-animal'). This particular species is Membranipora membranacea and I picked it up yesterday, in a rock-pool on the seashore at Seaburn, near Sunderland.

Membranipora forms extensive, fast-growing colonies resembling a lacy mat, that spread over the surface of kelp fronds. It's common on all coasts around Britain and has been introduced into shallow seas in other parts of the world where there is some concern that its rapid rate of growth could suppress the reproduction of some marine algae.

Each calcareous compartment, secreted by the animal inside, contains an individual animal (zooid) that extends a feeding arm called a lophophore, for filtering out plankton.

The zooids retract into a tube within their walled enclosure at the slighest hint of danger but when they're all extended they resemble a garden of transparent flowers, gently waving their arms..

Each walled enclosure has a small tower at the junction with its neighbours' walls and ......

....there are often gaps in the walls, which give the colony a degree of flexibility as the supporting kelp frond bends in the sea currents. The gaps tend to be most conspicuous in older sections of the colony.

There are at least two kinds of zooid - the flower-shaped feeding lophophores and these translucent cylindrical forms.

I'm not sure what their function is but my guess is that they provide additional surface area for oxygen uptake or maybe waste disposal.

You can find out more about bryozoans here or take a look for yourself - these are low-power micrographs (maximum magnification x50) but you can see the living zooids with a hand lens if you put a piece of the colony in a shallow dish of seawater. You can find them all year-round and fine specimens are often attached to kelps that are washed up on beaches after storms.

A Seaweed Microcosm....

If you want to explore exotic marine life in shallow seas you could jet off to a warm climate and scuba dive over a coral reef.......or you could just nip along to your nearest stretch of coastline (in our case Whitburn, near the mouth of the River Wear at Sunderland), collect a few small pieces of red seaweed and some seawater, take it home and examine it under the microscope.

This (below) is the piece of seaweed in question, floating in a rockpool.......

....and these (below) are just a few of the animals that I found living in it...

..First to break cover were these little crustaceans called isopods (which literally means 'equal legs' - all their legs are the same length - woodlice are terrestrial isopods). These are highly active little detritus feeders, breathing through gills at their tail end, and belong to a genus called Idotea..

..and they were swiftly followed by this little amphipod (meaning legs of two distinct lengths, long ones at the front, shorter at the back) which emerged from the waving weed fronds. Note the exquisite eyes of these little shrimp-like animals, known as gammarids...(more of those eyes in a future blog).....Whereas isopods tend to be flattened dorsiventrally (i.e. top-to-bottom), amphipods tend to be flatted laterally (side-to-side).

It soon became apparent that the thicker parts of the seaweed were covered with colonies of another phylum of animals called bryozoans (literally 'moss-animals'). These live colonially, interconnected, in little calcareous compartments. In the case of this species, each individual's shell was performated with holes, like an exquisite microscopic ceramic vase. The magnification used here is roughly x50Bryozoans (I haven't identified this species for certain yet, but I think it's Electra pilosa) feed by waving a tentacled arm called a lophophore, that looks a little like an old-fashioned wire egg whisk.

You can see extended lophophores (rather indistinctly, I'm afraid) in the following couple of photos.......

 ....The third phylum of animal to put in an appearance under the microscope (so far we've had crustaceans and bryozoans) was this exquite little sea slug, known as a nudibranch, which belongs to a genus called Eubranchus. Fully extended, this was about 3mm. long - a juvenile, that will probably grow to five or six times this size. Nudibranches are carnivores and it may well have been feeding on the lophophores of some of those bryozoans, although they typically feed on hydroid colonies (more about them in a future post). The back of this nudibranch is covered with strange, skittle shaped objects that wobble from side-to-side as it glides through the water. They're called cerata and are for gas exchange (nudibranch means 'naked gills' and that, in effect, is what these are). Remarkably, some species of nudibranch that feed on hydroids that are armed with stinging nematocysts (for more on nematocysts, see http://cabinetofcuriosities-greenfingers.blogspot.com/2009/09/flower-animal.html) can incorporate the nematocysts of their prey into the body wall of their own cerata, to protect themselves. Nudibranches detect their prey using incredibly sensitive organs called rhinophores, which are the top pair of tentacles at the head end. The pictures below are all of the same animal, but the lighting varies.

So there you have it.........a whole community of weird and wonderful microscopic animals living in a single frond of red seaweed in a rockpool. I spent a couple of very enjoyable hours photographing these but I've not doubt that I could have spent another day, extracting more microscopic marine life, before I exhausted the possibilities of this microcosm. There's a short video of the nudibranch on a separate post, above this one.You can find out more about all of these animals at http://www.marlin.ac.uk/species.php