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Wednesday, February 15, 2012

Pretty in pink

Many people haven't heard of bryozoans, but when viewed up close, they're one of the most attractive and captivating local animals. Bryozoans (or "moss animals") are colonial invertebrates that live on hard substrates (such as rocks and shells) or seaweeds.  Most bryozoans are marine, but a few live in fresh water.  

All of the bryozoans occurring on Bodega Head are marine.  One of the most beautiful is Eurystomella bilabiata.  Their encrusting colonies are pink or rose-red, and may be 5 cm or more across.  Members of the colony are very small, ~0.6 mm long.

A distinctive feature of bryozoans is the lophophore — a circle of ciliated tentacles used for feeding.  The cilia create downward currents that bring water and suspended particles towards the mouth.  Some bryozoans also use tentacle-flicking to direct potential food items inward.

The lophophore is retractable and when withdrawn, the opening is covered by an operculum (or trapdoor).  It looks like a little red hatch in the photo below.

When all of the colony members are withdrawn, the bryozoan looks like a hard crust with the bell-shaped trapdoors creating a very regular pattern.  (To a tidepool explorer's eye, a Eurystomella colony will appear like a relatively small, thin, rough pink patch on the rocks.)

The bryozoan above is joined by a few other animals — a pink chiton at the lower right (Tonicella sp.), a few small brown limpets (left side), a spirorbid tube worm with orange tentacles (upper center), and a barnacle (above the tube worm).  They are all living with encrusting coralline algae on a rock.

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