Adults of Sea Mats are so called as they create flat, dense colonies on seaweeds and rocks. Each individual secrets a tough "cell" in which the tiny creature lives, glued to its surrounding neighbours, clones of the original juvenile to settle there. They produce huge numbers of larvae with the most common type called a cyphonautes. These are liberated into the plankton and can spend several weeks growing and dispersing. This is why they are the most abundant larval form to be seen, sometimes in huge densities. 

​

A selection of cyphonautes are shown here including one that is lablled to show the various organs and the flow of water through the larva (blue arrow). The shape is a flattened triangle. They can be seen at most times of the year but some months, e.g. November and March, they can be especially common.