Seashells

Seashells are a typically used as decoration by all Syreni and are therefore not unexpected when worn on the body. Many different clubs, organizations etc. use them for more than just that though. Specific shells, worn in a specific way can be used as a signal between members of a group. But because there are so many groups (it can be just a group of friends or an investigative unit) who are using different signals it is not uncommon for Syreni to misinterpret a collection of shells worn by another Syreni.
And although they are supposed to be covert, there are some signals that became popular and known and used outside of the ingroup as well.
Like for example a closed New Zealander cockle worn at the centre of the back by homosexual Syreni signals that you want receive bareback. The left-handed turrid worn on the centre of the back can signal that a Syreni wants to give bareback if the left-handed turrid is worn in a way that it's mouth isn't visible, or it can signal that they are open to giving or receiving bareback if the mouth of the shell is visible. In the city, some heterosexual Syreni have started to use it as well, mostly females and some males but only if they are in a situation/place where their sexual orientation is clearly marked. And If someone wears a magical cone on the front of their left shoulder with the opening showing, it means that they are searching to buy Ziconotide. If they are worn with the opening hidden, it signals that they are selling (again on the front of the left shoulder). The same goes for the feathered cone which signals that one wants to either buy Nitrophoria or sell it.
All of this is called "shelling", a word generally equated to "signalling" or "trapping".