Polychaetes are common marine annelids (5300 species) with paddle-like appendages or parapodia bearing numerous bristles or chaetae and often gills. They have a distinct head region, often with sensory tentacles and simple eyes. Polychaetes are dioecious – each one produces only male or female gametes. Fertilisation of eggs occurs typically at swarming time when mature adults of a species arise synchronously to the sea’s surface and release their gametes. Fertilised eggs develop into trochophore larvae, which spend some time swimming in the plankton before maturing into bottom living adults.