Serpulids are sedentary animals and develop, not only a calcium based tube, but also, a cartilaginous plug on the dorsal most plume radiole called an operculum. This completes the impressive fortress which the serpulid creates for itself. The sabellid worm is different in that its soft papery tube collapses on itself when it retreats, effectively closing out most predators. Like most other animals who spend their lives anchored to a position, the serpulid worm employs a broadcast spawning strategy. This is when eggs and sperm are, simultaneously, released into the water column where fertilization happens. Shortly after fertilization, free-swimming larvae becomes a part of the ocean's great and diverse community of zooplankton. The length of time it remains planktonic varies between several days to two months. Serpulid blood is unusual because it contains two oxygen transporting molecules: hemoglobin and chlorocruorin.