An animal that cannot be seen. One head sits at the base of a living sponge and the body branches repeatedly through the host's internal canals, filling them like a root system, until a single worm ends in hundreds of tails - each capable of reaching an opening in the sponge surface.
Branching bodies are almost unknown among animals: this genus and one relative are the only segmented worms built that way, and how a nervous system and a gut cope with repeated forking is still being worked out. The worm appears to take food from water passing through the sponge and does the host no obvious harm. For reproduction it grows specialised swimming segments at the tips of branches, packed with eggs or sperm, which detach and swim off on their own. Nothing about it is visible on a dive, which is precisely why it belongs in a catalogue: the reef contains animals that no amount of looking will show you.
- never seen whole - only tail ends showing at sponge openings
- one head buried at the sponge base, hundreds of branching ends
- pale, thread-thin body inside the host's canals
- found only by cutting a sponge open
