Fan worm larva

On May 10th, 2012 I waded through the calm waters of Middle Cove at Cape Arago State Park, trailing a 153μm plankton net behind me.  While sorting through the plankton I found a polychaete larva from the family Serpulidae, subfamily Spirorbinae.  Serpulids belong to an order of annelids, Sabellida, commonly known as fan worms.  Adult Spirorbins live in small calcareous tubes approximately 2-5 mm in diameter typically attached to undersides of intertidal rocks. Adults are hermaphroditic and brood embryos inside their tube, then release lecithotrophic larvae that are only briefly planktonic (Strathmann 1987).

This metatrochophore larva has a prominent ciliated band anterior to the mouth, called the prototroch, which is used for swimming. At the anterior (up) is the apical tuft, and at the posterior end there is another cilary band, called telotroch. What helped us to identify this larva is the prominent collar located posterior to the prototroch (Kupriyanova et al. 2001). The collar is the widest portion of the larval body. This larva had  5-6 segments posterior to the collar, and three pairs of ocelli. 

To my surprise, when I went to photograph the larva a few days later, I could not find it at first.  No longer content with the planktonic life, it had settled on the bottom of the culture dish and built a calcareous tube (left). The tube is smooth, non transparent, and coils to the right (dextral). The characteristics of the tube helped us to further classify this specimen as likely belonging to the genus Circeis (Blake and Ruff 2007). 

You can see on the subsequent photograph that it got quite a bit longer in 4 days. The juvenile worm is caught peaking out of it’s tube in the first picture. One can discern a radiolar crown of tentacles used for feeding and respiration (Blake and Ruff 2007), red eyespots, and an operculum.  When disturbed the animal quickly retreats back into its tube, and shuts the operculum (below).

Blake JA, and Ruff RE.  2007. Polychaeta.  In: The Light and Smith Manual:  Intertidal Invertebrates from Central California to Oregon 4th Edition.  Edited by James Carlton. University of California Press, Berkeley.  

Kupriyanova, E.K., E. Nishi, H.A. ten Hove, & A.V. Rzhavsky 2001. A review of life history in serpulimorph polychaetes: ecological and evolutionary perspectives. Oceanography and Marine Biology: an Annual Review 39:1-101.

Strathmann, Megumi F. 1987.  Phylum Annelida, class Polychaeta. In: Reproduction and Development of Marine Invertebrates of the Northern Pacific Coast.  United States: University of Washington Press.