This sipunculid (peanut worm) pelagosphera larva was found in an open ocean plankton sample that was collected during our class boat trip on April 17, 2012 – a few miles south of the mouth of Coos Bay, OR. Pelagosphera larva is found only in the phylum Sipunculida. They develop from trochophore larvae and can be lecithotrophoic or planktotrophic. I have kept this larva for a few weeks and fed it unicellular algae Dunaliella tertiolecta and Rhodomonas lens. Since there is only one known species of peanut worm that has a long-lived, planktotrophic pelagosphera larva in the Pacific Northwest, this larva likely belongs to Phascolosoma agassizii.
The body of this pelagosphera larva can be divided into four regions: the head, the mid-region that includes the metatrochal ciliary band, the trunk, and the terminal organ. The head, mid-region and terminal organ can be simultaneously retracted into the trunk (left) when the larva is disturbed. A cuticle covers the epidermis of the entire larval trunk. This pelagosphera has numerous cuticular papillae covering the entire trunk. The digestive system can be seen through the cuticle and is indicated by the green hue in the esophagus, stomach and intestine.
The second picture (left) shows the pelagosphera with extended head, and the metatroch, which these larvae use to swim. The third picture shows the larvae with the head and lip extended. The head is covered by cilia on the ventral surface and divided into two ventral lobes by a ciliated groove that leads into the mouth. The lip is just below the head and is typically covered with cilia on the medial and proximal surfaces. The lip along with lip glands, gland cells of the oral cilature and buccal organ are believed to be involved in feeding.
The terminal organ (not shown) is usually retracted when the larva is swimming or inactive. The primary function of the terminal organ seems to be attachment to the substratum. I was not able to take a picture with the terminal organ of the pelagosphera visible.
Jaeckle, W & Rice, M. 2002. Phylum Sipuncula. In: Atlas of Marine Invertebrate Larvae. Edited by Craig M Young. Academic Press.
Johnson, K. 2001. Sipuncula: The Peanut Worms. In: An Identification Guide to the Larval Marine Invertebrates of the Pacific Northwest. Edited by Alan Shanks. OSU Press, Corvallis.