Figure 1: A sipunculan with its introvert completely retracted.
Photo credit: Alexandra Bolino

Recently in my invertebrate biology lab, I encountered an individual from the phylum Sipuncula. Sipunculans are marine organisms found in shallow waters. Their exact habitat in those shallow waters can vary from sandy burrows to coral reefs. Currently, there is some debate about whether sipunculans should be considered their own phylum, or are actually polycheates (Pechenik 2010). Sipunculans, unlike annelids, are unsegmented in their adult form and do not have setae. Some of the arguments concerning this debate reference the segmented ventral nervous system in sipunculan larvae, which, within lophotrochozoa, is associated with annelids (Kristof et al. 2008).

The first thing that I noticed about the individual that I observed was the “peanut-like” shape of its body, thus explaining its colloquial name, “the peanut worm.” (see figure 1). As I had already read about sipunculans before observing them, I was not surprised when the peanut-like shape of its trunk elongated and a narrow region about two times the length of its trunk extended  from its anterior end (see figure 2). This region is called an introvert and is used for feeding. Though I knew of the existence of the introvert, it was interesting to see how quickly the individual could fully extend and retract it. The worm was continuously extending and retracting its introvert, and though I assume that timing is variable, when I timed it, both extension and retraction took 19 seconds total.

Figure 2: A sipunculan with its introvert completely extended.
Photo credit: Alexandra Bolino.

The extension and retraction of the introvert is controlled by different muscles in the sipunculan trunk, which is divided into longitudinal muscles (Stephen et al. 1972). When the muscles in the body wall contract, the introvert extends out of the body. At this point, the sipunculan can feed using tentacles that surround the mouth, which is located at the apex of the long introvert (see figure 3). These tentacles are covered in mucus. They can be used to capture particles from surrounding water, or press into a substrate and shovel mud into the mouth (Pechenik 2010). Once food particles have been captured by the tentacles on the introvert, the introvert can be withdrawn, through the contraction of introvert retractor muscles, found in the middle of the trunk, and the particles can be ingested (Stephen et al. 1972).

Figure 3: The end of the introvert.
Photo credit: Alexandra Bolino

From the characteristic bands of pigmentation along the body, my lab partner and I were able to determine the worm was Phascolosoma agassizi, the most abundant peanut worm on the California coast (Cowles 2002). The trunk was covered in dark spots, and the introvert, a lined pattern of the same color. Though none of the texts I checked speculated on a reason for this pigmentation, I hypothesize that because this particular species is found under rocks and in sediment, this pigmentation may be useful in camouflage because it breaks up the otherwise peach color of the individual.

References:

Cowles D (2002). Phascolosoma agassizii. Keferstein, 1867. (Retrieved February 16, 2014, from http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Sipuncula/Phascolosoma_agassizii.html)

Kristof A, Wollesen T, Wanninger, A. 2008. Segmental mode of neural patterning in Sipuncula. Current Biology 18: 1129-1132.  DOI: 10.1016/j.cub.2008.06.066 2008

Pechenik JA (2010). The Sipunculans. pp. 314-318. Biology of invertebrates, 6^th Ed. McGraw-Hill (Ed.). New York, NY.

Stephen, A.C., & Edmonds S.J. (1972). The phyla Sipuncula and Echiura. pp. 528. British Museum of Natural History