How the Worm Turns (Into a Worm)

Those of you who have suffered through some of my posts on turrids may recall me discussing the subject of how differences in the mode of development of marine organisms relate to their classification. Features that were once considered of high significance are affected by whether the animal develops as a free-swimming larva or is nourished by a yolk supply provided in the egg, and may change more readily than previously thought. And indeed, it turns out that there are some cases where both developmental modes can be found in a single species.

Boccardia polybranchia, from here.

Boccardia is a genus of twenty-odd species of marine worm belonging to the family Spionidae. These are sedentary worms, living in tubes that they construct for themselves out of sediment bound together by mucus, or that they bore into substrates such as mollusc shells or coral. Boccardia and other spionids have a pair of long palps extending from the head that they use for feeding, sweeping them around to gather up detritus and such. Boccardia differs from other genera in the Spionidae in having branchiae (vascularised appendages that function as gills) starting on the second segment of the body, and two differentiated spine rows on the fifth segment with falcate spines in the upper row and bristle-tipped spines in the lower row (Williams 2001).

One of the best-studied Boccardia species is B. proboscidea, a species about one or two centimetres in length found around various parts of the Pacific, including along the western coast of North America. Boccardia proboscidea is very catholic in its habitat preferences: it can be found in the intertidal or shallow subtidal zones, and anywhere from mudflats to rubble to reefs to burrowed into the shells used by hermit crabs (Gibson et al. 1999). It also shows the aforementioned variation in larval development: some individuals hatch as small larvae and live and feed as plankton, others feed on the yolks from nurse eggs and don't hatch until they reach a more advanced stage of development. Whichever way the individual develops, the resulting adult seems to be more or less the same.

Nevertheless, it would be fair to wonder if this variation is as it appears. Combine the variation in development with the variation in habits, and you might wonder whether two or more morphologically similar species are being confused. However, not only are the adults of each larval type completely interfertile, but differently developing individuals may even come from a single egg case. Gibson et al. (1999) compared individuals of this species from two widely separated populations both morphologically and genetically, and found that while there were some differences between the populations, there was little or no difference between developmentally distinct individuals within each population. How and why this developmental variation is maintained seems to be an open question but there is some evidence that other spionids may show the same plasticity. After all, it doesn't matter how you get there, so long as you get there.

REFERENCES

Gibson, G., I. G. Paterson, H. Taylor & B. Woolridge. 1999. Molecular and morphological evidence of a single species, Boccardia proboscidea (Polychaeta: Spionidae), with multiple development modes. Marine Biology 134: 743–751.

Williams, J. D. 2001. Polydora and related genera associated with hermit crabs from the Indo-West Pacific (Polychaeta: Spionidae), with descriptions of two new species and a second polydorid egg predator of hermit crabs. Pacific Science 55 (4): 429-465.