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Department of Zoology, Michigan State University, East Lansing, Michigan 48824
Capitella sp. I, previously considered part of the Capitella capitata complex (1), is a small polychaete annelid that can be maintained in culture with ease (J.P. Grassle, Institute of Marine and Coastal Sciences, Rutgers University, pers. comm.). Lecithotrophic eggs are deposited in a maternal brood tube and can be readily harvested at different stages of development. Larvae emerge from the brood tube after approximately 8 days as many-segmented metatrochophores, each bearing a prototroch and telotroch, the classic trochophore stage being bypassed. The free-swimming metatrochophores are non-feeding and are competent to settle and metamorphose within a few hours of emergence. Metamorphosis in this species is not morphologically dramatic, but includes a pronounced elongation, loss of trochal bands and accompanying locomotory changes, and a transition from non-feeding to feeding. Postmetamorphic growth involves a general enlargement of the worm and addition of segments in a growth zone immediately anterior to the terminal pygidium.
The development of muscle patterns in soft-bodied bilaterian animals is not well understood, with most recent information coming from investigations of acoelomate flatworms (2,3) and the medicinal leech, a derived annelid (4,5). Segmentation between annelids and arthropods has traditionally been considered to be homologous; however, the recent assignment of annelids to the Lophotrochozoa and arthropods to the Ecdysozoa, brings this homology into question. A study of muscle development in a more ancestral annelid would be useful in furthering our understanding of the ontogenetic and evolutionary origins of segmentation, as well as the cellular interactions involved in muscle patterning and innervation during embryogenesis. To this end we are investigating early muscle development in Capitella sp. I. Staged embryos were removed from the brood tube and labeled with rhodamine-phalloidin following the procedure used by Reiter et al. (2) to detect actin filaments in developing muscle of turbellarian flatworms. Specimens were observed with an Olympus BX60 fluorescence microscope and imaged using an Olympus Magnifire digital camera (model S99860).
Muscle development proceeds from anterior to posterior. As the stomodeum forms, a ventral arc of muscle becomes apparent in the lower lip. Phalloidin binding continues dorsally until the mouth is surrounded (Fig. 1a). Approximately three days after fertilization, longitudinal muscles of the body wall begin to form. Initially eight longitudinal muscles appear in the following sequence: (1) four dorsal strands that will reach from the prostomium to the pygidium begin to develop; (2) two lateroventral muscles form at the lateral edges of the stomodeum (Fig. 1a), then come together at the apex of the prostomium (Fig. 1b); (3) these two lateroventral muscles also grow posteriorly, extending ventrally from the stomodeum into the pygidium (Fig. 1b); (4) medially, a second pair of midventral muscles (Fig. 1b) grows posteriorly from the stomodeum. Subsequently two additional lateral muscles form (Fig. 1b). Longitudinal muscles initially appear as thread-like single strands which thicken as more strands are added.
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As development continues, both longitudinal and circular muscles become more massive with the addition of more strands. Circular muscle formation continues posteriorly (Fig. 1c), filling the gap between the developing circular muscles and telotrochal bands. The number of telotrochal bands also increases.
During larval development additional muscles—longitudinal, intrasegmental, oblique, setal sac fibers, etc.—are added so that the muscle pattern in newly emerged metatrochophores is very complex (Fig. 1d). Since metamorphosis occurs without major structural changes, these larval muscles form the basis of the adult musculature.
Mesoderm formation in polychaetes is attributed to two teloblasts, derivatives of the 4d blastomere, which reside between the posterior ectoderm and the lining of the developing gut (6). Segmentation is believed to occur as successive blocks of mesoderm are formed. Currently segmentation in polychaetes is being investigated in a number of laboratories (7,8,9) using several genetic markers. Our results show that the circular muscles that differentiate in the larval trunk anterior to the telotroch are iterated sequentially from anterior to posterior. We suggest that the phalloidin-staining telotrochal bands are the nascent segmental muscles of the growth zone.
This work was supported in part by the Union College Faculty Research Fund. The authors gratefully acknowledge the generous assistance of Dr. William Eckberg in creating the figure.
Footnotes
1 Department of Biological Sciences, Union College, Schenectady, NY 12308 
Literature Cited
This article has been cited by other articles:
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  A. Wanninger Shaping the Things to Come: Ontogeny of Lophotrochozoan Neuromuscular Systems and the Tetraneuralia Concept Biol. Bull., June 1, 2009; 216(3): 293 - 306. [Abstract] [Full Text] [PDF]
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S. D. Hill and B. C. Boyer HNK-1/N-CAM Immunoreactivity Correlates with Ciliary Patterns During Development of the Polychaete Capitella sp. I Biol. Bull., October 1, 2003; 205(2): 182 - 184. [Full Text] [PDF]
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