Annelid Module

Annelids are the phylum of segmented worms, with over 17,000 described species in this clade. Annelids are characterized by their body plan, centralized nervous system, continued adult growth from addition of segments from a posterior growth zone, and adult regenerative abilities.  Embryologists observed their development over a hundred years ago and annelids have been used for evo-devo studies due to their superficial similarities to other segmented animals (e.g. vertebrates and arthropods).   Some annelids live in fresh water or are even terrestrial (e.g. leeches and earthworms), but most species of annelids live and reproduce in the marine environment, and many have an indirect life cycle, producing a free-swimming larval form called the trochophore. Two species that are emphasized at the MBL are Capitella teleta and Chaetopterus sp. Capitella is a small benthic worm with a cosmopolitan distribution and a high quality publically available sequenced genome (the first annelid to be sequenced!). There are distinct male, female, and hermaphrodite sexes. Chaetopterus is the parchment tube worm, has a highly tagmatized body plan, and is known for being able to regenerate both its head and tail.  Recent molecular phylogenomic studies also place it as one of the earliest branching members of the Annelida.

Annelids exhibit spiral development, which means that there is a highly stereotypic program of early cleavages. The cell lineage of several annelids has been described in detail, and every cell in the early stage embryo of Capitella and Chaetopterus can be uniquely identified. In addition, the fates of these cells are known and a comprehensive fate map that identifies the early embryonic origin of larval features is available for Capitella. Because this pattern of early development is highly conserved, homologous cells can be compared across species, and even other animals such as molluscs.   These observations provide interesting EvoDevo-type questions as to how such a highly conserved early cleavage program gives rise to such a highly diverse collection of adult body plans.

In this module students will be exposed to some of the diversity within annelids, both larval and adult forms. Students will learn animal handling techniques, how to obtain embryos from Capitella and how to set up an in vitro fertilization for the parchment tube worm, Chaetopterus. Students will have the opportunity to observe normal early development, and use immunohistochemical and staining methods to observe the developing larval nervous system and musculature across diverse larval forms.   These types of studies are essential before any understanding of how ontological differences arise during annelid evolution. Students can also perform classical embryological manipulations, laser deletions of individual blastomeres, pharmacological perturbations, microinjections, make time lapse movies, or a wide range of other experiments.