Annelids, or segmented worms, are a specious group of approximately 17,000 species which have diversified to such an extent that they have colonized terrestrial, aquatic, and marine systems. Almost all marine annelids fall into the class Polychaeta, which makes reference to the chitinous bristles known as chaetae present on each segment. In most any habitat you can imagine, all the way down to deep sea hydrothermal vents, polychaetes have found a way to persist. They do so by adopting a variety of life history strategies, and in having the morphological plasticity to adapt to change. The most generalized polychaetes are those that crawl along the benthos, but others have adapted to different ecological niches including burrowing, swimming, tube-dwelling, boring, filter-feeding, commensalism, and parasitism. Polychaetes occur across depth strata, from forms that live as plankton near the surface to an unidentified 3-cm specimen observed by the ROV Nereus at the bottom of the Challenger Deep, the deepest known spot in the Earth's oceans.
Because annelids have spread to such a wide extent, their evolutionary heritage is still tenuous. As such, many SEM studies have been aimed at identifying and understanding morphometric characters so as to further phylogenetic analysis. Establishing metrics for species ID that are consistent across the literature has been one facet of research where SEM has been implemented. Principal characters such as appendages and types of compound chaetae have been compared across families to address inconsistencies in the literature (Aungtonya 2003). To this same end, larval development is also relatively well-studied in this group (Eckelbarger & Chia 1976). Sperm morphology and the extent to which gametes and larval stages are modified from the ancestral polychaete state are another way of tracking phylogenetic trends.
Many tube-building polychaetes also form dense (70 000 to 180 000 worms‐m‐2), interwoven reef structures which provide necessary habitat for entire communities, and are in fact thought to be some of the most important reef-forming organisms (David W. Kirtley 1968). To better understand their formation and stability, SEM studies have been conducted to examine the structure and composition of the tubes themselves (Bubel et al. 1983, Aliani et al. 1995). Tubes are generally composed of two layers: an outer, 90 μm‐thick calcareous one and an inner, very thin organic membrane. Prismatic crystals are preset at the inner surface of the calcareous layer. Such studies have found that consolidation of the reef framework, which ensures the integrity of the structure, is accomplished through wedge‐shaped junctions which adhere the tubes together, and that small tubes of newly settled individuals encrust interweaving adults to cement the reefs.
SEM has also been especially useful in studies of deep sea hydrothermal species, of which very little is known. By examining a small section of tissue, it is possible to deduce many facets of the life history. For example, one study used light, scanning, and transmission electron microscopy to study morphology and diet within a single species Alvinella pompejana (Desbruyres et al. 1983). The digestive tracts were found to contain sulfide particles associated with organic matter and bacteria. Bacterial communities of different morphological types were also observed at different levels of the worm's outer teguments, suggesting differential partitioning of symbionts across the host’s surface. Lastly, an atypical (possibly bacteria-derived) nutritional source of carbon and nitrogen was indicated by the natural abundances of 13C:12C and 15N:14N in its tissues.
While not included in this discussion, a vast pool of research exists which examines the physiology, ecology, and life history of economically viable earthworm species (Oligochaeta), specifically as they relate to soil formation. Additionally, leeches (Hirudinae) have been well-studied due to their medical importance.