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Biology
Zoology Reading
Phylum Annelida  Polychaeta
The bristle worms show remarkable adaptive radiation of a basic body form which has enabled them to colonise a
range of different habitats. They include forms which are: active burrowers; sedentary burrowers; tube-dwelling in
sediment; tube-dwelling on a hard substrate; errant, beneath stones etc. on rocky shores; and pelagic forms. They
also show an associated range of feeding strategies, from active carnivores to suspension feeding and sediment
eating.
Polychaetes are annelid worms which have many bristle-like structures called setae that project from fleshy lobes
called parapodia. The parapodia themselves are arranged in pairs in a paddle-like fashion along each side of the
worm's body, one pair to each segment. The name "Polychaeta" is Latin for "many setae". This diverse group of
worms ranges in size from less than a millimeter to more than 2 meters in length. Their diversity in body form and
lifestyle has likely contributed to their great significance in marine ecosystems.
Polychaetes occur in nearly every habitat in the sea and are a tremendously important component of marine food
webs. As they are among the most abundant organisms in soft marine sediments, the presence of certain polychaete
species is often used to assess human-induced environmental impacts by employing knowledge of the biological
requirements of these species. Most of the approximately 8,000 species of polychaetes currently described are
found in marine and estuarine environments, although some species occur in fresh water and even in damp
terrestrial environments. New species are being discovered regularly, as well as new geographic distributions for
known species, due in part to the introduction of non-indigenous (or non-native) species from their naturally
occurring ranges to new parts of the world.
There are approximately 80 families in the Class Polychaeta. These families are often assigned to one of two
artificial (not taxonomically valid or "natural") groups, the Errantia and the Sedentaria. These two artificial
divisions of Polychaeta are not treated as official taxonomic groups because of the many inconsistencies that occur
within them; they are used for convenience to break up this very large class into functional groups. The group to
which a family is assigned is a function of the lifestyle and body plan of the worms in that family. In general,
Errantia includes worms with an errant (freely moving) lifestyle. These polychaetes tend to be grazers or carnivores
and have jaws, obvious parapodia and bodies that are relatively similar throughout their length. The Sedentaria are
mostly tube-dwelling or burrowing polychaetes having reduced parapodia with some of their setae modified into
hooks called uncini which help anchor the worms in their tubes. The heads of sedentary worms typically possess
tentacles or feathery gills called branchiae, which are used for feeding and respiration. Sedentary polychaetes are
filter feeders, deposit feeders, or sticky tentacle feeders that live a sessile lifestyle ("stuck in one place").
Identification of polychaete worms can be quite challenging and many groups are not well studied. Morphological
features including the structures of the prostomium and peristomium (the head and mouth regions), the shapes of
the branchiae, parapodia, setae, and of internal features such as the proboscis (a reversible pharynx used for feeding
in errant polychaetes) must usually be examined closely with microscopes to identify specimens. The best way to
begin learning the polychaetes is by learning to determine to which family an individual belongs by using
taxonomic keys. Families of Polychaeta share common morphological features which can indicate a good starting
point before eventually narrowing down to which genus and species a specimen belongs.
A diverse assortment of families occurs in San Francisco Bay, including species of both native and non-native
origin. Polychaetes are found in the Bay among the mussels and algae of dock fouling communities, in rocky
intertidal, benthic subtidal, interstitial (between sediment particles), commensal with other organisms, and across all
salinity gradients (from the Delta to the Golden Gate). As we survey the communities of the Bay, we hope to
uncover changes in distribution patterns and document introductions of polychaete species to the ecosystem.