One of the most distinctive groups of "soft corals," or
is the Pennatulacea, or "sea pens."
As is the case for all octocorals, sea pens are actually colonies of polyps. What distinguishes sea pens is polyp dimorphism. One polyp grows very large and loses its tentacles, forming the central axis. The base of this primary polyp forms a bulb which may be expanded or contracted; the sea pen uses this bulb (not visible in the picture to the left) to anchor itself. Branching off this primary polyp are various secondary polyps. Some, called autozooids, are typical feeding polyps. Others, the larger and fewer siphonozooids, serve as intakes for water, which circulates within the colony and helps keep it upright. Also supporting the colony are calcareous spicules and frequently a central axial rod of calcium carbonate.
In one group of sea pens, called the Subselliflorae, the secondary polyps are grouped into "polyp leaves," as in the Pacific species of Sarcoptilon shown here. The feather-like appearance of these species gives the sea pens their common name; they look something like old-fashioned quill pens. Most species, however, do not have polyp leaves, and look more like clubs, umbrellas, or pinwheels. The unusual "sea pansy," Renilla (shown below), has a primary polyp that is broad and flattened, with autozooids and siphonozooids on the upper surface. Renilla is actually able to crawl about on its leaflike primary polyp.
|Pennatulaceans have a sparse fossil record, but their spicules are sometimes found and identified as microfossils. They are fairly well documented from Cenozoic rocks. Rare fossils from earlier rocks point to a much longer evolutionary history, still poorly known. A Cambrian pennatulacean was recently described from the well-known Burgess Shale of the Canadian Rocky Mountains. Even older are the frondlike fossils from the Vendian, immediately before the Cambrian. Some of these look very much like pennatulaceans; others are less like living forms. Exactly what these fossils are is still not settled to the satisfaction of all. Examine Charnia and Pteridinium from the Vendian and see what you think.|
A searchable bibliography of the Octocorallia is now available from the National Museum of Natural History.