 |  |  |  |
Like corals, the worms’ primary food source is microplankton. They need plenty of wave and current action to bring them a constant supply of food and to wash away their metabolic wastes. Unlike corals the sabellariids can withstand several hours of exposure to the atmosphere during low tides. They can retain enough sea water within their tubes to maintain their body temperature and supply oxygen for several hours. During episodes of extreme high or low temperature they sometimes die in large numbers, but these events are rare on the southeast coast of Florida.
Anatomically, the Sabellariid share several important features. A dark, stout head is equipped with two rows of ciliated (hair-like) feeding tentacles along its underside. The head carries a coronal or circular array of hard, tooth-like grasping organs and a ring of paddle-shaped bristles (setae) that dress the "mortar" during construction and seal the end of the tube when the worm withdraws. Below the feeding tentacles is the mouth, with its pair of palpi that serve as small sensory and manipulative tentacles.
Farther down are three segments of the parathorax, with pad-like bundles of setae on either side; then about 35 abdominal segments with their tiny hooks (uncini) that anchor the body to the surrounding tube wall. Finally comes the cylindrical tail.
Young larvae have eyespots on both ends of the body. Presumably, with these simple visual organs they look about for a good site to attach themselves. They have been observed to "feel" the surface with their pair of grasping tentacles and "taste" the site with special sensory organs located around their mouth areas. The choice of a building site is important, because once a sabellariid starts building, it has little opportunity to change its mind.
The first portion built of the tube is a small, transparent mucous cylinder, about two millimeters long, just enough to encase the tiny creature. The worm collects small fragments of minerals, diatom frustules, sponge spicules, and other small objects of manageable size and implants these reinforcements in the delicate mucous tube.
During this crucial period, if the worm is broken loose from its attachment side with a portion of its tube intact (and if a reef fish doesn’t devour the worm forthwith), it could possibly repair the tube quickly and get a second start. This is unlikely in nature because the reefs usually teem with predators. So only the larvae and juveniles that attach themselves to a favorable site, rapidly develop their protective tube, and quickly set up housekeeping can be counted on to survive and continue the rest of their life cycle.
At first, the worms are not too choosy about the type of building materials selected, except to use particles of uniformly small size.
Later in their tube building the worms choose among a greater variety of materials. Into upper parts of the cylinder are set angular quartz grains, small fragments of broken mollusk shells, fecal pellets and other materials - all arranged in an overlapping spiral pattern that rises to the base of the flared opening. It is the grouping of these terminal apertures that gives the reef its honeycomb appearance.
Sabellariids are easily found on seashores around the world from as far north as Point Barrow, Alaska, to as far south as the Falkland Islands in the South Atlantic. They are found from the intertidal zone to almost 20,000 feet (5790 meters). Such wide distribution in modern sea, as well as the fact these reef builders have been traced back at least as far as Carboniferous times and probably to the Pre-Cambrian period (more than 600 million years ago) strongly suggests that they have done much to change seashores in the geologic past. Wherever they have found an abundance of tube-building materials, turbulent waters to promote reef growth, nutriment and other favorable ecological conditions, the sabellariids probably have played an important role in the accumulation of some sedimentary rocks and have helped control the development of coastlines of ancient seas.
Although a long-term, detailed study of the growth rates and life span of Phragmatopoma caudata has not been made, its colonies have been seen to settle on rubblestone groins and in six weeks build an encrustation ten inches thick around the base of the boulders. The growth rate of a Sabellaria alveolata reef was measured to be between 1,000 and 1,300 cubic yards in six months.
The durability of this sea worm’s product, and home, is best witnessed along Florida coastlines during the hurricane season, in late summer and fall. It is then that the tube builders’ engineering competence is severely tested. It is a time when ocean waves are extremely turbulent, removing natural beach materials, and often man-made structures. Except for a relatively few clumps and clusters the sabellariid worm reef have hardly changed through all the pounding.
The sabellariid worms offer several ecological benefits. They provide feeding and browsing grounds for a diverse community of marine organisms, including sea turtles and fishes and an ideal home for attaching plants, sponges and shelter-seeking animals such as the spiny lobster and stone crabs. Waves breaking over the reefs lose their energy in a cloud of spray. The reef impound a buildup of shell fragments, sand and other materials in quieter water on their landward side. It is held there until the next high intensity storm picks it up and puts it in motion. Through consolidation some of this impounded sand and shell fragments has eventually become beach rock.
The reef-building sabellariids are natural "surf zone engineers." More knowledge of their strategy in conquering and inhabiting this constantly changing habitat would be helpful in formulating intelligent decisions about the proper and efficient use and management of the precious coastal zone resources.
- by David W. Kirtley Ph.D.
| | | |
