MARINE WORM DIVERSITY:
Of the many phyla of worms, the segmented or annelid worms are the most diverse and perhaps the most beautiful. This group includes the familiar earthworm, the leech, and, in the marine environment, the class of polychaetes, comprising over 5000 species.
The annelid body is divided by partitions into compartments (segments) that, in part, restrict the flow of body fluids. This segmentation enables the burrowing annelid to dig much more efficiently than the nonsegmented worm. In this plate, three polychaete worms will be introduced.
Color Nereis and the enlarged views of its head region at right, Nereis is often an iridescent blue-green.
The clam worm, Nereis, is a widely distributed genus and is minimally specialized. Nereis has what may be considered the typical polychaete body, consisting of repeated identical body segments, each with a pair of lateral paddle-shaped appendages called parapodia. The parapodia are flattened projections of the body and are equipped with rods called setae. The setae project through the parapodia and are connected to muscles that enable them to retract or extend. As Nereis crawls along, the setae aid in gripping the substratum.
The head of Nereis consists of two segments, a prostomium and a peristomium. The prostomium is positioned in front of the mouth and bears several sensory structures; these include the light-sensitive eyes, as well as the antennae and palps, which appear to be receptors for both chemical and tactile senses. the peristomium, just behind the prosomium, contains the mouth and three pairs of tentacular cirri that also act as tactile receptors. As Nereis moves through the environment, these sensory structures concentrated in the head area provide information.
The mouth of Nereis holds an eversible proboscis. The proboscis remains folded in on itself until contracting body wall musculature increases pressure on the body fluid, which, in turn, everts the proboscis. The proboscis is armed with jaws that swing open and then clamp shut as the body fluid pressure is reduced and the retractor muscles pull the proboscis back in.
Nereid polychaetes consume a variety of foods; some are carnivores, some omnivorous scavengers, and some are herbivores. Species of Nereis move
about freely in many habitats, including the rocky intertidal zone, and burrow in mud and sand flats.
Now color Glycera and note its everted proboscis. The natural color of Glycera is a dark pink or light red. Also color the smaller drawing that shows the worm in its network of tunnels. Give the flooded galleries and the water above them a light blue color.
Glycera is a sand-flat-dwelling carnivore possessing a proboscis that is one-fifth its own body length and armed with four stout jaws, each with its own poison gland. Glycera constructs a nest of interconnecting burrows (galleries) in the substratum with many openings to the surface of the sand flat.
Glycera’s prostomium is conical and adorned with four short antennae. It is sensitive to changes in water pressure, as for example when prey move above the nest. Glycera feeds on polychaetes and other invertebrates. Some species of Glycera reach a length of more than 50 cm (20 in).
Next color the lug worm, Arenicola, and its burrow environment. The arrows indicating the flow of oxygenated water into the burrow and through the sand should also be colored. The natural color of lug worms ranges from pink to dark green.
The lug worm, Arenicola, lives on muddy sand flats and is a deposit feeder, not a carnivore. It excavates an L-shaped burrow and lies with its head in the toe of the L. The lug worm engulfs the sand at the toe of the burrow, and passes the sand through its digestive tract, thus removing any organic matter. As the lug worm swallows the sand at the toe of the burrow, new sand falls in to replace it, forming a distinct depression on the surface of the sand flat above. Arenicola’s burrow is easily identified by the pile of fecal mounds located at its opening. The lug worm ventilates its burrow using rhythmic peristaltic body contractions. Oxygenated water is pumped in from the surface through the burrow opening, flows over the gills on the body, and then disperses into the sand (aerated sand), thus aerating the upper layers of the sand flat. The worm’s feeding and burrowing activities provide circulation and re-exposure of the buried sediments and the nutrients they contain.