MARINE WORM DIVERSITY:
The fat innkeeper worm, Urechis caupo, is an interesting marine worm that inhabits semi-permanent burrows in sandy mud flats along the California coast. Normally about 20 cm (8 in) long, this round worm possesses a highly stretchable (distensible) proboscis and a pair of gold-colored, hooked bristles near its anterior end. A circle of similar bristles surrounds its anus.
Color the drawing of the innkeeper worm at the top of the plate a medium pink, which is its true color. Then color the diagram of peristaltic action. Here, the worm receives two colors, one for its body and one for the area of muscular contraction. Be sure to color the arrows that indicate the direction of the muscular contraction and water flow, using a light blue for the water. Now, color the worm in its mud flat habitat. Note that the sea water above the burrow and the sea water in the burrow receive shades of the same color, indicating that the burrow is flooded. Color the mud light brown. Color the proboscis of the worm in the burrow, then color over it with the light color chosen for the mucous net.
The innkeeper worm makes its burrow by digging with its proboscis and its anterior hooked bristles. The anal bristles and body movement carry the material backward and out of the burrow. Once the burrow is constructed, the innkeeper stays there unless disturbed.
The innkeeper worm pumps water through its burrow with waves of muscular contraction in a process called peristaltic action. These peristaltic waves create a moving “bottleneck” which travels down the worm’s body, pushing water before it and eventually out the burrow. To feed, the innkeeper secretes a mucous net placed snug against the burrow walls near one opening. the worm backs down the burrow, secreting the net as it goes. When the net is between
5 and 20 cm (2 – 8 in) long, the innkeeper stops, positions itself in the burrow, and begins drawing water through the very fine mesh of the net, using peristaltic action. When the net is laden with food particles, the worm moves back up the burrow, eating both the net and small particles of food; large chunks are discarded. Thus, the innkeeper is a filter feeder whose filter is outside its own body.
Color the other marine animals in the innkeeper’s burrow, noting their positions in the burrow. Also color the larger illustrations of these animals.
The large food particles discarded by the innkeeper worm do not go to waste. There are several marine animals that share the innkeeper’s home and take advantage of both the extra availability of food and the security of the burrow. This situation, in which two or more dissimilar organisms live together, is another example of symbiosis (Plate 91).
The small (2 cm, 0.8 in) pale brown pea crab and the translucent white polychaete worm (4 cm, 1.5 in) share the burrow and fight over the food discards. The polychaete worm remains in contact with the innkeeper in order to gain an advantage over the quicker pea crab.
Two other frequent residents of the burrow are a goby and a small clam. The mottled gray-green goby uses the burrow as a home base and forages for its own food out on the mud flat at high tide. The “tap-in” clam (1.7 cm, 0.7 in) is the innkeeper’s fourth guest. Instead of making its own shallow burrow at the surface of the mud flat where it might be washed away or eaten, the dull-white clam digs (“taps”) into the wall of the innkeeper’s burrow some distance below the opening. From here, the clam extends its short siphons into the water currents flowing through the large burrow, and from these currents it is able to siphon food.