Echiuran worms

Urechis caupo research

One of our favorite animals to study has been the echiuran worm Urechis caupo. This unusual animal lives in “U”-shaped burrows in estuarine mudflats along the California coast. Besides living in a sulfidic environment, this charming little critter has some rather useful properties for research. These include its extremely simple anatomy, its abundant coelomic fluid (“blood”), which contains cells with intracellular hemoglobin, its unusual specializations for gas exchange, and its unique mechanism of filter feeding.

Burrow irrigation behavior in the field

When Chris Osovitz was an undergraduate in our lab, he carried out a study of the burrow irrigation behavior of U. caupo in the field (Osovitz and Julian 2002). Chris put together an almost waterproof system for burrow irrigation (using a magnetic flow transducer) and recorded irrigation behavior for up to 3 hours following a low tide. We found that the average burrow irrigation rate was almost 300 ml per minute per worm, which was much greater than what had been expected based on lab studies. Furthermore, during feeding activity, a worm could reach a maximum sustained rate of 870 ml per minute. Based on previous estimates of population density (61 animals per square meter), we estimated that a population of U. caupo in one square meter of mudflat can collectively pump 23,000 l of seawater per day through their burrows, of which 13,000 l is filtered through the mucus net. That’s a lot of water.

MPEG files of Urechis caupo feeding behavior

U. caupo feeds by creating a mucus net, filtering water through it to capture particles and small organisms, and then eating the net. Jeffrey R. Judd created some videos of this behavior (below). The worm is in an artificial burrow constructed of clear acrylic. In these videos, the worm is in one of the vertical arms of the burrow. The red color in the feeding nets is caused by carmine particles that Jeff introduced into the water (you can see the particles gradually accumulate in the “Filtering” video).

Attachment (4.6 MB) | Spinning (6.0 MB) | Filtering (5.3 MB) | Withdrawal (1.5 MB) | Consumption (7.0 MB)

More information on U. caupo’s feeding behavior, including the methods used to obtain these videos, are in Julian et al. (2001).

Here are some excerpts from that paper that contain relevant text:

Urechis caupo is one of only a few tube-dwelling animals that capture suspended food particles with a secreted mucous net. The feeding behavior of U. caupo was described in detail by Fisher and MacGinitie (1928) and later expanded by Hall (1931) and MacGinitie (1945).

The feeding behavior of U. caupo is stereotyped and consistent (Fisher and MacGinitie 1928). The behavior can be divided into four functional sections: 1) attachment and secretion, 2) filtering, 3) withdrawal, and 4) grasp and consumption. A feeding episode starts with the worm moving up one of the vertical arms of its U-shaped burrow. U. caupo first expands its entire body, bringing a ring of mucous glands located posterior to the ventral setae into contact with the burrow wall. The proboscis is held upright in the center of the burrow and a series of large peristaltic waves, generated by contractions of the muscular body wall, are initiated. Simultaneously, the worm begins secretion of the mucous feeding net from the mucous glands while backing down the burrow. The waves of peristalsis are relatively uniform in size in the beginning, but subsequently the waves forward of the mucous glands inside the newly constructed net start to be reduced in size. The forward portion of the body inside the net is then contracted into a series of two or three folds that appear to position the net, keeping it taut during construction, and produce the funnel shape of the finished net. Net length varied between feeding episodes, ranging from 10 to 16 cm, and were similar in length to nets reported by Fisher and MacGinitie (1928) and MacGinitie (1945).

When the net is completed, the worm elongates and contracts the anterior portion of its body down to approximately 25% of its normal diameter. The feeding net completely blocks the burrow, as one end of the funnel-shaped net is attached to the burrow and the other remains attached to the worm at the ring of mucous glands. Once the net positioned, peristaltic contractions posterior to the net become large and vigorous, pulling water into the burrow and through the feeding net. As the net fills with particulate material, the peristaltic contractions increase in frequency and depth. The filtering periods observed in this study lasted 15 to 20 min. At the end of the feeding period, the worm breaks the connection between the net and its mucous glands with a quick backward motion. The worm then retreats a short distance down the burrow, flares and extends its proboscis and returns up the burrow grasping the net, consuming it as it continues to move up the burrow. The consumption of the net is very rapid, taking approximately 30 to 60 sec.

References

Fisher WK and MacGinitie GE (1928). A new echiuroid worm from California. Annals and Magazine of Natural History 1, 199-204.

Hall VE (1931). The muscular activity and oxygen consumption of Urechis caupo. Biological Bulletin 61, 400-416.

Julian D, Chang ML, Judd JR and Arp AJ (2001). Influence of environmental factors on burrow irrigation and oxygen consumption in the mudflat invertebrate Urechis caupo. Marine Biology 139, 163-173.

MacGinitie GE (1945). The size of mesh openings in mucous feeding nets of marine animals. Biological Bulletin 88, 107-111.

Osovitz CJ and Julian D (2002). Burrow irrigation behavior of Urechis caupo, a filter-feeding marine invertebrate, in its natural habitat. Marine Ecology-Progress Series 245, 149-155.