Looking at Trematodes

by
Duncan Parks
Scientist

Deformed Frogs Found in California

In 1986, Dr. Stan Sessions studied a natural population of Pacific Treefrogs near Santa Cruz, California. He was trying to explain the causes of their deformities. Most of all, he hoped to explain the occurrence of multiple legs, as shown in the frog in the picture below. This frog was collected from a pond near Santa Cruz. As you can see, it has three legs growing from the left side of its body.

ALL IMAGES ON THIS PAGE COURTESY OF DR. STAN SESSIONS

"Clearing and Staining"

Dr. Sessions used a technique called CLEARING AND STAINING to look at the skeleton of some frogs. Here's what a cleared-and-stained frog with extra legs looks like:

You can see that this frog has part of an extra leg on the left, and two extra legs on the right. One of the right legs has split to form two feet. (A scientific description of the CLEARING AND STAINING process is available if you are interested, although it is quite technical.)

The Culprit -- Trematodes!?!

Dr. Sessions noticed lots of black CYSTS

in the cleared-and-stained frogs he collected from Santa Cruz. He later identified these as TREMATODES. Trematodes are parasites that spend part of their life cycle as cysts in aquatic organisms like frog tadpoles. Trematodes are so small you need to use a microscope to see them clearly. Here is a highly magnified picture of a trematode swimming in water:

Trematodes burrow into the skin of the tadpoles near an opening called the CLOACA . The cloaca is at the back end of the tadpole. Here is a picture of a trematode actually burrowing in near the cloaca of a frog:

Dr. Sessions knew that the legs of an adult frog develop from tissues in the tadpoles that are very close to the cloaca. He thought that some of the trematode cysts might have burrowed into the tissues that were going to develop into legs. When he looked at the cleared-and-stained adult frogs (like the one in the photo below) he thought that the trematode cysts were interfering with the development of the legs.

Trematodes & Hind Leg Deformities

In this photo, trematode cysts seem to have stopped a frog's leg development completely. The cysts are the tiny black dots in the tissue around the leg stump.

Could trematode cysts cause the kind of deformities Dr. Sessions was investigating? A tadpole collected from a natural pond suggested they might. This tadpole is shown in the photo below:

In this photo, the left side of the tadpole has lots of trematode cysts, but there are no cysts on the right side. The left leg has been split in two, while the right leg looks normal. Notice how there are two sets of leg bones and foot bones on the left side. Dr. Sessions thought that the cysts had caused the left leg to split!

After seeing all this evidence, Dr. Sessions thought that the trematode parasites were causing some leg deformities. To test this hypothesis, he decided to perform The Bead Experiment which you will look at separately.

Evidence Information

KEYWORDS	deformed frogs, trematodes, cysts, parasite
SUBMITTER	Linda Shear