Background

One of the joys of photomicroscopy is the beauty of what we see within the small structures of the creation. At times the most accurate (or content-full) image is less than artistic, and sometimes we relax the scientific discipline for the sake of a good image. Occasionally the science and art come together in something that we find truly amazing. Several things came together recently to give me such a moment.

I had put half a dozen Daphnia pulex in a watch glass, intending to sort some out for observation; but got busy and did not return to them till the next day. While checking them under a dissecting microscope I noticed several flecks of debris, which turned out to be molted exo-skeletons. Not only were all of the normally hidden appendages exposed on these skeletons, but the shell was so thin, it easily compressed. Here were all the identification details in perfect condition, requiring neither the patience of waiting for a live daphnia to properly pose, nor having to smash and dissect the creature (with the remaining parts looking a bit worse for the treatment).

Images

Daphnia Anatomy Notes

It is helpful, when looking at a daphnia skeleton, to remember that they are small distant relatives of lobsters - a shell with legs and antenna attached. The daphnia has a head shell, which smoothly joins the carapace, which attaches at the back of the 'neck'. The body hangs from inside this carapace. The thorax has 4 to 6 pairs or swimming legs, which filter food particles and move them toward the mouth. The body ends in the post-abdominal claw. This claw is used to clean the thoracic legs, and is occasionally used for movement.

When daphnia molt, they usually split at the back of the 'head' into two parts. The 'head shield' or 'helmet' separates from the back clam-shell like carapace, while the antenna and other body appendages remain attached to the carapace. Occasionally these appendages remain within the carapace, but usually they are suspended outside.

The long swimming secondary antenna are easily identifiable, as they look the same as on a live daphnia. The post-abdominal process retains its characteristic shape, noted by the prominent 'claw'. The rest is mostly various other swimming appendages, prominent for all of their fine filtering hairs. The two dark object are grinding surfaces of the mouth.

Image Note

Images were shot with the camera operated through the computer. The 10x objective was used (100x microscope magnification), with 3x optical zoom on the camera. The camera flash and auto-focus light were off, but all other settings were set to 'automatic'.

The original images were shot in large-fine mode (2048x1536 pixel). To avoid sending 3 meg files over the web, the images have been reduced.

Darkfield

The (color) images follow standard darkfield procedures.

Light Field

The light-field images ('monochrome') follow standard procedures, but the light diaphragm was stopped way down. The shells are so thin, that the light must be greatly reduced for any details to be visible. Reducing the light to these low levels shifts the background color (from white to sepia/brown); there has been no post-processing manipulation of the images.

Technical Details

Microscope: Bausch & Lomb monocular, 10x ocular, 4x, 10x and 40x Nikon objectives.

Camera: Canon A70

References

Daphnia paleolimnology - daphnia skeleton (parts), extracted from lake sediments, are often used to identify species of past populations.

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Published in the July 2004 edition of Micscape Magazine.

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