You've got what, kyphosis?

Holopedium gibberum has a large mucous sheath that distinguishes it from other water fleas. This gelatinous material is made up of water and carbohydrate compounds. It is excreted by salivary glands within the labrum.

The jelly capsule provides protection against invertebrate predators as it restricts access to Holopedium. Beside this function we know that the envelope increases hydrodynamic resistance and lowers real density of organism, leading to attenuated sinking of animal. The sheath also has a significant impact on swimming, so that the animal has to exert greater effort. According to Stokes' law, which determines the drag forces on a small spherical object, area and density act as key factors. We can extend this statement to objects that are not globular, but we have to use correction numbers. Holopedium can molt jelly coat and even produce multiple new layers of it. Individuals without a sheath wouldn't be able to withstand sinking as well as those completely surrounded by mucus.

The same phenomenon can be demonstrated with floating producers, such as algae, and part of the bacterioplankton. They invest their energy into constructing structures that reduce the weight of their tissue or enhance their ability to float, deviating from a spherical shape. This involves forming air vacuoles and distinct protrusions from their cells, accumulating oils, as well as producing mucus. All of these adaptations enable them to remain in the illuminated upper layer of the water column, where they can rely on a metabolism dependent on sunlight.

Stenson, J. A. E. (1987): Variation in Capsule Size of Holopedium Gibberum (Zaddach): A Response to Invertebrate Predation. Ecology, 68(4): 928–934. Dostupné z: doi: 10.2307/1938364