Wooleater Echofin

Wooleater echofins do not often eat the cottonback's wool (or cotton) itself. Rather, they incidentally ingest the wool while trying to eat the microscopic organisms present on it.

Cottonbacks' wool can collect many tiny organisms from the surrounding water. The organisms that grow on its wool are presently too small and non-nutritive to be worth eating for the cottonback. Furthermore, it would be difficult to harvest them without pulling out the wool itself, which the cottonback needs to help it keep afloat. The bog echofin was in a prime spot to exploit this significant and diverse resource, and in doing so it developed into the wooleater echofin.

The wooleater echofin is more compact than its ancestor. It is laterally flattened like a flea, which helps it move through the wool. Using echolocation, it senses subtle differences in texture between "clean" wool strands and "dirty" wool strands. As it approaches, it emits calls more frequently, giving it a clearer image. Once it's certain the wool strand has food value, it snips off the wool strand and swallows whatever organisms are growing on it. It later coughs up the saliva-sodden strand, much like an owl coughing up pellets.

Wooleater echofins are presently unique for being the only organism to eat polyfees---very tiny parasites that mimic the host's skin. Wooleater echofins are apparently able to distinguish polyfees from the skin they mimic. This may be possible because, while polyfees are impressively similar to their surroundings visually, they stand out in "echolocation-vision". Polyfee-eating is most common among adult female wooleater echofins. Like a mosquito, it needs protein for its eggs, and eating polyfees provides that protein.

However, that's not the only way the adult female can obtain protein. It can also ingest wool soaked with sap-blood, such as from its host scratching a scab. (Since cottonbacks are "plents", they have sap-like blood. Their closest photosynthesizing ancestor is the puff-tongued plenthog.)

Pellet Strands

Newly "germinated" (since they reproduce through spores) wooleater echofins usually avoid sites with pellet-strands. The pellet-strands indicate that another wooleater echofin has been through the area recently, and thus is likely to have harvested all the food in the area. Occasionally newly germinated individuals do eat the pellet-strands. There are three hypotheses for this behavior. The first is that eating the saliva of an adult boosts its immune system. The second is that eating the saliva provides digestive enzymes, like a baby koala eating the excrement of its mother. The third is that it is deliberately transporting one "signpost" to another location to deceive others into avoiding food-rich locations. (This hypothesis is considered unlikely, since they do not seem particularly intelligent.)

(Information that did not fit neatly: The specific strain of floatfilm usually found on cottonbacks is either T. tetelan, a relative of T. tetelan, or another floatfilm strain that convergently developed adaptations similar to T. tetelan.)