Gefilte fish feed by filtering microscopic zooplankton through their spongy mass- hence the name. This is a passive form of nutrition for the most part, though there is evidence for an amoeba like 'swallowing' ('phaging' ) of larger particles. The larger, carrot like particles, (mistaken for light sensitive organs by earlier researchers- I.C. Carp et al ) are actually processed pieces of orange pigmented microcrustacea. It is permitted to eat these - they not considered 'shratzei hayam' because of the high degree of metabolic processing they have undergone. Zoologists are amazed that krill can be transformed into a mass indistinguishable from a piece of boiled carrot.
There are some sources in Jewish law which forbid the consumption of the gefilte fish based on the total lack of fins and scales. They claim it is really a relative of the sponges and they cite the mode of feeding as very telling evidence. Other sources refute this claim, since many communities have a strong tradition for the permissibility of this species. The fact that protein analysis demonstrates the presence of fish protein is another strong indicator of its kosher status. The detractors claim that protein analysis by itself does not overrule the flippers and scales rule, and that the fish protein could theoretically have been introduced to the genome by a virus. Jewish law deals with this problem neatly, claiming that if this is the case, then gefilte fish must be related to kneidlach, soup dumplings- an animal which has pretty much the same natural history but lacks the fish protein. There is also an irrefutable tradition for kneidlach.
Both gefilte fish and kneidlach are hunted using five foot ladles to pick them out from the jellyfish. They are then placed in rice water in order to leach out excess sea salt. They are too osmotically sensitive to be placed into tap water where they would quickly fall apart. Kneidlach can be safely transferred to hot soup later since it is osmotically similar to sea water. There is danger of cruelty to animals no sensory ganglia have ever been found in either of these species. Immersion in boiling water is, however, still considered slaughter on the Sabbath, and should be done before the Sabbath.
The reproduction of gefilte fish and kneidlach is as yet an unsolved mystery. No difference has yet been discerned between male and female gefilte fish, though some believe that male gefilte fish possess more orange pigment. Females could be entirely without the pigment. Others hold that this is simply natural variation. It is believed that they multiply by either binary fission (splitting into two pieces) or by fragmentation. Some private researchers hold that they have seen this phenomenon with kneidlach, but it is very doubtful since they are almost definitely subdued by water approaching boiling point. This phenomenon is probably a result of osmotic balance failure and the soup is way too watery.
Environmentalists have protested that there is a danger that wild stocks of gefilte fish and kneidlach are becoming depleted. They demand that we should restrict ourselves to one slice or chunk only for the first course. This also has the advantage that we won't spoil our appetite for the soup, chicken and rice. Kneidlach are also a potentially endangered species, as well as being rather fattening and should be eaten in moderation especially if we want to leave room for desert.