Dauer larva

• Non-Association Nematodes can live both on land and in water, residing in both soil and underwater sediment. However, as found by Rebecci et al. in their 2020 study, desiccation is a major selective force in only terrestrial environments, which the larva will combat by dauer dormancy.^[18] Phylogenetic analysis of nematodes suggests that parasitic lineages are derived overwhelmingly from terrestrial ancestors, even with lineages that reside in water.^[19][20] Both of these factors are supported by the dauer Hypothesis under the assumption that the dauer precedes the parasite, and is not influenced by earlier sources.

• Phoresy The next step in Crook's proposed plan is phoresy. Phoresy as a step for parasitism is not confined to nematode development and is seen similarly in
  Astigmata.^[21] Phoresy describes a non-parasitic relationship between two organisms, where one organism uses the other as a mode of transportation. In phoresy, there is a phoront, which is then transported species, and the vector, the mobile species through which the phoront travels. It is crucial to the evolution of parasitism due to both its initiation of close contact between the phoront and vector, as well as being a constraint on parasite size. The stressors of phoresy and parasitism are closely related, such as desiccation and starvation.^[17] This reliance on the vector mirrors the reliance on a host, both of which act on the affected organism's fitness. The need for phoresy is also much lower in marine environments, as marine nematodes can utilize currents as methods of low-effort transport.^[22]
  Therefore, an additional connection is formed between terrestrial species and eventual parasitism.

• Necromeny Necromeny is most effectively thought of as a parasitic extension of phoresy, in which the phoront will feed on the vector if it dies in transit, as well as using the body as a place for proliferation.^[16] However, necromeny has been found to select traits that reinterpret the vector not simply as transport, but also as a habitat. It is important to note that necromeny does not necessarily eliminate the further need for phoresy. Because of this, it is thought that developing nematodes rely on both environmental signals, as well as communication with other larvae while making the choice between continuing development on their vector (necromeny) or attempting to find a new one (phoresy). For example, it has been found that dauers can communicate with other dauers via pheromones, in which adult nematodes signal larvae to continue their development.^[23] This can create a habitat shift in group environments, and can further parasitic larvae development.

• Parasitsm: Through the development of phoresy to necromeny, developing larvae can officially reach a state of parasitism in their adulthood. In parasitic nematodes, there are two main methods of feeding: direct feeding and indirect feeding. In direct feeding, nematodes switch from their ancestral food source, such as bacteria, to their host vector's tissue. They utilize
  digestive enzymes for this process, by secreting them into the environment as opposed to internal use.^[24] However, in indirect feeding, nematodes weaponize bacteria to kill a host. For example, in George O. Poinar Jr's 1990 book on Nematodes and Biological Control, he describes Heterorhabditis, a genus of nematodes that harbors symbiotic bacteria that are highly pathogenic to hosts, but completely harmless to them. After the bacteria kill the host, they proliferate on the host's dead body. The Heterorhabditis then feeds on this new growth of bacteria for development.^[13]
  In both cases of feeding, the parasitic nematodes make direct use of the host's body, possible only through the evolutionary pathway aided by phoresy.

• Genetics of aging
• Polyphenism

References