I wonder whether improving the welfare of arthropods (e.g. insects and crustaceans) and nematodes (roundworms) is sufficiently important and neglected to greatly outweigh the low tractability, thus being underrated.
In What We Owe to the Future, William McAskill suggests using the number of neurons as “a very rough heuristic” for the “capacity for wellbeing”, presenting in Table 9.2 the total population and neurons of humans, farmed animals excluding arthropods, and wild fish. The table below extends that table to terrestrial and marine arthropods, and nematodes. I have also included additional columns for the neurons per animal, and total neurons relative to humans.
The capacity for welfare of arthropods and nematodes dominates based on neuron counts. Moreover, as a 1st approximation, it looks like animal welfare is really marine arthropod welfare.
Neurons per animal
Total neurons relative to humans
Farmed animals excluding arthropods
Wild Animal Initiative and Rethink Priorities (RP) have conducted research on the welfare of arthropods. For instance, 6 of the 11 species analysed in RP's Welfare Range Table are arthropods. Furthermore, the Insect Welfare Project (introduced here) and Shrimp Welfare Project aim to improve the welfare of farmed insects and shrimps, which are terrestrial and marine arthropods, respectively. These are great first steps, but they do not seem to quite reflect the scope of the capacity for welfare of arthropods. Terrestrial and marine arthropods have 20 k and 2 M times as many neurons as farmed animals excluding arthropods.
Nematodes have been mentioned by Wild Animal Initiative, Rethink Priorities and Open Philanthropy, but I did not find research on improving their welfare. So nematode welfare is potentially even more neglected than that of arthropods, while still accounting for 20 k times as many neurons as farmed animals excluding arthropods.
I understand improving the welfare of arthropods and nematodes is quite difficult, especially because virtually all of them are wild. However, given it is so important and neglected, I suppose trying is worth it, at least for the sake of the value of information.
I took the population size of terrestrial and marine arthropods, and nematodes from the 1st table of this post from rosehadshar. I computed the number of neurons of humans, farmed animals, and wild fish from the ratio between the total neurons and population size. I computed the number of neurons of terrestrial and marine arthropods from the geometric mean between the lower and upper bounds of 100 k and 1 M provided here by Brian Tomasik. I calculated the number of neurons of nematodes from the geometric mean between the lower and upper bounds of 100 and 1 k provided here by Animal Ethics.
Many models of consciousness requiring less than 10 neurons are described in Herzog 2007.
For example, according to this article from Abraham Rowe:
1 trillion to 1.2 trillion insects are raised on farms annually for food and animal feed.
These correspond to only 10^-9 (= 10^(9 - 18)) of the total number of terrestrial arthropods.