Are horseshoe crabs aquatic arachnids?

[Halichoeres]

Horseshoe crabs have been recognized as more closely related to arachnids than to true crabs for as long as I can remember, based on things like book lungs, chelicerae, and the use of copper as an oxygen carrier rather than iron. This new study takes a hard look at a huge genomic dataset and finds strong support for horseshoe crabs not only being related to arachnids but nested deeply within arachnids. In other words, they're the only arachnids to return to the sea!

Here's the tree from the paper, based on 1,499 genetic loci:


All of the green branches in this tree are clades that are conventionally regarded as arachnids. This shows the horseshoe crabs (Xiphosura) deep within that group, as the sister group to the Ricinulei (hooded tick spiders, which aren't true spiders), a rare group that now lives only in South America and Africa. Interestingly, both groups have a fossil record going back to the Carboniferous, and the Ricinulei have unusually thick cuticles--though not as thick as those of horseshoe crabs, obviously.

Paper (paywall, but inquire for pdf): https://academic.oup.com/sysbio/advance-article-abstract/doi/10.1093/sysbio/syz011/5319972

[Shonisaurus]

I remember that our Biology teacher at the Institute in the year 1985-1986 told us that crabs, including crustaceans such as prawns, prawns, lobsters and barnacles were relatives of arachnids, so the news did not surprise me.

The majority of crustaceans and shellfish whose food is the most expensive of the world gastronomy are familiar to the tarantulas, for example.

I saw some time ago a report of the last Indians of the Amazon where they hunted tarantulas, as a means of sustenance and then ate them as is logical and offered to the camera report and testify that its taste is almost identical to seafood as are necoras or the lobsters.

[Halichoeres]

Another study with what is probably better taxon sampling finds that terrestrial chelicerates are monophyletic after all, which would imply that horseshoe crabs are not arachnids and have always been marine.

https://www.nature.com/articles/s41467-019-10244-7

[Halichoeres]

Another sally in this debate, with a big molecular dataset and some careful fossil calibrations. The authors conclude that arachnids colonized land once, in the late Cambrian or early Ordovician. That age is based on a model of molecular evolution; if correct, we might eventually find fossils to support it, although to the best of my knowledge the earliest known fossil is Silurian in age.

Their phylogeny also finds pycnogonids outside of Arachnida:



Paywall, but let me know if you want the pdf: https://www.sciencedirect.com/science/article/abs/pii/S1467803920301201?via%3Dihub