Question: which ones are closely related

Naval Shipworm answered on 21 Nov 2017:

Great question! There are two pairs of species that are more closely related than others are, but these pairs are still separated by millions of years of evolution, and I am not sure which of the two pairs is the most “close” – this would require careful study of their divergence time, and I am not an expert on this!

The first pair, the Abyssal Grenadier and and Snake Pipefish, are both fish, and are definitely close to one another. Together with the Orkney Vole and Scottish Crossbill, the fish are vertebrates, so these are all reasonably closely related (but still, their last common ancestor was over a hundred million years ago!) Of the other species in the cryptic zone, their closest relative to these vertebrates would be the common starfish.

The second pair, Baltic Clam and Naval Shipworms, are also relatively closely related. Both of these species are bivalve (two-shelled) molluscs, and both are members of the “Heterodonta” subclass. Despite their closeness, they still have had many millions of years to differentiate from one another! Brachiopods also look a little like molluscs, but are actually different in body plan, in lots of interesting ways!

The “odd one out” is definitely the Twisted Wing Fly – it is more closely related to the Baltic Clam, Brachiopods and Naval Shipworms than to the vertebrates and starfish, but a long way from the others – closer to other insects, like beetles, as mentioned above!