Sibling species a cryptic plot twist

Hamish G. Spencer takes a look at sibling species.

The plot of Shakespeare’s Twelfth Night hinges on the inability of Duke Orsino and Countess Olivia, the characters with authority, to distinguish between Viola (disguised as a man and using the name Cesario) and her twin Sebastian. After the play’s climax, however, the difference is apparent to all and everyone (or almost everyone) lives happily ever after.

A similar confusion and revelation can occur in biology with what are known as sibling (or, more accurately, cryptic) species. Taxonomists, the authority figures of biological classification, at first fail to notice the differences, just like the duke and countess in Shakespeare’s tale.

Sibling/cryptic species are species that look very similar to each other, but are truly separate, in that they only breed with others of their own species.

Increasingly today, the distinctive nature of sibling species is often exposed by genetic studies. If just one species were present, different individuals would be genetically close. But the genetic variation of two sibling species will fall into two quite separate groups. And, as in Twelfth Night, after the big reveal, the differences between the siblings may become startlingly obvious.

Such a scenario has played out in some of my own research on marine snails living around the coast of Otago. A common species of topshell that frequents the mid-tidal zone of rocky shores here goes by the name of Diloma aridum. Indeed, this snail lives all around the coasts of Aotearoa New Zealand, including Rckohu/Chatham and Maungahuka/the Auckland Islands. It is especially abundant in small rock pools and under small boulders, where it grazes on small algae.

On a field trip to Kaka Point in South Otago, my colleagues and I found what we took to be Diloma aridum in a new habitat: on and under the holdfasts of rimurapa, the giant bullkelp (Durvillaea antarctica) at and below the low-tide mark.

Unlike true plants, which anchor themselves into the ground with roots through which they absorb water and nutrients, many marine seaweeds simply glue themselves, using a holdfast, to their rocky substrate. No nutrients, let alone water, are delivered via the holdfast; its sole purpose is to keep the alga in one place.

The holdfasts of rimurapa are particularly interesting, nevertheless, in that they are home to a host of different small animals: limpets, chitons, snails, worms, crustacea, bryozoans and more. These small creatures sometimes bore tunnels into the holdfasts, in which a wide variety of other organisms can shelter. Indeed, some of the holdfast species seem to be confined to this specialised habitat.

When we looked at the genetics of our new samples of Diloma, we discovered to our surprise that they were genetically distinct from Diloma aridum. We had found a new species, which we named Diloma durvillaea, after its algal home. The two species are sibling species in another sense too, being each other’s closest relative, "sister species", in the evolutionary tree of Diloma snails (see the figure).

Moreover, when we looked more carefully at the shells of our new species, we found we could easily distinguish them from Diloma aridum. The differences were obvious once we knew to look. The shell of our new species (see the figure) has a rounder profile than D. aridum, with obvious ribs spiralling around the outside of the shell. It is a dull jet black and mostly lacks the pale-yellow flecks that are common on D. aridum.

As far as we know, these sibling species do live happily ever after. They can both be found at the same locations (such as Kaka Point and St Clair), although they probably never meet, since they occur at different levels on the shore. D. durvillaea also has a less expansive range, living, so far as we know, only on Maungahuka/Auckland Islands and the eastern and southern coasts of Te Waipounamu/the South Island. It is absent from Rckohu/Chatham and Te Ika-a-Māui/the North Island.

The Twelfth Night scenario is surprisingly frequent in biological research in Aotearoa New Zealand. We are, it seems, still in an age of discovery when it comes to our biological taonga. Amazingly, at the same time in the same place, this same group of researchers also found a cryptic species of rimurapa/bullkelp. But that is another story with perhaps a different Shakespeare play to reference!

Hamish G. Spencer is sesquicentennial distinguished Professor in the Department of Zoology at the University of Otago