To care for our living treasures, we first need to know what they are.
If you want to start an argument among biologists, ask them what they really mean by "a species". There are probably more views about what constitutes a species than there are biologists!
That’s where genetic analysis can really help. Individuals of the same species are likely to be genetically similar to each other, whereas individuals of different species will be much more genetically distinct.
This distinction between genetic variation within and among species arises because of the way sexual organisms, whether plants or animals, reproduce. In producing the next generation, sexually reproducing organisms contribute their genes, which are thus shared among the offspring. Over many generations, these genes get shared widely across the members of the species, blending away most differences.
By contrast, members of different species do not (usually) share genes, and the differences between them grow over time as they adapt to their different ecological niches. In addition, these genetic differences are accentuated by newly arising mutations, rare random changes in the DNA that, provided they have little effect, can accumulate over time.
We can use the pattern of relative genetic similarity within a species and clear differences among species to identify cryptic species: genuine species that nonetheless look very similar. One example from my own work concerns a long-standing argument about some species of chiton.
Chitons are molluscs, shellfish if you like, with eight shelly plates, surrounded by a flexible girdle. They are exclusively marine and are normally found attached to rocky substrates, intertidally, subtidally, or even deeper. Aotearoa New Zealand has a surprising diversity of species: just over 60 of the 1000 or so living species are known to occur here.
A quick fossick in a rock pool will reveal our most common species, the snakeskin chiton or papatua, known scientifically as Sypharochiton pelliserpentis, which occurs all around our coasts. It lives in the rocks among oysters and barnacles, where its shell is often eroded and worn. When it lives deeper, just below low tide, its shell is often cleaner and beautifully marked (see photos). The amount of variation in shell sculpture and colour is considerable.
Some chitons from the low-tide zone, especially those found in the holdfasts of bull kelp/rimurapa, are also uneroded, with clear sculpturing on the shell and often with a brighter, sometimes blue, girdle. They were given the name Sypharochiton sinclairi, but more than 150 years of debate was not able to decide if they were just a form of S. pelliserpentis or a species in their own right.
Genetics to the rescue! By looking at the variation in a particular gene known to be informative about species status in many animals, we have resolved this argument. Sure enough, all the S. pellisperpentis we tested were genetically close to one another and the same applied to the individuals of S. sinclairi. But they were genetically quite unlike each other. In short, there were two species.
At the same time, we looked at two northern species, S. themeropis from the Kermadec Islands and S. aorangi from the Northland east coast. It turned out they were not different species as had been thought: all individuals we examined were genetically very similar.
You can see this information graphically in the chiton family tree, which also shows that S. pellisperpentis and S. sinclairi are possibly not each other’s nearest relatives. S. sinclairi was closer, genetically, to S. themeropis (although this relationship is not statistically robust; more data are needed).
You might ask, why do we care? The short answer is that you can’t look after something if you don’t know what it is. The first step in understanding biodiversity is the identifying and naming of species. Ecology, conservation, and zoology rely on species identity.
Indeed, being able to unambiguously identify these chitons showed us for the first time that, unexpectedly, both species can occur on and in the holdfasts of rimurapa. Although S. pelliserpentis was rare in this habitat, there is not an absolute ecological segregation. Our finding is surprising because competition between closely related species is often thought to mean they cannot occur in the same habitat.
Next time you come across a bull kelp holdfast washed up on a southern beach, have a look on the underside for any chitons. You might well find one (or even two) species of Sypharochiton. But can you tell the difference?
Hamish G. Spencer is Emeritus Professor in the Department of Zoology at the University of Otago. Each week in this column, writers address issues of sustainability.
