ECR Feature: Shahan Derkarabetian on the biogeography of opilionoid arachnids

Shahan is a postdoc at the Museum of Comparative Zoology, Harvard University. He is a systematic and evolutionary biologist with a keen interest in Opiliones, an order of arachnids. Shahan shares his recent work on the opilionoid family, Triaenonychidae, investigating the role of geological events and past climate on their geographical distribution.

Shahan Derkarabetian sifting leaf litter for triaenonychids in southern Australia. Photo courtesy Jennifer Trimble.

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Institute. Museum of Comparative Zoology, Harvard University

Academic life stage. Postdoc

Major research interests. Opiliones, Systematics, Evolutionary Biology

Current study system. My current and ongoing study system is a group of arachnids called Opiliones (a.k.a. harvesters, harvestmen, daddy-long-legs). A plethora of reasons make Opiliones interesting, including their diverse morphology and behaviour. There are Opiliones that are covered in sharp spines, some that are as green as moss, some that aggregate in groups numbering in the1000s, some that build mud nests, and so much more. Another reason is because there is still much to discover about their basic biology and ecology, and there are still many new species that need to be described, even from places that are considered well studied, like North America.

Recent paper in JBI. Derkarabetian S, Baker CM, Giribet G. 2021. Complex patterns of Gondwanan biogeography revealed in a dispersal-limited arachnid. DOI: 10.1111/jbi.14080

Motivation behind this paper. This study focused on one family of Opiliones, called Triaenonychidae, that we have been working on for many years. This group is widely distributed throughout the temperate forests of the southern hemisphere in continents associated with Gondwana. Our motivation for this study was to determine their biogeographic history through geologic time, and how geologic events and climate shaped their present-day distribution and diversity. Previous research suggested very complex patterns of geographic distribution not found in other studies of Gondwanan taxa, which is especially interesting given their low dispersal ability. We wanted to explore this complexity in more detail using modern phylogenomic approaches.

A representative of a new and undescribed genus of Triaenonychidae from southern Australia. Photo by Shahan Derkarabetian.

Key methodologies. We relied on excellent taxon sampling for this study; we included ~80% of the 100+ genera in this family. These specimens were either acquired through fieldwork we conducted ourselves over the last 20 years, or through borrowing specimens held in natural history museum collections all over the world. Using modern sequencing approaches we not only included all the freshly collected specimens but were also able to sequence DNA from historical museum specimens collected up to 100 years ago. Our thorough sampling allowed us to infer both large scale inter-continental patterns as well as more regional geographic patterns within continents.

Unexpected challenges. The big challenge for this research was integrating all the sources of information to form a cohesive picture of the biogeographic history of this group. We inferred a phylogeny with dated taxonomic splits and had to correlate these relationships and dates with both the geologic and paleoclimatic history. In order to tie these disparate sources of information together I had to do some deep dives into the literature for geologic reconstructions, paleoclimate reconstructions through time, and learn a bit about how plant fossils are used as indicators for past regional climates. Much of this was new to me as a scientist, so the challenge was becoming familiar with it enough to put it all together and tell the story.

Shahan Derkarabetian searching for triaenonychids under woody debris, in typical southern temperate forest habitat of Tasmania, Australia. Photo courtesy Marshal Hedin.

Major results. In our study system, we found unexpectedly complex phylogenetic relationships and patterns in the geographic distribution of different lineages across continental landmasses. For example, the species that are endemic to New Zealand were found across 5–6 genetically distinct lineages, suggesting multiple transitions to New Zealand. Previous studies have tended to report fewer numbers of lineages within continents. Yet as evidenced from New Zealand taxa in our study, there can be incredible intra-continental diversity. We think these differences between our study and previous works have to do with the biology of these organisms. Triaenonychidae are low-dispersal organisms, so they are more directly affected by geologic events. However, they are not completely without dispersal, so they do have those very rare events where there is some chance of a long-distance dispersal event happening. This particular balance of dispersal ability led to geographic patterns and diversity shaped by both geology and rare long-distance dispersal across small and large geographic ranges.

Next steps. We want to take more detailed looks at the biogeography of smaller more regional groups within the Triaenonychidae with better species level sampling. The biological characteristics of triaenonychids, specifically low dispersal ability and specific microhabitat preferences, make them great candidates for biogeography at large and small geographic scales. As such, we expect to find equally new and complex geographic patterns within continental regions. Another aspect we were unable to explore is rare long-distance dispersal. For example, there is a species endemic to the isolated Crozet Islands, for which we were unable to obtain samples for this study. With newly acquired samples, we can ask: how and when did that species get there, and where did it come from?

If you could study any organism on Earth, what would it be? I think I would still study Opiliones, but I would want to study the species that existed 200, 300, and even 400 million years ago. I do not mean as fossils, but as they existed in those times, which would mean I would need a time machine. There were many time periods that were critical to the formation of much of the modern-day diversity in Opiliones, and it would be very cool to see what they looked like, where they were distributed, and how diversity formed through geologic time.

The joys of fieldwork in a campervan. Photo by Shahan Derkarabetian.

Anything else you would like to share? Fieldwork! This was of course the best part of doing this type of research. For this project I did quite a bit of fieldwork in south-eastern Australia, including Tasmania. Collecting Opiliones gives us a good idea of their preferred habitat, which helps infer biogeographic history. It also provides hints at the true amount of diversity that exists, especially what still needs to be described. Fieldwork often leads to new species discoveries, and Opiliones are no exception. From a two-week trip through southern Australia, we collected about 30–35 species, only five of which are formally named and described. As a taxonomist, this means I still have a lot of work to do!

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