The paper in Communications Biology is here: http://go.nature.com/2EUy7CB
of the biggest questions in the history of life on Earth is why some
groups of organisms have enjoyed such evolutionary success that there
are now, literally, millions of species living today, whereas their
close relatives that
at similar times in the past have produced only a handful of species.
This was the question we had in mind when we started designing this
Caridean shrimp are members of decapod crustaceans, a group of largely aquatic arthropods that includes many familiar species such as crabs, hermit crabs, lobsters, crayfish, shrimp and prawns. Decapod crustaceans all have 10 legs (decapod literally means 10 legs) and a hard external shell. Carideans first appear in the fossil record in the Lower Jurassic, approximately 190 million years ago. There are now more than 3,500 species worldwide. Contrast this with Glypheidea, another group of decapod crustaceans that are even older than shrimp, and yet today only contain two living species. Why then, is there such a difference in the success of these closely related groups?
We found that transitions into freshwater habitats did correlate with increased speciation rates but the evolution of symbiotic lifestyles correlated with decreased speciation rates. The freshwater result was no big surprise. It is well known that the colonisation of new habitats can lead to increases in speciation as advantage is taken of the new habitat and lack of competitors, and our study supported this widely held hypothesis. A reduction in speciation with the evolution of symbiotic ecologies was, however, something of a surprise as we had hypothesised that a new mode of life might also result in new opportunities for speciation, much like the freshwater transitions. After some further research we realised that the poorly understood nature of the symbiotic relationships might be masking the truth behind this result, as it turns out that many symbiotic shrimp are parasitic, which means that they might impact on the fitness of their host and hence these relationships might inhibit rather than promote speciation.During their evolutionary history shrimp have spread from their ancestral marine habitats into freshwater lakes, streams and rivers while many marine species have evolved to live in symbiosis with other organisms, such as corals, fishes, tunicates and sponges. We hypothesised that these newly evolved lifestyles might have contributed to the relative success of caridean shrimp. To study this though we first needed a better understanding of their evolutionary relationships (phylogeny). Although many people have studied shrimp evolution no-one had pieced together all the evidence to give an overall summary of the relationships of the whole group. This was therefore the starting point of our work and we synthesised all the evidence to date for shrimp evolutionary relationships into what is termed a “supertree”. Our supertree gives a broad overview of all the information we have so far on shrimp phylogeny. Once we had our supertree we were able to explore the effects of newly evolved lifestyles on speciation within caridean shrimp. We did this by collecting ecological information telling us whether the species in our study lived in marine or freshwater habitats and, for the marine species, whether they were free-living or symbiotic. Mapping this information onto the phylogenetic tree allowed us to work out when these lifestyles evolved and whether there was any correlation with increases in the number of species.
main implication of our findings are very much tied in with the
complicated nature of the relationship between symbiosis and
speciation. Although we have no information currently on the
extinction risk of marine carideans, many of the symbiotic species
are closely associated with coral reefs, which are delicately
fragile ecosystems, that are
at elevated risk from the effects of anthropogenic climate change.
Symbiotic carideans could therefore be at particularly high risk of
extinction as their ability to recover from
species losses could
be greatly inhibited by the lower rates of speciation they exhibit.