Over the past ten years, MV curator Tim O’Hara has been snooping through museum collections all over the world, collecting data about brittle stars for a major mapping exercise. He compiled nearly 7000 samples from 250 common species of brittle stars from 24 different museums and discovered something quite unexpected about their distribution.
Brittle stars, or ophiuroids, are echinoderms closely related to sea stars. They have five long, flexible arms attached to a central body. Unlike sea stars, brittle stars are quite active and fast-moving. They are ideal for this kind of large-scale mapping study because they are found all over the globe in a variety of habitats.
A brittle star (Conocladus australis) from southern Australia wrapped around a whip-coral.
Image: Julian Finn
Source: Museum Victoria
Biogeographers – scientists that study the patterns of distribution of life – have long observed that certain species are associated with particular environments. This makes sense; an animal has particular requirements of temperature, salinity, depth, food availability, and won’t survive where these conditions don’t exist. However in the deep-sea, environmental factors are not very variable - deep water is cold and dark everywhere. Correspondingly, it has been assumed that the fauna in the deep-sea won’t vary much, or at most, certain species would be confined to particular oceans.
It turns out this assumption is not necessarily true. Tim's brittle star study found that there are distinct bands of species distribution not only in shallow water environments, where conditions can be very variable and distinct, but in the deep-sea. Deep-sea brittle stars are found in the same latitudinal bands as their shallow-water relatives, and it’s not yet clear why.
Map showing the overlapping distribution of tropical, temperate and polar brittle stars.
Image: Tim O'Hara
Source: Museum Victoria
Tim thinks the pattern he's discovered might be related to the life history of brittle stars. As he explains, the distinct bands might be due to the way currents disperse larvae. “A lot of these animals have very yolky eggs and there’s a theory that in cold water, eggs go into suspended animation and float on the currents for perhaps a year. Some don’t need to feed – they have all the energy they need to go through metamorphosis to juvenile stage.”
“It’s a funny strategy that an animal would just throw eggs into the current and hope for the best, but obviously it’s successful because they get around. We’re doing a lot of genetic work at the moment over this study area and we’re getting things that are almost identical 7000km apart.”
Tim’s study, co-authored by Ashley Rowden and Nicholas Bax, was published in Current Biology. This project was generated as part of the Marine Biodiversity Hub, a multi-institutional research program funded by the Australian Government’s Department of Sustainability, Environment, Water, Population and Communities.
O'Hara,Timothy D., Ashley A. Rowden, Nicholas J. Bax. A Southern Hemisphere Bathyal Fauna Is Distributed in Latitudinal Bands, Current Biology, 8 February 2011 (Vol. 21, Issue 3, pp. 226-230)
Marine Biodiversity Hub
Discussion of this study elsewhere:
The Age: 'Scientists discover deep-sea creatures play in the same band'