New research published in the journal Molecular Ecology has discovered that the genetic structure of populations of the southern rock lobster (Jasus edwardsii) at two Australian sites is unexpectedly and radically diverse – a phenomenon known as “chaotic genetic patchiness”.
The long-lived lobster has a two-part life cycle that can span more than 30 years. Mature females lay up to half a million eggs, which hatch into a larval stage. The larvae spend one to two years at sea slowly developing and maturing. It’s not smooth sailing: the mortality rate can reach as high as 98%. Finally, the survivors will settle on a coastal reef where they will stay, often being “recruited” into a pre-existing community.
It has long been known that populations of J. edwardsii were variable in both number and composition. The rate at which juveniles are recruited into them is also fluid, and a number of hypothetical mechanisms have been proposed to explain these characteristics.
Now Cecilia Villacorta-Rath, a PhD student at the University of Tasmania’s Institute for Marine and Antarctic Studies, has led a research team to study the genetic make-up of lobster populations in Tasmania and South Australia.
“By studying their DNA, we have shown that young lobsters settling on reefs are a random assortment of unrelated individuals from a large number of different sites,” she says. “Some locations along the coast seem likely to be more important than others, but these locations vary from year to year – it’s chaotic.”
The team found little evidence of natural selection at work once the larvae had settled, and furthermore concluded that non-biological causes were most likely behind the chaotic patchwork of genes found in J. edwardsii populations. In particular, the researchers suspect that the vagaries of ocean currents might be responsible for a single reef being populated with individuals from many and varied locations.
The researchers’ conclusion, then, is that the southern rock lobster is currently nearly unaffected by the directional evolution of natural selection. Instead it has left its fate to the whim of time and tide.
There are practical lessons for conservation and fisheries management in the work, too. It is important to manage all lobster fisheries, rather than concentrating on just a few sites.
“Controlling catch across the fishery with regional quotas or size limits would be less risky than relying on a limited number of Marine Protected Areas (MPAs), for example, as the main source of juvenile lobsters,” says co-author Caleb Gardner.