The neurobiology of house-hunting at sea

Max-Planck-researchers study the life-cycle of marine ragworms


Tübingen, 11th April 2013. As everyone who’s been through the house-hunting process knows, location is key when you’re searching for your ideal home. Likewise, for marine invertebrates like corals, worms, snails and clams - which often begin their lives as free-swimming planktonic larvae before settling to the seabed - finding the ideal place to settle is imperative for their future survival. But how do these microscopic larvae navigate the vast oceans to find their ideal future home? Studying larvae of the ragworm Platynereis dumerilii, scientists at the Max Planck Institute for Developmental Biology in Tübingen have now discovered some of the internal signals that trigger the complex life-cycle transition of larval settlement. As Gáspár Jékely and his team report in the current issue of 'PNAS - Proceedings of the National Academy of Sciences', a small neuronal signalling peptide plays a key role in initiating the Platynereis larval "house-hunting" process and eventual settlement to the seabed.

"Platynereis is one of the few marine species that are easy to grow in the laboratory and the larvae have a simple nervous system," Gáspár Jékely is pointing out the benefits of his favourite animal. In Platynereis, as in many other marine larvae, the transition of larvae from a planktonic to a ground-dwelling, or benthic, lifestyle is controlled by a simple organ, the apical organ, located in the larva’s head. "This organ is composed of several different types of neurons with chemosensory functions," explains Jékely, who leads the research group "Neurobiology of marine zooplankton" at the MPI in Tübingen.

While the role of the apical organ in initiating larval settlement was undisputed, it was unclear which signalling molecules the apical organ uses to achieve the life-cycle transition of larval settlement. In extensive experiments, testing a wide variety of neuropeptides, Jékely’s team finally found the perfect candidate - the myoinhibitory peptide (MIP). As the scientists observed, adding synthetic MIP to the water clearly affected the ciliary band that surrounds the Platynereis larva like a belt and is used for swimming. After exposure to synthetic MIP, the usually constant beating of the larval cilia was more frequently interrupted by long pauses. Immediately the larvae began to sink and upon reaching the bottom, they crawled around the culture dish in an exploratory manner. "It's amazing how a single neuropeptide can initiate such a complex behavioural change," says Markus Conzelmann, one of the lead authors on the paper.

As molecular tests revealed, some of the neurons in the apical organ of Platynereis larvae produce high amounts of MIP. "At the same time, these cells have chemosensory function - so they can respond directly to an environmental stimulus by releasing MIP," explains Elizabeth Williams, the paper’s other lead author. Which substances the larvae may respond to is still unclear. It is, however, likely that they detect chemicals from food sources on the ocean bed that point the way to a favourable habitat in which the larvae can grow to maturity. The receptor through which MIP exerts its effects, identified with the help of researchers from the MPI for Heart and Lung Research, is located on adjacent neurons of the apical organ. These cells, too, have neurosecretory function. Jékely therefore assumes that MIP is at the top of a still unknown neuroendocrine (hormonal) signalling cascade.

Studying Platynereis is all the more interesting as marine annelid worms evolve slowly and have retained many ancient characters. We know from the fossil record that annelids already existed in the Cambrian period more than 500 million years ago, and for millions of years these worms have lived in a relatively stable marine environment. The newly discovered Platynereis signalling system can be traced even further back in evolution. "Peptides related to MIP also regulate transitions in the life-cycle of distantly related cnidarians like corals and sea anemones", explains Elizabeth Williams. Outside of the ocean, the MIP receptor-ligand pair is also found in insects. In some insect species, MIP and its receptor are involved in the regulation of hormones and thus in insect metamorphosis, another important animal life-cycle transition. The MIP receptor-ligand pair has been conserved during evolution as a key control mechanism. This highlights its significance for the development of individual larvae. After all, it is vital for the planktonic larvae to find a suitable habitat within a few days. The larvae of many species don’t feed while swimming in the plankton, but are nourished by a small supply of maternal yolk. "For those larvae, settlement is a matter of life and death", says Gáspár Jékely. Finally, the researcher points to the larger context of his work: Tiny as a single larva is, the migration and settlement of swimming marine larvae as a whole is of great ecological importance and determines the dispersal range and success of many marine animals.



Original Publication:
Markus Conzelmann, Elizabeth A. Williams, Sorin Tunaru, Nadine Randel, Réza Shahidi, Albina Asadulina, Jürgen Berger, Stefan Offermanns, and Gáspár Jékely
Conserved MIP receptor–ligand pair regulates Platynereis larval settlement
PNAS 2013 ; published ahead of print April 8, 2013, doi:10.1073/pnas.1220285110
www.pnas.org/cgi/doi/10.1073/pnas.1220285110


Contact:

Gáspár Jékely
Phone: +49 7071 601-1310
E-mail: Opens window for sending emailgaspar.jekely[at]tuebingen.mpg.de


PR (Public Relations)
Phone: +49 7071 601-302
E-mail: Opens window for sending emailpresse-eb[at]tuebingen.mpg.de




Apical organ. Picture: MPI for Developmental Biology.

Apical organ. Picture: MPI for Developmental Biology.

Platynereis larva. Picture: MPI for Developmental Biology.

Platynereis larva. Picture: MPI for Developmental Biology.

Platynereis larva. Picture: MPI for Developmental Biology.

Platynereis larva. Picture: MPI for Developmental Biology.