Crowned heads and their kin
Many a zoologist’s dream is a kind of checklist of all the characteristic features of an animal species – one that even lets you distinguish closely related species with certainty. Animal morphology – observing appearance, shape and structure – is often a helpful tool for such things. The “crown of thorns”, a weird spiky structure on the head of certain water fleas is one such example of a unique specific trait so far used to make such distinctions. And it would probably continue to be used, if it were not for the fact that a genetic study of these miniscule aquatic crustaceans, led by Dr. Christian Laforsch from LMU and Dr. Adam Petrusek from Charles University in Prague have now shown that even such prominent characteristics as these can be totally misleading. “The thorns around the animals’ heads only develop if they happen to be sharing their habitat with predatory tadpole shrimp,” Laforsch reports. “This means that, while the crown does offer protection against enemies, it is not characteristic to any of those species we studied. Not only have we detected a new, fascinating defense mechanism; we have also proven how valuable genetic data is to the study of ecological and evolutionary biological relationships.”
Distinguishing related species by external traits is often difficult, even for the experts. So-called cryptic species, in particular, are so similar that they can hardly be distinguished at all. Only at the genetic level, in the DNA, can they be uniquely classified by their genetic makeup. Which is why, for example, the genetic data of animals is now also used in taxonomical and ecological studies when studying the biodiversity of a particular habitat. This so-called “barcoding” allows us to record differences between species using selected genetic markers.
These can even be contradictory to morphological criteria hitherto used for classifying species. Dr. Christian Laforsch of the chair of Evolutionary Ecology research group at the Department of Biology II and Geobio Center Munich directed a study in which his associate Andreas Haas and colleagues Dr. Adam Petrusek (Prague/Czechia), Professor Ralph Tollrian (Bochum/Germany) and PD Dr. Klaus Schwenk (Frankfurt/Germany) detected such an error regarding two species of water flea.
The researchers showed that the crown of thorns found on the heads of some animals is not typical to one species only, but in fact develops only when it is needed, and irrespective of the species within a certain group of water fleas. The development of different appearances in different environments is referred to as phenotypic plasticity. ”Water fleas of the Daphnia genus can defend themselves excellently against predators,” states Laforsch. ”If the predators only occur some of the time, then the water fleas only develop their defense when it is needed. If no predators are present, then the costs for this protection are saved.”
Classical taxonomy, however, is the science of identifying species by the organism’s appearance alone. This is why genetic methods are gaining more ground in present day research. There are even efforts to characterize animal species by characteristic gene sequences, the so-called “barcodes”, to allow their identification by genetics alone. In the present study of the phylogenies of water fleas, the genetic data revealed that the crown of thorns so far used to distinguish the species Daphnia atkinsoni and Daphnia bolivari occurs in several genetically distinguishable lines, but provides no evidence of any relation.
”Yet the trait was linked to occurrence in certain habitats,” states Laforsch. ”The Daphnia with the pronounced crown of thorns coexisted in their habitat with the predatory notostracan Triops cancriformis. These animals have earned the title of “living fossils”, seeing as their basic blueprint hasn’t changed for more than 220 million years. If Daphnia detect chemical substances from this predator in the water, then they grow the thorns, making them not the most appetizing prey on the menu. Triops namely has bristles and sense organs around the mouth that get hooked up in the horns when they attack. Usually, the predator will simply let a Daphnia with a crown of thorns go.”
Overall, this is a fascinating example of the workings of evolution – in this case, the adaptation of prey to its predators. This previously unrecognized form of defense probably occurs all around us. After all, Daphnia species with crowns of thorns have been found in Europe as well as Greenland and South Africa. There, too, they coexist in their habitats with predatory notostracans. The study also underlines the importance of DNA barcodes and other molecular data for the classification of species, and for deciphering as yet unknown ecological links and interactions between species. (suwe)
A 'crown of thorns', an exceptional inducible defense, protects Daphnia against an ancient predator,
Petrusek, A., Tollrian, R., Schwenk, K., Haas, A. Laforsch, C.,
PNAS early edition, 14 January 2009
Dr. Christian Laforsch
Tel.: 089 / 2180 – 74252
Fax: 089 / 2180 – 74204