Marine larvae in Cretaceous amber-important insight into the evolution of parasitic life habits of epicaridean isopods
Résumé
Isopods-generally known as woodlice-are crustaceans which not only managed to establish a terrestrial lifestyle from primary marine ancestors, but also show a great diversity in numerous aquatic lineages. Isopods are found in all water depths, occurring in the deep sea as well as in freshwater. Ecologically they perform various functions, from decomposing dead plant matter to hyperparasitism. Fossil isopods are relatively rare and in most cases are not very informative from a paleoecological view. Non-compressed, three-dimensional fossils, at best from Konservat-Lagerstätten, provide more details and may provide deeper insights into the ecology via functional morphology. Even better comparable to modern forms are specimens preserved in amber. There are quite some records of terrestrial as well as very few marine isopods from amber deposits from all over the world. However, we present here new findings that make a special case. Most isopods have offspring very similar to the adults, yet the new specimens are true larvae. Additionally, they represent the so far oldest record of a lineage of obligate parasitic isopods with a complex life cycle. The specimens are larvae of Epicaridea. As their name suggests, adults are found on shrimps (and other crustaceans) on which they feed as adults. Compared to other parasitic isopods the larvae of epicarideans are very small and feed on a different organisms than their parents, namely on small copepod crustaceans. Their development including the host change involves three distinct larval stages. Our findings represent the last true larval stage that searches for the final host while being part of the plankton. With the help of fluorescence microscopy we were able to reveal delicate structures on the very small fossils (ca. 0.5 mm body length) down to single sensory setae. We compare the morphology of our fossils with the available information on modern forms as well as to the only other fossil record of epicaridean larvae. Our findings provide an exceptional glimpse on the "morphology through time" for this very special isopod group.