Friday, 15 May 2015

How The Bilaterian Got Its Tentacles

The animal kingdom can be split into two broad categories: those with bilateral symmetry and those without. Those without are creatures like jellyfish and corals. Those with bilateral symmetry, the bilaterians, include ourselves and most groups of phyla, including arthropods, molluscs and the plethora of worms which inhabit most of Earth's habitats. The earliest animals did not possess bilateral symmetry and were most akin to sponges. We have fossils very similar in form to jellyfish which are close in age to the proposed date for when such creatures evolved. Bilaterians have a proposed date of origin, but what is the first bilaterian is more contentious. A recent study, however, has revealed that this ancestor had tentacles.

Previously there were two competing hypotheses regarding their anatomy. The first suggests that this was a worm-like creature devoid of appendages, possessing a simple nervous system, yet importantly lacking the body cavity known as the coelom, a structure involved in digestion. After the first groups of bilaterians diverged away from their common ancestor they developed coeloms. The other hypothesis states that the first bilaterian was rather more complex, with a well developed nervous system, appendages and a coelom. In later groups the coelom underwent adaptations, and in some cases, was lost altogether. Originally the coelom was one of many gastric chambers in a non-bilaterian jellyfish-like ancestor.

A team led by Professor Elena Temereva of the Department of Invertebrate Zoology at Lomonosov Moscow State University has found strong evidence in support of the second hypothesis. Their work focused on a species of brachiopod called Lingula anatina, a species virtually identical to its 500 million year old counterparts. Lingula is a brachiopod and brachiopods alongside three other groups of invertebrates, the phoronids, entoprocts and bryozoans, possess a feeding structure, the lophophore, a ring of small tentacles. As such they have been grouped together under the banner of the lophophorates.

Several phylogenetic studies have previously suggested that the lophophorate group is not actually biologically valid and is an assemblage of physiologically similar creatures which do not share an immediate common ancestor. Temereva's team showed otherwise, using immunocytochemistry techniques, laser confocal microscopy, 3D-reconstruction and transmission electron microscopy. 'Our study shows that there is a group of the lophophore animals among the bilateria, which includes the largest variety of types of animals.' Considering that we can now say that the two major groups of bilaterians possess tentacles, it is likely that the common ancestor of both groups - the first bilaterian - possessed tentacles also.