Size does matter


 

18 November 2010
 
Size does matter to Dr Anastassya Maiorova, a post-doctoral researcher with the Russian Academy of Sciences, who is studying sipunculids, a group of unsegmented marine worms, on the CReefs expedition.
 
While some sipunculid species are only two-millimetres long, and most are less than 10 centimetres, some species, such as the Sipunculus nudus, eaten as a delicacy in China, have been known to grow to 60 centimetres.
 
But, Dr Maiorova, explains, scientists only measure the worms from the posterior terminal point of the body to the prominent anal opening – which, in sipunculids, is just below the introvert.
 
"Sipunculids have what is called an introvert, which is like a neck, with tentacles and a mouth at the tip. When they are hiding, in shells or coral or sand, they can extend their introvert for feeding. When they are threatened, they can withdraw it," Dr Maiorova explains.
 
"So we don't include the introvert when we measure the sipunculid, because it can be stretched out to up to 10 times the length of the body," she says.
 
Dr Maiorova is exploring the relationships between genera and species within the sipunculid phylum by comparing specimens' DNA with their morphology.
 
The morphology includes the shapes and features of the body surface, including the papillae (spikes which allow some of the sipunculids to attach to the inside of shells or coral); the internal anatomy; and the hooks and tentacular crown.
 
"I am most interested in the organisation of the tentacular apparatus, which is very important to the taxonomy of the species. So I am looking for the species in which tentacles are arranged in different ways" she says.
 
Fossil records suggest that the morphology of sipunculids has changed relatively little since the Cambrian period. Debate among taxonomists about the evolutionary classification of sipunculids, however, has only recently been resolved. Scientific papers published in the past year show that sipunculids are much more closely related to annelids, which are segmented worms, than to other phyla.
 
Dr Maiorova's work may contribute to better understanding of these evolutionary relationships.