- 28 April 2010 by Michael Marshall
Species: Symbion pandora
Habitat: In the eastern Atlantic Ocean – on the mouthparts of Norway lobsters (also known as Dublin Bay prawns or langoustines)
There's no question that discovering a new species is very cool. But how about discovering a new phylum?
A phylum is a broad division in taxonomy: all vertebrates, for example, from fish to humans, are in the chordate phylum. In 1995, Peter Funch andReinhardt Møbjerg Kristensen, both then at the University of Copenhagen, Denmark, discovered an animal so unlike any other that a new phylum –Cycliophora – had to be created just for it.
Symbion pandora, as they called the new creature, is a tiny animal with a complex body and a bizarre life cycle. It still baffles biologists 15 years after it was formally described, and the latest work on its nervous system isn't helping matters.
Opening Pandora's tube
Symbion is a tiny animal about half a millimetre long, shaped like a bulbous tube with a ring of tiny hairs – cilia – at one end.
They live on the hairy mouthparts of Norway lobsters, with tens or even hundreds per lobster. They feed on bits of leftover food and seem to beharmless to their hosts.
The ring of cilia drives food particles into a funnel, which delivers them into a U-shaped digestive tract. Any indigestible fragments are ejected through the anus – with the help of a muscular sphincter.
Symbions have no legs, but keep themselves in place with a short stalk ending in an adhesive disc that attaches to their host lobster.
Things start to get complicated when you consider their life cycle. Let's start with a feeding animal living on a lobster's mouthparts: this individual – it's hard to assign a sex – can then produce one of three kinds of offspring: a "Pandora" larva, a "Prometheus" larva or a female.
The Pandora larva develops into another feeding adult – a straightforward case of asexual reproduction. By contrast, the female remains inside the adult and awaits a male – but, attentive readers will be crying, what male?
The answer lies in the Prometheus larva. This attaches itself to another feeding adult, then produces two or three males from within itself. These dwarf males, which are even more internally complex than the other stages, seek out the females and fertilise them – though the details are unknown.
Once the female has been fertilised, she leaves the adult's body and hunkers down in a sheltered region of the lobster's mouthparts. Her body, no longer needed, turns into a hard cyst. Inside this, a fertilised egg develops into yet another stage: the chordoid larva.
In due course this larva hatches and swims off to colonise another lobster. Once it has attached itself to one, it develops into another adult and the cycle begins again.
Mysterious origins
It seems there are two other species of symbion: one lives on American lobsters, and another, which may not be a distinct species, lives on European lobsters.
Nobody knows how the symbion phylum arose in evolutionary history, or where it fits into the evolutionary tree/thicket/web (you can choose your own metaphor).
Genes and brains
Studies of their genes suggest that they may be related to the entoprocts and bryozoans, two groups of marine animals that look like goblets on long stalks. The "goblets" are topped with rings of cilia called crowns with which the animals feed – rather like the symbions do.
There is just one problem: no one is really sure where the entoprocts belong in the tree of life either.
Another way of looking at the problem is to compare their nervous systemswith those of other animals. The symbion phylum (along with the entoprocts) is part of a larger group called the Lophotrochozoa – so which of these do their nervous systems look like?
Bizarrely, the answer may be "none". The latest study, by Kristensen and colleagues, shows that the various symbion larvae have nervous systems quite unlike those of other lophotrochozoans. They seem to be missing several of the key components – perhaps because they lost them at some point in their evolution.
It seems the symbions will keep their aura of mystery for a while yet.
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