Every so often someone writes a paper with a really eye-catching title. I've come across two of these this week: When Zombies Attack! Mathematical modelling of an outbreak of zombie infection (no, honestly, I'm not making it up! You can read the original paper here) and Deep-sea, swimming worms with luminescent 'bombs' (Osborn, Haddock, Pleijel, Madin & Rouse, 2009). I'm not going to say anything more about the zombies - if you're interested, there is some commentary on this Respectful Insolence post. But I thought the worms were really cool.
Polychaetes are segmented worms that belong to the same phylum (Annelida) as earthworms. You may well have seen a polychaete if you've investigated a rock pool. When I was a kid we called them 'sea centipedes' because they do appear to have an awful lot of legs! But these are not really legs. The name 'polychaete' means 'many-haired', and most body segments of these worms (apart from a few at the head end) have a lobe on each side, and each lobe (or 'parapod') has a bunch of bristles sticking out of it.
Polychaete lifestyles are quite variable - the ones I knew as a child ranged around the nooks and crannies of rockpools, looking for something to munch on. But others live in burrows, and still others are free-swimming. The ones described by Osborn et al. are deep-sea worms that were found at depths ranging from 1863m to 3793m, living either close to the sea floor or around 400m above it. And as you'd expect from that description, all of them swim very well, using a sinuous side-to-side movement of their bodies that's aided by the many long bristles protruding from their parapodia. And they're blind.
But what I love about these little critters (they're between 18 & 93mm long, depending on species) is the fact that five species are described as having luminescent 'bombs' :-) Bioluminescence is well-known in a number of deep-sea organisms: many fish, & squid, for example. It can serve a number of functions - as a lure to draw in prey; in species recognition & communication; in camouflage. Since the worms described by the research team are blind, they can't be communicating using light signals, so what's going on?
All five species have "four pairs of lateral, ellipsoidal organs on anterior segments that produced brilliant green bioluminescence when autotomised" (Osborn et al. 2009). Autotomised???? Autotomy is the term used to describe the event where an animal spontaneously loses a body part - the way some lizard species can shed part of their tails when attacked. It may be that these worms shed their little glowing oval organs when under attack - since they burst into intense luminescence when released, these 'bombs' would certainly have the potential to act as distractors, drawing the attention of a potential predator while the worm makes its escape unseen.
To paraphrase the opening lines of the Star Trek shows: "Deep ocean - the final frontier" There's just so much we don't know about this last great unexplored earthly ecosystem.
K.J.Osborn, S.H.D.Haddock, F.Pleijel, L.P.Madin & G.W.Rouse (2009). Deep-sea, swimming worms with luminescent 'bombs'. Science 325: 694