‘Worms’ is a very general term that we tend to use for the variety of invertebrates that are soft-bodied & have a tubular body with a mouth at one end & an anus at the other. The familiar earthworm belongs to a group of worms called the annelids – worms with segmented bodies. There are about 10,000 species of annelids, which as well as earthworms include polychaete worms (we used to call them ‘sea centipedes’ when I was a kid, because the clumps of stiff hairs that bristle from each segment look a bit like legs) & leeches. Most polychaetes live in the oceans, where their lifestyles range from active predator to sedentary filter-feeder. I thought the most unusual polychaete was the Pompeii worm, which lives around hydrothermal vents along mid-ocean rifts. But now I find there’s one with an equally odd lifestyle – the bone-eating worm Osedax.
These are truly amazing little animals. In the depths of the ocean they find the carcasses of dead whales – & feed on the bones. And their feeding method is quite extraordinary – with no mouth or stomach, the worms use thread-like ‘roots’ that infiltrate the bones. The job of obtaining nutrients from the bones is done by endosymbiotic bacteria (belonging to the taxon Oceanospirallales) living in the worm’s ‘roots’.
Photo: Greg Rouse / South Australian Museum and Univ. of Adelaide
Osedax has what’s best described as an opportunistic lifestyle – the availability of dead whales isn’t exactly predictable. On the vast scale of the ocean floor, whale carcasses are far & few between, & so you’d expect a particular set of features in the organisms that have adapted to exploit this particular niche: they should grow fast, mature early, produce large quantities of gametes which are simply released into the water, have no parental care, & spend long periods of time in the dispersal phase. In the lab, Osedax females produce 335 eggs/day (on average) & the dispersal phase – when larvae are actively swimming in the water column – lasts up to 16 days (Rouse et al. 2008).
Now, to get from ‘eggs’ to ‘larvae’ you need ‘fertilisation’. But where does the sperm come from? All the adult Osedax found on deep-sea whale bones are female. But when Rouse & his colleagues looked closely, they found dwarf males living within the gelatinous tubes surrounding the female’s bodies. These tiny males look quite simlar to late-stage Osedax larvae, but these larvae make sperm. What determines whether a larva develops into a male or a female?
Rouse et al. hypothesise that if a larva is going to become female, it has to land on whale bone that’s in a suitable state for worm colonisation, AND pick up an infection with Oceanospirillales bacteria. (You’d think that these would be passed on from the mother, but it seems that they’re picked up from the seawater.) Then the presence of adult females may be enough to trigger late-arriving larvae to become male. Given the difficulty of finding both habitat & potential mates in the deep dark ocean, where both are hard to come by, this sort of reproductive strategy makes a lot of evolutionary sense.
G.W. Rouse, N.G. Wilson, S.K. Goffredi, S.B. Johnson, T. Smart, C.Widmer, C.M Young & R.C.Vrijenhoek (2008) Spawning and development in Osedax boneworms (Siboglinidae, Annelida). Marine Biology doi:10.1007/s00227-008-1091-z (published on-line 10 December 2008)