Strange underwater sex– sexual strategies of the bone-eating worm

Going deep today, folks.

Bone-eating worms. On bone. Photo courtesy of Greg Rouse via BBC Nature.

When you’re a bone-eating worm without a gut or an anus, and you live 10,000 feet under the ocean where it is big, dark and you’re stuck in one place, it can be hard to not only eat, but to mate. In such an instance one sexual strategy is for males to permanently move in with a female. This is the case of most bone-eating worms, Osedax spp.

First, let’s talk about this ‘bone-eating’ business. Osedax, which means ‘bone-devourer’, is a fingertip long worm that is found on the skeletons of whales and other vertebrates that have given up life and sank to the bottom. It’s strange enough to image that a worm eats bones, but even stranger to consider that it has no mouth to do so. The worm relies instead on acid. When a worm settles on a bone it sets out ‘roots’, fingerlike projections that secure it to the bone (FYI: the ovaries are in the roots too – where else would you keep your ovaries other than next to the acid?). From these ‘roots’ an acid is released and the nutritious lipids are pried loose from the bone and absorbed by the worm.

Things get even stranger when we talk about sex of a bone-eating worm (I never thought I’d ever write such a sentence!). When a set of bones makes its way to the bottom of the ocean from say a whale or someone made to walk the plank and all the flesh is gone, the larvae of the bone-eating worm, which have been floating on the currents begin to settle. The larvae are undifferentiated in terms of sex – they are neither male nor female. If a larva lands on the bone itself, it becomes female. If it lands on another worm its male. But males never develop into adults and therefore remain very small (up to 1000x smaller than females). The dwarf males live within the female’s tube and use the rest of the energy reserves in their yolk sacs to produce nothing but sperm. Each female can have up to 100 males in her harem.

The large differences between male and female body size is found in other animals such as the blanket octopus, Tremoctopus violaceus, where females are 40,000 times bigger than males. The most famous example is the deep-sea anglerfish, Ceratias holboelli, where the tiny male bites the female and fuses his mouth to her body. His organs shut down and all he does is produce sperm. Here are some true facts about anglerfish by zefrank.

As I mentioned above, having small males that attach to the females is a sexual strategy when mates are scarce (another strategy is to collect sperm and store it for later). For bone-eating worms, however, it may also be an ecological strategy. In an environment of scarce resources (in this case skeletons), stopping males from reaching maturity prevents them from competing with females for food.

Osedax rubiplumus. Large worm is female with an itty-bitty male harem in her tube.

We have to pause to admit that this a strange worm. One that has no mouth but eats bones using acid released from its ‘roots’ and where females collect a harem of immature males that do nothing but release sperm. All at the bottom of the ocean.

Now let’s add one more twist.

The strategy of having tiny males and big females is a recent evolutionary trend for the bone worm; their ancestors had males and females of similar sizes. A new study led by Greg Rouse from the Scripps Institute of Oceanography, however, suggests that the trend is reversing back to its ancestral ways. Evolutionary reversals are very rare in the animal kingdom. A new species of bone-eating worm, Osedax priapus, has been found to have independent and fully developed males of the same size as females. The males of this species compete for the same resource as females and now have the advantage of mating with multiple females. The reason for the shift back to ancient sex? More bones scattered on the seafloor and therefore less competition for habitat. Again, ecology is driving evolution at the bottom of the ocean. We don’t yet have a clear hypothesis why there are more bones in Davy Jones’ Locker.

Nature is simply amazing. And one day if you decide to become a marine biologist you can say things like this [these are direct quotations from the paper]:

“…sunken bones are most likely a limited resource.”
“…a bone-eating adult male can access multiple females, whereas a dwarf male is confined to a single female’s tube.”

When my bones are laid to rest, put me where the worms can have me. I would rather be habitat than bones in a box.

The Science:

Greg W. Rouse et al. A Dwarf Male Reversal in Bone-Eating Worms. Current Biology, published online December 11, 2014; doi: 10.1016/j.cub.2014.11.032