Alvin

The Deep Submergence Vehicle Alvin is the best known marine research vessel. It was commissioned by the United States Navy in 1964, but it calls Woods Hole Oceanographic Institution home. For its 49th birthday, it's received a refit that will increase its dive range by two kilometers and give the people inside more room and greater vision. Unfortunately, due to the limitations of its batteries, Alvin won't be able to reach its rated depth for another few years. Lithium-ion batteries are considered to be too great a fire risk at the moment.

Alvin returning to the surface carrying samples (photo Wikipedia)

Alvin sprang to fame in 1977 when scientists inside it made the first observations of hydrothermal vent communities off the Galapagos Islands. These were the first communities of multicellular organisms ever discovered that were able to survive in isolation from the sun. To marine biologists, the discovery of hydrothermal vent communities was more exciting than the moon landings less than a decade earlier. And it was Alvin, like Apollo 11, that made it possible. Unlike Apollo 11, Alvin continues to make discoveries and has contributed more to marine science than any other vehicle.

A stepping stone of rotting wood

Many of the animals living at hydrothermal vents and cold seeps carry chemosynthetic bacterial symbionts in their body, which convert methane or hydrogen sulfide into food. Some have lost the ability to feed on anything other than what the bacteria living inside their tissues provide them. Almost all cannot survive without a sufficient supply of methane or hydrogen sulfide. One hypothesis is that decomposing organic matter that has sunk from the surface, like whale carcasses, seaweed, and wood could serve as a food source, providing stepping stones between vents or seeps.

A field of mussels at a cold seep (photo Wikipedia)

Animals more typically found at vents and seeps are known to colonise the remains of whales in the deep sea. Smith et al. (1989), were the first to report vent animals colonising whale skeletons. They also provided a conservative estimate of the distribution of whale carcasses on the ocean floor and suggested that they would provide a persistent and abundant habitat for cold seep and hydrothermal vent animals. Adding sunken wood and seaweed to the list only increases the amount of available habitat.

A whale skeleton in the deep sea with patrolling hagfish (photo Wikipedia)

A gap in our understanding, though, is how enough methane or hydrogen sulfide is produced to support populations of chemosynthetic animals by decaying wood. The deep sea is a cold place, which is not conducive to the rapid breakdown of organic material. A large amount of wood would be required to provide the surface area necessary to produce enough gas. One hypothesis though, is that the surface area of wood might be increased by larger organisms, such as wood-boring bivalves, breaking it up first.

Researchers tested this idea by depositing wood logs on the Eastern Mediterranean seafloor at 1700 meters down and returned a year later to examine the animals and bacteria the had colonised the wood. They also measured the chemicals in the water released by the bacteria breaking down the wood. Using underwater robots, they observed that wood-boring bivalves had indeed broken the wood into smaller pieces, which were further broken down by other organisms. 

The activity of the organisms digesting the wood reduced the amount of dissolved oxygen, resulting in anoxic conditions that allowed sulfate-reducing bacteria to move in and produce hydrogen sulfide. The hydrogen sulfide then attracted a species of mussel, which usually found at cold seeps where it gains energy from symbiotic chemosynthetic bacteria. The mussels seemed to preferentially colonise cavities under the bark of the wood, presumably because sulfide levels were higher there.

The chemosynthetic mussel Idas modiolaeformis was found in the sunken wood piles (photo from Bienhold et al. 2013)

So it appears that wood boring organisms are able to pave the way for chemosynthetic organisms to colonise sunken wood in the deep sea. Their burrows, feces and the chips of wood that they produce all increase the surface area of material available for hydrogen sulfide producing bacteria to digest. Moreover, their activity and the activity of other organisms produce the anoxic conditions required for sulfate reduction, which is necessary to support chemosynthetic life.

A hypothetical succession of animals on submerged wood in the deep sea over a year. Initially wood-boring bivalves move in, followed by predators and detritus feeders (e.g. polychaetes and sipunculids). The respiration of the colonisers creates anoxic niches that allow the chemosynthetic mussels to move in (diagram from Bienhold et al. 2013).

Smith, C., Kukert, H., Wheatcroft, R., Jumars, P., & Deming, J. (1989). Vent fauna on whale remains Nature, 341 (6237), 27-28 DOI: 10.1038/341027a0  

Bienhold, C., Pop Ristova, P., Wenzhöfer, F., Dittmar, T., & Boetius, A. (2013). How Deep-Sea Wood Falls Sustain Chemosynthetic Life PLoS ONE, 8 (1) DOI: 10.1371/journal.pone.0053590

They came from the deep

Polychaetes are annelid worms that are mostly found in marine environments. Earth worms are annelids too, but they're oligochaetes. The oligochaetes are mostly found in freshwater and terrestrial systems.

The major differences between polychaetes and oligochaetes that can be used to distinguish them are the parapodia and chaetae. Polychaetes have them and oligochaetes don't. However, some aquatic oligochaetes have structures that look very similar to parapodia, and the parapodia and chaetae are hard to find on some polychaetes. There are exceptions to every rule!

Cross section of a polychaete showing a typical body plan.

Parapodia are fleshy outgrowths that are generally used for locomotion. In polychaetes the parapodia divide into upper and lower lobes. The upper lobe is called the notopod (pl. notopodia) and the lower lobe is called the neuropod (pl. neuropodia). Out of the lobes grow the chaetae, which are bristle-like in appearance. It's the many chaetae that give polychaetes their name.

Recently, the Telegraph newspaper published some electron micrograph images of some polychaetes that were collected from a hydrothermal vent. They make the already alien appearance of the worms even more strange. The false colour (EM images are black and white) does not help things.

Portrait of a polychaete. I'm not sure, but I think it's a Nereid polychaete (rag worm).

Another polychaete. This one is a Polynoid polychaete (scale worm). I wonder if it's predatory?

The reporting of these images has been pretty poor. Stories I have seen make it sound like these worms are only found in the deep-sea, which is not true. And that they eat the bacteria at hydrothermal vents, which is also not true. The images are of worms collected from deep hydrothermal vents, but there are many shallow water examples of both rag worms and scale worms. And both groups are generally predatory.

Rag worms and scale worms feed by rapidly everting their pharynx, which has some hardened mandibles attached. Prey that are caught in the jaws are dragged back into the mouth. In the upper image the rag worm still has its jaws hidden on the inside, while the scale worm in the lower image has partially exposed its jaws.

The everted jaws of Perinereis cultrifera, a Nereid polychaete (photo estran 22)

The other thing that was reported badly in some places involves the rag worm image. The structures that are coloured in a glowing pink are not eyes, they're sensory palps. They function as organs of smell, not sight. They eyes are on the upper surface of the animal and the image is of the underside. 

The head of a polychaete. The eyes are the four black spots at the left-hand end. The pharynx can be seen on the inside of the transparent body (photo Wim van Egmond).