Figure 1: An example of a deep-sea hydrothermal vent habitat, which shows the vivid red headed giant tube worms round the base of a black smoker. (Photo Credit: NASA).

The deep sea (depths below 200m) appears to be a foreboding, almost barren environment with vast expanses of nothing but lifeless open water and sediment, a thought by early scientist such as Edward Forbes, who hypothesized that life would not exist below 550m. But there actually a large number of refuges and niches in the abyss. One such refuge being hydrothermal vents. These tectonically influenced habitats pollute the surrounding waters with super-heated water up to 350ºC, heavy metals such as lead, and toxic sulphides ions. Taken together, these two environments make for an unlikely home. However, these oases in the depths tend to be very large ecosystems capable of supporting several metre-long tube worms, clams the size of footballs, and a plethora of crustacean species, to name a few, an example of such an environment can be seen in figure 1. There are many bizarre methods in which organisms adapt inhabit the hydrothermal habitats in the deep. The oddest of these deep hydrothermal vent dwelling creatures must be a mollusc, which incorporates fool’s gold into its own shell.

Figure 2: A scaly-foot gastropod collected from Longqi field, on the Southwest Indian Ridge. Scale bar is 1 cm. (Photo Credit: Chong Chen, Katsuyuki Uematsu, Katrin Linse & Julia D. Sigwart).

The Oddity

The mollusc in question is the scaly-foot gastropod (Chrysomallon squamiferum) as shown in figure 2, is a snail species that can only live in the presence of hydrothermal vents and is only currently known from hydrothermal vents in the Indian Ocean. But what makes this snail such an oddity? Normally the shells of snails are made up of two layers; the inner calcified layer (which you may find empty, washed up on beaches) constructed from either calcite or aragonite, and the organic outer layer which promotes growth called the periostracum.

However, in the case of the scaly-foot gastropod there are three layers that make up the shell. The inner layer is the same as a normal shell as it is the calcified layer, except it is made with only aragonite. The middle layer which is an equivalent to the outer organic periostracum layer found on a normal shell. Finally, the third and outer layer consists of iron sulfides, such as pyrite, or fool’s gold, giving the shells a vivid metallic gold colour, leading to the genus name Chrysomallon, from the Greek for “golden haired”. This is further accentuated with another unique feature, the presence of hundreds of dermal sclerites, a sort of scale armour covering the foot of the snail, a trait which is unique to this species. These sclerites like the shell, are made up of 3 layers. The first consisting of soft tissue core which is the link to the rest of the foot, which are coated by a conchiolin layer, a series of proteins that are secreted by the snail’s mantle, and finally the pyrite and greigite (another form of iron sulfide) layer creating the illusion of golden chainmail coating the foot of the animal making it the golden knight of the deep.

Figure 3: The three locations, indicated by numbers (1, 2, 3) in order of discovery, where the scaly-foot gastropod resides in the Indian Ocean, off the coast of Africa and Madagascar. (Photo Credit: NOAA).

They Like It Deep and Hot

Only able to inhabit in proximity to hydrothermal vents, where there are high concentrations of hydrogen sulfide. The scaly-foot gastropod is only found in depths between 2400 – 2800 m in three locations in the Indian Ocean. The locations as indicated by figure 3 are the Kairei hydrothermal vent field [1] on the Central Indian Ridge where it was first discovered around the bases of the black smokers in 2001, the Solitaire field [2] also on the Central Indian Ridge, and the Longqi field [3] on the Southwest Indian Ridge. When the species has been located on the hydrothermal vents, they are always on or around the bases of the black smoker chimneys, or directly on the diffusing flow sites adjacent to both the acidic and reducing vent flow, in water temperatures that fluctuate between 2-10ºC.

The Ultimate Diet

Compared to other gastropod species, the scaly-foot gastropod has an incredibly reduced digestive system along with a reduction in the size and function of radula, its feeding apparatus. So how does an animal with no capable method of feeding survive? The scaly-foot gastropod receives nutrition through bacterial symbiosis. A relationship in which the host gastropod and nutrient sourcing bacteria are highly reliant upon each other’s presence. Meaning they would either struggle to, or not function at all without the other. Evidence of this is shown in the scaly-foot gastropod, as it has an enlarged esophageal gland (an important section of the digestive tract in a gastropod), and a unique, complex network of blood vessels from this gland to the foot, which provides an efficient path for gases and metabolic exchanges from the environment to the symbiont bacteria housed in the esophageal gland, thus giving the gastropod the ability to gain nutrient without consumption.

 

Facing Threats

This unique animal that has adapted to the extreme habitat of deep-sea hydrothermal vents, and incorporated fool’s gold into its shell and sclerites could become under threat by man. Deep-sea mining has become in recent years a new process of mineral extraction. The process also targets areas in which there are active or extict hydrothermal vent sites, due to their high deposits of precious and useful metals such as silver, gold, and copper. In 2011, China was granted a 15 year commercial mining explorition license in the area surrounding Longqi. Additionally in 2015, Germany was also granted a commercial mining explorition license by the International Seabed Authority in the area of Kairei, also lasting for 15 years and since the gastropod is not registered as a threatened species, this exploration can occur. This means that two out of the three locations that the scaly-foot gastropod is found have an encroaching threat that could significantly damage the population of this unique species.

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