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Chrysomallon squamiferum the Scaly Foot Gastropod  sports an iron shell and roof-tile pedal scales but most unique is the internal body structure evolved solely to support a strain of symbiotic chemosynthetic bacteria. This novel and endemic species of gastropod lives in the extreme environments of the deep ocean hydrothermal vent sites of the Central Indian Ridge.

APPLIED SNAIL SCIENCE

Material Scientists at the Massachusetts Institute of Technology (M.I.T.) intrigued by C. squamiferum, with its iron pyrites (FeS2) and greigite (Fe3S4) shell, devised experiments to test the armour capacity of the casing to survive a range of destructive measures. Unique, amongst gastropods, is the tri-layered construction of the shell (Figure 1. below) comprising a thin hard outer layer (OL) protecting a thick organic mid layer (ML) and a thicker inner calcareous layer (IL).

Stress and shock tests indicated a remarkably resilient encasement.  Shocks and fractures were mitigated due to the protective elastic properties of the organic middle layer. Percussion energy was dissipated thereby protecting shell breakage and increasing overall strength.

This is great news for Material Scientists looking at bio-inspired-product-designs. The consumer market is increasingly geared towards safety and so understanding fracture resistance and developing sustainable mechanical loads are a worthwhile pursuit – think cars, aeroplanes, paints, and military vehicle and personal Armor applications.

Figure 1. Photograph of Chrysomallon squamiferum (Image, Anders Warén); Schematic and optimal micrograph of the tri-layered shell structure with rigid outerand inner most layers sandwiching the novel organic - shock dissipating - middle layer (Imagery, Haimin Yao. et al,.)
Figure 1. Photograph of Chrysomallon squamiferum (Image, Anders Warén); Schematic and optimal micrograph of the tri-layered shell structure with rigid outer and inner most layers sandwiching the novel, organic – shock dissipating – middle layer (Imagery, Haimin Yao. et al,.)

BUT WHAT ABOUT THE METAL SHELL?

Recent discovery on the Central and South West Indian Ridge revealed three populations of SFG’s within 2500 km of each other. The populations from the Kairei, Solitaiere and Longqi sites identified genetically as the same species but the Solitaire snail were white, lacking the mineral deposition on their shell, on sclerites or within their flesh. Future research might look at the mechanisms that may produce iron pyrites deposition in one gastropod but not for the others.

In Photograph 1. below, shows a comparison between the two morphotypes with the black snail showing what seems to be an increased metal and element load, possibly an effect of localized vent stream toxicity.

Figure 2. The two morphotypes of the Scaly- foot gastropod, Iron Pyrites covered snail showing black shell and scleries reacting to elemental mapping of iron and sulphur (green). (Photographs Nakamura et al., 2012)
Photograph 1. The two morphotypes of the Scaly- foot gastropod, Iron Pyrites covered snail showing black shell and scleries reacting to elemental mapping of iron and sulphur (green). (Photographs Nakamura et al., 2012)

RESIDENT RISK

Whilst the M.I.T study and its application are headline grabbing news, the extreme environment – and morphological adaptations that allow our snail to survive – need more than a mention.

Hydrothermal vent habitats are toxic. Super-heated waters ladened with dissolved metals, compounds, sulphides, and gases spew into cold (2 °C), highly pressured (~2500m depth), bottom waters creating local chemical re-mineralisation, heavy metal toxicity and high acidity. These are conditions that the majority of marine organisms would find highly challenging. However as one can see in the clip below (by marum – Zentrum für Marine Umweltwissenschaften), a multitude of sessile and vagile organisms are shown to exist and thrive under such seemingly extreme conditions.

Clip 1. Organisms utilising the chemical rich waters of various hydrothermal venting habitats.

Clip 2. C. squamiferum present at 1:08 minutes showing an abundance of motile individuals swarming around the black smoker vent (courtesy of thesearethevoyages.net)

As light does not penetrate to this depth, nutrition is not photosynthetic, but  chemosynthetic. Bacteria, as primary producers, use hydrogen or hydrogen sulphide to reduce carbon compounds into organic matter, e.g.,

12H2S + 6CO2 → C6H12O6 (=carbohydrate) + 6H2O + 12

for cell growth and as protection externally.

Some organisms consume bacteria. Many bacteria strains have a symbiotic relationship with their hosts similar to warm water corals and zooxanthella. All organisms living in vent systems have evolved and adapted their morphology and life history strategies to survive the toxic waters.

ENSLAVEMENT

It could be argued that the symbiotic thioautotrophic gammaproteobacterial inhabitants of C. squamiferum have had such an effect on the host that the internal morphology has changed specifically to accommodate them.

The C. squamiferum’s heart size is a huge 4% of the body volume of the snail, an unprecedented size when compared to most animals – human heart’s are around 1.3 %. The heart pumps blood to an enlarged ctenidium (gills) for dissolved gas exchange to occur. The ctenidium surface area is further increased due to protuberant gill leaflets filling the mantel cavity. Dissolved gasses including oxygen for the host snail and hydrogen sulphide are then transported to the enlarged and greatly vascularised oesophageal gland, that houses the bacteria. The bacteria require the snail to provide nutrient rich solutes to create organic compounds to nourish the snail. The snail has to maintain a strong position in the hydrothermal waters, to ensure optimal gas flow over its ctenidium, and to pump that nutrient rich blood back to the oesophageal bacteria for gas exchange and processing – though how is unreported. The radula in juveniles is thought to be useable however there is a lack of wear on adult radulas. A reduced stomach is present but not seemingly utilised in adults. Lack of radial wear and reduced digestive capacity suggests enhanced reliance on the endosymbionts. The big-hearted snail and friendly bacteria are able to coexist and thrive in the others company.

Further morphological adaptations are not necessarily bacterially driven. In the dark there is little need for sensory appendages such as eyes so there are no brain structures such as ganglia. C. squamiferum is a simultaneous hermaphrodite with the snails gonads arranged below the shell spire.  This may provide greater internal spatial development and subsequently increased fecundity.

THE FUTURE 

Further investigations along the hydrothermal vent systems of the Indian Ridge territory may discover new populations of C. squamiferum and additional novel organisms. Through the mapping and investigation of deep sea sites one is able to ascertain value that can be translated into benefit for our present and future populations. The discovery of new organisms is always exciting and may also lead to bio-inspired design from research lead by M.I.T. and other organisations.

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