Deepstaria enigmatica and the Enigma of Gigantism in Deep Antarctica
Craig R. Smith once commented:
‘The fauna of abyssal seafloor habitats is generally poorly sampled and poorly described by taxonomist, with greater than 90% of the polychaete worms, copepods, isopods, and nematodes collected in any given sample typically being new to science’.
Whilst his statement was built upon criticisms in his own field of expertise, it characterises a general frustration in the lack of knowledge surrounding deep ocean life.
Abyssopelagic organisms and deep-sea dwellers are some of the most bizarre life forms ever discovered. Their environment has remained relatively unchanged for thousands of years due to its isolation from extreme climatic events and minor change in selection pressures as a result. A caveat to this statement is that recent studies show that anthropogenic climate change is predicted to reduce deep benthic organism populations by 5% in the next hundred years due to a fall in detritus making its way down the water column.
Life in the abyss
These extreme selection pressures that define the deep seas include absence external light sources, extremely cold temperatures ranging from 0-3°C when excluding hydrothermal vents . Abyssal salinities vary from 34.6 and 35.0 salinity units which is marginally less than surface water and slightly more than depths of around 1000m, the typical trough in a halocline. Pressure increases by 1 atmosphere every 10 metres of depth, therefore at 2000m below surface level the pressure would be 200 atmospheres. Energy production in the deep ocean is limited to detritus descending from the surface and chemosynthetic bacteria. These bacteria are also a prime example of how primitive deep-sea organisms can be, fossilised bacteria 1.8 billion years old remains unchanged from contemporary specimens.. In Antarctica there is far less biomass falling to the deep sea as surface productivity is much lower than in the tropics. Much how intense pressure creates diamonds on land, these environmental conditions can create astonishing biota.
The 10-metre-wide behemoth, Deepstaria enigmatica resembles a thin sheet blowing in the wind as much as it is comparable to other members of the Cnidaria phylum. Due to its unique morphology, it was the first described member of the Deepstaria genus. Frederick Stratten Russell first identified the species in 1968. He named the genus as an homage to the Deepstar 4000 submersible and its creator Jacques Cousteau. The limited sightings still indicate widespread distribution but is typically found in Antarctic seas with a depth range of 829 to 1830 meters below surface. Some noteworthy physiological characteristics include:
- Thin and wide bell (up to 10 metres in width).
- Lack of tentacles.
- Hexagonal mesh like digestive system.
- 4-5 Long oral arms.
- Mesoglea 1-2 cm thick.
The size of the Deepstaria enigmatica is strongly influenced by its extreme habitat, but perhaps the trend of gigantism found in other deep dwelling marine organisms provides explanations for its enormity. The phenomenon of gigantism in deep sea environment is present in many classes of marine invertebrates. Other examples would be the Japanese spider crab (Macrocheira kaempferi) which is the largest crustacean and Colossal squid (Mesonychoteuthis hamiltoni) which are the largest cephalopods. Both giants are the amongst the deepest of divers in their phylum/sub phylum. Giant species main morphological difference than its surface dwelling relatives is size. There are several theories for this trend, but none are yet to be solidified as the sole cause of large size.
Deep and polar water is exceptionally cold. In cooler temperatures oxygen is increasingly soluble in water. At 4 bars of pressure and a temperature of below 5°C, oxygen solubility is above 40mg/l. The increased solubility of oxygen can lead to higher respiration of organisms used for growth and development, especially in larval stages. Temperature also affects the duration of larval developments: Lower temperatures result in longer duration of development. Being in development stages for a prolonged period with high abundance of oxygen could allow these organisms to achieve gigantic size.
The increased scarcity of food sources as less detritus sinks down the water column means that the average age of sexual maturity increases, this means decades of growth can be achieved before growth rates begin to decrease after sexual maturity.
Polymath Carl Bergmann theorised that species of greater size are found in colder environments, and species of smaller size are found in warmer ones in his eponymous ‘Bergmann’s rule’. Russian biologist S. F. Timofeev applied Bergmann’s principle to deep-water crustaceans and suggested that cold temperatures lead to bigger cell size and life span. This increased life span allows more time for growth and allows more individuals to exhibit gigantism. Furthermore, the size of a crustacean’s body increases along the temperature gradient, both by latitude and by depth. The phenomenon is applicable to:
- Amphipods – Alicella (Alicella gigantea)
- Cephalopods – Colossal squid (Mesonychoteuthis hamiltoni)
- Decapods – Japanese spider crab (Macrocheira kaempferi)
- Euphausiids – Antarctic krill (Euphausia superba)
- Isopods – Giant Isopod (Bathynomus giganteus)
- Mysids – Giant red mysid (Gnathophausia ingens)
- Cnidaria – (Deepstaria enigmatica)?
The extreme environmental conditions of cold waters in the deep and/or Antarctic region mould the selection pressures in ways science does not fully understand but the correlation is clear. Deepstaria enigmatica is yet another example of that trend, even if future research suggests it’s causation for large size differs from that of other crustaceans and invertebrates.