Deepstaria enigmatica and the Enigma of Gigantism in Deep Antarctica
The 10-metre-wide behemoth, Deepstaria enigmatica is an oddity 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.
Life in the abyss
These extreme selection pressures that define the deep seas include a lack of 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.
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 counterparts 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.
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’. Originally this rule was created through the observations of terrestrial, endothermic mammals. This is because larger animals have reduced relative heat loss due to a lower surface area to volume ratio. However, the rule can still be applied to many aquatic ectothermic organisms in the ocean. Russian biologist S. F. Timofeev applied Bergmann’s principle to deep-water crustaceans and suggested that cold temperatures lead to slower cell division, bigger cell size and life span. An increased life span allows more time for growth and allows more individuals to exhibit gigantism because the age of sexual maturity increases. Decades of growth can be achieved before growth rates begin to decrease after sexual maturity. 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 such as Alicella gigantea, which are found on the abyssal plain. Mysids display this trend as shown by the Giant red mysid (Gnathophausia ingens). Perhaps Deepstaria enigmatica can be used as an example for gigantism in Medusae.
Max Kleiber’s observations in the early 1930’s lead to the development of Kleiber’s Law, an allometric law that states that an animal is likely to have a metabolism equal to ¾ of its mass. It is a trend which is observed across the whole animal kingdom. Metabolic rate increases at 75% the rate of mass increasing, resulting in larger animals being able to spend energy more efficiently. As previously mentioned, energy production in deep waters is mostly limited to small amounts of sinking detritus. Highly efficient physiology is a prerequisite in a high food scarcity environment. A large size is beneficial because large organisms have more reserve storage than smaller organisms, which means a lower proportion of body mass requires maintenance costs, so their overall proportionate metabolic rates are better. Regarding food availability, Deepstaria enigmatica most likely uses its large bell as a mechanism for feeding similar to its close relative Deepstaria reticulum. The bell traps detritus and prey in its stomach to be absorbed by a high surface area hexagonal mesh digestive system.
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 its’ causation for large size differs from that of other crustaceans and invertebrates. 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. There’s little doubt that this will reduce overall biomass in the deep-sea communities but there is evidence to suggest that this could increase the maximum size of known gigantic organisms. 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.