Introduction

Mussel bed on the edge of a deep sea brine pool. Author: NOAA 

Cold seep communities offer a contrast to the desert that is most of the deep-sea bed with low diversity, low population densities and low biomass. Hydrocarbon vents on the other hand offer high population density, high biomass with high diversity [1].

The cold seeps of the Gulf of Mexico often occur alongside pockmarks (or craters) which are filled with hypersaline brine. These brine pools occurred due to tectonic deformation of the Louann salt which is a Jurassic evaporite deposit. This deposit created structural complexity in the north of the Gulf of Mexico [1].

Cold seeps occur alongside the brine pools like salt diapirs (definition) and growth faults. These faults are the routes the hydrocarbons take to the seeps. The salt diapirs occur throughout the recent sedimentary strata, thus brine seepage occurs all through the Gulf of Mexico [1].

The dominant organisms which inhabit these environments are Seep Mussels such as Bathymodiolus childressi found around the brine pools in the Gulf of Mexico and are among the many organisms which recieve their energy from the cold seeps of hydrocarbons [2]. A member  of the genus Bathymodiolus which sees members of this family occur in deep seas worldwide.

 

Cold seeps

Cold seeps were first discovered in the Gulf of Mexico in the 1980s, [3] resulting in them now being some of the most studied cold seeps in the world. Cold seeps are locations where hydrocarbons primarily methane (natural gas) bubble up from the sediment. Many of these cold seeps also have brine filled pockmarks. These are pools of water which have high levels of dissolved salt, making the water much denser than the regular salt water while also making it toxic to almost everything which enters it.

 

Feeding

Seep Mussel (Bathymodiolus childressi) with a cut throat eel.  Author: NOAA

In the absence of light and very low levels of suspended particles to feed on, seep mussels are adapted for chemosynthesis gaining nutrition from the Hydrocarbons emanating from the seeps. They do this by relying on a symbiotic relationship they have with intracellular (located within cells) methanotrophic (organisms which use methane as their only source of carbon) bacteria that the mussels host in their gills having co-evolved with the mussel over time [4].

 

Predation

Predators do feed on the mussels present and can be put into two categories: feeding by vagrant species and feeding by resident community consumers.

Resident community consumers of the mussels like the sea snail Buccinum canetae stay within the seep community grazing on the seep mussels like Bathimodious within the seep mussel community [5].

Vagrant species are species which appear a good distance outside their normal range such as the spider crab Rochina crassa and the squat lobster Eumunida picta. Which is found throughout the western Atlantic including cold seeps in the Gulf of Mexico. It has been thought to feed on the mussels around the seeps [5]. This behavior is believed to perhaps provide substantial trophic export outside the community.

 

Reproduction and larvae

Studies of B. childressi larvae with induced spawning has shown its eggs to be negatively buoyant with larvae hatching by 40 hours into free swimming blastulae with shells beginning to develop by the 12th day. By studying the egg size and the morphology of the larvae’s shell it is thought B.childressi larvae to be planktotrophic. Wide distributions of this larvae observed in the Gulf of Mexico and amphi-Atlantic suggest the larvae spend long periods in plankton with some individuals being thought to spend more than a year in plankton [6].

 

Future threats

There are multiple future threats to cold seep communities in the Gulf of Mexico. Amongst these is the mining of seafloor massive sulphide deposits as they have the potential to contain large amounts of valuable commercial metals like gold [7].

The exploitation of hydrocarbon pockets by the oil and gas industry could also potentially impact this habitat as the industry is likely to be active in these areas in the coming years as seep communities very often occur alongside hydrocarbon reservoirs [8].

These activities present a strong possibility of large direct damage to the seafloor and seep structure as well as the production of sediment plumes which could possibly smother filter feeders killing them. The potentially increased noise levels could disturb and change the organisms’ behavior. The alteration of the fluid dynamics of the seep thus changing the physical environment of the seep.

Summary

Deep sea submersible surveys a mussel bed in the Gulf of Mexico Author: NOAA

Cold seeps are a unique deep-water environment which operate independently of energy from the sun. Relying on chemosynthesis as its source of energy, offering many unique species like Bathimodious while also having to cope with hyper salinity although they are known for their tolerance of aromatic compound and toxic sulphides thus giving belief that it may be possible for other chemosynthetic fauna to do the same. While it is not possible to put a value on this environment, it remains at risk from future exploitation of the rich natural resources it sits upon.

Video from BBC’s Blue Planet 2 showing a cold seep community in the Gulf of Mexico

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