Pachycara caribbaeum: A new species of Eelpout from vent fields in the Caribbean.

Eelpout, Zoarcidae., are bottom-dwelling Eel-like marine fish related to Sticklebacks and Scorpionfish. Members of the genus Pachycara are found at bathyal to abyssal depths (between one and six kilometers below the surface of the sea) in all oceans except the Arctic. There are currently twenty seven described species, of which three have been described from hydrothermal vent fields, areas where superheated water laden with minerals that has percolated through hot volcanic rocks reaches the surface, supporting whole ecosystems that get their energy from chemotrophic Bacteria (Bacteria that gain energy by breaking down chemicals from the vents rather than photosynthesis).

In a paper published in the journal Zootaxa on 13 January 2016, Eric Anderson of the South African Institute for Aquatic Biodiversity, Russell Somerville of Ocean and Earth Science at the University ofSouthampton and Jonathan Copely also of Ocean and Earth Science at the University of Southampton and of the Natural History Museum, describe a new species of Pachycara from the Von Damm Vent Field on the Mid-Cayman Spreading Centre and an un-named methane seep (area where methane is escaping from sedimentary rocks on the sea-floor, also often supporting ecosystems based upon chemotrophic Bacteria) close to Tobago.

The new species is named Pachycara caribbaeum, in reference to the area where it was found. The species is described from two male and one female specimens recovered from the Von Damm Vent Field at depths of 2309-2313 m by deep-sea submersibles, none of which are judged to be mature. The males are 117 and 199 mm in length, the female 197 mm.

Pachycara caribbaeum in the Von Damm Vent Field. Anderson et al. (2016).

Pachycara caribbaeum was found close to areas of visible diffuse flow venting in the Von Damm Vent Field, with temperatures ranging from 5-20ºC with several potential prey species in the vicinity. This suggests that the species does not have the tolerance of high temperatures seen in some vent-dwelling organisms and avoids areas of higher temperatures. He species was also observed at a methane seep close to Tobago, though the individuals seen here were not as large or as numerous as those seen in the Von Damm Vent Field, suggesting this was a less optimal environment. The species has not been seen on the roughly 2000 km of seafloor that separates the two sites, nor in the Beebe Vent Field which lies close to the Von Damm Vent Field (about 30 km) away, but much deeper (about 5 km compared to the 2.3 km depth at the Von Damm Vent Field.

Locations (indicated by stars) of Pachycara caribbaeu; West, Von Damm Vent Field (depth ca.2300 m), east, methane seep near Tobago (depth 1049 m). Scale bar is 1000 km. Anderson et al. (2016).

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Deepest hydrothermal vent communities yet found discovered in the Caribbean.

Hydrothermal vents occur on the boundaries between plates in the deep sea, as water percolates through super-heated volcanic rocks soaking up minerals, and is released back into the ocean. They are often called 'black-smokers' because of the dark colouration of the mineral-rich water. The water coming from hydrothermal vents often has temperatures of hundreds of degrees centigrade, well above the boiling point of water at the surface, but remains a liquid in the high pressure environment of the deep sea. The water is also often highly acidic, and laden with potentially toxic metal ions. Despite this deep-sea hydrothermal vents have been shown to support whole communities of animals, witch depend not on energy from green, photosynthesizing plants for survival, but rather of bacteria able to generate energy by consuming the toxic chemicals from the hydrothermal vents.

On 10 January 2012 a team lead by Douglas Connelly of The National Oceanography Centre at the University of Southampton and Jonathan Copley of the Department of Ocean and Earth Science, also at the University of Southampton, published a paper on Nature Communications in which they describe the discovery of two systems of hydrothermal vents on the Mid-Cayman Ridge, 4960 m and 2300 m beneath the Caribbean Sea.

The location of the new hydrothermal vent sites. From Connelly et al. (2012).

The Von Damm Vent field is located on the upper slopes of an undersea mountain, Mount Dent, 13 km to the west of the active spreading centre, at a depth of 2300 m. The water at the vent is rich in hydrogen sulphide (H₂S), and while enriched in metals relative to the surrounding water, is metal poor compared to other deep-sea vents; it appears colourless at the vent rather than black.

The Beebe Vent Field is located 4960 m beneath the surface, 20% deeper than any previousy discovered vent. The water issuing from the vent is not as rich in hydrogen sulphide as the water at the Von Damm site, but it is extremely rich in copper and iron. This creates a visible plume of black water that reaches 1100 m above the vents. The Southampton researchers estimate that for the column to remain discrete from the surrounding water for this far the water must be emerging at about 500 °C (they did not have the means to take a direct temperature reading at this depth).

(a & b) A colony of shrimps on a vent at the Von Damm Vent Field; the water issuing from the vent is colourless. (c & d) Sulphide chimneys issuing black 'smoke'; coloured water at the Beebe Vent Field, which gets its colour from dissolved copper and iron ions. From Connelly et al. (2012)

Both sites had thriving biological communities, dominated by dense colonies of shrimps, containing at least two species of shrimp. The Von Damm site also had extensive mussel beds. Fish, snails, sea anemones and squat lobsters were also present at both sites. This is similar to the fauna found on deep-sea vents on the Mid-Atlantic Ridge.

(a) A a dense colony of shrimps at the Beebe Vent Field. (b) Sea anemones and microbial mats at the Beebe Vent Field. (c) Dead mussel shells on Mount Dent. (d) Possible tube-worm casts on Mount Dent.

See also New deep-sea hydrothermal vent ecosystem discovered in the Southern Ocean.