Deep-sea mussels (Bathymodiolus) are the distant cousins of the blue mussels found on our coasts (Mytilus edulis). The species of this genus form large mussel beds in diffuse vent areas where the seawater is 5-10°C. The Azores deep-sea mussel (Bathymodiolus azoricus) can reach 15 cm in length.
This deep-sea mussels lives in symbiosis with bacteria that develop in their gill cells, and can synthesise organic matter using the dissolved carbon dioxide gas and the chemical energy released from the oxidation of mineral compounds (hydrogen sulphide or methane). It is attached to the substrate by strong byssus filaments. However, this mussel is relatively mobile and can move rather quickly using its strong, supple foot, up to 2.5 cm/h! This speed was measured from the analysis of video footage filmed on the observatories.
Gastropods belong to phylum Mollusca (molluscs) and are highly represented in hydrothermal vents: there are more than 100 endemic species around the world. The genus Leptodrilus has only been described in chemosynthetic ecosystems and includes 15 species distributed in all the world’s oceans. They are “patelliform” species, similar to the dish-shaped limpets that are found along our coasts. They are grazers: their rasping tongue, called a radula, has tiny teeth that scrapes the bacterial films that form on the mineral substrates or on mussel shells and worm tubes. These species are often found in high densities in tube worm colonies, called tube-worm bushes, in the Pacific Ocean where their abundance can reach more than 10,000 individuals on just a few tubes.
Lepetodrilus fucensis is characteristic, as its name implies, of the Juan de Fuca Ridge, and forms clusters of individuals bunched on top of each other hanging from Ridgeia tubes. It looks a lot like a Smurf hat!! Its colonisation success has been attributed to the broad flexibility of its diet. This species can graze on bacterial mats, live in symbiosis with chemosynthetic bacteria and can filter-feed like bivalves. These three feeding options means that it can easily adapt to changes in its environment.
The species Lepetodrilus atlanticus is found in the Atlantic Ocean. In contrast to L. fucensis, L. atlanticus lives in sparser densities and does not form the clusters that are so characteristic in the Pacific. Therefore, it is more difficult to observe on the video images and your chances of spotting it are low! However, advanced spies will be able to detect it on close-up images…
This gastropod belongs to the large phylum Mollusca (molluscs), just like the deep-sea mussels and lepetodrilid limpets. It is closely related to the whelk found on our beaches or seafood platters. It is the largest species of gastropod found in the hydrothermal vents in the North Pacific Ocean. It can measure up to 3 cm in length and is thus easy to spot on the video images. It is a carnivorous necrophage (i.e. feeds on dead animals) and eats its prey using its radula (the name of the gastropod tongue), which is lined with tiny teeth. It is mobile, using its large foot to crawl along the substrate. It is an opportunistic species that often aggregates to reproduce or to feed on a cadaver that it finds using highly developed sensory organs (called osphradia). For this reason, it is often observed in dwindling communities where it can find a high number of dead animals.
Other small, solitary, gastropods can be seen on images for which the zoom level is high enough. In the Pacific the two species Provanna variabilis and Depressigyra globulus resemble snails with their coiled shells. They graze on bacterial mats that develop on the tube of Ridgeia using their little tongue called the radula.
In the Atlantic an other specie of limpet, Pseudorimula midatlantica colonizes the surface of mussels' shells where they also feed on bacteria.
This tube worm is closely related to Riftia pachyptila, the emblematic hydrothermal vent species. These tube worms were long considered to belong to their own phylogenetic branch, but morphological studies (discovered with the first discovery of hydrothermal vents) show that they belong phylum Annelida (segmented worms) just like most marine worms. Although its sister species R. pachyptila can reach up to 2 m in length, R. piscesae is smaller and measures from 20 cm to 1 m in length.
This worm lives in a chitinous tube that it secretes. The tube helps protect it from predators and difficult environmental conditions. One of its strangest features is that it does not have a mouth, gut or anus. It thus has a strange way of feeding: its body cavity is filled with a large sack, called the trophosome, loaded with symbiotic chemosynthetic bacteria. These bacteria use the chemical energy contained in the hydrothermal fluids and that circulate in the worm’s vascular system. Bathed in the hot hydrothermal fluids, the gill-like structures on the worm’s characteristic red “plume” take up the chemical elements found in the vent fluids, including hydrogen sulphide. These worms and bacteria live in symbiosis because they each depend on the other species for survival. The red colour of its plumes is due to the presence of haemoglobin, essential for transporting oxygen from the environment to the bacteria.
These worms colonise vent areas at temperatures of between 5° and 20°C. Ridgeia have developed a surprising morphological adaptation to their environment with the existence of two distinct morphotypes. In diffuse vent areas, individuals are long and thin and can measure up to 1 m whereas in active vent areas, where temperatures can reach 20°C, individuals are short and fat and often do not exceed 20 cm.
They live in colonies and form so-called bushes. They are called engineering species because they provide habitat for smaller species that will use the worm bushes as refuges and food sources.
Polynoid polychaetes (scale worms)
Polychaete worms are exclusively marine annelid worms. The term ‘annelid’ means that they are composed of multiple, successive segments called metameres that give them their long shape. The term polychaete comes from the numerous hairs that are found on each segment. In contrast, the common earthworm in our gardens is an oligochaete because it does not have any hairs. Polychaetes are only found in marine environments. Within the polychaetes, polynoids are a highly diverse family that includes species with elytra — plates or scales — that more or less cover the body. There are at least 24 species of scale worms associated with hydrothermal vents (called endemic species) that are found in high abundance in animal communities. They are generally small, making their observation on video images difficult, but the most advanced spies will learn to spot them rapidly with a little experience!
They are carnivorous or grazing species depending which family they belong to. They use their tubular mouthparts, called a proboscis, that they evert to attack their prey and graze on the substrate. This diversity of food sources allows several species to cohabit in the same area, partitioning thus the resources: the species occupy different ecological niches.
The individuals observed in the middle of tube-worm bushes belong primarily to two species: Lepidonotopodium piscesae and Brachinotogluma sp. but they are impossible to differentiate based only on an image. Only sampling and identification in the laboratory can tell them apart. The differences often lie in the shape and the number of hairs as well as the morphology of their mouth parts.
There are also several species known in the Atlantic Ocean, including Lepidonotopodium jouinae, Branchinotogluma sp., but they are difficult to observe on TEMPO images.
Ophiuroids (brittle stars)
Ophiuroids, also known as brittle stars, belong to phylum Echinodermata (echinoderms), along with sea urchins, sea stars, sea cucumbers and sea lilies. They can be identified by their five slender, flexible arms that they use to move. All of their organs are located in the central disk; on the underside is found the mouth, usually armed with five toothed jaws. The species that are found in the Lucky Strike mussel beds are not well known and are no larger than a couple of millimetres. Only two species have been described in hydrothermal fields on the Mid-Atlantic Ridge and it is very difficult to tell them apart in a video image. Thus, we know little about their biology and their ecology.
This species is a pycnogonid (sea spider), which belongs to phylum Arthropoda (animals with jointed appendages). Pycnogonids are closely related to spiders, and have a very similar morphology! We know little about this species. They live in groups, which helps increase the efficiency of oxygen circulation in their lymph. A recent study conducted in our laboratory showed that their behaviour follows the rhythm of the tides, thereby anticipating the environmental changes caused by the tides. Even in the depths, tides can influence animal behaviour!! Tides act directly on current direction and intensity. Thus, we think that according to the environmental conditions, pycnogonids migrate to the centre of the tube-worm bush to feed and hide from predators and then move back to the surface of the bush to find more favourable conditions when the influence of the hydrothermal fluid is too intense.
Here’s an interesting fact: the fathers nurture the young — they carry their young in their feet until they are developed enough to fend for themselves!
No common name known to date. This species is only known on the Mid-Atlantic Ridge hydrothermal vents, but another species belonging to the same genus was recently discovered on a hydrothermal site in the Indian Ocean (near Rodrigues).
Of roughly 3 cm in length, it is found in groups along the pits formed by active chimneys, benefiting from hydrothermal fluid emissions at 5° to 10°C, or on mussel beds. It feeds on organic particles found in the water column, such as debris left by mussels or animal remains. Experimental observations in aquariums show that they feed on exuviae (exoskeletons shed after moulting) left on the bottom. Some individuals travel away from the active zones at distances of up to several meters. They thus become prey to fish or even hydrozoans (sessile polyps attached to the rocky substrate).
This Bythograeidae crab is characteristic of hydrothermal sites on the Mid-Atlantic Ridge. This highly mobile species is found in a wide variety of habitats, from smokers to diffuse vent areas where mussel beds develop. This distribution is attributed to its opportunistic behaviour, because it feeds on cadavers such as mussels, shrimp or shrimp exuviae (exoskeletons shed after moulting). Although it is a poor predator, it can also feed on live mussels after breaking their shells with its claws.
Segonzacia crabs are solitary and territorial animals. They measure 3 to 6 cm and are white to pinkish yellow in colour.
Spider crab Macroregonia macrochira
This species is not a spider, but a crustacean that belongs to the same family as the spider crabs that are fished along our shores. Their name comes from their morphology that is similar to that of spiders with long legs and thin claws. Although this North Pacific species is also found in habitats other than hydrothermal vents, it frequents the deep sea where it feeds on various hydrothermal organisms. The abundance in this habitat means that it can find food easily without expending too much energy. With a carapace exceeding 10 cm, it is easy to spot when it ventures in front of the camera!
Pinkish in colour, this small, slender fish belongs to the Zoarcidae family, a group that is highly represented in hydrothermal communities. In contrast to other fish that live around chimneys, this species is endemic to the hydrothermal ecosystem. It can reach a length of 40 cm and feeds on amphipods, isopods and polychaetes. We know little about the biology of this species.
This greyish black fish with a long, tapered body of around 65 cm is observed in the North Atlantic and the western Mediterranean Sea from 900 to 2830 m depth. It is not specific to hydrothermal vents, but is often observed there because it can find food in abundance, such as Segonzacia mesatlantica crabs and Alvinocaridae shrimp. We know little about its biology or ecology.
Chimaera Hydrolagus sp.
This ghost shark lives at depths ranging from 1200 to 2500 m and can reach 1 m in length and has a wide distribution on both sides of the North Atlantic, from New England to Canada and from the European coasts to Greenland. Light in colour, it is easily recognised by the characteristic markings around its eyes. It is a bentho-pelagic fish, meaning that it lives both on the seafloor and in the water column. They are often observed swimming above hydrothermal sites where it can enjoy the abundant resources. Shells and Bathymodiolus mussel parts have been observed in the stomachs of some individuals, but their biology and ecology are not well known.
These mats are made of bacterial microorganisms. These microorganisms are at the base of the hydrothermal ecosystem because they, like terrestrial plants, produce primary matter from chemical energy stored in the reduced compounds found in hydrothermal fluids, such as hydrogen sulphide (H2S), methane (CH4) and hydrogen (H2). These microorganisms are found in symbiosis with larger macrofaunal species (symbiotic relationship) but also as free-living forms, invisible to the naked eye. Some bacteria associate and form mats that are easily visible. They are found in diffuse vent areas under the influence of hydrothermal fluid that they depend on.