Newly explored habitats

Shallow-water marine caves and the deep sea

Marine caves located in the littoral zone offer a permanently dark, stable, quiet environment with limited food resources. In fact, they resemble, at least to some extent, the deep sea. Because these caves can be visited by SCUBA divers, they have tremendous potential as accessible analogues of deep-sea habitats. One important difference is that the water temperature in shallow-water marine caves is usually much higher than near the ocean floor. One important exception to this is the Northern part of the Mediterranean, where, during the winter, the temperature of the deep water (~13°C) is similar to that of the surface waters.

Most of the well-studied caves are located in the North Western Mediterranean. They have a temperature profile that ascends from the outside to the inside: warm summer water is being trapped in the upper parts of the cave. However, one particular cave, the 3PP cave near Marseilles, has an inverse profile which traps cold (~13-15°C) water the entire year round. Similar caves of this type have now been discovered. It has been suggested that such marine caves can act as a refuge during unusually hot summers.

DEEPSETS discoveries

Partly within the MarBEF DEEPSETS project framework, a detailed study of the 3PP cave fauna was made.

The most striking and best-studied examples were the carnivorous sponge Asbestopluma hypogea and the hexactinellid sponge Oopsacas minuta, which are both also found in the bathyal Mediterranean. Similar examples exist for less conspicuous taxa such as bryozoans and brachiopods. It appears that caves, particularly these cold water caves, are home to an interesting combination fauna: successfully-established true deep-sea species and mobile shallow water taxa which use these caves as a shelter from predators.

Recent research investigated the sediment-dwelling Foraminifera and meiobenthos; mainly harpacticoid copepods, nematodes and annelids. Preliminary results showed a strong gradient in meiofaunal composition from the cave’s entrance to the darkest parts, with more deep-sea components in the darker inner regions of the cave. Little is known about the temporal stability of these caves. Seasonal differences in the littoral zone naturally affect the cave entrance. However, seasonal fluctuations also penetrate into the darkest parts of caves, where allochthonous organic matter may display considerable intra-annual variations. In addition, the daily movements in and out of the cave of some residents (fish, mysids) may transport organic matter and thereby transmit a temporal signal. Longer temporal trends are largely unknown, but some marine caves have been affected by current warming trends. After a series of unusually hot summers, one Mediterranean-cave mysid species has been replaced by a similar mysid in the majority of caves of the North Western Mediterranean. There are also indications that some deep-sea taxa (particularly sponges), which are probably living near their thermal limit in the ‘cold-water’ marine caves, are showing signs of mortality during warm winters. Other Mediterranean caves are home to new planktonic species of copepods, such as Stephos vivesi.

Biodiversity and hydrothermal vents

Analyses of high-definition photographs and video records conducted revealed detailed information about the spatial distribution of biotic assemblages on the Eiffel Tower hydrothermal edifice (a large hydrothermal vent). This edifice is situated on the Mid-Atlantic Ridge, south of the Azores. The faunal assemblages consists of bivalves, decapods and other smaller fauna ranging from polychaetes and gastropods to bacteria.

The distribution of these assemblages on the surface of the edifice is very patchy and is related to the position of thermal vents and resulting temperature gradients. This distribution remained rather constant during the years in which the edifice was observed (1994, 1998,2001, 2002, 2005, 2006 and 2008).

The east side of the Eiffel Tower hydrothermal construct in the Lucky Strike vent field (Mid-Atlantic Ridge). The map shows the distribution of different faunal assemblages and substrates during 2006. The assemblages are characterised by different animals species and the presence or absence of bacterial mats. The main attached animals present are the mussel Bathymodiolus azoricus (Assemblages 1, 2a, 2b) and the shrimps Mirocaris fortunata and Chorocaris chacei (Assemblage 3). Substratum 1b is a bare surface with visible bacterial mats; Substratum 2 is a bare surface with whitish or greyish mineral precipitation and possible bacteria. (Adapted from Cuvelier et al., 2009) .