Preliminary study of hyperbenthos in Heraklion Bay (Cretan Sea)

Figure 1. Lateral and front schematic view of the hyperbenthic sledge: C1: collector 1; C2: collector 2; C3: collector 3; P.H.: pressuring house containing the battery package and the controller; Pn1: plankton net 1; Pn2: plankton net 2; Pn3: plankton net 3; E.M.S.: electro-mechanical opening-closing system; D1: door 1; D2: door 2; D3: door 3

The hyperbenthos is a term applied to the association of small sized bottom-dependent animals (mainly Crustaceans) that have good swimming ability and perform, with varying amplitude, intensity and regularity, seasonal or daily vertical migrations above the seabed (Brunel et al., 1978). Beyer (1958) was the first referring to "hyperbenthos" that has to be used in preference to "suprabenthos", because the Greek noun benthos should be preceded by a Greek prefix (hyper-), rather than by its Latin equivalents (supra-, super-).

 The terms "hyperbenthos" and "suprabenthos" are used mainly in temperate and northern seas, whereas "swarming", "resident" or "demersal zooplankton" and "benthopelagic plankton" or "benthic boundary layer fauna" are usually preferred to designate the same fauna in tropical areas and the deep sea respectively.

There has been much taxonomic interest in the hyperbenthos. Most of the hyperbenthic species are present in much higher densities than in either the overlaying water layers or in the adjacent sediment and most of them are not there accidentally. Beyer (1958) discussed the species found in abundance in the hyperbenthos but which were rare or absent elsewhere. 

More recently, there has been recognition of a distinction between the truly hyperbenthic species and a variety of "visiting" or "immigrant" animals that can be classified as endobenthic, epibenthic or planktonic. There has been also increased interest in the role of hyperbenthos in the functioning of marine ecosystems, mostly because it has been found that many demersal fish and epibenthic crustaceans feed, for at least part of their life, on hyperbenthic animals. Furthermore, there is renewed interest in pre-recruit studies as many larval and early post-larval fish and crustaceans have a hyperbenthic phase. On the other hand, studies of benthic-pelagic coupling related to energy fluxes rarely include samples taken within a few centimeters above the seabed and may, therefore, underestimate significantly the flux of particulate organic material (Mees & Jones, 1997).

The often highly mobile hyperbenthic animals living immediately above the seabed are only occasionally caught by conventional benthic or pelagic sampling gears thus a plethora of hyperbenthic sampling devices have been constructed and used with varying success. The choice of sampling equipment in use depends largely on local conditions e.g. size of the ship, power and capabilities of the lifting gear, degree of exposure, depth, bottom relief and sediment structure and the type of sample required for the specific research topic under investigation (Eleftheriou & Holme, 1984).

Most of the published information refers to temperate and cold-water environments. In recent years there have been considerable development in hyperbenthic research in the tropics (mostly in coral reef lagoons) and the deep sea. Nevertheless, Mediterranean Sea hyperbenthic studies are scarce and limited to the western basin of this enclosed region referring either to the surf zone or to deep environments (Cartes 1998; Cartes & Sorbe, 1999; San Vicente & Sorbe, 1999; Cartes et al., 2001). As far as the Eastern Mediterranean Sea is concerned, there is total absence of information on hyperbenthic communities. In the 1990s a number of studies (Karakassis, 1991; Tselepides 1992; Tselepides & Eleftheriou, 1992; Koutsoubas et al., 1992; Karakassis & Eleftheriou, 1997) contributed significantly to our knowledge of the macrobenthic fauna from the Cretan Sea. Even so, our knowledge of the hyperbenthic fauna remains nil. 

In the framework of a multidisciplinary European research project conducted by the Department of Technology and Management of the Marine Environment of the Institute of Marine Biology of Crete (I.M.B.C.), a sampling program of 5 successive cruises of the R.V. Philia has been performed in the continental shelf and upper slope of Heraklion Bay (Cretan Sea). The investigation of the hyperbenthic diversity of this particular area was included among the aims of the project. 

Heraklion Bay is situated in the north coast of Crete. Fieldwork included collection of water column, sediment, plankton and macrobenthic samples, as well as measurements of physicochemical and hydrographic parameters. For the collection of hyperbenthic samples, in particular, a modified three-level towed sledge (see Figure) was used. Technical characteristics are given in Shand & Priestley (1999).

Preliminary results reveal that the hyperbenthic sledge really samples a specific fauna. The hyperbenthic samples comprise even different taxonomic groups comparing to the macrobenthic and macrozooplanktonic ones. Mysidacea, Cumacea, Amphipoda, Decapoda, Copepoda and Isopoda are among the most abundant taxonomic groups of the hyperbenthic habitat of Herakion Bay.

 Further analysis to species level will provide new information on the life cycles and classification of macrofaunal species in endobenthic, epibenthic, and hyperbenthic ones. Considering that numerous benthic species -up to now regarded as rare in the area- may prove to be abundant when proper sampling methodology has been used and it is expected that results of this project will significantly increase our knowledge concerning the biodiversity of the benthic environment in the continental shelf of the Eastern Mediterranean.

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