THE DEEP-SEA FLOOR: AN OVERVIEW
25
of the 180-km transect are relatively similar (of the
species expected in a random sample of 200 individuals
at each of the two localities, 79% were shared)
(Grassle and Maciolek, 1992). Despite this similarity,
the endemics are a source of faunal heterogeneity
among stations.
The pattern of a few widespread species and many
endemics also occurs in the ocean basins. For example,
at the scale of the Atlantic, 11 of 109 species and
subspecies of protobranch bivalves have been found
in six or more of the 15 deep basins, but 48 have
been found in only one, and the remaining species tend
to occur in two adjacent basins (Allen and Sanders,
1996). At least for protobranchs, the distributions of
most species tend to be much smaller than pan-Atlantic,
and the protobranch faunas of adjacent basins tend to
be similar – for instance, the Sierra Leone and Angola
Basins share 20 of a total of 40 species. The Norwegian
Basin is an exception. It shares no protobranch species
with any other Atlantic basin. The reason for this
difference is not known, but the Norwegian Basin
became anoxic during the last glaciation (Schnitker,
1979) and lost most of its metazoan fauna. It is unclear
why some species occur in several basins and others
occur in only one, but ecological differences among the
basins, dispersal limitations, and history are all likely
to play a role.
Patterns with depth
The faunal break at the shelf/slope transition has
been confirmed repeatedly (Sanders and Hessler, 1969;
Haedrich et al., 1975; Rex, 1977; Carney and Carey,
1982). Below that depth, regions of relatively slow
faunal change (= zones) are separated by bands of more
rapid faunal change. The depths of zone boundaries
vary among taxa and locations, but in the North
Atlantic appear to be at about 0.5, 1, 1.5, and 2 km
(Gage and Tyler, 1991). Below 2 km, the rate of change
of the fauna slows, and zonation does not appear to
be as marked, perhaps in part because of the smaller
sampling effort at these depths.
Although the depth range of an individual species
does not often extend from 200 m into the trenches or
even from 200 m to the abyssal plain, some species
have extensive ranges. For example, the brittle star
Ophiomusium lymani off North Carolina has been
found from 1372 to 3987 m (Grassle et al., 1975).
Species are not equally abundant throughout their
ranges, tending to be rare at the extremes as expected,
if conditions become less suitable as their range
boundaries are approached (Grassle et al.: their table 6).
The depth range of a species may vary along isobaths.
For example, in contrast to its depth range off North
Carolina, O. lymani was found only between 1705 and
2170 m on a transect from Massachusetts to Bermuda
(Schoener, 1969). Many species appear to have very
small depth ranges, having been collected at only one
station along a transect. Because many transects have
had stations at depth increments of a few hundreds of
meters, the depth ranges of such species must be less.
Contrary to expectation, representatives of deep-sea
taxa are found on the shelf in the Arctic, Antarctic,
and Mediterranean seas and in Scandinavian fjords.
For example, species of asellote isopods of deepsea genera have been found at shelf depths in the
Bay of Naples (Schiecke and Fresi, 1969) and off
Sweden (Hessler and Str¨ omberg, 1989); foraminifer
species of deep-sea genera have been found at SCUBAdiving depths in Antarctica (Gooday et al., 1996);
and a sponge from a bathyal–abyssal subphylum has
been found in a cave at 18 m in the Mediterranean
Sea (Vacelet et al., 1994). This pattern could arise if
representatives of taxa that had evolved in the deep
sea entered shallow water. Alternatively, the shallowwater representatives of these taxa could be relics in the
original environment of groups that invaded the deep
sea. At least for the shelf representatives of asellote
isopods, the former appears to be the case (Hessler and
Thistle, 1975). The environments where representatives
of deep-sea taxa are found in shallow water have deepsea temperatures at shelf depths (cold at the poles but
warm in the Mediterranean), suggesting that ordinarily
the temperature gradient (or a correlate) separates the
deep-sea fauna from that of the shelf.
Combined along- and across-isobath patterns
Relatively few deep-sea studies have had sample
coverage adequate to address along- and across-isobath
patterns in the distribution of species. For protobranchs
at the scale of the Atlantic, Allen and Sanders (1996)
found that the similarity of the fauna among stations
was much greater along isobaths than across isobaths.
In particular, the faunas of stations separated by
16 500 km along an isobath were as similar as those
of stations separated by 0.8 km across isobaths. The
pattern of greater similarity along than across isobaths
has also been found at the 100-km, within-oceanbasin scale (Carney et al., 1983; Grassle and Maciolek,
1992). At the 100-km scale, recent work on benthic
decapods from the Mediterranean slope has revealed
25
of the 180-km transect are relatively similar (of the
species expected in a random sample of 200 individuals
at each of the two localities, 79% were shared)
(Grassle and Maciolek, 1992). Despite this similarity,
the endemics are a source of faunal heterogeneity
among stations.
The pattern of a few widespread species and many
endemics also occurs in the ocean basins. For example,
at the scale of the Atlantic, 11 of 109 species and
subspecies of protobranch bivalves have been found
in six or more of the 15 deep basins, but 48 have
been found in only one, and the remaining species tend
to occur in two adjacent basins (Allen and Sanders,
1996). At least for protobranchs, the distributions of
most species tend to be much smaller than pan-Atlantic,
and the protobranch faunas of adjacent basins tend to
be similar – for instance, the Sierra Leone and Angola
Basins share 20 of a total of 40 species. The Norwegian
Basin is an exception. It shares no protobranch species
with any other Atlantic basin. The reason for this
difference is not known, but the Norwegian Basin
became anoxic during the last glaciation (Schnitker,
1979) and lost most of its metazoan fauna. It is unclear
why some species occur in several basins and others
occur in only one, but ecological differences among the
basins, dispersal limitations, and history are all likely
to play a role.
Patterns with depth
The faunal break at the shelf/slope transition has
been confirmed repeatedly (Sanders and Hessler, 1969;
Haedrich et al., 1975; Rex, 1977; Carney and Carey,
1982). Below that depth, regions of relatively slow
faunal change (= zones) are separated by bands of more
rapid faunal change. The depths of zone boundaries
vary among taxa and locations, but in the North
Atlantic appear to be at about 0.5, 1, 1.5, and 2 km
(Gage and Tyler, 1991). Below 2 km, the rate of change
of the fauna slows, and zonation does not appear to
be as marked, perhaps in part because of the smaller
sampling effort at these depths.
Although the depth range of an individual species
does not often extend from 200 m into the trenches or
even from 200 m to the abyssal plain, some species
have extensive ranges. For example, the brittle star
Ophiomusium lymani off North Carolina has been
found from 1372 to 3987 m (Grassle et al., 1975).
Species are not equally abundant throughout their
ranges, tending to be rare at the extremes as expected,
if conditions become less suitable as their range
boundaries are approached (Grassle et al.: their table 6).
The depth range of a species may vary along isobaths.
For example, in contrast to its depth range off North
Carolina, O. lymani was found only between 1705 and
2170 m on a transect from Massachusetts to Bermuda
(Schoener, 1969). Many species appear to have very
small depth ranges, having been collected at only one
station along a transect. Because many transects have
had stations at depth increments of a few hundreds of
meters, the depth ranges of such species must be less.
Contrary to expectation, representatives of deep-sea
taxa are found on the shelf in the Arctic, Antarctic,
and Mediterranean seas and in Scandinavian fjords.
For example, species of asellote isopods of deepsea genera have been found at shelf depths in the
Bay of Naples (Schiecke and Fresi, 1969) and off
Sweden (Hessler and Str¨ omberg, 1989); foraminifer
species of deep-sea genera have been found at SCUBAdiving depths in Antarctica (Gooday et al., 1996);
and a sponge from a bathyal–abyssal subphylum has
been found in a cave at 18 m in the Mediterranean
Sea (Vacelet et al., 1994). This pattern could arise if
representatives of taxa that had evolved in the deep
sea entered shallow water. Alternatively, the shallowwater representatives of these taxa could be relics in the
original environment of groups that invaded the deep
sea. At least for the shelf representatives of asellote
isopods, the former appears to be the case (Hessler and
Thistle, 1975). The environments where representatives
of deep-sea taxa are found in shallow water have deepsea temperatures at shelf depths (cold at the poles but
warm in the Mediterranean), suggesting that ordinarily
the temperature gradient (or a correlate) separates the
deep-sea fauna from that of the shelf.
Combined along- and across-isobath patterns
Relatively few deep-sea studies have had sample
coverage adequate to address along- and across-isobath
patterns in the distribution of species. For protobranchs
at the scale of the Atlantic, Allen and Sanders (1996)
found that the similarity of the fauna among stations
was much greater along isobaths than across isobaths.
In particular, the faunas of stations separated by
16 500 km along an isobath were as similar as those
of stations separated by 0.8 km across isobaths. The
pattern of greater similarity along than across isobaths
has also been found at the 100-km, within-oceanbasin scale (Carney et al., 1983; Grassle and Maciolek,
1992). At the 100-km scale, recent work on benthic
decapods from the Mediterranean slope has revealed