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AMER. ZOOL., 30:45-64 (1990)
Aspects of the Ecology of the Deep-water Fauna of the
Gulf of Mexico1
WILLIS E. PEQUEGNAT, BENNY J. GALLAWAY, AND LINDA H. PEQUEGNAT
LGL Ecological Research Associates, Inc., 1410 Cavitt Street, Bryan, Texas 77801
SYNOPSIS. Recent transects across the continental slope off western Louisiana, the Mississippi River delta, and the Florida peninsula in the general depth range of 300-3,000
m have provided information on habitat variables and on faunal composition, density,
and depth zonation. In the meiofauna (retained by 63 /im screens) nematodes, harpacticoid
copepods, nauplii, polychaetes, ostracods, and kinorynchs were numerically dominant, in
that order, and together these groups made up 98% of the fauna. The macrofauna
(retained by 0.3 mm screens) was dominated by polychaetes, ostracods, bivalves, tanaids,
bryozoans, and isopods in that order, and together these made up 86% of the fauna.
Densities of both groups were highest on the Central Transect, and densities of both
tended to decrease with depth. Between the depths of 300 m and 3,000 m there was a
threefold decrease in meiofaunal and a twofold decrease in macrofaunal density. Among
the megafauna (collected by otter trawl) invertebrate densities, dominated by crustaceans,
were four to five times as great as fish densities at all depths and on all transects. Densities
were greatest on the Eastern and least on the Central Transect, and on all transects they
decreased with depth. On the slope off Louisiana and East Texas, in the depth range of
400-900 m, dense biological communities have been encountered at about 40 locations
aggregated around oil and gas seeps. These organisms include clusters of large tube worms
(vestimentiferans), vesicomyid clams, mussels, galatheid crabs, bresiliid shrimps, neogastropods, limpets, and fishes. This community is trophically dependent upon chemoautotrophic bacteria (which utilize hydrogen sulfide), although some mussels directly utilize
methane as a carbon source. This community is closely related to that of the hydrothermal
vent systems of the East Pacific Rise and to the seep communities at the base of the Florida
escarpment. The megafauna of the northern and eastern Gulf of Mexico falls naturally
into the following depth distribution pattern: Shelf/Slope Transition Zone (118-475 m),
Archibenthal Zone—Horizon A (500-775 m), Archibenthal Zone—Horizon B (800-975
m), Upper Abyssal Zone (1,000-2,275 m), Mesoabyssal Zone (2,300-3,225 m), and Lower
Abyssal Zone (3,250-3,850 m). Biological characteristics of each zone are discussed.
in the deep waters and sediments of the
Mediterranean are very low, resulting in a
very impoverished fauna (Riedl, 1983). The
nutrient depletion results from the fact that
t h e onl
y natural source of ocean water of
the Mediterranean comes from the Atlantic over the Gibraltar Sill which has an
effective depth of 300 m or less (Fairbridge, 1966). Because these shallow waters
have already supported phytoplankton
growth in the Atlantic, they are depleted
of much of their nitrate, phosphate, and
silicate. Yucatan Strait is the Gulfs principal connection with the Atlantic (via the
Caribbean Sea), but the sill here has an
effective depth of 1,500 to 1,900 m
(McLellan and Nowlin, 1963), which does
not block the input of nutrients but does
prevent the input of the Caribbean's 2°C
1
From the Special Session on Ecology of the Gulf of bottom waters
INTRODUCTION
This paper presents some aspects of the
ecology of the offshelf benthic invertebrates and demersal fishes of the northern
and western Gulf of Mexico. T h e Gulf is
often described as a Mediterranean-type
sea, but it is much smaller (1.6 vs. 2.8 million km2), shallower (3,850 vs. 5,090 m),
and has a much more typical deep-sea fauna
than the Mediterranean Sea. In fact, the
Mediterranean does not have an abyssal
habitat in spite of its depth. The temperatures of its deepest waters range between
12.7 and 14.5°C at salinities between 38.4
and 39 ppt, whereas the abyssal bottom
watersintheGulfreach4.35°Candasalinity of 34.97 ppt. Moreover, nutrient levels
2 l ^^^ M «Kn l ££SS
Our collections of the meiofauna and
of the American Society of Zoologists, 27-30 December 1987, at New Orleans, Louisiana.
macrofauna were limited geographically to
the northern Gulf between 86° and 93° west
45
46
W. E. PEQUEGNAT ET AL.
longitude, and bathymetrically between
about 300 and 3,000 m depth. Sampling
for these groups was carried out under the
direction of LGL Ecological Research
Associates, Inc. (LGL) through Contract
Nos. 14-12-0001-30046 and 14-12-000130212 with the U.S. Dept. of the Interior
Minerals Management Service (MMS). The
discussion of the megafauna is based upon
the LGL collections and to a lesser extent
upon those reported upon by TerEco Corporation under Contract No. AA851-CT112 with MMS (Pequegnat, 1983).
of the Archibenthal Zone (800-975 m); the
Upper Abyssal Zone (1,000-2,275 m); the
Mesoabyssal Zone (2,300-3,225 m); and the
Lower Abyssal Zone (3,250-3,850 m). The
purpose of this strategy was to evaluate the
predictive value of the Pequegnat zonation
scheme.
During this study 40 habitat variables
were measured as potential factors affecting the distribution of the biota by region,
depth, season, and years. To analyze this
data set by inspection is difficult and time
consuming; however, Principal Component Analysis (PCA) enables one to transMATERIALS AND METHODS
form the set into smaller combinations that
LGL's sampling was conducted on five account for most of the variance of the
cruises in the years 1983 (Cruise I, fall), larger set. The output of PCA permitted
1984 (Cruise II, spring; and Cruise III, fall), us to group stations in terms of their physand Cruises IV and V in spring of 1985. A ical/chemical attributes and then to comtotal of 59 stations was sampled along three pare these with various biological classifidown-slope transects (Cruises I, II, III) in cations of the same stations (see under
the western, central, and eastern Gulf (Fig. Megafauna).
1), and isobathymetrically at three depths
on the Eastern Transect (Cruise IV) and
RESULTS
between the Western and Central Transects (WC on Fig. 1) during Cruise V. Sam- Environment of the study area
ples for analysis of meiofauna and macroThe Gulf of Mexico shares more deepfauna, as well as a suite of sediment sea species with the Atlantic, even to the
characteristics and inclusions [e.g., texture, latter's eastern boundary in the Bay of Biscarbonate, total organic carbon (TOC) and cay off the coasts of France and Spain, than
hydrocarbons], were taken in a box corer it does with the Mediterranean. This is not
that measured 24.5 x 24.5 x 44 cm. It to imply that the Gulf does not have its
was fitted with six metal coring tubes that unique oceanographic characteristics, for
measured 43.5 x 3.5 cm in internal diam- indeed it does. The two factors that seem
eter and were used for meiofaunal analy- to account for most of these characteristics
ses. The megafauna was sampled with are the East Gulf Loop Current, which flows
trawls, having gapes of 9 m (LGL) and 20 into its eastern half, and the Mississippi
m (TerEco), with dredges, and with a ben- River System, whose waters pour into the
thic camera system. The latter is not dis- Gulf a little east of the middle of the northcussed in this paper; suffice it to say that ern boundary and then spread westward
photos taken along trawling lines produced over the continental shelf.
higher megafaunal densities than the trawl
The Loop Current, which is an extenand in some cases revealed a holothurian sion of one limb of the Gulf Stream, enters
species that was never brought up by trawls the Gulf from the Caribbean via the Yucaeven though it was undoubtedly the most tan Channel at speeds up to four knots and
common species in its depth range. Sam- exits into the Atlantic via the Florida Straits,
pling depths were not randomly spaced where it joins the main Gulf Stream. This
down the slope but were the approximate current and its branches drive the major
midpoint of previously defined biological surface circulation system of the Gulf and
zones with faunal assemblages (Pequegnat, account for some of its biological features.
1983): (1) the Shelf/Slope Transition Zone It brings larvae, pelagic fishes, plant mate(118-475 m); (2) Horizon A of the Archi- rial, and heat into the eastern Gulf in parbenthal Zone (500-775 m); (3) Horizon B ticular. But the Loop also has a major influ-
..; ||, )„ , , l..','..'.l. ..j . ,i ' . " V ' I
30°-
-30°
-28°
•26*
92°
FIG.
90°
88°
86°
1. Station locations and sediment map of the MMS/LGL Northern Gulf of Mexico Continental Slope Study.
48
W. E. PEQUEGNAT ET AL.
ence on the western part as well. When
fully extended to the north, the Loop may
cut off anticyclonic (clockwise) rings or
gyres that slowly migrate into the western
Gulf while spinning at substantial speeds.
The Loop also entrains water on its northwestern edge that eventually spins off as a
cyclonic (counter-clockwise) ring that also
moves westward. As a result, the western
Gulf displays a northern cyclonic ring, a
midposition anticyclonic ring, both attributable to the Loop, and a southern cyclonic
gyre in the Gulf of Campeche (Nowlin and
McLellan, 1967).
The Mississippi River System exerts an
important ecological influence on the Gulf
because it brings great amounts of freshwater, fine sediments, plant material and
various other hydrocarbon material to the
delta and thence to the Gulf. As the freshwater flows over the saline waters, it creates
a geostrophic current that because of the
Coriolis effect turns to the right or westward. The river also discharges huge
amounts of sediment into the Gulf each
day. Some of it is transported due west
where it falls to the bottom, creating rich
fishing grounds, but more of it moves
southwestward to the edge of the delta and
from time to time cascades down the slope
onto the Mississippi Fan or onto the abyssal
plain to the west, much as it did in the
geologic past.
Distribution of sediment types
The Gulf can be divided into two major
sediment provinces, carbonate to the east
of DeSoto Canyon and southward along
the Florida coast, and terrigenous to the
west of DeSoto Canyon past Louisiana and
Texas thence southward along the Mexican coast around to Campeche Canyon
adjacent to the monolith of carbonate
called Campeche Bank. Most of the sampling reported here was done in the terrigenous regime west of 86° west longitude.
As shown in Figure 1, sediment type distribution had considerable regional variation. In the study area, the most common
sediment type was silty-clay. It varied
regionally having slightly higher percentages of sand in the eastern Gulf than in the
western or central areas, and in the latter
there were higher percentages of silt than
clay at the deepest stations. Clay sediments
were usually texturally uniform and predominated at stations shallower than 1,226
m in the western and central Gulf. Sandy
clay and sand-silt-clay sediments occurred
in patches, both shallow and deep on the
Western and Eastern Transects.
Hydrocarbons in sediments
Sediments on the Gulfs slope contain a
mixture of hydrocarbons derived from terrestrial, petrogenic, and planktonic sources.
The molecular-level alkane distribution
and concentrations were fairly uniform
across the slope except in the vicinity of
hydrocarbon seeps where concentrations
were very high. The relative importance
of one or another of the above sources also
varied with season and depth. The concentrations of extractable organic matter
(EOM), aliphatic hydrocarbons, and aliphatic unresolved-complex-mixture (UCM)
were much lower than found in sediments
of the Gulfs coastal and shelf sediments.
EOM is a composite of biogenic and petrogenic material; its concentrations were
lowest on the Eastern Transect and nearly
equal on the Western and Central Transects. The UCM concentrations were similar on all transects, being only slightly elevated in Western Transect sediments. The
amounts of terrestrial hydrocarbons
decreased both to the west and east of the
Central Transect, being inversely related
to distances from the Mississippi River delta
and steepness of the slope at a given station. The concentrations of planktonic
hydrocarbons tended to be higher on the
Eastern Transect, but perhaps this was due
to a relatively low terrestrial input which
will mask other inputs. Overall the highest
aliphatic hydrocarbon concentrations were
found in clayey sediments, especially on the
Central Transect, where reservoired
petroleum seeps upward from deeper layers and particles derived from river/land
sources are transported to the slope (see
section on Seep Communities).
Hydrography
Hydrographic data collected on temperature, salinity, dissolved oxygen, and
nutrients were relatively uniform across the
49
ECOLOGY OF DEEP-WATER FAUNA
TABLE 1.
Overall abundance and estimated biomass of the meiofaunal collection.
Taxa
Overall abundance
Mean density
per 10 cm1
Standard error
Nematoda
Harpactacoida
Nauplii
Polychaeta
Ostracoda
Kinorhyncha
All other taxa
Total
135,167
41,826
30,119
9,648
6,118
2,222
5,648
230,748
414.62
128.3
92.39
29.6
18.77
6.82
0.91
707.82
10.25
3.11
2.75
1.56
0.58
0.41
0.13
16.2
northern Gulf except for a double oxygen
minimum layer in the eastern region. Plots
of temperature/salinity curves against
depth revealed three quite uniform physical environments in the water column: a
shallow zone (300 to 600 m), an intermediate zone (600 to 1,000-1,200 m), and a
deep zone from there to the abyssal plain.
The shallow zone varied most in temperature/salinity conditions.
Meiofauna
Meiofauna were denned in this study as
metazoa retained on a 63 jim screen; counts
of Foraminifera were noted but not
included in the present analysis. A total of
43 major groups were identified of which
the Nematoda, Harpacticoidea, Polychaeta, Ostracoda, Kinorhyncha and the
temporary crustacean naupliar larvae comprised 98% of the collection. A total of
about 231,000 individuals was examined,
with the nematodes, harpacticoids, nauplii,
polychaetes, ostracods and kinorhynchs
accounting for 58, 18, 13, 4 and about 1%
of total individuals (Table 1). The same
proportions were maintained in mean density (per 10 cm2), ranging from 415 nematodes down to 7 kinorhynchs; however, wet
weight relationships did not follow the same
pattern (Fig. 2). The total wet weight of
the entire collection was 1,151,283 fig of
which polychaetes accounted for 46%,
ostracods for 20%, harpacticoids for 10%,
nematodes for 9% and nauplii for 5%.
Although there were some indications
that spring samples on the Central Transect had higher densities than the fall samples of 1983 (Cruise I) and 1984 (Cruise
III), and that the spring samples of 1985
on the Eastern Transect had relatively high
Wet weight
multiplier
0.85
2.8
2.15
55.25
39.15
2.8
NA
Approximate
wet weight n%
114,892
117,113
64,756
533,052
239,520
6,222
75,730
1,151,283
densities, we did not detect any marked
seasonal differences in meiofaunal density
(Fig. 3). Although this is consistent with
figures published by Thiel (1983) for the
deep sea, it is noted that Pequegnat and
Sikora (1979) and others have reported
seasonality in abundance of permanent
meiofauna collected from coastal and estuarine environments.
Comparisons showed that there were
significant differences in meiofaunal density (P = 0.05) among the three transects.
Densities ranged from 125 to 1,141 organisms/10 cm2, with generally higher densities on the Central than on the Eastern
and Western Transects. There were also
significant differences in density among
stations within the three transects, some of
which resulted from the proximity of stations to oil seeps, but it was not possible to
establish a clear and predictable pattern of
significant change throughout the 59 sampling stations.
Regressions of the logs of the numbers
of meiofauna and macrofauna per m2 plotted against depth revealed (a) that the density of meiofauna was about two orders of
magnitude greater than that of the macrofauna, (b) that the densities of both
groups decreased with increasing depth,
and (c) that there was a threefold decrease
in the density of the meiofauna and a twofold decrease for macrofauna between
depths of about 300 and 3,000 m. The
decrease in density of meiofauna with
increasing depth has been reported in the
Gulf by Rowe and Menzel (1971) and Rowe
et al. (1974) and in the eastern Atlantic by
Thiel (1983). Our results support Thiel's
data on meiofaunal decrease with depth,
but we found a decreasing ratio of meio-
50
i
W. E. PEQUEGNAT ET AL.
*
•o
VI
W2
V3
V4
W5
C1
C2
C3
C4
C5
E1
E2
E3
H
•
H
E^
H
•
3000 n
E4
E5
Kinorhyncha
Ostracoda
Polychaeta
Nauplii
Harpacticoida
Nematoda
2500 -
x:
CO
<D
2000 -
Wet\
•>-
O
1500
proxi
1
-
1000 U
500 -
359 604 854 1410 2506 354 598
838 1390 2389 354 627 846 1350 2827
Depth (m)
FIG. 2. Densities and approximate wet weights for the six most abundant meiofaunal groups collected during
Cruise II, Western, Central, and Eastern Transects.
fauna to macrofauna with depth instead of
his reported increasing ratio.
Regressions of the logs of approximate
wet-weight biomass per m2 of meiofauna
and macrofauna revealed that within the
depth range studied the weight of the
meiofauna was only slightly greater than
that of the macrofauna.
Regressions of the number of meiofaunal individuals of nematodes and harpacticoids per sample replicate against percent sand and percent clay showed that both
MEIOFAUNA DENSITY - Number/10 cm 2
REGION • SEASON • YEAR
COMPARISONS
Station-Mean Depth (m)
1OOO
\
500
7//
/ S /
fff
-/O«Z
'
. S)W/A'& /S&*
/**/.£. /«SW
V
/$&+
M£l
^F
-sr*r/<xv
trssr
FIG. 3. Meiofauna density (no./cm!) at the West, Central, and East Transect stations during four sampling periods.
52
W. E. PEQUEGNAT ET AL.
TABLE 2.
Relative abundance of major macrofaunal groups.
Number of
taxa
Taxonomic group
Abundance
Polychaeta
Ostracoda
Bivalvia
Tanaidacea
Bryozoa
Isopoda
Amphipoda
Aplacophora
Nemertea
Ophiuroidea
Sipuncula
Cumacea
Porifera
Scaphopoda
Scyphozoa
Gastropoda
Holothuroidea
Oligochaeta
Ascidiacea
Hydrozoa
Brachiopoda
Arachnida : Acarina
Kinorhyncha
Echinoidea
Priapulida
Scleractinea
Decapoda
Mystacocarida
Pogonophora/Vestiment.
Echiura
Actiniaria
Alcyonaria
Turbellaria
Crinoidea
Pycnogonida
Hemichordata
Asteroidea
Misc. Anthozoa
Mysidacea
Archiannelida
Total
24,313
4,960
3,645
3,610
3,049
2,327
1,285
626
19
55
186
99
133
886
630
603
570
521
424
382
331
276
250
247
136
103
60
49
33
33
25
12
11
9
8
8
8
7
7
4
2
2
2
1
1
21
17
37
86
39
10
1
53
13
9
18
15
2
1
3
6
1
6
13
1
1
1
3
2
1
2
3
1
2
1
1
1
48,970
1,569
79
62
went down as the percentage of sand rose
to 40% and went up as the percentage of
clay rose to about 75%. However, the correlations were low (r = 0.17 to 0.38), suggesting that other factors were involved.
Macrofauna
Macrofauna are denned in this study as
those organisms that were collected with
the box corer and retained on a 0.3 mm
sieve, instead of the 0.42 or 0.5 mm sieves
often employed by others. This must be
borne in mind when comparing densities.
79
Number of
species
414
18
41
168
82
119
50
0
20
13
31
76
22
5
0
8
4
6
11
8
2
0
0
3
0
4
10
0
0
0
0
1
0
2
3
0
0
0
0
0
1,121
Number of
genera
163
1
10
13
12
8
11
0
0
3
3
8
10
2
0
27
4
1
3
3
0
0
2
1
0
1
1
0
0
0
0
0
0
0
0
0
1
0
0
0
288
Number of
other taxa
49
0
4
5
5
6
18
1
1
1
3
2
7
3
1
18
5
2
4
4
0
1
1
2
1
1
2
1
1
1
3
()
()
160
The box core samples contained 8 phyla,
which were separated into 1,569 differentiable taxa. Species identifications were
attempted in all cases except for nematodes, harpacticoids, Aplacophora, and
Scyphozoa. Aside from these, 71% of the
taxa were identified to species and an additional 18% to genus but not to species. One
characteristic of the Gulf macrofauna,
which was remarked upon by most taxonomists, was their small size as compared
with the western North Atlantic.
Table 2 shows the 40 major macrofaunal
53
ECOLOGY OF DEEP-WATER FAUNA
groups in decreasing order of total numerical abundance for all five cruises. The first
six groups account for 86% of the total
numerical abundance, and the first 20
account for 99% of the total number of
individuals collected. At the species level,
most macrofaunal taxa were represented
by very few individuals. Excluding the
polychaetes, 550 species were collected five
or fewer times, yielding only from 1 to 5
individuals total at all 59 stations.
The overall regional, seasonal and annual
patterns by depth of macrofaunal abundance are shown by Figure 4. In the spring
of 1984 when all three transects were sampled, abundance was somewhat higher on
the Central Transect than on either the
Eastern or Western Transects. Moreover,
annual differences appear to be less than
regional and seasonal variations in abundance. Clearly spring abundances were
greater than fall abundance levels. On the
Eastern and Western Transects, an overall
decline of macrofaunal density with depth
was clearly indicated, but on the Central
Transect there were major peaks at 620
and 1,400 m depths, possibly as a result of
the proximity of oil and gas seeps.
Regional, seasonal and annual patterns
of macrofaunal diversity by depth also
showed distinct trends. Although the trends
were not pronounced, diversity appeared
to decrease from east to west and to have
been somewhat higher in fall than in spring
on the Central Transect. Differences in fall
diversity levels between years on the Central Transect were marginally higher than
spring 1984 levels. There appears to be a
tendency of slight diversity increase
between the shallowest station and some of
the sequentially deeper stations down to
1,400 m, at which point there was a marked
decrease down to the deepest station on
each transect. Diversity indices such as H"
often suffer the criticism that they can be
biased by sample size. Rarefaction, where
the sample data are used to estimate the
expected number of species represented by
a given sample size, is one approach towards
eliminating this bias. Trends in expected
number of species, E(S), for a sample of 50
individuals, paralleled the data obtained
from use of the H" diversity index.
Selected macrofaunal taxa
Polychaeta. A total of 24,313 polychaetes
was collected at the 59 stations of the five
cruises. These individuals represented 414
identified to species, 163 to genus only, and
49 only to family. Densities ranged from
167 to 2,905/m 2 . Density decreased with
increasing depth, except in the vicinity of
seep communities, and a sharp reduction
occurred at and below 2,000 m. Maximum
densities were obtained along the Central
Transect. Mean polychaete densities
arranged by depth for all stations sampled
show the following pattern of decreasing
densities with depth:
Depth
Range (m)
Density
(No./m2)
298-492
500-900
1,500-2,000
2,000-2,845
1,982
1,787
1,441
482
East-west differences in density showed
higher densities on the Western Transect
vs. the Eastern at depths shallower than
350 m, but the reverse was true at depths
of 540 m and more. At all depths, polychaete densities were substantially higher
in spring than in fall. Some 150 species
were represented by 10 or more individuals (264 species had less than 10 individuals). The 50 species that had more than
50 individuals were ranked as abundant;
64% of these ranged from the shallowest
to deepest stations, making them poor zone
markers.
Deposit feeders, selective and non-selective, were the most abundant polychaetes
in terms of total counts and number of
taxa; however, more families of carnivores
were collected than any other category, but
their abundances were less than either
deposit feeders or omnivores. Having 196
taxa, the deposit feeders were the most
diverse group, whereas the scavengers were
the least diverse.
Polychaetes were the most diverse group
of macrofauna sampled. A test of depthrelated changes was made during the fall
cruise of 1984 (Cruise III) on the Central
MACROFAUNA DENSITY, TOTAL
REGION-SEASON-YEAR
COMPARISONS
Station -Moan Depth (m)
I
2
3
A
5
355
62O
B5O
IAOO
26O2
FIG. 4. Comparative levels of macrofaunal densities by region, season, year and selected depth interval.
5000
ECOLOGY OF DEEP-WATER FAUNA
Transect where 11 stations including the
original five were sampled. Diversity was
uniformly high (mean of 55 species per station) down to station 4 at 1,465 m; it then
dropped to a mean of 22 species per station
down to 3,000 m.
Tanaidacea. The tanaidaceans were represented by 186 taxa of which 168 were
identified to the species level. In general
the number of species of tanaidaceans was
greatest between depths of 600 and 1,000
m, and lowest at the deepest and shallowest
stations. Density ranged from 28/m 2 to
512/m 2 , and as with diversity, was highest
at intermediate depths between 500 and
1,500 m. The four most common genera
were Pseudotanais, Mesotanais, Leptognathia
and Apseudes.
Isopoda. The isopods were represented
by 133 taxa of which 119 were ranked as
species, and most of which were new. As
with other macrofaunal crustacean groups,
the highest diversities of isopods occurred
at intermediate depths around 700-800 m.
Density ranged from 28-580/m 2 , with the
highest mean densities being found
between 1,000 and 1,500 m, which is somewhat deeper than for other macrofaunal
Crustacea. The four most commonly collected genera were Gnathia, Ischnomesus,
Prochelator, and Macrostylus.
Cumacea. The cumaceans were represented by 86 taxa of which 76 were identified to species and 8 to genus. Like most
other macrofaunal Crustacea, the Cumacea attain highest diversities at depths less
than 900 m. Their density pattern of highest mean densities at intermediate depths
is similar to that of amphipods and tanaidaceans; however, this is in the 500-900
m depth range unlike that of the isopods
at 1,000-1,500 m. Like other crustacean
groups, the cumaceans exhibited a sharp
decline in density below 2,000 m. The four
most common species are Procampylaspis
acanthoma, Cumella antipai, Campylaspis spinosa, and Cumella erecta.
Amphipoda. The Amphipoda were seventh in overall abundance and sixth in total
number of taxa with 79 taxa of which 50
were identified to species. Amphipods
reached their highest species diversity at
the shallowest depths and their lowest
55
diversity at the deepest stations. Diversity
on the Eastern Transect (45 taxa) was much
higher than on the Western Transect (11
taxa). Amphipod population densities
ranged up to 232/m 2 with a mean of 102/
m2, and maximum densities were reached
at stations within the 500-1,000 m depth
range. The four most common amphipod
genera were Pardisynopia, Byblis, Melita, and
Metaphoxus.
Three other groups are worthy of mention, viz., the gastropods, bivalves, and ascidians.
Although the gastropods and bivalves
were represented by nearly equal numbers
of taxa (53 vs. 55), only eight of the former
could be identified to species and 27 to
genus, whereas 41 of the bivalves were
identified to species and an additional 10
to genus. This resulted from the fact that
many gastropods were tiny immature specimens. Gastropod densities averaged about
10% of bivalve densities and greatest densities were achieved at depths of less than
900 m. Bivalve densities were highest at
1,000-1,500 m. Finally, the ascidians were
represented by 18 taxa, of which 11 were
identified to species and an additional three
to genus. The population densities of ascidians ranged from 1 to 49/m 2 , with the latter being found at a depth of 1,390 m. Drs.
Claude and Francoise Monniot (1989)
report that the ascidean density found by
LGL on the slope of the northern Gulf is
the highest known for ascidians in the deep
sea.
Megafauna
The megafauna includes those invertebrates and fishes that were captured in the
otter trawl. The fishes include both demersal and benthopelagic species. In the present studies megafaunal invertebrates were
between four and five times more abundant than fishes at all depths on all transects in terms of average density. The density of the megafauna on the Central
Transect was 3,241 individuals per hectare, which was about half the density
observed on the Western Transect (6,267/
ha) and a third of that found on the Eastern
Transect (9,463/ha). Bathymetrically, the
densities of the megafauna per trawl haul
56
W. E. PEQUEGNAT ET AL.
dropped from 185/ha at 1,250-m depth to
as low as 11/ha at 2,000 or more meters.
1988). The chemosynthetic organisms
contribute most of the elevated biomass
evident at the seep communities, but much
Seep communities
of the species diversity is attributable to the
Soft bottoms. In the region south of Lou- opportunistic aggregation of slope fauna
isiana and eastern Texas, the continental that are common but less abundant elseslope is characterized by the presence of where in the area.
scattered areas of seismic "wipe-out zones."
Solid bottom. Dense biological communiA wipe-out is an area in which the strati- ties of large epifaunal taxa similar to those
fication of the sedimentary facies has been found along ridge-crest vents at the East
obliterated by upward movement and Pacific Rise were discovered at depths of
seepage of petroleum and/or natural gas. 3,000 m at the base of the Florida EscarpMany of these active zones are character- ment by Paulina/. (1984). The most abunized by the presence of dense biological dant organisms of this seep community are
communities composed of large tube worm vestimentiferans and mussels, along with
"thickets," over 1 m in height and up to 2 vesicomyid bivalves, galatheid crabs, and
m in width, extensive beds of clams and limpets. The fauna is apparently nourished
mussels, and various other associated by sulfide-rich hypersaline waters seeping
species, such as galatheid crabs, bresiliid out at near ambient temperatures onto the
shrimps, neogastropods, limpets, and fishes. sea floor. These Gulf communities reveal
Various chemoautotrophic bacteria are the that hydrothermal vents and solid subprimary producers in this complex food strata are not necessary factors in the
web, driven by chemical compounds asso- development of seep communities.
ciated with the seepage of oil and gas
through the sea floor (Childress et al, Distribution, abundance and diversity
1986). They derive energy from hydrogen of megafauna
sulfide produced by bacterial degradation
Fishes. Fish density (no./ha) was markof oil in the sediment, or from seeping oil edly higher on the Eastern Transect than
and gas that contain sulfide. Childress et al. on the Central and Western Transects, with
(1986) have also demonstrated that the density on the Western Transect slightly
mussels can oxidize methane as a carbon higher than that observed on the Central
source.
Transect (Fig. 5). Based upon data from
Chemosynthetic assemblages have been the Central Transect, fish density was
found at some 40 locations in the north- higher in fall than in spring, and there was
central Gulf at water depths of from 400 little difference between fall collections in
to 900 m, but it is suspected that they exist 1983 and 1984. On all transects, fish denat greater depths as well. Both thermo- sity declined with increasing depth, but the
genic and biogenic hydrocarbons are trend was irregular on the Eastern Traninvolved. The taxa known to support che- sect. On this transect, however, there was
moautotrophic symbionts are two genera a marked similarity in density between the
of Vestimentifera, Lamellibrachia and common stations sampled in spring of 1984
Escarpia, two genera of vesicomyid bivalves, and 1985. More intense sampling of the
Calyptogena ponderosa and Vesicomya cordata, Central Transect (7 stations were added)
and a mytilid bivalve, Bathymodiolus sp. during fall 1984 revealed two abundance
(LGL Ecological Research Associates and peaks at depths of about 600 and 1,200 m,
Texas A&M University, 1986, personal whereas very few fishes were trawled at any
communication). In addition a clam in the of the five stations sampled between depths
family Limidae, Acesta bullisi, is usually of 1,200 and 2,500 m.
found attached to the obturacular plumes
In contrast to fish abundance, diversity
of the tube worm Lamellibrachia sp. Also a levels offishesdiffered little among regions,
new species of caridean shrimp, Alvinocaris seasons, or years, but did decline with
stactophila associated with the vestimenti- increasing depth in each region and seaferan worms has been described (Williams, son. There was no significant variation in
FISH DENSITY
300,
REGION-SEASON-YEAR COMPARISONS
Station-Mean Depth (m)
I
2
3
4
5
355
620
850
1400
2602
s
D
M
Y1984
w
7
y-/-SPRING 1984/)-}-}
c
z
FALL 1983
WEST
CENTRAL
7
5
k
{-
—STATION NUMBER
• REGION
FIG. 5. Comparative levels offish densities by region, season, year, and selected depth interval.
58
W. E. PEQUEGNAT ET AL.
fish diversity along the isobaths in either
the eastern or west-central Gulf area.
Interestingly, the community structure and
diversity of fish populations along the continental slope of the Middle Atlantic, as
reported by Musick (1976) are quite similar.
Invertebrates. Density patterns of megafaunal invertebrates were similar to those
noted for fishes, where levels in the eastern
Gulf were far greater than those observed
on either the Central or Western Transects
(Fig. 6). In addition, density observed on
the Central Transect during fall was higher
than spring and overall there was little difference between the fall levels of 1983 and
1984. Density by depth differed markedly
from that observed for fish. In at least three
periods the density at the deepest stations
was as high or higher than at shallow stations, with mid-depth stations being characterized by lowest density levels.
Decapod crustaceans dominated the
megafaunal invertebrate collections (129
of the 163 species total), so that diversity
of this group was used for a direct comparison to fish diversity patterns. As with
fish, we found no distinct regional, seasonal
or annual differences in decapod diversity;
however, we did find that maximum diversity often occurred at mid-depth stations
as opposed to shallower sites.
(1986) contains descriptions of six faunal
zones on the slope. All of these findings
agree that the replacement of species with
depth is not uniform, i.e., the rate of
exchange of species per depth increment
varies predictably on all vertical aspects of
the slope. One may regard zones as large
bands that show little change in faunal
composition that are separated by narrower bands where the rate of replacement
of critical species is high. If this concept is
correct, one should see far fewer exchanges
of species sampling along isobaths than
sampling vertically across isobaths. Also,
sampling in the same area but along successively deeper isobaths should show
decreasing degrees of faunal similarity
when compared with the samples from the
shallowest isobath. Clearly depth will be an
important determining factor, but so will
physico-chemical factors as well.
To test these assertions, on Cruise IV at
the Eastern Transect, trawl and physicochemical samples were taken at 15 stations
along three isobaths having mean depths
of 350, 625 and 850 m, and at one station
at 2,900 m depth. The data derived from
the 40 physico-chemical parameters measured at these stations were subjected to
Principal Component Analysis (PCA) to see
what order, if any, existed among the stations (Fig. 7). Principal Components 1 and
2 accounted for 59% of the total sample
BATHYMETRIC ZONATION OF THE
variance and yielded four station groups
MEGAFAUNA
separated on the x-axis mainly by bottom
Benthic biologists have considerable temperature, dissolved oxygen, and silt and
interest in whether or not the megafauna clay content and on the y-axis by hydrois arrayed on the continental slope in dis- carbon levels and bottom and surface parcernible depth-related zones. Some sup- ticulate organic carbon concentrations (Fig.
port this perception, but others believe that 7). In general, the groupings reflected four
observed abrupt changes in faunal assem- depth related environments: one repreblages are artifacts of sampling, i.e., that sented by stations shallower than 500 m
the limits of the zones were determined by (Shelf/Slope Transition); one by stations
the depths at which sampling was done. To about 650 m in depth (Archibenthal—
the contrary, Haedrich et al. (1975, 1980) Horizon A); one by stations about 850 m
identified four faunal zones on the U.S. in depth (Archibenthal—Horizon B), and
North Atlantic slope, and in 1983 Hecker the last by a single station located at 2,900
et al. designated five faunal zones on the m depth (Mesoabyssal).
Atlantic slope. In 1983 also, Pequegnat
Two methods were employed to test the
identified five major zones on the slope and biological aspect of the zonal concept with
rise of the northern Gulf of Mexico. Cruise IV data on the Eastern Transect, as
Recently a report on the benthic fauna of noted above. First, the stations were clusthe north Atlantic issued by Battelle et al. tered, using the average linkage method
INVERTEBRATE DENSITY
1000
REGION SEASON YEAR COMPARISONS
800 "o
Station-Mean Depth (m)
600 •£
I
2
3
4
5
355
620
85O
I4OO
26O2
400 Sg
200
w
8
0
c
filCG/O/V
FIG. 6. Comparative levels of invertebrate densities by region, season, year, and selected depth interval.
60
W. E. PEQUEGNAT ET AL.
PLOT OF FIRST AND SECOND PRINCIPAL COMPONENTS FOR CRUISE 4
2.0-
1.0-
1o
o
a.
w
o
u
a.
in
a,
S
J
§
u
a.
0.0-
u
O
O
m
1.0-
a:
a,
-2.0-
-3.0-5.0
-4:0
-3:0
-2.0
-1.0
0.0
IJO
2.0
3.0
4.0
5.0
6.0
BOT TEMP. S I L T - ^
*
BOT DO. CLAY
PRINCIPAL
COMPONENT 1
(45%)
FIG. 7. Principal Component Analysis plots and groupings of East Transect stations in Spring 1985.
on presence-and-absence data on those
species having an overall abundance of 10
or more individuals. The resulting dendrograms for invertebrates and fishes
showed a clear separation of the faunal
assemblages from one isobath to another
(Figs. 8 and 9). Also, as predicted, the deepest series of stations differed from the shallow series more than did the intermediate
stations, and the 2,900-m station revealed
no similarity with any others. The second
analytical method for detecting zonal
boundaries of the megafauna employs the
chi-square test (Gage, 1986). The method
used first-plus-last captures at between-station intervals on a transect having a gradient of increasing depth. Where areas of
faunal homogeneity are separated by narrow regions of species exchanges, upper
and lower limits should occur concurrently
more frequently than expected on the basis
of chance. Comparisons were made
between expected values for collection
intervals and those actually observed using
the chi-square test and obtaining probability levels. The chi-square values, which
can be thought of as indices of faunal
change for each depth interval, are then
plotted on the abscissa. Peaks in the graph
mark depth intervals of maxima in the rate
of faunal change, while valleys below the
alpha line are interpreted as being homogeneous zones. Applying the method to fish
species on the Central Transect (Fig. 10),
peaks are seen between 450 and 550, at
750, 1,050, and 2,250 m, which correspond to the breaks between zones established previously (Pequegnat, 1983). The
analysis of fish data on the Western Transect is not quite as definitive but corresponds depthwise to values obtained on the
Central Transect (Fig. 11). Note particularly the strong peak at 1,000 m.
61
ECOLOGY OF DEEP-WATER FAUNA
CRUISE 4 (EAST) — INVERTEBRATES
1.000
CRUISE 4 (EAST) — FISH
AVERAGE LINKAGE METHOD TREE DIAGRAM
AVERAGE LINKAGE METHOD TREE DIAGRAM
SIMILARITY
SIMILARITY
0.500
0.500
El
—f—
0.000
—I
E1B
E1C
E1A
E2
E2A
E2B
E2C
E2D
E2E
E3
E3A
E3B
E3C
E3D
FIG. 8. Dendrograms for megafaunal invertebrate
species similarities at East transect stations in Spring
1985.
Fie. 9. Dendrogram for fish species similarities at
East Transect stations in Spring 1985.
gobioides, which feeds upon Natantia, fish,
and small crustaceans; the grenadier, Coelorinchus caribbaeus, which feeds heavily
In establishing the concept of zonation upon polychaetes, amphipods, calanoids,
of the deep-sea megafauna, the statistical and Natantia; and the right-eyed flounder,
methods dealt simply with species as num- Poecilopsetta beani, which feeds upon
bers. In this section a few of the mega- amphipods, mysids, and calanoids. The
faunal species forming the assemblages that starfish were dominated by two genera,
are characteristic of the zones are desig- Astropecten of which the species A. nitidus
was most abundant, and Luidia with the
nated.
very common L. elegans. Echinoids were
ShelfI Slope Transition Zone (118-475 m)
represented by the shallow-water genus
Demersal fish abundance together with Brissopsis {atlantica and alta). Brachyura
a rich group of asteroids and brachyurans, were common with Lyreidus bairdii, Acanthe majority of which are predatory, marks thocarpus alexandri, and Benthochascon
this as a very productive zone. Among the schmitti being most abundant. Penaeopsis
most common fishes are the batfish, serrata was by far the most abundant
Dibranchus atlanticus, which feeds upon penaeid shrimp and Parapandalus willisi the
amphipods, polychaetes, isopods, and most common caridean. Among galathecumaceans; the percophid, Bembrops ids, the genus Munida (especially longipes
ZONAL DISTRIBUTION OF
FAUNAL ASSEMBLAGES
62
W. E. PEQUEGNAT ET AL.
S
§ 10 H
5
500
1000
1500
2000
2900
3000
DEPTH (m)
FIG. 10. Chi-square values for fish species taken on the Central Transect. Peaks above alpha line are depth
intervals of high rate of species change.
and forceps) was common, while the genus maximum populations here is less than half
that in Horizon A. This presages a major
Munidopsis was taken only occasionally.
zonal change. Two dominant species are
Archibenthal Zone—Horizon A
the macrourid Nezumia aequalis, which
(500-775 m)
feeds on Natantia, amphipods, calanoids,
Demersal fishes are abundantly repre- and tanaidaceans, and Bathygadus melanosented here, but there is a reduction in the branchus which feeds on benthopelagic
number of species and in those with max- crustaceans. There are remarkable reducimum populations. Among the most com- tions in asteroids and echinoids and
mon fishes are the grenadiers Coelorinchus brachyurans, the latter being represented
coelorhynchus and Bathygaclus macrops. by Geryon quinquedens.
Asteroids are very well represented with
four species of Cheiraster and the largest Upper Abyssal Zone (1,000-2,275 m)
known starfish, Midgardia xandaros, which
Even though the upper abyssal's bathyis a suspension feeder. The Brissopsis echi- metric range is about three times that of
noids are scarce, but their place has been the Archibenthal Zone, it has only half as
taken by Phormosoma placenta and Plesiodia-many fish species; however, the number of
dema antillarum. Caridean shrimp species demersal fish species attaining maximum
have doubled in number here, with Ple- populations is over twice that of Horizon
sionika holthuisi the dominant. Prominent B. The dominant species is Gadomus loncrabs are Bathyplax typhla and Rochinia gijilis which feeds primarily on calanoid
crassa. Munida valida is very abundant, and copepods. There is a major increase in the
Munidopsis robusta represents that deep- number of species of large sea cucumbers,
water genus.
with Mesothuria lactea and Benthodytes sanquinolenta heading the list. Galatheids are
Archibenthal Zone—Horizon B
represented by 11 species of the genus
(800-975 m)
Munidopsis and only one of Munida. The
Although the total number of demersal number of brachyurans continues to drop
fish species has been reduced only mod- with only four species here compared with
erately, the number of species that reach 35 in the Shelf/Slope Transition. Deep
ECOLOGY OF DEEP-WATER FAUNA
63
20 -I
10 Chl-squart for alpha = .029
1000
2000
1500
3000
DEPTH (m)
FIG. 11. Chi-square values for fish species taken on the Western Transect.
water carideans, such as Nematocarcinus macrolepis. Asteroids are represented by
rotundus and Glyphocrangon aculeata, and theDytaster insignis and Ampheraster alaminos,
polychelid Stereomastis sculpta are charac- and sea cucumbers by Benthodytes typica and
Psychropotes semperiana. Two galatheids
teristic of the zone.
here have very wide distributions in the
Mesoabyssal Zone (2,300-3,225 m)
Atlantic, viz., Munidopsis bermudezi and M.
sundi.
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