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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. A very sharp faunal break occurs here between the Upper Abyssal and the MesoREFERENCES abyssal Zone, in part because of the presence of escarpments. The number of Batelle New England Research Laboratory, LamontDoherty Geological Observatory, and Woods demersal fish species having maximum Hole Oceanographic Institution. 1986. Study of populations in the zone drops from 49 in the biological processes on the U.S. North Atlanthe Upper Abyssal to 5 here. Similar reductic Slope and Rise. Rep. to U.S. Dept. of Interior, Minerals Management Service, Contract No. 14tions of species are noted in other groups 12-0001-30064. OCS Study MMS 86-0022. 201 as well. The depauparate fish fauna is reppp. + Append. resented by Dicrolene kanazawai and Bas- Childress, J. J., C. R. Fisher, J. M. Brooks, M. C. sozetus normalis, the holothurians by ProKennicuttll, R. Bidigare.and A. Anderson. 1986. tankyra brychia and Psychropotes depressa. A methanotrophic marine molluscan (Bivalvia, Lower Abyssal Zone (3,250-3,850 m) This zone incorporates the continental rise at the base of the escarpments and also includes the abyssal plain. Here it accumulates organic matter in and on its sediment which has many unique species. Although the fauna must be considered depauparate it is richer than would be expected by the drop in diversity observed between the Upper Abyssal and Mesoabyssal Zones. 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