We studied patterns of nematode distribution along
the western Indian continental margin to determine the influence of habitat
heterogeneity and low oxygen levels on the community's taxonomic and
functional structure. A single transect, perpendicular to the coast at
14
The continental margin exhibits great topographical heterogeneity in
geomorphological features (e.g. canyons and seamounts, shelf, slope, rise,
marginal highs) and their related environmental conditions (e.g. depth,
pressure, temperature, salinity, light, dissolved oxygen, sediment
characteristics) (Levin et al., 2001). This heterogeneous environment is of
great ecological interest because of its wide range of gradients in living
conditions and the high faunal diversity it supports on local, regional, and
global scales (Vanreusel et al., 2010). The western Indian continental
margin, located in the eastern Arabian Sea of the northern Indian Ocean,
includes a series of complex environments including shelf, slope, and a
permanent oxygen-depleted zone. In general, oxygen minimum zones (OMZs) are
defined as layers of the water column where dissolved oxygen (DO)
concentrations fall below 0.5 mL L
The entire western Indian continental margin (shelf to slope) supports a
high benthic biodiversity (Ingole et al., 2010). Free-living nematodes are
prominent members of the meiobenthos along the ocean continental margin,
often constituting > 90 % of all metazoa (Vincx et al., 1994).
Moreover, they are more tolerant to anoxic conditions than macro- and other meiofauna (Giere, 1993; Moodley et al., 1997). Studies like those of Jensen (1987)
and Murrell and Fleeger (1989) reported effects of low-oxygen
conditions on nematofauna in sandy sediments at shallow depths and from the
Gulf of Mexico, respectively, and showed that nematode abundance was
unaffected by oxygen concentration. Cook et al. (2000) studied nematodes
from the Arabian Sea OMZ and concluded that nutrient input is more important
than oxygen concentration, but in another experimental study in the tidal
flats, hypoxia caused a dramatic change in the nematode community
composition, including a reduction in diversity and abundance of all dominant
nematodes except for the genus
Several transitional settings in the western Indian continental margin, including the shelf, slope (long stretch of OMZ), and basin, provide multiple oxygen and other environmental gradients. These settings allowed us to investigate how oxygen and environmental factors affect and modulate the structure and function of nematode communities at the species level. An exclusive study from the Indian western continental margin with reference to nematode tolerance in the OMZ is therefore important. Furthermore, nematode community structure from the western Indian continental margin (including the OMZ) of the Arabian Sea remains unstudied. In general, information on nematode species data is missing from the Arabian Sea, and the present study can provide important basic information on nematode community structure.
Traditional taxonomy-based methods of nematode community analyses may not, however, fully account for nematodes' diverse roles in ecosystem function (Schratzberger et al., 2007), so the inclusion of functional analysis has been recommended (de Jonge et al., 2006).
Many previous studies (e.g. Bremner et al., 2003; Boström et al., 2006) on macrobenthic invertebrates have shown that linking the taxonomic and functional aspects can reveal different relationships between assemblages, and for an exploration of the relationship between biodiversity and ecosystem function, nematodes could be considered the ideal model organisms (Danovaro et al., 2008).
Nematode species in functional groups share several morphological traits (Chalcraft and Resetarits Jr., 2003), and the use of these traits may provide additional information on changes in biodiversity and also facilitate better comparison with other geographical regions (Bremner, 2008). Giere (1993) showed that a classification based on buccal structures of marine nematodes had applications in a variety of marine habitats. Thistle and Sherman (1985) found that nematode tail shape formed an important biological trait especially in locomotion and reproduction. Furthermore, body size is known to influence many traits of an animal, such as its life history, physiology, and energy requirements, whereas species morphotype represents adaptations to life in oxygen-poor and sulfide-rich sediments (Jensen, 1987). Bongers (1990) and Bongers et al. (1991, 1995) classified the genera (on a scale of 1 to 5) from r-selected “colonisers” to K-selected “persisters”, according to their life history, to assess the condition of terrestrial and freshwater habitats but also of marine environments from the shelf.
In the study here, we examined the nematode community structure along a habitat gradient using traditional diversity measures, taxonomic properties, and the functional-group concept based on biological traits so as to answer the following questions.
Does a heterogeneous gradient like the shelf, slope, and deep basin of the western Indian continental margin affect the marine nematode community structurally and functionally?
What are the patterns and drivers of variation in nematode composition and diversity (structural and functional) along the western Indian margin; i.e. are oxygen levels the main driver, or are other factors (sediment, productivity, etc.) more important?
The bottom topography of the Arabian Sea includes the Carlsberg Ridge in the
southern part, the Murray Ridge in the northern part, and an unsloped region
in between (from 14 to 21
Surface circulation is controlled by the seasonal variation in various types
of winds. During the southwest monsoon, biological productivity in the Arabian Sea
lies mainly around the centres of seasonal upwelling off the Arabian
Peninsula, Somalia, and southwest India (Qasim, 1977). The Arabian Sea is
characterised by a very pronounced midwater oxygen minimum zone (OMZ)
between 150 and 1250 m; it is over 1000 m thick and extends vertically from
the bottom of the euphotic layer (
A single transect was selected perpendicular to the coast of the Arabian Sea
at 14
Map showing the depth contours and positions of the eight sampling stations (left; black circles) on the coast of the Arabian Sea (marked as red circle on the map on the right).
Dissolved oxygen was analysed by Winkler's method (Strickland and Parsons,
1968). Sample bottles were stored upright in an ice chest in a dark location
and were analysed after a period of 8–9 h. Chl
Meiofauna samples were washed over a 500
Nematode species were classified according to their buccal morphology, tail shape, adult length, adult shape, and life history.
Individual nematode species were assigned to four feeding categories according to Wieser (1953): selective deposit feeders (1A), non-selective deposit feeders (1B), epigrowth feeders (2A), and predators or omnivores (2B).
Thistle and Sherman (1985) developed a functional-trait scheme based on tail shape. Tail types are diverse and variable and have been suggested as an effective method of characterising nematode communities. Nematodes were assigned to four tail-shape groups, which are common in free-living marine nematodes: short or round, elongated or filiform, conical, and clavate.
Total length and the maximum body width for adult nematodes were measured, and from the results we calculated the length : width ratio. Soetaert et al. (2002) suggested three body-shape morphologies: stout, slender, and long and thin. The length : width ratio is a measure of a nematode's body shape; long and thin animals have high ratios, and stout animals have low ratios. From the measured length, each species was therefore assigned to one of four length groups (< 1, 1–2, 2–4, and > 4 mm) and three shape categories (stout, with a length : width ratio < 18; slender, with a length : width ratio of 18–72; and long and thin, with a length : width ratio > 72).
Nematodes were allocated to life-history groups according to Bongers (1990) and Bongers et al. (1991); genera are classified on a scale of 1 to 5 from colonisers (1: short life cycle, high reproduction rates, high colonisation ability, tolerance of various types of disturbance) to persisters (5: long life cycles, low colonisation ability, few offspring, sensitivity to disturbance). The species belonging to the monhysterid family were assigned to the C–P 2 class (“general opportunists”) as advised by Bongers et al. (1995) and later restated by Pape et al. (2013); no nematodes belonged to C–P class 1 (“enrichment opportunists”).
A biological-traits matrix was constructed by assignment to each nematode species of its affinity to each trait category and the percentage (%) contribution of each species.
The similarity analysis at the species level was based on two types of similarity measures: the Bray–Curtis dissimilarity (standardised, square-root transformed) (Bray and Curtis, 1957), based on the relative abundances of nematode genera, and ordination using the Jaccard similarity index based on presence or absence (Clarke, 1993). Differences between the samples were visualised by means of non-metric multidimensional scaling (nMDS) plots.
We applied a permutational multivariate analysis of variance (PERMANOVA) designed with two factors – “station” (all stations in the zone combined) nested in “zones” (shelf, slope, and deep basin) – to detect the statistical significance of differences in pairwise comparisons of nematode communities from different zones (PERMANOVA; Anderson, 2005; Anderson et al., 2008).
Diversity was expressed in terms of the expected number of species in a
sample, ES (51). The other diversity indices were calculated by means of
Margalef's index (Margalef, 1968) for species richness (
Environmental variables were then subjected to principal-component analysis (PCA) for identification of the spatial patterns based on environmental data. A lower triangular Euclidean distance matrix relating to the ordination was constructed (Clarke and Green, 1988). Before calculation of the Euclidean distance resemblance matrix, the data were checked for uniform distribution and normalised (by subtraction of the mean and division by the standard deviation, for each variable) before analysis.
Analysis of collinearity was tested by means of a draftsman plot and the
associated standard product moment correlation coefficients between all
pairs of variables, and pairs with correlations
To identify the species that would characterise the three zones compared
(shelf, slope, and basin), we performed the Indicator Species Analysis or
IndVal (Dufrêne and Legendre, 1997) using multi-level pattern analysis
(De Cáceres et al., 2010) in the R environment (R Development Core Team,
2015) with the “indicspecies” function. The statistical significance of the
relationship between the species and site was tested by means of Monte Carlo
randomisations with 1000 permutations. The details of the method are
described by Dufrêne and Legendre (1997). All the analyses were
performed with the procedures in the R software (R Development Core Team,
2015; Dimitriadou et al., 2011) and the PERMANOVA
The sediment texture was variable, as the shelf region and upper slopes
(525 m) were characterised by silty and sandy facies, whereas the sediments
on the mid slope (1001 m), lower slope (1524 m), and in the basin were characterised
by clayish silts. The salinity of the bottom water varied little, but
temperature decreased with increasing depth. The bottom-water DO ranged from
0.08 to 2.3 mL L
Locations and depths of sampling stations and sediment texture.
The OMZ was also characterised by higher values of sediment Chl
Principal-component analysis derived from the contribution of parameters in each benthic zone. PC 1 and 2 accounted for 81 % of the total variation present.
A total of 110 nematode species belonging to 24 families were found along
the transect. Nematode density was higher at the shelf stations than on the
slope or in the basin. The lowest average density was observed in the basin:
62.9
nMDS based on nematode species abundance (Bray–Curtis) and presence or absence
data illustrates clearly the extent to which the three zones differ (Figs. 3
and 4). The nMDS plot based on the Bray–Curtis similarity measure indicated
three groups of samples (Fig. 3). The largest group included the slope
stations and the deepest shelf station – those stations that form the OMZ.
The PERMANOVA community results indicated significant differences between
the three zones (
nMDS ordination based on nematode species abundance according to the Bray–Curtis similarity index.
Results of pairwise comparison (samples from different zones)
PERMANOVA analyses (based on Bray–Curtis similarity measure). Data were
fourth-root transformed; resemblance was calculated according to
Bray and Curtis. The values indicate differences significant at
Average relative abundances (%) of the most abundant species belonging to each of three benthic zones. Only the top five dominant species from each zone are presented.
Thirty-eight species were identified that were found only in the shelf
region. Of these,
nMDS ordination based on nematode species presence or absence according to the Jaccard similarity index.
Only seven species were found exclusively in the OMZ:
The 10 most abundant species recorded from the oxygen minimum zone (OMZ) and from areas outside that zone.
The IndVal index produced a list of indicator species for each group of
sites: four genera were indicative of the shelf area and five genera of the
slope (Table 5).
nMDS ordination based on nematode species' biological traits according to the Bray–Curtis similarity index.
Schematic model of the functional traits in each benthic zone.
List of indicator species calculated by IndVal index; number of genera associated with the shelf group: 4; number of genera associated with slope groups: 5; number of genera associated with the basin: 7.
The three benthic zones differed significantly in the diversity indices
(Margalef's index:
Mean and standard error for diversity indices of the nematode
communities at species level.
Functional attributes of nematode communities in the Arabian Sea are listed in Tables 7 and 8. The geographic separation was less pronounced in the ordinations derived from functional characteristics of nematode communities, although the basin and slope samples were clustered into groups, but shelf stations were randomly distributed (Fig. 5). As shown by the results of pairwise PERMANOVA, three zones containing nematode assemblages (shelf vs. deep basin; slope vs. deep basin) differed significantly. Assemblages were dominated by deposit feeders (1B, 1A) followed by predators (2B), whereas proportions of epigrowth feeders (2A) were comparatively low (Table 7). The shelf area was dominated by non-selective deposit feeders (1B) and predators (2B), whereas the slope stations showed the highest proportions (29–48 %) of predators or omnivores (group 2B). Similarly, the dominance of predators (2B) was seen in OMZ stations from 102 to 1001 m. The shelf and basin areas were dominated by deposit feeders (mainly the non-selective 1B group).
Functional trait matrix showing the percentages of all individuals belonging to four nematode feeding categories (1A: no buccal cavity or a fine tubular one, selective deposit (bacterial) feeders; 1B: large but unarmed buccal cavity, non-selective deposit feeders; 2A: buccal cavity with scraping tooth or teeth, epigrowth (diatom) feeders; 2B: buccal cavity with large jaws, predators or omnivores) and four tail-shape categories.
Functional trait matrix showing the percentages of all individuals belonging to four body-length categories, three body-shape categories, and five life-history strategies (coloniser–persister score).
Clavate and conical were the most prevalent tail shapes at the majority of stations (Table 7); the average proportion of conical tails was higher on the slope and at the OMZ stations (33–49 %). Clavate tails dominated at all stations, but slightly less on the slope and inside the OMZ (1001 m) than on the shelf and in the basin area. Average adult lengths of nematodes ranged from 1 to 4 mm, and the majority of all recorded individuals occurred in the two intermediate length classes (1–2 and 2–4 mm). The OMZ stations showed the highest proportion of individuals from the 1–2 mm category (36–62 %); those from the 2–4 mm size category were the second most dominant (28–39 %). Moreover, the basin was also dominated by these two length categories.
Distance-based redundancy (dbRDA) bubble plot illustrating the DistLM model based on the species assemblage data and fitted environmental variables with their vectors (strength and direction of effect of the variable on the ordination plot). Axis legends include percentage of variation explained by the fitted model and percentage of total variation explained by the axis.
Distance-based redundancy (dbRDA) bubble plot illustrating the DistLM model based on the species functional assemblage data and fitted environmental variables with their vectors (strength and direction of effect of the variable on the ordination plot). Axis legends include percentage of variation explained by the fitted model and percentage of total variation explained by the axis.
The majority of the nematodes were slender, although their proportion decreased with depth (from 82 % on the shelf to 76 % in the basin), whereas stout animals accounted for between 2 and 13 %. Long and thin animals continuously decreased with increasing depth as far as the lower slope (at the core of OMZ), but they were more abundant (17–24 %) in the basin than those of slender or stout shape were. Over 76 % of all identified individuals attained a C–P score of 2 to 3. The shelf, slope, and basin regions mostly harboured colonisers in the range of 2–3. Extreme persisters (C–P score of 5) and extreme colonisers were completely absent (Table 8).
The information on a particular functional group was not a simple reflection of the species abundance data. Ecological information captured by adult length, adult shape, and life-history strategy was less site-specific and thus differed notably from information contained in taxonomic groups.
Moreover, the biological-trait matrix revealed no particular notable relationships between traits. Certain trends were observed, however; for example, 1–2 and 2–4 mm lengths were generally correlated with slender animals and with colonisers (C–P score 2–3), whereas nematodes with a length > 4 mm had higher C–P scores and a long and thin body shape. Some other combinations occurred, such as non-selective feeders with clavate tail shape, whereas predators followed a trend similar to that of the conical tail-shape category (Table 9).
Biological-trait matrix along with contribution (%) of species at each zone.
Continued.
1A: selective deposit feeders; 1B: non-selective deposit feeders; 2A: epigrowth feeders; 2B: predators (Wieser, 1953). R: round. E/F: elongated or filiform; CO: conical; CA: clavate. ST: stout; SL: slender; L/T: long and thin; C–P: coloniser–persister score (Bongers et al., 1991, 1995; Pape et al., 2013); 1: species exhibiting trait; 0: species not exhibiting trait.
The schematic model represents the exact trend of each functional trait at each benthic zone. The relative abundance of each trait was plotted as an area graph, and a schematic figure was prepared to show the pattern according to the zones. For example, the shelf region favours the dominance of clavate tail shape, epigrowth feeding, slender body shape, and coloniser lifestyle with length in the range of 1–2 mm. Similarly, the OMZ shows a higher abundance of some traits, such as round tail shape, stout body shape, 2–4 mm length, and greater prevalence of omnivores and deposit feeders. A coloniser lifestyle (C–P 2–3) was more frequent, whereas persisters (C–P 4) decreased to lower numbers in the OMZ. In the basin, however, the sediment characteristics changed from silty sand to clay. Feeding habit and tail shape also differed in the basin (Fig. 6).
Results from BIOENV analyses: Spearman rank correlation
(rho) and significance
level (
The RELATE analyses indicated that patterns based on environmental variables
were significantly related to the patterns inherent in nematode community
structure both taxonomically (
Result of distance-based linear model (DistLM) analyses showing the influence of environmental parameters on nematode species abundance data and Bray–Curtis similarity of square-root-transformed abundance.
SS: sum of squares; F: pseudo-F; P: p value; Prop: proportion of explanation; Cumul: cumulative proportion of explanation; res.df: residual degree of freedom.
Result of distance-based linear model (DistLM) analyses showing the influence of environmental parameters on nematode species functional biological traits (Bray–Curtis similarity of square-root-transformed abundance).
SS: sum of squares; F: pseudo-F; P: p value; Prop: proportion of explanation; Cumul: cumulative proportion of explanation; res.df: residual degree of freedom.
Analyses with distance-based linear models (DistLM) indicated that the six
abiotic variables related to the variation in nematode community structure
explained 97 % of its total variation, although not all variables were
significant. Similarly, for the functional traits of the eight variables, six
were used by the DistLM procedure to construct the best-fitting model,
together explaining 96 % of total variation. The DistLM analysis allowed
the identification of those abiotic variables that were best correlated with
the observed distribution patterns of taxonomic and functional traits of
nematode species (Tables 11, 12). Results based on the abundance and
presence or absence data were virtually identical; therefore, only the results
based on the abundance data and functional traits are reported. Variables
such as the C
Nematode density was higher in the shelf regions than on the slope or in the basin, but the lowest density was observed in the basin (2001–2546 m), where oxygen content was highest. Nematode densities and biomass along the regular slope generally decrease with water depth, surface primary productivity, and distance offshore (Soltwedel, 2000).
The role of DO is crucial in structuring meiofaunal communities, and oxygen
limitation may directly control meiofauna composition within the OMZ
(Neira et al., 2001). Of the eight stations presented here, three are true
OMZ stations, where DO contents are less than 0.5 mL L
In our results, nematode densities were lower than those reported by Cook et al. (2000)
for the bathyal Oman margin (range: 494–2495 ind 10 cm
Moreover, in general, nematode abundance did not follow any particular
gradient though it was lowest at the deep basin stations. Generally, depth
can have a major influence on nematode density (Soltwedel, 2000; Udalov et
al., 2005). The pattern of quantitative distribution of nematode abundance
has been described for intertidal and upper subtidal (< 50 m) zones.
The average worldwide nematode density is 1530 ind 10 cm
The nMDS based on nematode species abundance and presence or absence separated
the three zones, and the difference was confirmed by PERMANOVA (
Our study did not reveal that nematode species have developed any obvious
adaptations, but they must have a certain tolerance for anoxic conditions.
The dominant species in the OMZ, such as
Even though the majority of the species identified in our study were common
and recorded from each of the three benthic zones, the IndVal index
identified some species that can serve as indicators of the shelf, slope,
and basin. The IndVal index is an important tool for ecosystem monitoring
and assessments because it expresses a value for species or genera that can
characterise the particular environment (Dufrêne and Legendre, 1997).
The genus
The IndVal index reaches its maximum (100 %) when individuals of the target species are observed at all the sites of one group and at no sites of any other (Dufrêne and Legendre, 1997). For those species that were found in equal proportions in each of the three zones, the association with the set of all the sites cannot be statistically tested because no external group is available for comparison, so these species cannot be treated as indicators of a particular habitat but can be considered cosmopolitan.
Species diversity was higher on the shelf than on the slope or in the basin.
This difference appears to be partly due to the bottom-water DO gradient,
which includes values that are below the oxygen tolerance of many nematode
species. Therefore very few species were present on the slope, especially
inside the OMZ. Moreover, the results of a macrofauna study from this area
have shown that different physiographic provinces and an oxygen gradient
have a greater influence on the species composition and diversity than do
other oceanographic conditions (Ingole et al., 2010). The species number and
diversity were positively correlated with DO and negatively correlated with
Chl
Marine organisms are often challenged by fluctuations in the aquatic environment, which they must survive, regulate, tolerate, or resist (Odiete, 1999). The different gradients of biological, chemical, and physical properties in the aquatic environment complicate the situation. One major challenge is the depletion of DO, but some organisms are able to cope by means of extreme changes in their body morphology and physiology (Schneider and Bush-Brown, 2003). In our study the distinct features of the three zones were also reflected in the functional-trait approach.
Deposit feeders were more abundant in shelf and basin areas than at most of
the slope-area (OMZ) stations, where predators or omnivores were most abundant.
Epigrowth feeders were more abundant at lower shelf stations and at some
stations of the OMZ region, probably because of the higher organic content
of the sediment. OMZs often support mats of large sulfur-oxidising bacteria,
including
Several previous researchers have speculated that
The dominance of the deposit feeders in silty sediments of the basin is in
agreement with results from most previous deep-sea studies (Tietjen, 1984;
Jensen, 1988; Netto et al., 2005). Jensen (1988) found that the deep-sea
fauna in the Norwegian Sea was dominated by deposit feeders and included few
predators and scavengers. Deposit feeders were also the dominant group in
the Puerto Rico trench and the Hatteras abyssal plain. The selective deposit
feeders (microvores) are the main consumers of deep-sea bacteria (Ingels et
al., 2011) because deposit-feeding nematodes have minute buccal cavities,
restricting them to small particulate food or dissolved organic matter. The
relative proportion of each of the four Wieser feeding types in a community
depends on the nature of the available food, which may perhaps explain their
prominence on exposed substrata (Platt and Warwick, 1980). BIOENV analysis
showed that feeding types were well correlated with C
In nematodes, the tail plays an important role in locomotion, which in turn
depends on sediment type (Riemann, 1974). Our BIOENV analysis revealed that
tail type in nematodes was significantly correlated with sediment texture
and C
Vanhove et al. (1995) and Soetaert et al. (2002) noted that length and width are important functional attributes for describing chemical stress, metabolic rate, the ability to move or migrate, and vulnerability to predation. In fact, the size of animals is an integrative feature strongly correlated with their morphology, locomotion, feeding mode, and other characteristics. Most authors have related nematode length and width to granulometry, in concordance with the BIOENV analysis, which revealed that body shape was significantly correlated with sand and clay.
In our case, slender nematodes were dominant in the shelf region, and their lengths were mainly in the ranges of 2–4 and 1–2 mm. The slope area harboured more long and thin nematodes, as was the case in earlier studies (Jensen, 1987). Pronounced body elongation in nematodes and a greater surface : volume ratio in thiobiotic species are adaptive characteristics related to low oxygen partial pressure and epidermal uptake of dissolved organic matter (Jensen, 1987). Further, increased length under suboxic or anoxic conditions reflects increased mobility. The increasing proportions of long and thin nematodes from the shelf to deeper stations possibly signify that a large body size could facilitate easy burrowing through the sediment, but below the OMZ, the long and thin nematodes were even more abundant, suggesting a relationship with fine clay and silt sediments where only an incomplete interstitial system exists. In a general comparison, however, the organisms inhabiting the oxygenated environments are found to be of a size and shape different from those of organisms from the OMZs (Schneider and Bush-Brown, 2003).
Some biological traits, usually less accessible but with direct functional
roles (Hodgson et al., 1999) such as life history, are widely used in
freshwater and terrestrial habitats (Bongers, 1990). Over 75 % of all
identified individuals attained a C–P score of 2 to 3. Animals from the OMZ
represented a higher contribution to this range. Under eutrophic conditions,
colonisers are more numerous, whereas the number of persisters remains
constant, but persisters (scores 3–4) were more often seen in deeper
waters, perhaps because most deep-water nematodes are deposit feeders, which
often score 3–4 on the C–P scale (Bongers et al., 1991). Bongers and Ferris (1999)
also distinguished between two types of opportunists: enrichment and general
opportunists. The family Monhysteridae was
represented by only two species (
The trait matrix revealed that some combinations of functional groups were significantly correlated. A stout body shape was positively correlated with a clavate, long tail and non-selective feeding type because these traits showed the same increasing trend from shelf to basin. A similar trend was evident for long and thin tail shape with a length of 4 mm and a clavate shape with non-selective feeding, which showed very high positive correlation (0.8). In general, large organisms were more prominent at eutrophic sites, whereas smaller organisms became more dominant in oligotrophic environments (Thiel, 1975). This pattern clearly explains the abundance of long and thin nematodes in the slope region of the Arabian Sea. The analysis of a combination of biological traits seems to be a more reliable approach for assessing the functional structure of nematode communities than relying on single functional groups (Schratzberger et al., 2007). Our schematic model presents the trait responses in different zones. The outcome confirmed that functional-trait analysis is of additional ecological importance, and the information captured by the biological-trait matrix was not a simple reflection of the information contained in taxonomy. Nematodes with different functional characteristics differ in their abilities to respond to environmental stresses and disturbance, thereby providing resilience to the community. Environmental conditions thus influence the importance of functional complementarity in structuring communities (Hooper et al., 2005).
The Arabian Sea OMZ extends over 285 000 km
Results of BIOENV analysis revealed the best correlation between the
functional trait and individual parameter, i.e. the importance of
C
Free-living nematodes are an abundant and diverse component of the meiofauna on the western continental margin of the Arabian Sea. They follow the general trends of decreasing nematode abundance and diversity with increasing depth observed among other benthic taxa along the continental shelf and in the deep sea. We aimed to determine the nematode community structure and functional traits in the Indian western continental margin, including the OMZ. The effects of heterogeneous environments were more evident in taxonomic groups than in functional traits. Although numerous factors could be important for the formation and persistence of nematode communities, the presence of the OMZ seems to be paramount. Along with organic content, DO plays a vital role in structuring nematode communities and certain functional adaptations among different species may play an important role in determining the ecosystem function. From our study, no single parameter can be determined to cause the observed patterns in the three benthic zones. Our results support the previous finding that nematodes are the taxon most tolerant of environmental variation but that within the nematode community only a few species can tolerate oxygen minima. Our study has provided the first insight into the ecosystem functional diversity from coastal to deep-sea nematode fauna from the Arabian Sea, but only further work will reveal whether this pattern is broadly applicable.
We thank the Director, CSIR-National Institute of Oceanography, Goa, India, for the use of scientific facilities and the Ministry of Earth Sciences (Government of India) for ship-board facilities. Edited by: L. Levin