The depth distribution of recent marine Ostracoda from the southern Strait of Magellan

From 16 sediment samples collected from the Chilean part of the Strait of Magellan, 2338 Ostracoda were recovered. These represent 61 species belonging to 45 genera and 16 families. Previous work in the Tierra del Fuego Province has shown the faunas to be highly endemic, resulting from the relative isolation of the region and its particular climatic and oceanographical characteristics. The fauna of the Strait of Magellan is similar to those previously described with one notable exception: the occurrence of deep-water, psychrospheric species at shallow depths. Species of Bradleya, Agrenocythere, Poseidonamicus, Bythoceratina, and Legitimocythere, usually recorded from bathyal to abyssal depths of more than 1000 m, were found together in the same samples with a typical, shelf fauna. Such unusual depth distribution of psychrospheric species may have resulted from the extremely cold temperature and low salinity of the water in the southern Strait of Magellan, coupled with the upwelling of cold, deep water masses.


INTRODUCTION
The Strait of Magellan lies within the subantarctic region of the South Atlantic Ocean. This region has a northernmost limit of latitude 5IoS and can be subdivided into two main provinces, the Chilean and the Magellanic. The ostracod faunas studied were collected from the latter province, which extends north to 42"s and comprises Tierra del Fuego, the Patagonian con-tinental shelf, and the Falkland Islands. The area sampled incorporates the southern end of the Strait of Magellan within the Tierra del Fuego region of southern Chile (Fig. 1).
Although the Argentine and Chilean continental shelves comprise one of the most extensive areas of continental shelf in the world, few publications prior to the 1960s discussed the benthic Ostracoda from this region and the western South Atlantic and eastern South Pacific as a whole. The only early works available formed part of wider studies conducted by various researchers and research vessels such the HMS Challenger expedition to the South Atlantic (Brady, 1880) and Skogsberg's (1928) description of material collected by the Swedish Magellan expedition of 1896 and the Swedish Antarctic expedition of 1901 to 1903. Subsequent discussion of Ostracoda from the South Atlantic and Antarctic was quite limited until publications by Hartmann (1987Hartmann ( , 1988Hartmann ( , 1989Hartmann ( , 1990Hartmann ( , 1991Hartmann ( , 1993 and Hartmann & Hartmann-Schroeder (1962), Whatley & Moguilevsky (1975), and Whatley et al. (1987and Whatley et al. ( , 1988and Whatley et al. ( , 1995); however few publications have specifically discussed the marine Ostracoda of the Strait of Magellan. Kaesler et al. (1979) listed 33 species of Ostracoda from the northern part of the Strait in an analysis of the effects of petroleum pollution on the microfauna of the region.

Methods
Samples were collected in October 1969, by personnel of the University of Kansas and the Smithsonian Institution aboard the research vessel RV Hero. Sixteen samples of bottom sediment were collected from the southern part of the Strait of Magellan (Fig. 2) using either a Peterson grab or bolapipe dredge in shallow water and a Sanders epibenthic sled dredge to recover deeper-water sediments. The samples were collected at depths ranging from 2 to 916m. The sediments were processed at the University of Kansas before being sent to Aberystwyth, where the ostracod valves and carapaces were further sorted and identified. Sample sizes were variable (Table 1). Analysis of hierarchical diversity was used in addition to the more traditional means of studying samples of modern sediments and their contained Ostracoda. Pielou (1966aPielou ( , 1966bPielou ( , 1966cPielou ( , 1969 discussed in detail the use of indices of species diversity from information theory and recommended strongly the use of Brillouin's (1962) equation. Later (Pielou, 1979, pointed out that diversity may be partitioned hierarchically and that the components of the diversity at successively higher levels in the taxonomic hierarchy are additive and sum to the species diversity. This aspect of species diversity has not been widely used. Mulvany & Kaesler (1976) presented a Fortran IV program for computing hierarchical diversities. Later, Kaesler & Herricks (1 979) applied the methodology in an investigation of the community structure of aquatic insects and fishes in polluted streams; and Kaesler & Mulvany (1977) used hierarchical diversity to study communities of Ostracoda. Here, diversity has been partitioned into four, additive levels: superfamilial diversity, familial diversity, generic diversity, and species diversity. The superfamilial level also includes ostracods of the suborder Platycopina, which have typically been grouped only into families and suborders but not superfamilies.

Ecology
The Magellanic province in southern Chile lies along the Strait of Magellan to the east of the Andes and includes the Chilean part of Tierra del Fuego (Fig. 1)  within the southern hemisphere temperate zone. Climatic variations are common across the area, depending on variability of relief and proximity to the ocean. The land mass is divided into the Andes Mountains to the west and areas of low, flat, Tertiary rocks that bound the Atlantic seaboard to the east. To the north of the study area, southern Patagonia lies in a cool temperate zone of prevailing westerly winds and fairly frequent low-pressure storm systems. The area is typical of a cold, montane climate, characterized by continuously cool or cold weather with no warm season. This climate is controlled by the exposure to polar cyclones in the high latitudes of southern Chile. To the west of the mountains, precipitation reaches an annual maximum of 500 cm, and temperatures average little more than 8OC in the summer and 3.8OC in the winter. Drier sites in the lee of the mountains, such as Punta Arenas (Fig. 2), may receive as little as 50cm of rain annually (Collier ef al., 1992). The sampling area is confined to Boltovsky's (1968) zone four, the subantarctic zone, and extends from 50°9 55" S (sample 1) to 53O56 0 S (sample 10). The fauna of this zone was described as consisting entirely of cold-water or cold, temperate-water species. The waters are brought from high latitudes by the Cape Horn Current and its western branch, the Falkland Current. The Antarctic Circumpolar Current (west wind drift) and the Falkland Current bring cold, subantarctic waters to the Argentinian shelf. Offshore, parts of this current reach as far north as latitudes 34O to 3SoS, but coastal areas are strongly influenced as far north as 32OS. The surface temperature of the Falkland Current waters is usually between 6OC and 10°C, and the salinity is between 33.5%0 and 34%, but variation up to 18.5OC and 34.7?& is common. North of 33.5"s in the summer and 34.5OS in the winter, the Falklands Current is present only on the bottom, but upwelling can occasionally bring it to the surface at lower latitudes.

DISTRIBUTION OF OSTRACODA General
Sixty-one species of Ostracoda totaling 2338 individuals and belonging to 45 genera and 16 families were recovered from the 16 samples (Table 2). In all the samples, a high abundance of species is matched by high generic diversity. The genus:species ratio is approximately one for each sample. Familial diversity, however, is appreciably lower, as is to be expected ( Table 1). The Hemicytheridae are the most common and diverse family, with at least one of 16 species found in 14 of the 16 samples (Fig. 4). The Cytheruridae, with 11 species present, and the Trachyleberididae, with eight species, were nearly equally abundant. Many families had few species, in particular, the Cytherellidae (I), the Pontocyprididae (1) and the Cytherideidae (1). Although few species of these families are present, individuals are sometimes quite abundant, in particular the Cytherellidae, which occur in seven samples (Fig. 4). Procythereis iganderssoni Skogsberg, 1928 (Hemicytheridae), the most common species, occurs in 11 of the 16 samples. Austroaurila theeli (Skogsberg, 1928) (Hemicytheridae) and Keijia falklundi (Brady, 1880) (Pectocytheridae), the next most widespread species, were found in nine samples, while several species occurred in six or seven samples. Using the depth zonation established by Whatley (1983), the environments from which samples were collected can be classified as littoral (CrlOm), shelf (1&500m), or slope environments (50CrlOOO m). Most species were not abundant in samples from depths greater than 527 m, with the exception of Bradleya dictyon (Brady, 1880); Krithe sp. cf. K. producta Brady, 1880; and Procythereis torquata Skogsberg, 1928 (Fig. 3).

Littoral environment
Two samples of fine sand and mud were collected from the littoral environment, samples 12 and 16. Both contained species belonging to the families Hemicythendae, Cytheruridae, and Trachyleberididae (Fig. 4). Species of these three families common to the two samples were Procythereis iganderssoni, Austrotrachyleberis antarctica Neale, 1967, and Henryhowella sp. Sample 16 also contained members of the families Bairdiidae, Krithidae, Pectocytheridae, and Thaerocytheridae, while sample 12, the shallowest sample, collected at a depth of only 2 m, was slightly more diverse and included, in addition to the families mentioned above, members of the families Leptocytheridae, Loxoconchidae, and Cytherellidae but no species of the Bairdiidae. While the Cytheruridae are typical of littoral environments, other characteristic littoral families, such as the Xestoleberididae and the Paradoxostomatidae, were absent from these samples.
Depth also influenced the species distribution of Ostracoda on the continental shelf. Species richness, which is greatest between depths of 50 and 300m, decreases considerably below 400m, with most species not found below 527m (Fig. 5). The distribution of some of the shelf species is strictly depth controlled, allowing three, broad, faunal depth ranges to be identified. The first depth range is occupied by the shallow-shelf species that are confined to less than 50m, including Australicytheridea dispersapunctata Whatley et a/., 1987; Austroaurila impluta (Brady, 1880); Bairdopillata hirsutu (Brady, 1880); Hemicytherura stationis (Miiller, 1908); Leptocythere mosleyi (Brady, 1880); Argilloecia sp. cf. A. meridionalis (Chapman, 1914); and species of the superfamily Bairdiacea, with the exception of Bairdopillata sp. cf. B. simplex (Brady, 1880). The second depth is occupied by species that range from less than 50 m to approximately 500 m, including Austroaurila theeli, Austrotrachyleberis antarctica, Falklundia ephippiata (Skogsberg, 1928), and Kuiperiana meridionalis. The final depth zone is occupied by species that range from shallow-shelf to slope environments, such as Bradleya dictyon, Krithe sp. cf. K. producta, and Procythereis torquata (Fig. 5).

Continental slope environment
One sample was collected from this zone at a depth of 6 9 6 916m from a muddy substrate. Only four species were present, giving this locality the lowest species and generic richness in the study (Table 1). As would be expected at such depths, two psychrospheric species were present, Bradleya dictyon and Krithe sp. cf. K. producta.  1971) because they inhabit cold, deep water that is at temperatures below 8-10°C. Deep-water, psychrospheric faunas are less diverse than their shallow-water counterparts, and individuals are usually larger and rarer than those of shallowwater environments (Benson, 1984). Furthermore, a common characteristic of these faunas is their tendency to become sightless at depths greater than 600m (Benson, 1972). At 12 sampling sites in the study area, blind psychrospheric species including Bythoceratina scaberrima (Brady, 1886); Bradleya dictyon; Bradleya normani (Brady, 1880); Agrenocythere hazelae (van den Bold, 1946); Legitimocythere acanthoderma (Brady, 1880); Poseidonamicus sp. (Whatley ef al., in press); and Henryhowella asperrima (Reuss, 1850), were found inhabiting shelf environments alongside shallow-water taxa (Fig. 6).

Hierarchical diversity
The advantages of computing indices of species diversity from information theory are twofold. First, an index of diversity combines information on the number of taxa and the evenness with which individuals are distributed among taxa, providing better insight into the structure of the community than species richness alone. Second, the index is not tied to the occurrence of specific taxa. Thus it allows two or more samples to be compared with respect to their community structure even when they have no species in common. Figure 7 shows the results of partitioning species diversity among four categories in the taxonomic hierarchy. Indices of species diversity were computed for only eleven of the sixteen samples. Samples that yielded fewer than 100 ostracods were excluded because Brillouin's (1 962) index is dependent on sample size.
Species diversity, the uppermost broken line in Fig. 7, was nearly steady across the study area, typically ranging from about 1.6 to 2.3. A pronounced exception to this generalization is the species diversity of sample 14, which is only 0.509. Sample 14, collected at a depth of 74m, yielded 125 ostracod specimens but only seven species. This species richness was lower than that of any other samples except for samples 2 and 17 (Table 1). The low species richness of these two samples is no doubt related to their low overall yield of ostracods, 11 and 32 individuals, respectively. Of the 125 specimens from sample 14, 106 were Bradleya dictyon, a characteristically deep-water species. The unevenness of the distribution of individuals among species and the small number of other species (6) contributed to the low species diversity. Some characteristic of the environment, perhaps low oxygen, kept typically shallow-water ostracods out of the sample but did not prevent B. dictyon from proliferating.
Generic diversity closely tracks and in some samples is exactly equal to species diversity. This results when the numbers of species and genera in a sample are the same. With a few notable exceptions, familial diversity more or less bisects the distance between the curves of generic and superfamilial diversity. Sample 14 has a familial diversity only slightly less than the generic and species diversity. The superfamilial diversity of sample 14 is 0.039, the result of its having only one specimen that is not in the superfamily Cytheracea, a single specimen of Bairdoppilata sp. cf. B . simplex. Because of the abundance of psycrospheric species throughout the study area, the diversities at all levels generally increase with depth from the low values of sample 14 collected at a depth of 74m.

DISCUSSION
The fauna displays several important characteristics. The species are small, very abundant, and, importantly, most are sighted. Apart from the low species and generic richness mentioned above, the traits of the fauna are not characteristic of deep water. Despite the obvious shelf nature of the fauna, psychrospheric species of Ostracoda were recorded from 12 sampling sites in the area. Sample 1 had six psychrospheric species comprising 86% of the ostracod fauna, most of which were present as adults, apart from Agrenocythere hazelae, which was recovered only as instars. Sample 3 had four psychrospheric species, comprising 29% of the fauna, including a single valve of Bythoceratina scaberrima, a rare element in the deep sea that is characteristic of depths from 1000 to 3000m. Six psychrospheric species were recovered from sample 8, including Agrenocythere hazelae (the dominant species comprising 57% of the fauna) and Henryhowella asperrima, which accounted for 10% of the total fauna. The other four species comprised 24%. Bradleya normani and Henryhowella dasyderma occur in sample 13, forming about 3% of the total population. Five psychrospheric species occurred in sample 14. Bradleya dictyon formed 85% of the total population, while the other species accounted for 11 %. This sample is unique among the 16 samples: its fauna comprised almost entirely psychrospheric species. Sample 16 contained three of these deep-water species forming 28% of the fauna, while sample 17, the only truly deep-water sample (collected from 696 to 916m), provided specimens of Bradleya dictyon and Krithe sp. cf. K. producta, the latter comprising 87% of the ostracod fauna.
The relative abundance of a species vaned greatly between samples. Poseidonamicus sp. and Pennyella dorsoserrata (Brady, 1880) were recovered only as single valves, while Bythoceratina scaberrima yielded just two adult valves. Other species, such as Agrenocythere hazelae, while dominating the faunas in which they were recorded, were not distributed widely, having been found at only two sites. Similarly, Legitimocythere acanthoderma was a dominant species in sample 1 but was not found elsewhere in the study area. Henryhowella asperrima, which was found only in sample 8, comprised 10% of the fauna. Elsewhere, Henryhowella dasyderma and a new species of Henryhowella were the common species of Henryhowella encountered. By far the three most abundant psychrospheric species were Krithe sp. cf. K . producta, Bradleya normani, and Bradleya dictyon. Bradleya normani, which was recovered from five samples, was the dominant species at sites 1 and 16. Bradleya dictyon was extremely abundant in the two samples from which it was recorded, and, finally, Krithe sp. cf. K . producta formed a high percentage of the total fauna of six of the samples examined. The depth ranges recorded for the psychosperic species in this study are illustrated in Figs 5 and 6. It has been mentioned previously that deep-water species occur at relatively shallow depths in this region, due to the low water temperatures in the area, which is influenced by cold, polar currents. For example, Chadwick (unpublished MSc thesis, Aberystwyth, 1986) recorded Bradleya normani and Henryhowella dasyderma at shallower depths than previously reported. Benson (1974) stated that Bradleya dictyon is generally restricted to water depths in excess of 1750m, yet this species is recorded at 74m (site 14) in the southern Strait of Magellan. Furthermore, the genera Agrenocythere, Bradleya, and Poseidonamicus usually only occur at abyssal depths (Benson, 1974). Agrenocythere is considered typical of depths of about 1500m (Benson, 1972) but has been discovered at depths as great as 2957m (Benson and Peypouquet, 1983). Poseidonamicus species are normally found between 2500 and 4000 m depth with the exception of a sighted species of Poseidonamicus discovered living on the continental and upper slope of southwestern Africa (Whatley & Dingle, 1989). Species belonging to both genera were identified in the study area in the samples collected as shallow as 36m depth and occur with typical shelf species. Coles & Whatley (1989) have recorded Legitimocythere at depths of 5725 m in the North Atlantic, but there have been occasional recordings from Recent environments as shallow as 341 m (Cronin, 1983). Legitimocythere acanthoderma has been recorded at a depth of 2515m (Brady, 1880), 3175m (Benson, 1974) from DSDP Site356, and 2086m (Ducasse and Peypouquet, 1979) from DSDP Site 43. This species was found at a depth of 456m in the Strait of Magellan. The deep-water species Bradleya normani and B. dictyon, which are commonly used to identify abyssal assemblages, were also recorded at shallower depths than usual (9.3 m for Bradleya normani and 14m for B. dictyon).
Upwelling is particularly common on the western sides of continents. This phenomenon seemingly operates in the area of the Strait of Magellan studied and impacts the environment. Upwelling of deep, cold water masses probably resulted in the occurrence of these blind, deep-water genera on the continental shelf of the Strait of Magellan. Apart from the effect of upwelling on the depth distribution of the psychrospheric species, the specialized environmental conditions existing in the study area may have been influential, too. Continuous cold weather caused by exposure to polar cyclones results in a very low annual temperature range of 3.8-8OC. Water temperatures are also extremely low due to the influence of polar currents, and salinity is usually from 33.5 to 34%. Such extremes of

Generic
& Species diversity diversity diversity diversity Fig. 7. Additive, hierarchical components of diversity from information theory resolved into four levels of the taxonomic hierarchy: superfamilies, families, genera, and species. Superfamilial diversity also includes ostracods of the suborder Platycopina, which is typically not divided into superfamilies.
temperature may have resulted in an environment i n which it is possible for psychrospheric species to exist at much shallower depths than normal.
Index of diversity from information theory (H)