Taxonomy and ecology of the Family Cytheruridae (Ostracoda) in Recent sediments from the northern Rio de Janeiro coast, Brazil

A study of the family Cytheruridae in Recent sediments from the coast of northern Rio de Janeiro State revealed the presence of four new species: (Hemicytherura auriculata, Oculocytheropteron delicatum, Oculocytheropteron circumcostatum and Semicytherura caudata,) and four previously described species (Semicytherura rugosoreticulata Whatley, Chadwick, Coxill & Toy, 1988, Oculocytheropteron macropunctatum Whatley, Chadwick, Coxill & Toy, 1988, Oculocytheropteron reticulopunctatum Whatley, Chadwick, Coxill & Toy, 1988 and Paracytheridea bulbosa Purper & Ornellas, 1989). A further two species Cytheropteron sp. and Kangarina sp. are left in open nomenclature due to paucity of the material. The ecology and distribution of the fauna suggest that the presence of many of these species, which also occur in colder waters off the coast of Uruguay and Argentina, is probably due to the upwelling of the South Atlantic Central Water in this area.


INTRODUCTION
The cytherurids are one of the most diverse and abundant families of cytheracean marine and brackish water Ostracoda. Their diversity in either shallow or deep water changes little from the poles to the equator and, in addition, they commonly represent one of the most abundant families. One of the characteristics of the Cytheruridae is their small size but, because they frequently have very species-specific ornamentation, they are easily recognized. Notwithstanding this, they are often left in open nomenclature or dealt with solely at the generic level.
In Brazil, only Ornellas & Fallavena (1978) and Purper & Ornellas (1987a, b; have made detailed specific studies of cytherurids. The former workers described a new species, Cytherura purperae, a mixohaline species from southern Brazil, while Purper & Ornellas (1987a, b; studied the taxonomy and zoogeography of the genus Paracytheridea along the Brazilian coast. Dias-Brito et al. (1988) and Coimbra et al. (1992) studied Sepetiba and Tamandare Bays, respectively, and recorded the occurrence of cytherurids. Pinto et al. (1978) also surveyed the Ostracoda along the Brazilian coast, including the cytherurids. Whatley et al. (1988) monographed the Cytheruridae from the Antarctic to southern Brazil and described a total of 34 species belonging to eight genera. This paper deals with the taxonomy and ecology of the entire cytherurid fauna present in a specific geographical area of the Brazilian coast. The northern coast of Rio de Janeiro State was chosen because it records the greatest intensity of upwelling of cold water masses from the south. This area coincides with the southern portion of the zoogeographical 'transition zone' of the ostracod fauna of the Brazilian continental shelf as defined by Coimbra & Ornellas (1989).

MATERIAL AND METHODS
The 47 samples studied were collected during the GEOCOSTA RIO 11 expedition and were made available to the authors by the Department of Geography of the Universidade Federal do Rio de Janeiro, where the project 'Levantamento Fisico Ambiental da Plataforma Continental Interna entre as cidades do Rio de Janeiro e Cab0 Frio' was developed.
The samples were collected in March 1986. All the ostracods were picked from dry sediment samples, each weighing 20 g, and mounted using conventional micropalaeontological techniques.
The constancy ( C ) of the species (the frequency of the species in the samples, expressed as a percentage) was calculated using the following formula: C = p.lOO/P, where p = number of samples in which the species occurs and P = total number of analysed samples. Dominance (D) was calculated using the formula D = t.lOO/T, where t = number of total specimens of each species and T = total number of specimens (Tinoco, 1989).
The diversity index was calculated using the Shannon-Wiener information function. The diversity index is expressed by the equation H (5') = -Cpi.lnpi , where p, = the proportion of each species i and lnpi = natural logarithm of pI.
The percentage of sediment types in each water depth interval was calculated using the 'Idrisis' program for Windows.
The type material, together with a series of homotypes and hypotypes, is deposited at the Museu de Paleontologia da Universidade Federal do Rio Grande do Sul, Porto Alegre, UFRGS, under numbers MP-0-1483 to MP-0-1503 and MP-O-1462. MP-0-1463.

STUDY AREA
A rectangular area (35x 15 km) with depths ranging from 20 to 75m, of the continental shelf of Rio de Janeiro State, between Cab0 Frio Island and Saquarema Lagoon (22"55'S-23"05'S and 42"00'W42" 20'W), was studied. This area is about 80 m north of the city of Rio de Janeiro, immediately offshore of Lagoon Araruama (Fig. 1).

Sedimentology
Mainly relict sands, resulting from the lack of input of continental sediments and the presence of a barrier beach, cover  the study area. The erosion of the unfossiliferous Barreiras Group (Plio/Pleistocene) from alluvial deposits and sediments produces the relict sands. These sands are also the result of reworking by waves and currents. Grain size analysis shows a decreasing gradient both towards the east and towards deeper waters. Between the coast and a depth of 50m, medium sand predominates (80%); an exception is in the far east of this area, near the town of Arraial do Cabo, where very fine sand predominates (15%) within this depth range. Coarse sands occur in the west of the area and in isolated parts of the more central portion of the studied area. At depths greater than 50m, fine Medium sand Fine sand silt Coarse sand @@ Very fine sand sand predominates. Very fine sand occurs in the western, central and eastern areas, but only in isolated patches between 60 and 70m depth. Silt-grade sediments occur in the far southeast and also in a single sample from the central portion of this area, always at depths greater than 65 m (Figs. 2-9).

Physical oceanography
The main water masses affecting the area of study are: coastal water with temperatures > 18°C and salinities < 35%; South Atlantic Central Water with temperatures between 6 and 18" and salinities between 34.6 and 35.8%; and Surface Tropical

Water with temperatures
18°C and a salinity >35.9%. The Coastal Water results from the mixing of water coming from the continental drainage of superficial tropical water and of South Atlantic Central Water. The South Atlantic Central Water is produced by a mixture of Tropical Water and Subantarctic Water which occurs in the subtropical convergence (Weber,1 994).
The hydrology of the Rio de Janeiro coast between Cab0 Frio and Ponta de Joatinga (southern Rio de Janeiro State) is related mainly to the intensity and variability of the upwelling process, which is greatest between Cab0 Frio Island and Saquarema Lagoon. The prevailing winds are east-to-west as a consequence of the Coriolis force. This causes drift of the superficial coastal water towards the ocean, which is equalized by the elevation of the South Atlantic Central Water coming from the continental slope between 200 and 300 m.
The process of downwelling, which represents the return of warm and more saline waters of the Brazil Current to the coast, occurs during the passage of cold fronts with southwesterly winds. In winter, the hydrological structure is typical of downwelling, with a homothermic water column down to 50m. Homothermy with cold water is more common in spring and in some summer months, due to the maximum upwelling of the cold South Atlantic Central Water. With intensive upwelling, the surface temperature of the ocean can reach between 13 and 18OC, and 5°C lower between 20 and 30 m. More details of the physicakhemical conditions and the influence of upwelling in the area can be found in Weber (1994). [non] 1965 Hemicytherura howei (Puri) Pooser: 49, pl. 9, figs 1, 3.

SYSTEMATIC DESCRIPTIONS
[non] 1979 Cytherura howei (Puri) Cronin: 144 and 145, pl. 14, Derivation of name. L. from the rather ear-like aspect of the right valve of this species in lateral view. Diagnosis. Carapace with the right valve ear-like in lateral view and ornamentation formed by coarse and rounded reticulae. Large median rib that is interrupted medianly. Holotype. Carapace, MP-0-1490. Paratypes. Carapace, MP-0-1491. Material. Nineteen adult valves and five juvenile valves. Type locality. Rio de Janeiro coast, GEOCOSTA RIO 11, sample 46. Age. Recent. Description. Thick-shelled carapace, right valve ear-like in lateral view. Anterior margin angular and quadridentate. Apex below mid-height. Ventral margin with a distinct oral concavity. Posterior margin bluntly caudate in right valve, less pronounced in left valve. Dorsal margin strongly convex in right valve and stronger than in left valve. Ornament strongly reticulate, formed by circular reticulae. Dorsal region delicately punctate. Large median rib interrupted by reticulate ornament medianly. Two strong ventral ribs extend from the mid-anterior margin, parallel to the ventral margin. Two prominent depressions: one anteroventral and another postero-ventral occur below the median rib. Internal features typical of the genus. Sexual dimorphism not evident.  Remarks. The present material is identical with Hemicytherura howei (Puri) (in Sanguinetti, 1979) from the Miocene of the Pelotas Basin. However, a comparative study of the present species with type material of Kangarina howei Puri, 1953, demonstrated that they are distinct species. Hemicytherura auriculata sp. nov. has less prominent ribs, the caudal process is shorter and straighter and it is bigger than Kangarina howei Puri, 1953  Material. Eighty-three adult valves and seven juvenile valves. Type locality. Rio de Janeiro coast, GEOCOSTA RIO 11, sample 43. Age. Recent. Description. Carapace inflated, thin-shelled and subovate in lateral view. Slightly inequivalve. Cardinal angles not prominent; a few specimens are more accentuated in the posterior region. Anterior margin rounded. Posterior margin with large and upturned caudal process above mid-height in RV. Dorsal margin strongly arched. Ventral margin sinuous and slightly concave in the antero-median region. Ornament consists of sinuous parallel horizontal ribs interconnected with short riblets in the dorsal region. A median rib curves dorsally, almost forming a posterior loop. Internal features as for genus. Sexual dimorphism strongly developed, males more elongate and more inflated than females.  , 1988), but the ventral rib is less strongly developed, the caudal process is more upturned, and the ornament is less punctate. The species occurs in the Quaternary of the Pelotas Basin, Chui Formation, -Rio Grande do Sul, Brazil and off the Rio de Janeiro coast. It occurs in GEOCOSTARIOII,samples 18,20,27,28,33,36,37,39,40,41,42,43,45,46,47 and 48. Semicytherura caudata sp. nov. is more numerous than either Hemicytherura auriculata or Semicytherura rugosoreticulata in the studied area. It is distributed between 42 and 72m depth and is most abundant in medium sand. However, it is also found more rarely on very fine sands (Fig. 4). Remarks. This species is similar to Hemicytherura chuiensis Kotzian (in Bertels, Kotzian & Madeira-Falcetta, 1982) from the Quaternary of Rio Grande do Sul. However, it differs in that the puncta of the central area are much more spaced and fewer in number, and the ventral rib is more convex in the posterior region. It occurs along the Rio de Janeiro coast, GEOCOSTA RIO 11,in samples 20,22,23,27,33,35,36,37,39,40,42,43,45,46,47,48,49,51,52,53,54,55,56 and 57 at depths ranging between 40 and 72m, and mainly on fine sand sediments (Fig. 3).  fig. 1. LV, external view, x89. fig. 2. RV, external view, x85. fig.  3. RV, external view, detail of anterior region, x 131. fig. 4. LV, internal view, x98. fig. 5. LV, internal view, detail of posterior hinge, x263. fig. 6. LV, internal view, detail of anterior hinge, x263. fig. 7. LV, internal view, detail of muscle scar, x 194. figs 8, 9. Oculocytheropteron macropunctatum Whatley, Chadwick, Coxill & Toy, 1988. Homotype. MP-0-1494, female. fig. 8. LV, external view, x74. fig. 9. RV, external view, x77. figs 10, 11. Oculocytheropteron reticulopunctatum Whatley, Chadwick, Coxill & Toy, 1988   samples 19,20,27,33,34,36,37,39,40,42,43,45,49,51,52,55,56 and 57, with a bathymetric distribution between 42 and 74m, being more abundant between 51 and 72m. It prefers fine and very fine sandy sediments (Fig. 5).
The results show that ostracods are more abundant between 51 and 70m, where 88% of the total number of specimens occur (Table 1). Fine sand predominates in this depth range (Fig. 8).
The abundance of the fauna is inversely proportional to grain size and directly proportional to depth range. Only two samples were analysed between 71 and 80m, although the number of specimens found (95) in this depth range is considerably higher than from samples shallower than 51 m (Table 1, Fig. 8).
Between 20 and 40 m, where medium sand predominates, few specimens occur (less than 1 YO). Paracytheridea bulbosa and Oculocytheropteron delicatum occur in shallower water than other species; Paracytheridea bulbosa occurs in almost all samples with a large depth range, between 27.5 and 74m, but is more abundant between 45 and 51 m on medium and very fine sand. Although Oculocytheropteron delicatum occurs between 40 and 74m, it is more abundant between 60 and 70m. Semicytherura caudata and Oculocytheropteron circumcostatum are more abundant between 50 and 60m, where the sediment size ranges from medium to very fine sand (Figs 4 and 5, Table  1). Hemicytherura auriculata, Semicytherura rugosoreticulata, Oculocytheropteron macropunctatum and Oculocytheropteron reticulopunctatum are more abundant between 60 and 70m, in sediments of fine and very fine sands. The highest diversity is between 71 and 80m, where the fauna is homogeneously distributed. The lowest diversity occurs between 20 and 30m, closer to the coast, where a single species is represented by only one specimen (Table 1). The highest number of specimens recovered, per genus, was for Oculocytheropteron. 0. macropunctatum predominates over all other species and, together with 0. delicatum, it has the highest percentage of constancy (Fig. 9). Table 2 shows the number of specimens per species in each sample. The samples are listed in depth order. In Table 2, j represents the presence of juveniles. Table 1  Many specimens exhibit dissolution and overgrowth of calcium carbonate. This can probably be explained by the effect of upwelling in this area. Normally, upwelled waters contain higher concentrations of nutrients (such as phosphates, silicates and nitrates) than surface water, which has been depleted as a result of biological consumption. Thus dissolution results from the corrosiveness of the water, due to a charge in the water's carbonate ion content, low temperature, and high hydrostatic pressure, increased water flow through the sediments and high pco2. Carbon dioxide in the water changes to carbonic acid, which eventually dissolves the calcium carbonate; carbon dioxide is produced by the respiration of benthonic organisms, which increases in upwelled waters (Kennet, 1982: 466).
Although dissolved and overgrown specimens might, in many circumstances, be thought to be the product of reworking of older sediments, in this area, given that the sediments are derived from the unfossiliferous Barreiras Group, this is considered unlikely.