Occurrence of Pteria colymbus ( Bivalvia : Pteriidae ) in Argentine waters

The family Pteriidae is well represented in tropical and subtropical continental shelf regions. Two names belonging to the genus Pteria are usually recorded in the literature from the southwestern Atlantic: P. colymbus (Röding) and P. hirundo (Linnaeus). The study of living specimens sampled off Buenos Aires coast aboard of the R/V Puerto Deseado allows updating the geographic range of the Atlantic wing oyster Pteria colymbus in the region. Type materials of related species are illustrated. Ecological requirements and shell morphology description of P. colymbus are provided. This study constitutes the first revision of this species from Argentine waters.

The main objective of this study is to clarify the presence of Pteria colymbus in the southwestern Atlantic as the unique living Pteriidae in Argentine waters.

MATERIALS AND METHODS
New specimens of P. colymbus were collected off Buenos Aires coast with a bottom dredge aboard the R/V Puerto Deseado on October 2009 (Fig. 1).All specimens were fixed in 10% formalin and preserved in 70% ethanol.In addition, morphometric parameters of new records and type material were measured with a digital caliper and listed in the Table 1.They are the anteroposterior length (APL), the greatest distance between the anterior and posterior over the dorsal margin of the shell and the dorsoventral height (DVH), the greatest dorsoventral distance measured from hinge line to the ventral edge.The relation APL/DVH is provided.Physical parameters were recorded and listed in Table 2.The latitude/longitude data were registered with a GPS Garmin.The sampling depth at each station was provided by the crew of the R/V Puerto Deseado and a Niskin bottle was used to collect seawater and to measure bottom temperature and salinity.Associated species are mentioned in Table 3 and identified based on the traditional catalogues and type material examination.Finally, material deposited at invertebrate collection of Museo Argentino de Ciencias Naturales (MACN-In) and Linnean Society of London (LSL) was revised.New collected specimens were housed at MACN-In and invertebrate collection of Centro Nacional Patagónico (CNP-Inv).

RESULTS
Despite the 32 stations sampled along Buenos Aires province coast by the R/V Puerto Deseado, only in the station 22 living specimens of Pteria colymbus were collected.This area is charac-terized by a sandy bottom with broken shells influenced by the Río de la Plata estuary (Fig. 1).Nevertheless, it is suitable to host banks of Mytilidae, which allows also the presence of the byssate Pteria colymbus.All the specimens of P. colymbus were fixed to each other by the byssus forming a sort of nest initially attached to empty shells of pectinids and venerids or small rocks (Fig. 14).This work updates the distribution of Röding's name from North Carolina (N 35° 20´ -W 76° 00´), United States of America, to off Buenos Aires province coast (S 37° 35´ -W 55° 30´), Argentina in 100 m depth.
Pteria colymbus is characterized by a shell obliquely oval, fragile, thin, large, up to 80 mm of shell length, external surface smooth in the umbonal area, with very fine flat spines in radial rows, more aligned in the anterior part; concentric ridges present over all external surface; anterior auricle short, solid, with straight anterior edge; posterior auricle elongated; brown periostracum disposed in radial rows; internally nacreous, with wide non-nacreous margin; hinge line straight with one anterior tooth and one posterior socket in the left valve; one anterior socket and posterior ridge in the right valve.

DISCUSSION
In local literature, Doello Jurado (1938) adopted the name of Linnaeus and considered that the species living in the southwestern Atlantic is the same than the European taxon.However, he observed shell differences that attributed to intraspecific variation.After the analysis of the type material deposited in the LSL, we conclude that the single specimens of Linnean collection belongs to the European species P. hirundo (Figs.6-7) and the complete specimen to the Atlantic species P. colymbus (Figs.2-5).The shell morphology 14) coincides with the specimens collected by the R/V Puerto Deseado.
The conditions on which the new records of P. colymbus specimens were collected suggest a mixed bottom substrate dominated by sand and mud with a large sediment deposition coming from Rio de la Plata estuary.All revised lots deposited in the malacological collections were sampled along the mouth of Rio de la Plata estuary in a bathymetric range from 50 to 130 m depth (Fig. 1).This could suggest similar living conditions of all examined and sampled specimens of P. colymbus in the region.The association between pterids and other organisms were widely registered in several regions of the world.Mytilids, gorgonians and seagrass association were registered in the literature (e.g.Hedley, 1924;Mikkelsen et al., 2004;Morton, 1995;Temkin, 2006;Villafranca & Giménez, 2004; among others).These results coincide with the observed during our fieldwork where the specimens of P. colymbus were collected with a bottom dredge associated to banks of the mussel Mytilus edulis d´Orbigny.In addition, species belonging to Porifera, Cnidaria, Arthropoda, other Mollusca and Echinodermata were also sampled by the dredge at same station that specimens of P. colymbus (Table 3).The population abundance was not estimated during field work.However, specimens of P. colymbus were collected by the dredge only in one station of the field work off Buenos Aires coast.This might suggested a low abundance of the species.This study revised the taxonomic status of pterid species living in Argentine waters by the exa-mination and illustration of the type material.Additionally, expands the range distribution and information of associated fauna of Pteria colymbus.

Table 1 :
Morphometric measurements of new records of P. colymbus during fieldwork

Table 2 :
Physical Parameters from the station where Pteria colymbus was collected