GENUS OCTACANTHIUM (ZYGNEMAPHYCEAE, DESMIDIACEAE) IN THE STATE OF SÃO PAULO, BRAZIL

A floristic survey was based on the study of 148 sample units, from which 75 were already deposited at the Herbário Científico do Estado “Maria Eneyda P. Kauffmann Fidalgo” of the Instituto de Botânica, São Paulo State Department of Environment, and 73 were collected during the period 2000-2001. Three taxa of Octacanthium (Desmidiales, Zygnemaphyceae) were identified: O. bifidum (Brébisson) Compère var. bifidum, O. mucronulatum (Nordstedt) Compère var. mucronulatum, and O. octocorne (Ralfs) Compère var. octocorne.


INTRODUCTION
The genus Octacanthium was proposed by Compère (1996) to include all species formerly classified in the Sect.Octacanthium of genus Arthrodesmus.It presently includes nine worldwide known species.
First report of the occurrence of representatives of this genus in Brazil is in Nordstedt (1869).Octacanthium mucronulatum (Nordst.)Comp.(as Arthrodesmus mucronulatus Nordst.) is the only species cited in that paper, based on material collected in the municipality of Lagoa Santa, state of Minas Gerais.Notice above was accompanied by a brief description and illustration of the material studied.
All knowledge of Octacanthium for the state of São Paulo is in just two papers.Borge (1918) was the first to refer to the genus for the state of São Paulo.The paper was based on material collected by Alberto Löfgren in the municipality of Pirassununga, and just O. octocorne (Ralfs) Comp.(as Arthrodesmus octocornis Ralfs) was cited.
The state of São Paulo's geographical area covered by the two papers above is very restricted, representing less than 10% of that of the state.Furthermore, there has never been a publication prior to the present paper that presented such a detailed genus inventory, considering the state of São Paulo's geographical area.In conclusion, in both the size of the geographical area now covered, as well as the scientific depth of the taxonomical treatment, present research provides new information.

MATERIAL AND METHODS
The study area is that of the state of São Paulo (Fig. 1).One hundred and forty eight sample units were studied, which were collected from 110 localities in the state of São Paulo.Below are the 8 sample units in which representatives of Octacanthium were found, listed according to increasing access to the herbarium code numbers.
SP188207: Municipality of Porangaba, intersection of roads right  Sampling of plankton material was performed by using a 20 µm mesh nylon plankton net.Net was used as many times in the system's superficial layer (± 30 cm depth), as was needed to obtain a reasonable amount of material.Measure of such a 'reasonable amount' was taken by the naked eye according to the appearance of a greenish to greenish-brown mass that accumulated at the bottom of the collecting vial.Such a slimy mass is, in general, very rich in desmid material.Samplings were carried out at the littoral zone of both lotic and lentic systems, where there are usually plenty of attached aquatic plants, totally or partially submersed, making such environments natural plankton traps.
Periphytic material was collected by manual squeezing of totally or partially submersed plants (bryophytes, pteridophytes, and phanerogams), and by the gathering of complete specimens or of submersed parts of emergent specimens.
Sample fixation and preservation was performed immediately after collection, in the field, with formaldehyde 3-5% aqueous solution.Immediate fixation aimed at preserving cell morphology as much as possible to that at the moment of collection.Concentration of material in the vial may cause acceleration of the cell division rate in certain species, especially in those of desmids and, consequently, production of many anomalous phenotypes because the alga has not got enough time to fully develop all its structures before undergoing a new division.Since apparent morphology is the basis for the sorting out of species, varieties, and taxonomic formae in Octacanthium, it is mandatory that such results of malformations be avoided and, if they occur, that they are not mistaken for regular expressions of the intra-population morphological variation spectrum.
All material studied is deposited at the Instituto de Botânica herbarium (SP).All information pertaining to the sampling localities obtained after installation of the BIOTA / FAPESP Program obligatorily followed the program's standard forms, and location of the collecting sites was defined by GPS.
Genus Octacanthium was proposed by Compère (1996) who, in fact, erected to the genus level the former Sect.Octacanthium Hansg. of Arthrodesmus Ehr.ex Ralfs.
Present study sample units in which the species was encountered: none.
Specimens collected in the state of São Paulo came from two localities, i.e.Açude do Jacaré, a small, artificial reservoir located at the Moji Guaçu Biological Reserve, municipality of Moji Guaçu, and from a pond located between km 74 and 75 of road SP-88, municipality of São Paulo.Populations studied were formed by just a few individuals each, which showed morphological variation regarding the isthmus, being more (fig.5) or less (fig.4) delimited, and also regarding the shape of the upper margin of semicell.In the first case (fig.5), the isthmus is very well marked due to the basal margins of both semicells forming a more or less straight angle.In the second case (fig.4), such an angle does not exist and the isthmus is not that much delimited, the sinus being just a curved, concave line that goes from the spine-bearing angle of one semicell to that of the opposite semicell.There is also variation in what respects the degree of curvature of the upper margin of semicells, which may be either shallower (fig.5) or deeper (fig.4).

Octacanthium mucronulatum (Nordstedt)
Compère var.mucronulatum a short, simple spine tipping each lower angle of semicells and just a mucron at each upper angle (Nordstedt, 1869: 232).The original illustration of this species was published 17 years later (Nordstedt, 1887: pl. 4, fig. 58), together with all other illustrations of the 1869 paper and a few corrections to some figure captions.
Octacanthium mucronulatum (Nordst.)Comp. is a very polymorphic species.Strengthening Bicudo (1975), Bicudo & Azevedo (1977) observed great morphological variation within the species in the three sample units studied.Latter authors observed some specimens with a supernumerary spine on each side of semicell, contiguous and immediately below the regular spine of the upper angle, some other specimens with no spine at the upper angles in spite of the persistent angular projection at the place formerly occupied by the spine, and a few specimens in which there was a disappearance of both the spine usually located at the upper angles of semicells and the angular projection, resulting in an uniformly convex upper margin.In a very extreme condition, Bicudo & Azevedo (1977) observed the complete lack of the spines of the lower angles of the semicells.Specimens with spines only at the lower angles of both semicells and upper margin of semicells broad and uniformly convex, could be easily taken for Staurodesmus convergens (Ehr.ex Ralfs) Teil.
Populations studied from Itatinga, Paraguaçu Paulista, Pitangueiras, Rio Claro, and São Paulo also showed considerable morphological variation, mainly in what concerns the lack of the spines of the upper angles of semicells (fig. 10-12) or the presence of supernumerary spines on the upper (fig.9) and lower (fig.9) angles of semicells.Also observed was the smoothing of the upper angle of semicells (fig.11, 13) and the presence of an uniformly convex upper margin (fig.13) which gives imperceptible continuation to the spines of the lower (now the only ones) angles of semicells.Material from the state of São Paulo showed variation mainly of the isthmus, which was sometimes long and very much cylindrical (fig.3) and some other times short and almost not cylindrical (fig.2).It is also important to note the little metrical variation detected by Bicudo & Azevedo (1977) in the three populations they studied and which were collected from three different localities (Atibaia, Lorena, and São Paulo) in the state of São Paulo.

Octacanthium octocorne (Ralfs
Material from the municipality of São Paulo, collected from a display of aquatic plants at the São Paulo Botanical Gardens, presented frequently zygospores.

CONCLUSIONS
The This reinforces the statement that sexuality is of rare occurrence in nature in the Desmidiaceae.4.The fairly great metric variability observed both in the material studied and in the literature proved the impossibility of using just measurements for identification of species and infraspecific taxa in the genus.Due to the high incidence of polymorphism mainly in Octacanthium mucronulatum (Nordst.)Comp., it is nearly impossible to precisely identify anomalous specimens of this species without a very careful population analysis.Polymorphism hides which characteristics are good for diagnosis and which ones are not.Consequently, a robust identification depends on a very cautious study of the greatest possible number of specimens.