ABSTRACT. Morphomctric varia tion and osteologica l analysis in Ramllogaster arcuata (Jenyns) a nd Plafanichthys plafalla (Regan) (Teleostei, C lupeidae). Rallll10gasrer arclIara (Jenyns, 1842) and P/aranichrhys p/afana (Regan, 19 17), found ai lhe estuary or Lagoa dos Patos and coastal lagoon (Lagoa Mirim) at the South of Brazil , are two sympatric clupeid fi shes with very similar features. In order to clarify lhe ir taxonomic and geographical ituation , delimiting ilS occurrence in fres h water , a comparati ve stud y of the bony struclures of the head, anal and dorsal fin s and ventral scutes of lhese Iwo sardines was deve loped. The osteological stud y with clear and stain technique included more information about lhe differenti ati on of the group from the bones or lhe supramaxilla. demary, pterygiophore and epipleural ribs. Variat ions on lhe bod y shape of lhose species were analyzed lhrough the multi variate mo rphometry. in order 10 obtain differentialion patterns between the gro ups. Thirty spec ime ns were selec led from each species. The meas urements were oblained by lrllss network , by using 10 homologoll s anat omical landmarks. The analys is of the principal componenl s, adj usted by Burn aby's orthogona l projection, indicated two main group , separated along the PC I. whi ch ex plained 63.2% of the variation between groups. The diagnostic characters that differenliate R. arcuara from P. plarana , were re laled to segments of distance of lhe head , longitudinal growlh, body heighl and caudal pedunde. Furthermo re, two groups o f P. p/afano were found . In spite oflheir morphologica l similarities, lhey presented a differentiation gradient between lhe estuary poplll ati on of the Patos Lagoon and lhe Mirim Lagoon, poss ibly resul ting from lwo eco logica ll y different environments. KEY WORDS . Clupeidae, multi variate morpho metry, morphological varialion , osteology A fa mília Clupeidae é representada por peixes de pequeno porte. São habitantes típicos de regiões costei ras em todos os oceanos, estando presentes também em ambientes estuarinos ou até mesmo em água doce (WH ITEHEAD (9 85).No sistema estuari al da Lagoa dos Patos e nas lagoas costeiras no sul do estado do Ri o Grande do Sul (Fig. I), dois cl upeídeos destacam-se pel a grande semelhança morfológica. Platanichthys plafana (Regan, 1917) é citada para estuá-ri os, ri os e lagoas costei ras, enquanto Ramnogaster arcuata (Jenyns, 1842) está restri ta a águas coste iras e es tuarinas ( WH ITEHEAD 1985) ( Fig. 2A e B).
Spectral analysis revealed oscillations associated to atmospheric phenomena in the extreme south of Brazil. Cycles from 36 to 77 days, linked to Madden -Julian Oscillation, from 18 to 28 days, related to the bipolar South Atlantic Convergence Zone-Subtropics mode and 10-day oscillations due to the passage of frontal systems were detected.
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