Acta Protozool. (2016) 55: 221–229 www.ejournals.eu/Acta-Protozoologica doi:10.4467/16890027AP.16.021.6008 ACTA PROTOZOOLOGICA Morphological and Morphometric Description of a Novel Shelled Amoeba Arcella gandalf sp. nov. (Amoebozoa:Arcellinida) from Brazilian Continental Waters Jordana C. FÉRES 1 , Alfredo L. PORFÍRIO-SOUSA 2 , Giulia M. RIBEIRO 2 , Gustavo M. ROCHA 1 , José Mauro STERZA 1 , Maria Beatriz G. SOUZA 2 , Carlos Eduardo A. SOARES 3 , Daniel J. G. LAHR 2 1 Ethica Ambiental Serviços e Consultoria Ltda, Vila Velha, Espírito Santo, Brazil; 2 Department of Zoology, Institute of Biosciences, University of São Paulo, Brazil; 3 Department of Biology, State University of Maringá, Paraná, Brazil Abstract. Arcellinida are free-living lobose amoebae that produce an outer shell (test). Here, we describe a conspicuous new species, Ar- cella gandalf sp. nov, from Brazilian continental waters, along with a morphological and biometrical characterization. Test diameter and test height are on average 81 and 71 respectively. This new species has an apical conical extension, which differentiates it from other Arcella species. A. gandalf seems to be closely-related to A. brasiliensis, due to the distinct marginal ring (test brim) present only in these two species. Since A. gandalf is easily identifed by morphological features and due to its apparent geographic restriction to South America, we discuss its possible use as a new fagship species. Key words: Testate amoeba, new species, morphology, morphometry, fagship species. Address for correspondence: Daniel J. G. Lahr, Rua do Matão, Travessa 14, 101, Cidade Universitária, São Paulo, SP, Brazil, 05588-090; E-mail: dlahr@ib.usp.br INTRODUCTION Arcellinida are free-living lobose amoebae that produce an outer shell (test). These organisms are cur- rently classifed in the Amoebozoa: Tubulinea (Adl et al. 2005) and are largest and most diverse group of tes- tate amoebae, with an estimated 700 to 1,300 nominal species (Meisterfeld 2002, Kosakyan et al. 2016), but this number may increase signifcantly with recent dis- coveries of widespread cryptic/pseudo-cryptic species (Kosakyan et al. 2013, Oliverio et al. 2015). Thus, the real richness of testate lobose amoebae is diffcult to estimate. This is mainly due to sparse sampling and un- revised nomenclature, with a large number of synony- mous, homonymous and ambiguous taxa (Kosakyan et al. 2016). Furthermore, many species of testate amoe- bae show high intraspecifc morphological variability, suspected to be induced by environmental conditions (Wanner and Meisterfeld 1994, Bobrov and Mazei 2004, Porfírio-Sousa et al. 2016). Arcella Ehrenberg 1832 is one of the largest gen- era, with more than 130 described taxa (Tsyganov and Mazei 2006). Taxonomy in this genus is gener- ally based on shell morphology and morphometry. Ar- cella has a hemispheric shell in side view, and circular in apertural view (Pearl and Dunbar 1903). Tests are