SHORT COMMUNICATION Genetic diversity of the Pennaria disticha Goldfuss, 1820 (Cnidaria, Hydrozoa) complex: new insights from Brazil Claudia F. Vaga 1,2 & Marcelo V. Kitahara 1,3 & Karine B. Nascimento 1,2 & Alvaro E. Migotto 1 Received: 22 May 2020 /Revised: 17 July 2020 /Accepted: 20 July 2020 # Senckenberg Gesellschaft für Naturforschung 2020 Abstract Analyzing the mitochondrial 16S gene from specimens collected along the Brazilian coast, we expanded the current genetic diversity and phylogeny of the Christmas tree hydroid, Pennaria disticha Goldfuss, 1820 complex. A total of seven clades were retrieved, of which three were recovered among the newly added specimens—two were already known from different locations in previous studies and a new one was formed by a specimen collected near an isolated coastal island in the São Paulo state— demonstrating that P. disticha also represents a complex of cryptic species in the southwestern Atlantic. Species delimitation analyses recovered three to five clades as separate species within P. disticha; moreover, the haplotype network confirmed the already suggested scenario of multiple invasive trajectories and helped untangle the genetic relationships between clades. The data presented herein, besides broadening P. disticha genetic diversity, reinforce the importance of barcoding analyses of local populations in cryptic species studies. Keywords Phylogenetic . Barcoding . 16S . Southwestern Atlantic . Cryptic species Introduction Species are considered ‘cryptic’ if their morphological bound- aries are unclear (Bickford et al. 2007; Pfenninger and Schwenk 2007) resulting in more than one species being assigned under one species name. In the last decades, molec- ular approaches, such as phylogenetics and DNA barcoding, have shed light on cryptic lineages across different habitats and taxa (Hebert et al. 2004; Bekker et al. 2016; Maggioni et al. 2016; Bringloe and Saunders 2019). As different species might respond differently to the environmental variables (Heads 2015), the identification of cryptic species has serious implications for nature conservation (Bickford et al. 2007). The identification of cryptic species has also become of cru- cial importance when it involves the detection of exotic spe- cies (Geoffroy et al. 2012; Stouthamer et al. 2017; Bezeng and van der Bank 2019; Hernández-Triana et al. 2019). During the last decades, biological invasions have worldwide become one of the major causes of the deterioration of natural com- munities and loss of biodiversity (Bax et al. 2003; Simberloff et al. 2013; Chan and Briski 2017). In particular, shipping in combination with ballast water and biofouling, on both re- gional and global scales, is the most significant vector of non-native species introduction in the marine realm (Rocha et al. 2013; Bailey 2015; Verna et al. 2016; Peters et al. 2019). The Christmas tree hydroid, Pennaria disticha Goldfuss, 1820, is a conspicuous species that forms large pinnate colo- nies on hard substrates. Considered to have a wide geograph- ical distribution in tropical and temperate waters (Schuchert 2006; Calder 2019), it has a complex taxonomical history (synonymy in Calder 1988). Until recently, P. disticha has been interpreted as showing a high intraspecific variability, which led to the description of a series of nominal species in Communicated by B. W. Hoeksema Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12526-020-01102-7) contains supplementary material, which is available to authorized users. * Claudia F. Vaga claudia.vaga90@gmail.com 1 Center for Marine Biology, University of São Paulo, Rodovia Manoel Hipólito do Rego, km 131,5, Pitangueiras, São Sebastião, São Paulo 11612-109, Brazil 2 Department of Zoology, Institute of Bioscience, University of São Paulo, Rua do Matão, 321, Travessa 14, Cidade Universitária, São Paulo 05508-900, Brazil 3 Department of Marine Sciences, Federal University of São Paulo, Campus Baixada Santista, R. Carvalho de Mendonça, 144, Santos, São Paulo 11070-100, Brazil Marine Biodiversity (2020) 50:68 https://doi.org/10.1007/s12526-020-01102-7