We describe a new cryptic species of leopard frog from the New York City metropolitan area and surrounding coastal regions. This species is morphologically similar to two largely parapatric eastern congeners, Rana sphenocephala and R. pipiens. We primarily use bioacoustic and molecular data to characterize the new species, but also examine other lines of evidence. This discovery is unexpected in one of the largest and most densely populated urban parts of the world. It also demonstrates that new vertebrate species can still be found periodically even in well-studied locales rarely associated with undocumented biodiversity. The new species typically occurs in expansive open-canopied wetlands interspersed with upland patches, but centuries of loss and impact to these habitats give some cause for conservation concern. Other concerns include regional extirpations, fragmented extant populations, and a restricted overall geographic distribution. We assign a type locality within New York City and report a narrow and largely coastal lowland distribution from central Connecticut to northern New Jersey (based on genetic data) and south to North Carolina (based on call data).

Funding: Financial support for field work and analysis was primarily provided by a Rutgers Graduate School of New Brunswick Bevier Dissertation Fellowship and a New Jersey Department of Environmental Protection Conserve Wildlife Matching Grant to JAF, an NIH NIEHS Center grant P30ES005022 to JB, and a Hudson River Foundation grant to JB and JAF. Partial support was also provided by the Rutgers Graduate Program in Ecology and Evolution, Brookhaven National Laboratory, the Foundation for Ecological Research in the Northeast, and the Tiko Fund. Funding for genetics laboratory work for the holotype, performed at the Louisiana State University Museum of Natural Science, was provided by NSF DEB-1146033 (to Christopher C. Austin). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Introduction

In order to develop clear understandings of species and their ecologies, distributions, and conservation needs, they must first be properly identified and accurately delimited [1]. Such efforts can be complicated, however, by the presence of cryptic species – species that, due to morphological similarity, have been incorrectly included with one or more other species under a single species classification [2]. Identifying cryptic species can be difficult though, which presents taxonomic and conservation challenges. These challenges can be further exacerbated in heavily altered environments and areas where extirpations and habitat loss have led to insufficient numbers of individuals or populations for sampling. Nonetheless, a cryptic species discovery can have important implications for multiple species, including the new species itself and its cryptic congeners [1]. Further, cryptic species can be found in unexpected locales [3], and in some regions, reflect surprisingly high levels of diversity [4]. Left undetected, however, cryptic species can remain concealed among other species, which can be problematic if seemingly common or widespread nominal species actually contain hidden component species that are range-restricted, rare, or even extinct [1], [2].

Considerable effort has been given to identifying and cataloging new species, cryptic and otherwise, over the past few decades. In the case of amphibians, these efforts carry added urgency in the face of severe global declines and extinctions and also reveal strongholds of undocumented species, often in areas of tropical species richness or poorly known composition [4], [5]. In contrast, far less attention or discovery has been associated with urban areas and other highly developed or well-documented regions, especially those outside the tropics. Among anurans, for example, only two truly novel species (that is, taxa that were not previously recognized as subspecies) have been reported from the continental United States (US) and Canada since 1986 [3], [6], [7]. In this paper we describe the most recent of these, a cryptic leopard frog lineage that was first identified from the New York City region in 2012 [3]. Few examples of undescribed vertebrate diversity exist in the recent literature from highly urbanized regions and areas with well-established taxonomic infrastructures.

The species we describe here was first identified by Newman et al. [3] via molecular data. It constitutes the newest member of the Rana pipiens complex and occupies parts of the lower Northeast and mid-Atlantic US within the densely populated and heavily industrialized Interstate-95 (I-95) corridor. This is one of the largest human population centers on earth [8] and a region where endemic vertebrate species are rare. The long-term concealment and recent discovery of a novel anuran here is both surprising and biogeographically significant, and illustrates how new species can occur almost anywhere. It also raises potentially important conservation concerns: amphibians can be sensitive to disease, contaminants, and environmental perturbations, and their low vagility can be particularly problematic in fragmented and urban landscapes [9]. Also worrisome are enigmatic declines that have led to disappearances of leopard frogs from parts of the Northeast and mid-Atlantic US [10]–[13]; this includes some relatively non-urbanized coastal, suburban, and agricultural regions in southeastern New York (NY) [3], [14], southern Connecticut (CT) [11], and presumably parts of northeastern Pennsylvania (PA) where they were reported historically, but not in recent decades [15]–[20].

Here, we expand upon the initial genetic results presented by Newman et al. [3] to name, diagnose, and describe the new species. We present several lines of supporting evidence, but focus on bioacoustic signals and molecular data. We also provide a brief history of relevant taxonomic confusion within the R. pipiens complex, comparisons to similar species, and information on distribution, ecology, and conservation status.