On the identification of paedomorphic and overwintering larval newts based on cloacal shape: review and guidelines

Current Zoology, 2017, 63(2), 165–173 doi: 10.1093/cz/zow054 Advance Access Publication Date: 9 May 2016 Article Article On the identification of paedomorphic and overwintering larval newts based on cloacal shape: review and guidelines € * Mathieu DENOEL Laboratory of Fish and Amphibian Ethology, Behavioural Biology Unit, FOCUS, University of Liège, 22 Quai van Benden, 4020 Liège, Belgium *Address correspondence to M. Deno€el. E-mail: . Received on 2 February 2016; accepted on 15 April 2016 Abstract Paedomorphosis is an alternative process to metamorphosis in which adults retain larval traits at the adult stage. It is frequent in newts and salamanders, where larvae reach sexual maturity without losing their gills. However, in some populations, larvae overwinter in water, while remaining immature. These alternative ontogenetic processes are of particular interest in various research fields, but have different causes and consequences, as only paedomorphosis allows metamorphosis to be bypassed before maturity. It is thus relevant to efficiently identify paedomorphs versus overwintering larvae. In this context, the aim of this article was 3-fold: firstly, to perform a meta-analysis of the identification procedures carried out in the literature; secondly, to determine the effectiveness of body size to make inferences about adulthood by surveying natural newt populations of Lissotriton helveticus and Ichthyosaura alpestris, and thirdly, to propose easy guidelines for an accurate distinction between large larvae and paedomorphs based on an external sexual trait, which is essential for reproduction—the cloaca. More than half of the studies in the literature do not mention the diagnostic criteria used for determining adulthood. The criteria mentioned were the presence of mature gonads (10%), eggs laid (4%), courtship behavior (10%), and external morphological sexual traits (39%) including the cloaca (24%). Body-size thresholds should not be used as a proxy for paedomorphosis, because overwintering larvae can reach a larger size than paedomorphs within the same populations. In contrast, diagnosis based on cloacal external morphology is recommended, as it can be processed by the rapid visual assessment of all caught specimens, thus providing straightforward data at the individual level for both sexes. Key words: amphibians, cloacal morphology, facultative paedomorphosis, neoteny, review, secondary sexual traits, sexual dimorphism, sexual maturity. Many species of newts and salamanders are biphasic and show the successive use of aquatic and terrestrial habitats across life stages (Griffiths 1996; Petranka 1998). This involves the metamorphosis of an aquatic gilled larva into a terrestrial juvenile phenotype. One of the features of this shift is the resorption of the external gills and the closure of gill slits (Ivanovic et al. 2011). However, in some populations, a part of the aquatic larvae do not metamorphose and acquire sexual maturity while retaining larval morphology, C The Author (2016). Published by Oxford University Press. V including gills (Dzukic et al. 1990; Breuil 1992; Whiteman 1994; Deno€el et al. 2005). The polymorphism, named facultative paedomorphosis, therefore produces two alternative adult phenotypes: paedomorphs and metamorphs. The term “paedomorphosis” is here used to describe the acquisition of sexual reproduction in larvae, that is, in gilled individuals (Garstang 1922; McKinney and McNamara 1991; Deno€el et al. 2005). Although paedomorphosis is rarer than metamorphosis, it is found in a large number of species 165 This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact 166 and is often locally abundant in the geographic areas where it is expressed. In newts, facultative paedomorphosis is particularly observed in Ichthyosaura alpestris, Lissotriton helveticus, and Lissotriton vulgaris in Europe (Dzukic et al. 1990; Andreone and Dore 1991; Deno€el 2007; Deno€el et al. 2009a) and in Notophthalmus viridescens in North America (Wilbur and Collins 1973; Takahashi et al. 2011). It is also present in other families, such as ambystomatid salamanders (Sexton and Bizer 1978; Shaffer and Voss 1996; Whiteman et al. 2012). However, the persistence of aquatic larvae for longer than usual does not necessarily involve a strictly speaking paedomorphic process (i.e., involving maturity). Indeed, in some populations, larvae do not metamorphose before winter and are then found the year after they hatched from eggs. These larvae are larger than the typical larval cohorts born after winter and coexist with them (Ernst 1952; Grossenbacher 1979; Deno€el et al. 2016). They are called overwintering (immature) larvae (Harris 1987) and they can metamorphose during their first year, that is, before the second winter. However, they can also spend more than one winter in water before metamorphosing, or can be present at intermediate developmental stages before the acquisition of sexual maturity in a paedomorphic stage (Sexton and Bizer 1978; Breuil 1992; Wissinger et al. 2010). As they are larger than the usual size at metamorphosis, they can be seen as branchiate juveniles in comparison with the terrestrial juvenile stage that becomes a metamorph at adulthood (Fasola and Canova 1992; Deno€el and Joly 2001) and can also reach similar sizes to some paedomorphs, but cannot reproduce. Both phenomena, that is, the overwintering of larvae and paedomorphosis, are interesting processes in ecology and evolution as well as in conservation, because they represent examples of intraspecific diversity (Emel and Bonett 2011; Deno€el and Winandy 2015; Deno€el et al. 2016). However, it is essential to distinguish between both states, because they can be produced by various processes and have different implications. Overwintering larvae can be the result of late breeding, second breeding as a maximization on autumnal rains or slow development that prevents metamorphosis before winter (Ernst 1952; Andreone and Dore 1992; Wissinger and Whiteman 1992). In contrast, paedomorphosis involves maturity and can be regarded as a longer-term strategy. Facultative paedomorphosis has indeed been shown to be adaptive in newts and salamanders in allowing either early reproduction or an optimal use of resources and adaptation to local environmental conditions (Semlitsch 1987; Deno€el et al. 2005; Deno€el and Ficetola 2014). Because maturity is a key life-history trait (Ryan and Semlitsch 1998), the presence of a sexual trait would be expected to be a major diagnostic trait for separating these processes. However, this is often not the case, as body size is also used to describe paedomorphs (see e.g., van Gelder 1973). Therefo (...truncated)