Morphological complexity, plasticity, and species diagnosability in the application of old species names in DNA‐based taxonomies

A paper by Belton et al. (2013) published in this issue of the Journal of Phycology addresses species boundaries in the Caulerpa racemosa–peltata complex. Caulerpa is a member of the siphonous green algae (order Bryopsidales), which consist of a single giant cell that forms a simple tube or one that branches to form a range of morphologies, from very simple branched tubes to much more complex architectures consisting of a medulla and cortex that can display elaborate macromorphological features (Hillis-Colinvaux 1984, Verbruggen et al. 2009a). In Caulerpa, species display a complex morphology consisting of a stolon bearing root-like rhizoids and upright stalks (rachis) with lateral branchlets (ramuli; Fig. 1H). In “paradigm” C. racemosa the branchlets are spherical (Fig. 1E), whereas in C. peltata they are umbrella-like (Fig. 1A), although in reality one finds all sorts of intermediates between these morphologies (Fig. 1, A–E) as well as some other morphologies (Fig. 1, F–G). Furthermore, culture studies have provided evidence for habitatinduced phenotypic plasticity of the branchlets and the overall thallus appearance (Calvert 1976, Ohba and Enomoto 1987, Ohba et al. 1992). It is therefore no surprise that the C. racemosa–peltata complex has long troubled algal taxonomists. Two centuries of taxonomic work on the complex have resulted in a Gordian knot of more than 50 formally described species and intraspecific taxa that have been merged back into racemosa and peltata, with several additional aberrant morphological variations on the same theme being described as separate taxonomic entities. Some workers have recognized the plasticity induced by microhabitat and chosen a system with few species and some ecomorphs (ecads) within them. Over the last few decades, molecular work showed that the C. racemosa–peltata complex consists of multiple clusters that are likely to correspond to species (e.g., Fam a et al. 2002, de Senerpont-Domis et al. 2003, Sauvage et al. 2013). The new work by Belton et al. (2013) applies objective methods to detect species boundaries in DNA data. Put simply, the method they use starts from a large haplotype tree and detects the transition between the type of branching one would expect to see above the species level (i.e., a Yule model) and the type of branching one would expect to see within species (i.e., a coalescent model). This transition should thus correspond to the species boundary and can be used to define species-level clusters (Pons et al. 2006, Fujita et al. 2012, Carstens et al. 2013, Payo et al. 2013). This method, used in combination with a second approach based on branch support, implied that the C. racemosa–peltata complex consists of 11 species. But, accurate as it may be, the resulting DNAbased taxonomy does not resolve the taxonomic conundrum; it is only the first step. The toughest job is to choose appropriate names for the 11 species that are recovered with the DNA work. With several dozen existing species and variety names to choose from, and knowing that the species exhibit morphological plasticity, this is clearly a very difficult task. In fact, the discrepancy between the characters we use currently to discover species (mostly DNA) and the fact that we need to give new species names that take into account all the existing names which were based on a different set of features (predominantly morphological), has created much uncertainty and decision paralysis (De Clerck et al. 2013). Some have used DNA sequencing of type specimens as a solution (Hughey et al. 2002, Hayden et al. 2003, Gabrielson et al. 2011), although others have identified serious problems with this approach (Saunders and McDevit 2012). The poor preservation of many type specimens and the limited accessibility of types for destructive DNA work mean that this approach will not be feasible across the board, and we will more than likely continue to rely on morphological information to resolve the remaining problems. So the question of how likely we are to be able to assign old names to new taxa based on morphological comparison is a very relevant one to ask. In this article, I aim to quantify how the morphological complexity of a taxon affects the diagnosability of its species (i.e., identification success at the J. Phycol. 50, 26–31 (2014) © 2013 Phycological Society of America DOI: 10.1111/jpy.12155