Taxonomic Reshuffling of the Cladophoraceae

The notion that some groups of algae suffer from a “low-morphology problem” that masks their true species diversity has been recognized for at least two decades (van Oppen et al. 1996). This problem has become dramatically evident in recent years as DNA sequence data have taken center stage in species delimitation. The issue is usually associated with algal groups having simple morphologies and a limited set of morphological characters useful for morpho-taxonomic circumscriptions, and in taxa experiencing habitat-induced phenotypic plasticity (Verbruggen 2014). The Cladophoraceae, a family of small filamentous green seaweeds, is clearly impacted by the lowmorphology problem and is an excellent example to illustrate these difficulties. The paper of Boedeker et al. (2016) published in this issue represents a milestone in the systematic biology of this widespread group of chlorophytes. The authors propose a new classification of the Cladophoraceae based on a robust molecular phylogeny. The impressive sampling effort and geographic coverage lead to strong conclusions, making this study a major taxonomic advancement for green algae in general, not just the Cladophoraceae. The Cladophoraceae contains three genera that are familiar to most marine biologists: Cladophora, Chaetomorpha, and Rhizoclonium. They all have thalli consisting of uniseriate filaments formed by large multinucleate cells. The distinction between genera is (apparently) straightforward: Cladophora has branched filaments, Chaetomorpha has unbranched thick filaments, and Rhizoclonium has unbranched thin filaments. In the field, algae of these genera are generally easy to recognize as green filamentous patches, bushes, or rope-like structures occurring in a wide range of habitats (Fig. 1). The Cladophoraceae is one of the few algal taxa that has crossed the borders between sea and freshwater, because it includes species distributed in marine, brackish, and freshwater environments (e.g., in the Mekong river in Laos, where Cladophora is harvested for consumption). Some species thrive in marine intertidal habitats and coastal lagoons, where they occasionally produce large blooms in situations where nutrients are abundant and competition is reduced (Zulkifly et al. 2013). From a geographic point of view, the family has a cosmopolitan distribution, ranging from tropical to polar waters, with generally higher diversity in temperate regions. The Cladophoraceae is one of the longest-known and studied taxa of green algae (the first few species were described as early as the late 18th century), yet is still recognized as a taxonomically challenging group. Species delimitation has been particularly problematic, as demonstrated by the plethora of specific and intraspecific names that exist (1,072 listed in AlgaeBase only for Cladophora, Guiry and Guiry 2016). A poor understanding of the biology of these algae has contributed to this nomenclatural inflation. It is now known that thallus morphology is greatly affected by phenotypic plasticity related to environmental conditions (Leliaert and Boedeker 2007) and that this has been the major cause of taxonomic misinterpretations. Some morphological characters considered taxonomically relevant may vary in relation to habitat. An excellent example is found in one of the most detailed treatments available for this group: the monograph of van den Hoek (1963) for European Cladophora. This study, which is still considered a key reference for the family, reports that several common Cladophora species exhibit different apical cell sizes depending on whether the alga grows in well-lit or shaded sites, yet apical cell size used to be an important taxonomic character. At the same time, the shortage of morphological characters available for these algae poses a limit to the number of species that can be diagnosed on morphological basis. It is therefore not surprising that convergent evolution of morphological traits has been common in the Cladophoraceae; this phenomenon emerged in early molecular studies of the family (Bakker et al. 1994, Hanyuda et al. 2002, Leliaert et al. 2003) and is now confirmed with more substantial evidence. The study by Boedeker et al. (2016) is an example of good practice in the establishment of a natural classification of a taxonomically difficult group of algae. The authors use sequences of the LSU and SSU rDNA genes obtained from a large number of J. Phycol. 52, 901–904 (2016) © 2016 Phycological Society of America DOI: 10.1111/jpy.12459