Where Should We Draw the Lines Between Dinocyst “Species”? Morphological Continua in Black Sea Dinocysts
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J. Micropalaeontology, 38, 55–65, 2019 https://doi.org/10.5194/jm-38-55-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Where should we draw the lines between dinocyst “species”? Morphological continua in Black Sea dinocysts Thomas M. Hoyle1, Manuel Sala-Pérez2, and Francesca Sangiorgi1 1Department of Earth Sciences, Utrecht University, Utrecht, 3584 CB, the Netherlands 2BRIDGE, School of Geographical Sciences and Cabot Institute, University of Bristol, Bristol, BS8 1SS, UK Correspondence: Thomas M. Hoyle ([email protected]) Received: 24 October 2018 – Revised: 15 March 2019 – Accepted: 3 April 2019 – Published: 26 April 2019 Abstract. The morphology of dinoflagellate cysts (dinocysts) is related not only to the genetics of the motile dinoflagellate from which it derives, but is also dependent on a range of environmental factors including salin- ity, temperature and nutrient status. Although this knowledge improves our understanding of the drivers behind dinocyst morphological variations, it makes the taxonomy governing their description somewhat complex. In basins such as the Black Sea, where environmental change can be extreme and occurs on relatively short (millen- nial) timescales, taxonomy becomes particularly challenging. Morphological continua can be observed between described forms, displaying a large range of intermediate phenotypes that do not necessarily correspond to any genetic difference. As these morphological nuances may preserve information about palaeoenvironments, it is important to find a systematic method of characterising morphotypes. Here, we show a dinocyst matrix within which dinocysts are described according to their similarity to (or difference from) described forms based on key descriptive parameters. In the example set out here, cyst shape and degree of process and/or ectophragm devel- opment are taken as two key parameters in Pyxidinopsis psilata and Spiniferites cruciformis, and can allow the description of intermediate forms even though the definitions do not overlap. We review some frequently occurring morphotypes and propose that using matrices to show the gradual varia- tion between endmember forms is the most pragmatic approach until cyst–theca studies and genetic sequencing can be used to demonstrate relationships between genotypes and morphotypes. As prior studies propose salinity to be a primary driver of intraspecific variability, the endmembers presented may represent salinity extremes within an overall brackish environment. Although we cannot assign each morphotype to a value or a range of an environmental parameter (e.g. salinity) as the different morphotypes can occur in the same sample, using this matrix allows preservation of information about morphological variability without creating taxonomic categories that are likely to require alteration if genetic evidence becomes available. 1 Introduction ellates in fresh (lake) waters (Evitt et al., 1985; Kouli et al., 2001; McCarthy et al., 2011) and brackish water environ- Organic-walled dinoflagellate cysts (dinocysts) are the rest- ments, such as the Sea of Marmara, Black Sea and Caspian ing phase in the life cycle of single celled eukaryotic plank- Sea (Mudie et al., 2017). Here, surface water parameters ton of the phylum Dinoflagellata (e.g. Fensome et al., 1993; such as salinity can be highly variable, from < 1 psu in the Head, 1996). They can preserve easily in anoxic environ- Volga delta to ∼ 30 psu in the Sea of Marmara (Tuzhilkin and ments and therefore have potential to retain important bios- Kosarev, 2005; Yalçın et al., 2017). tratigraphic and palaeoenvironmental information (de Vernal Dinocysts’ morphological variation in general exhibits and Marret, 2007). Most dinocysts are encountered in marine a relationship with water properties. In particular, process environments, although they are also produced by dinoflag- length variation has been observed to mostly change with Published by Copernicus Publications on behalf of The Micropalaeontological Society. 56 T. M. Hoyle et al.: Where should we draw the lines between dinocyst “species”? water density (as a function of salinity and temperature) 2 Materials and methods (Mertens et al., 2011, 2012a). However, other parameters have been mentioned as drivers of intraspecific variation, Thirty-two samples were selected from the Deep Sea Drilling such as water column deoxygenation (Pross, 2001), temper- Project (DSDP) Site 379 in the central part of the Black ature and nutrients, which have a relationship with intraspe- Sea (Location: 43◦00.290 N, 36◦00.680 E; at 2165 m water cific cyst body size variation (Mertens, 2013; Mousing et al., depth; drilling date: May 1975) (Shipboard Scientific Staff, 2017). Process development has been considered an environ- 1978). Sampling was carried out at the international core mental adaptation, for example as an aid to clustering in or- repository at the University of Bremen. Sampling was spread der to promote sinking (Mertens et al., 2009). across the interval 45–615 m (of 624.5 m), representing the Dinocysts from the Black Sea have rather a complex tax- last ∼ 1:2 Ma. Details of sampling levels are given in the fig- onomy due to a combination of factors. They have evolved in ure captions and a table is provided in supplementary infor- a sporadically isolated setting, presenting uncertainties as to mation which shows all samples and their lithologies accord- their genetic affinities that have only recently been broached ing to the original logs (Shipboard Scientific Staff, 1978). (Mertens et al., 2017). They have also been subject to con- The entire record, in combination with other proxies, will be stantly changing environmental conditions (annually and on presented in Hoyle (2019), Hoyle et al. (2019) and Bista et longer timescales) through recent geological time, a factor al. (2019). that is known to drive variations in cyst morphology (Mertens Samples were processed for palynological analysis at et al., 2012b; Rochon et al., 2009). Utrecht University using cold HCl (30 %) to remove carbon- A diverse range of genera, species and subspecies has been ates and cold HF (38 %) to dissolve silicates. Samples were described from the Black Sea region, including some poten- subjected to five minutes in an ultrasonic bath and sieved tially endemic forms (Marret et al., 2004; Mudie et al., 2001; through meshes (125 and 10 µm) and the 10–125 µm fraction Wall and Dale, 1973). The ecological affinities of many of was retrieved. The residues were mounted on slides using these dinocysts have recently been refined by comparison of glycerine jelly and sealed with lacquer. surface sample assemblages with measured sea surface con- Identifications were made using several reference texts ditions, allowing more accurate interpretations of past water (Marret and Zonneveld, 2003; Mudie et al., 2001, 2002, column parameters (Mudie et al., 2017). However, problems 2017; Rochon et al., 1999, 2002; Shumilovskikh et al., 2013; often arise in the study of Black Sea dinocysts due to the Soliman and Riding, 2017; Süto-Szentai,˝ 2000, 2010; Wall apparent phenotypic plasticity; many morphotypes fall along and Dale, 1973; Zonneveld et al., 2013). morphological gradients between formally described species Transmitted light photographs were taken using a Leica (Wall and Dale, 1973) but cannot correctly be ascribed taxo- DM2500 LED differential interference contrast microscope nomically. with a Leica MC 170 HD camera. Transmitted light pho- Here, we review the morphological variability found in tographs were all taken at magnification ×630. Scanning some of the dinocysts retrieved from the late Quaternary electron microscope (SEM) photographs were taken using of the Black Sea. We provide examples of several unde- a JEOL JCM-6000 benchtop SEM after adhering the paly- scribed dinocysts and suggest using variability matrices as nological residues to a stub and coating with platinum (6 nm a method of grouping dinocysts with similar morpholo- thick) using a JEOL JFC-2300HR high-resolution fine coater gies whilst maintaining morphological information that may with a JEOL FC-TM20 thickness controller. Examples are be indicative of palaeoenvironments. This approach is sug- taken from representative samples, but the morphotypes ob- gested here as an alternative to creating unnecessary and served generally occur throughout the sediment sequence. potentially misleading taxonomic categories. A strict taxo- nomic approach, although generally vital in palaeontologi- cal work, may not be sufficiently flexible to deal with the 3 Results morphological plasticity observed in the Black Sea (and the wider Pontocaspian region) during the Quaternary. Among the dinocysts retrieved from all the samples analysed, We provide an example of how morphotypes may fall we focus on presenting some selected forms to demonstrate along gradients between described taxa and that continua the morphological variability encountered in the late Quater- exist between different genera. We have chosen Pyxidinop- nary sediments (Figs. 1 and 2). sis psilata and Spiniferites cruciformis (Wall and Dale in Figure 1 shows transmitted light photographs of sev- Wall et al., 1973) as endmembers of our review. Despite ad- eral dinocysts that cannot be directly attributable to exist- vancements in modern DNA extraction techniques, obtaining ing species. Several round cysts with smooth outer walls marine plankton DNA from pre-Holocene fossil samples re- and precingular archeopyle, resembling Pyxidinopsis