AlgaeTraits: a trait database for (European) seaweeds

. The analysis of biological and ecological traits has a long history in evolutionary and ecological research. However, trait data are often scattered and standardised terminology that transcends taxonomic and biogeographical context are generally missing. As part of the development of a global trait database of marine species, we collated trait information for European seaweeds and structured the data within the standardised framework of the World Register of Marine Species (WoRMS). We collected 45 175 trait records for 21 biologically and ecologically relevant traits of seaweeds. This resulted in a trait database for 1745 European seaweed species of which more than half (56 %) of the records were documented at the species level, while the remaining 44 % were documented at a higher taxonomic level and subsequently inherited at lower levels. The trait database for European seaweeds will serve as a foundation for future research on diversity and evolution of seaweeds and their responses to global changes. The data will contribute to developing detailed trait-based ecosystem models and will be an important tool to inform marine conservation policies. The data are publicly accessible through the AlgaeTraits portal, https://doi.org/10.14284/574 (AlgaeTraits, 2022).


Introduction
Trait-based approaches, focusing on diversity of organismal characteristics rather than species diversity, are an effective data source to answer important biological questions (Costello et al., 2015;Beauchard et al., 2017;Degen et al., 2018). Traditionally, traits have been and still are of significant value for taxonomic research (Voultsiadou et al., 2017). Moreover, as they enable the description and investigation of complex ecosystems in relatively simple ways without having to laboriously study each individual component species, they are also integral to the study of the ecological and evolutionary dynamics of populations, species, communities, and ecosystems (Violle et al., 2007;Kattge et al., 2011;Degen et al., 2018;Schleuning et al., 2020). For example, the analysis of trait data can assist in estimating responses to multiple stressors, including anthropogenic and climate change impacts (Degen et al., 2018;Schleuning et al., 2020), and may provide crucial information needed to develop effective management strategies to counter negative consequences of climate change (Bremner, 2008).
Although the importance of trait-based approaches is widely recognised, there remains a general lack of accessible, standardised, and harmonised trait data for aquatic organisms (Costello et al., 2015;Beauchard et al., 2017;Degen et al., 2018;Martini et al., 2021). Recent initiatives have intended to resolve this gap and made significant progress for some marine animals, including fishes, polychaetes, copepods, and macroinvertebrates (reviewed in Martini et al., 2021), but aquatic photoautotrophs, including marine macroalgae (or seaweeds), remain underrepresented.
Seaweed traits and functional forms have been used for at least one century as a tool to answer various ecological and evolutionary questions. Since the early 1900s, traits have been explored to formulate life-form classification schemes for algae which intended to reflect habitat requirements and responses to environmental fluctuations (Chapman and Chapman, 1976;Feldmann, 1966) or describe different reproductive strategies (De Wreede and Kinger, 1988;Bell, 1994) used to explain various evolutionary questions (Heesch et al., 2021). In the 1980s, Littler and Littler (1980) and Littler et al. (1983) used the adaptive value of morphological traits to categorise algal species into morphofunctional groups, which cluster species with similar ecological functioning. As the concept of morpho-functional groups is relatively easy to apply and can limit processing time and associated costs, it has been frequently applied in seaweed ecology (Veiga et al., 2013;Vélez-Rubio et al., 2021). As a result, multiple variations of the original six morphofunctional groups have been developed and applied in a wide range of contexts. For example, they have been applied to understand the distribution of communities along spatial (Steneck and Dethier, 1994;Wieters et al., 2012;Gaspar et al., 2017) or environmental scales (Balata et al., 2007;Gaspar et al., 2017;Gómez et al., 2019;Gómez and Huovinen, 2020), to understand the potential of communities to resist invasion (Arenas et al., 2006), and to evaluate the ecological status of coastal waters (Orfanidis et al., 2011). Yet, other recent research indicates that current morpho-functional groups may not capture enough variation to describe ecological functioning (Mauffrey et al., 2020;Ryznar et al., 2020) and call for further development towards a broader trait-based approach that includes non-morphological and other traits (Mauffrey et al., 2020).
Seaweed trait information remains largely scattered, not widely available, and not semantically standardised, which has hampered the development and application of phycological trait-based approaches over broad taxonomic, spatial, or temporal scales. To overcome such drawbacks and enhance the development of a common ontology favouring the use and comparability of trait-based approaches on seaweeds, we present AlgaeTraits, a seaweed trait database for 1745 species occurring along the Atlantic and Mediterranean coasts of Europe. We expand the spectrum of commonly used functional traits (i.e. measurable or quantifiable properties of individuals; McGill et al., 2006) to include data on habitat, seasonality, morphology, life cycle, life history, and biogeographical range (Fig. 1, Table A1). In total, we describe 21 traits linked to taxonomic information, covering 9 of the 10 previously prioritised traits in Costello et al. (2015). These data are available on the Algae-Traits data portal https://doi.org/10.14284/574 (AlgaeTraits, 2022), which is a subregister of the World Register of Marine species (WoRMS, 2022;Marine Species Traits, 2022). Here we introduce AlgaeTraits by (i) presenting the methodologies used to collect data, (ii) highlighting trait coverage for the current and first version of the database, and (iii) exploring trait variability for a selected subset of traits.

Trait collection
Trait data collection started with an extensive literature review in 2013, supported by the Biology project of the European Marine Observation and Data Network (EMODnet). In 2015, the data were further refined with expert opinions (Supplement S1). In total, more than 200 references were used and 33 experts (all co-authors on this article) contributed by filling out a survey. For specific traits where interspecific variation is known to be minimal, trait information was documented in the World Register of Marine Species (WoRMS, 2022) at the genus level. The database behind WoRMS (Aphia, Vandepitte et al., 2015Vandepitte et al., , 2018, see further) is built in such a way that the information at the genus level is then automatically inherited to all species within this genus, if no information at the species level was provided or available. Traits that may have substantial interspecific variation, such as blooming, seasonality, wave exposure, zonation, and body size (Table A1), were not inherited from the genus to species level and were only documented at the species level. Contributions were quality-checked to verify whether they conformed to the survey and the database format.

Trait data
We included 21 traits for marine seaweeds: 8 ecological traits, 4 morphological traits, 6 life cycle traits, and 3 life history related traits (Fig. 1, Table A1). Ecological traits relate to the habitat of the seaweed with respect to environment (marine, freshwater, brackish), tidal zonation, various degrees of wave exposure, environmental position (substrate), tolerance to organic pollutants, vertical space use, and seasonality. In the few instances where seaweed species are known for their potential to form blooms, this has been indicated as well (Fig. 1, Table A1). Morphological traits describe the thallus based on body shape, body size, calcification, and cytomorphology ( Fig. 1, Table A1). Body size is the only quantitative trait included in AlgaeTraits and can be documented as a mean, maximum, or minimum value to describe the thallus length or diameter. Life-history-related traits describe patterns that influence demography and population dynamics, while life cycle traits relate to the various stages an organism undergoes from one stage in its development (e.g. fertilisation) to the same stage in the next generation (Albecker et al., 2021) (Fig. 1, Table A1). In addition to these 21 traits, species distributions were documented as ecoregions defined by Spalding et al. (2007) and can be used to subset the database according to geographic region. Moreover, WoRMS also displays species' occurrences of the Ocean Biodiversity Information System (OBIS; https://www.obis.org, last access: 2 March 2022) (Fig. 1). Contrary to the other traits, environment and distribution were not uploaded as traits (i.e. attributes) in the database system (Aphia; see further) but as environment flags and distributions. This implies that environment and distribution are slightly differently visualised on the AlgaeTraits data portal (https://www.algaetraits.org, last access: 10 September 2022).

Taxonomy
The total number of described seaweeds is estimated at 9250 species on a global scale (Appeltans et al., 2012;Guiry, 2012), of which at least 1800 species occur in Europe (Costello et al., 2022). The taxonomic classification of these species is not always straightforward and is continuously updated. To provide the best possible taxonomic accuracy, the trait data were implemented in WoRMS. WoRMS aims to provide a complete taxonomic authoritative list of all currently published names of marine species and is synchronised with AlgaeBase, the most complete database on global algal taxonomic information (Guiry and Guiry, 2022).

Metadata
Every trait value is linked to a source (expert or literature) and a note field that can be used to include relevant metadata related to specific trait values. As values for the traits asexual reproduction, macroalgal blooming, body size, and seasonality may display considerable geographic variation, Figure 1. (a) Relation of AlgaeTraits to other databases. AlgaeTraits is a subregister of the World Register of Marine Species (WoRMS), which aims to provide a comprehensive authoritative list of all published names of marine organisms and other biological information including traits. AlgaeBase, the most complete list on global algal taxonomy, is used as the main source for algal taxonomy in WoRMS. WoRMS and AlgaeTraits display OBIS occurrence data. All traits included in WoRMS and AlgaeTraits were collected under the EMODnet project. (b) All 21 traits currently included in AlgaeTraits can be categorised under morphology, life history, life cycle, and ecology.
we assigned a specific locality to these trait values. Localities are defined as georegions obtained from the Marine Regions platform, which provides a hierarchical, standardised list of georeferenced marine regions and areas (https: //www.marineregions.org, last access: 6 April 2022) As mentioned above, life cycles of seaweeds can be complex and often consist of different life stages with radically different morphological, physiological, and ecological characteristics (Fig. 2). When relevant, trait values are documented for the specific life cycle stage they apply to (i.e. sporophyte, gametophyte, microthallus, macrothallus).

Database structure and management
The AlgaeTraits database is a thematic subregister of WoRMS, part of the Aphia platform. The Aphia platform is an MS SQL (Microsoft Structured Query Language) database specifically built to include taxonomic data and related information such as biological traits. In total, Aphia contains more than 400 data fields, which are maintained by more than 500 experts under guidance of the WoRMS steering committee, which takes the lead on setting priorities and future directions and coordination of the experts . Both editors and database users are supported by a data management team that includes technical and scientific staff hosted at the Flanders Marine Institute (VLIZ) and can be contacted through info@marinespecies.org. This data management team is committed to safeguarding the integrity and online access of the database.  ) can be formed on the same or separate thalli (mono-vs. di-; light blue indicates production of male and female gametes on separate individuals), and sex determination can happen in both the haploid and diploid life phase (-oicous vs. -oecious). The form of the gametes can be (1) identical in size and morphology (isogamous), (2) different in size but both motile with flagella (anisogamous), or (3) different in morphology and size with a bigger non-motile female gamete lacking a flagella and smaller motile male gametes with flagella (oogamous), except for red algae in which the male gamete is also non-motile. n -haploid, 2n -diploid, F! -fertilisation, R! -meiosis.

Results
Twenty-one traits were documented resulting in 45 195 entries for 2830 accepted taxa according to AlgaeBase, among which 681 were at the genus level and 1742 at the species level. Fifty-six percent of the trait entries for species were inherited from the genus level. Environment and cytomorphology were the traits most documented for species, while the least information was provided for the traits that show substantial variation within genera and that were not inherited from genus to species level (Fig. 3).
3.1 Morphology-related traits 3.1.1 Body shape Seaweeds are known for their wide variation in body shapes, as illustrated in Fig. 4. For example, seaweeds can exist as filaments (filamentous) or be leaf-like (foliose) or cushionlike (saccate). They can trail closely along the surface (prostrate), be upright (erect), or take many other shapes (Table A1, Fig. 4). Body shape had a high data coverage of 96 % (1678 species) at the species level (Figs. 3, 5-7).

Body size
Seaweed body size was described in several dimensions (thallus length, diameter, width, thickness, height) and varied from a few micrometres in thickness or width (e.g. filamentous, turf forming species such as Hapalospongidion macrocarpum) to several metres long (e.g. kelp such as Saccharina latissima). Compared to other traits, body size had a relatively low data coverage of 28 % (490 species) for European accepted seaweed species (Figs. 3, 5-7).

Calcification
Several species in different taxonomic groups have calcified thalli . In the green seaweed orders Dasycladales and Bryopsidales, calcification can occur as intraor extracellular aragonite deposition, and both orders contain calcified articulated and calcified non-articulated species (Fig. 8). In European brown seaweeds, calcification is restricted to Padina (Dictyotales), where the surface of several species is covered with aragonite crystals (Benita et al., 2018) (Figs. 4a, 9). Calcification in various forms is present in four European orders of red seaweeds: in the Corallinales and Sporolithales as calcite in the cell walls (Figs. 4d, 10), whereas the Peyssonneliales and Nemaliales deposit aragonite (Pentecost, 1980). Calcification had a high data coverage of 96 % (1670 species) on a European level (Figs. 3, 5-7).

Cytomorphology
Cytomorphology was the trait best documented on a European level and reached a data coverage of 99 % (1726 species) (Figs. 4,(5)(6)(7). Although the trait which distinguishes unicellular from multicellular organisms and therefore might be perceived as trivial, in a seaweed context, it does set apart macroscopic multicellular thalli from a series of exotic cytomorphologies, including coenocytic and siphonal growth forms. Especially in green seaweeds (e.g. Bryopsidales and Dasycladales), the morphology of the thallus is decoupled from the formation of cells, which may result in thalli tens of centimetres tall and differentiated in blade-like structures, stolons and rhizoids while still being essentially unicellular.

Environmental position
Environmental position was well documented on the European level with 94 % (1636 species) (Figs. 3, 5-7). Most seaweeds grow attached to rock (epilithic) or other macrophytes (epiphytic), while some other species grow on animals (epizoic) or within rocks (endolithic), macrophytes (endophytic), or animals (endozoic) ( Table A1). Many seaweeds are not very specific with respect to the substrate onto which they are attached. The same species may grow epilithically or epiphytically, but some species show high substrate specificity. Several diminutive algal species (e.g. Acrochaete, Acrochaetium, Laminariocolax and Myrionema) grow exclusively epiphytically, endophytically, or even endozoically on a variety of hosts. At least for some species (e.g. Vertebrata lanosa being associated with Ascophyllum nodosum), high substrate specificity has been demonstrated (Garbary, 2017).

Macroalgal blooming
Macroalgal blooming indicates the demonstrated capacity of a species to produce blooms. This trait had relatively low data coverage of 13 % (233 species) on a European level (Figs. 3,(5)(6)(7). Notorious examples of blooming species include Ulva prolifera, Cladophora glomerata, Caulerpa cylindracea, and holopelagic Sargassum and Rugulopteryx okamurae, and events have been often linked to eutrophication (Charlier et al., 2008;Pierucci et al., 2019;Smetacek and Zingone, 2013). The trait, however, shows considerable variation with geographic location. This is exemplified by several non-native species, which bloom in the invaded region while showing non such behaviour in their regions of origin. This variation was incorporated in the database by linking entries for macroalgal blooming to geographic localities.

Seasonality
Seasonality had a relatively low data coverage of 14 % on a European level (249 species) (Figs. 3, 5-7). Seaweeds with large geographical ranges adapt to local seasonal conditions by adjusting their phenology (Lüning, 1991). It is therefore possible for a species to be present in one season in one part of its range while being absent elsewhere.

Tolerance to organic pollutants
Some seaweeds have a lower tolerance to turbidity or nutrient concentrations than others (Table A1). For example, Choristocarpus tenellus is mainly observed in waters with low nutrient levels and high visibility (oligotrophic, clear water; Table A1), while Ulva spp. are mainly observed from waters with mid to high nutrient concentration (mesotrophic, eutrophic; Table A1). Tolerance to organic pollutants was covered for 39 % of European accepted seaweeds (678 species) (Figs. 3, 5-7).

Wave exposure
Some seaweed species are solely known from habitats with low-energy wave forces (sheltered; Table A1) such as Chaetomorpha adrianii or from habitats with solely highenergy wave forces (exposed; Table A1) such as Valonia utricularis. Other seaweeds can occur in a variety of wave exposures including sheltered semi-exposed and exposed such as the non-native Sargassum muticum. Wave exposure had a relatively low coverage of 18 % on the European level (315 species) (Figs. 3, 5-7). The taxonomic coverage is restricted to marine representatives. The order Charales is included to accommodate a limited number of species that occur in brackish habitats. The left side shows a phylogenetic tree at the ordinal level. Next to the tree the number of species with trait information available in the database is indicated as the total number (no. of species), the percentage relative to the total number of European species (% Europe), and the percentage relative to the total number of global species (% global). The heatmap indicates the percentage of trait coverage relative to the total number of European species per order per trait included in the database. At the bottom of the figure, average species coverage per trait is presented as a percentage of the total number of European (% Europe) and global (% global) species. Oltmannsiellopsid. refers to Oltmannsiellopsidales.

Zonation
Many species are physiologically adapted to endure the stress of tidal differences and daily changes between desiccation and submersion (e.g. Pelvetia canaliculata). Other species will only occur below the low water mark and do not endure such high variation in desiccation, salinity, temperature, or other stressors under normal conditions (e.g. Alaria esculenta). Zonation had a relatively low data coverage of 18 % on a European level (316 species) (Figs. 3, 5-7).

Life cycle
Information on life cycle was available for 95 % of European seaweed species (1660 species) (Figs. 3, 5-7). Many seaweeds have a biphasic or haplodiplontic life cycle in which a diploid sporophytic and a haploid gametophytic life phase al-ternate. These can have a similar (isomorphic; e.g. Chondrus crispus) or distinct (heteromorphic) body shape (e.g. Laminariales) (Fig. 2). Haplodiplontic life cycles are the dominant type of life cycle in the brown (Fig. 9) and red ( Fig. 10) seaweeds. Other seaweeds have a monophasic life cycle that is either diploid (diplontic) or haploid (haplontic) (Fig. 2). Variation in the life cycle has been documented in several species either in the lab (e.g. in reds, Maggs, 1988;in Ectocarpus, Coelho et al., 2012) or in the field (e.g. in Gracilaria, Destombe et al., 1989). However, the knowledge about the processes driving this variation (e.g. epigenetic (plastic) or genetic bases) is not known.

Asexual reproduction
In many seaweeds, sexual reproduction occurs alongside asexual reproduction (i.e. partial clonality) that does not involve fusion of gametes or meiosis and usually results in Figure 6. Data coverage for brown seaweeds (Phaeophyceae). The left side shows a phylogenetic tree at the ordinal level. Next to the tree the number of species with trait information available in the database is indicated as the total number (no. of species), the percentage relative to the total number of European species (% Europe), and the percentage relative to the total number of global species (% global). The heatmap indicates the percentage of trait coverage relative to the total number of European species per order per trait included in the database. At the bottom of the figure, average species coverage per trait is presented as a percentage of the total number of European (% Europe) and global (% global) species. progeny with an identical genetic constitution to the parent and to each other (Table A1). Asexual reproduction is widely spread in green, brown, and red seaweeds (Figs. 8-10) and can happen through fragmentation, direct development of spores, parthenogenesis, or other mechanisms. Asexual reproduction had a high data coverage of 95 % on a European level (1649 species) (Figs. 3, 5-7).

Dispersion mode
Seaweed dispersal is heavily influenced by ocean currents and water motion (but in red seaweeds, animal mediated transport of male gametes could be important; Lavaut et al., 2022) and is in most cases limited in spatial scale. But dis-persal over longer distances is also possible when the species can for example drift, or the dispersal is mediated by vectors such as boat hulls (Table 1A). Dispersion mode has a moderate data coverage of 80 % on the European level (1401 species) (Figs. 3, 5-7).

Gamete type
Gamete type varies among and within orders of green and brown seaweeds (Figs. 8,9). In red seaweeds, gamete type is more conserved: male gamete is unflagellated (spermatia), and fertilisation takes place in the female organ called the carpogonium. This type of reproduction is considered oogamous when information is available. This trait was documented for 95 % of the European seaweeds (1661 species) (Figs. 3, 5-7).

Gametophyte arrangements
In seaweeds, the male and female gametes can be formed on the same or separate thalli (mono-vs. di-); sex determination can happen in both the haploid and diploid life phase depending on the taxon (-oicous vs. -oecious) (Fig. 3, Table A1). For example, in fucoids, sex determination occurs in the diploiddominant stage, whereas, in all red seaweeds, sex determination occurs in the haploid stage. Gametophyte arrangement was documented for 93 % of the European seaweed species (1626 species) (Figs. 3, 5-7).

Spawning
For the majority of the brown and green seaweeds, fertilisation occurs in the water column, but in red seaweeds, male gametes are not flagellated and fertilisation occurs on the female gametophyte and gives rise to the "third phase" of the life cycle called the carposporophyte. Spawning has been documented for 95 % of European species (1658 species) (Figs. 3, 5-7).

Generation time
Of all traits, least information was collected for the trait generation time (Fig. 4b). In seaweeds, generation time can vary from just a few weeks in Ulva (Wichard et al., 2015) to multiple decades such as in Gracilaria (Engel et al., 2001) or Ascophyllum (Åberg, 1992). Generation time was documented for only 7 % of the European seaweed (117 species) (Figs. 3, 5-7).

Life span
Life span varies within and among European green, brown, and red seaweeds (Figs. 8-10). Perennial macroalgae can live up to multiple years such as for several years. Many kelp species such as Laminariales or Gracilaria gracilis can live for more than 50 years (Engel et al., 2001); Ascophyllum nodosum can live for more than 120 years (Åberg, 1992). In contrast, annuals live only a few months, such as several small filamentous species (e.g. Chaetomorpha). At the European level, data coverage was high for life span, 95 % (1652 species) (Figs. 3, 5-7).

Reproductive frequency
Data coverage was limited for reproductive frequency, with data available for only 25 % for the European species (427 species) (Figs. 3, 5-7).

Discussion
AlgaeTraits includes 21 traits and 1742 European seaweed species, and it is structured within a general framework and ontology aiming to describe all marine species (Costello et al., 2015). With a mean coverage of ∼ 60 % per trait, this database will be a solid tool for a variety of biological research and related fields, including marine conservation, nature-based solutions, and aquaculture. For example, traits can be used to monitor community and ecosystem changes (McGill et al., 2006;Vélez-Rubio et al., 2021) or to identify conservation priorities (Albouy et al., 2017;Cardeccia et al., 2018;Esmaeili et al., 2022). In addition, they can be incorporated into predictive modelling to assess eco-evolutionary consequences of climate change (Schleuning et al., 2020), can contribute to predicting the invasiveness of species (Nyberg and Wallentinus, 2005;Quell et al., 2021), can be used in research aiming to better understand the driving forces of evolutionary trait history (Heesch et al., 2021) or the mechanisms of community assembly (Weiss and Ray, 2019), and can even help to assess ecosystem service vulnerability (Díaz et al., 2013;Stevenson, 2014).
The AlgaeTraits database complements recent efforts to collect and publicly provide well-structured and organised seaweed trait data. Just as MarLIN (2006) and SeaTraIn (2022) we focussed on biologically important traits. But contrary to these other initiatives, AlgaeTraits includes all European seaweed species and presents a different, yet overlapping, set of traits. MarLIN (2006) covers over 40 traits, of which 12 overlap with AlgaeTraits, but it is limited to only 30 seaweed species. SeaTrain (2022) covers around 10 traits, of which 2 traits overlap with AlgaeTraits, and includes 96 seaweed species. As to taxonomic coverage (1742 species) and inclusion of functional important traits as prioritised by Costello et al. (2015), AlgaeTraits is the most extensive seaweed trait database published so far.
In this first version of AlgaeTraits, not all traits are available for all species yet, and the current list of traits is not yet complete. For example, other key eco-evolutionary traits that might be considered to be included in the database are tolerance to light, tolerance to temperature, tolerance to grazing, tolerance to sedimentation, or tolerance to epiphytism. There is currently no possibility to describe parasitic algal life forms in the database, ignoring a relatively diverse group of red algae. Completion and further refining and expanding the database will be an ongoing effort of the Al-gaeTraits editor community. AlgaeTraits currently has 30 trained thematic editors who actively update the trait information on a voluntary basis through the online editing inter-face (https://www.algaetraits.org,lastaccess:10/09/2022) and gradually work on the expansion of this European database to a global database. The AlgaeTraits editorial community will also need to report to the WoRMS steering committee. As the database expands, adding new traits or trait values can be considered under thorough consideration and discussion with the WoRMS steering committee and data management team.
For now, traits are included at the species level in Al-gaeTraits. However, multiple traits can exhibit substantial intraspecific variation (Kattge et al., 2011). For example, morphological seaweed traits can differ among populations depending on underlying genetic patterns (Serisawa et al., 2003) or depending on environmental conditions such as wave exposure with rather smaller and more slender individuals in exposed than sheltered localities (Ruuskanen et al., 1999;Fowler-Walker et al., 2006;Kim et al., 2022). But also habitat-preference, life-history (Araújo et al., 2011), or life-cycle traits can vary intraspecifically. Some seaweeds reproduce sexually under normal conditions but change to asexual reproduction under specific environmental conditions (Demes and Graham, 2011;Murúa et al., 2017) or at the limits of their distribution (i.e. geographic parthenogenesis: Oppliger et al., 2014;Hoshino et al., 2021). Intraspecific trait variation is thus caused by both genetic variation and phenotypic plasticity and can be a response to improve performance under specific environmental biotic or abiotic conditions (Kattge et al., 2011). At the moment, we considered intraspecific variation by assigning localities (or life stages) to the trait values when possible or specifying detailed information in the note field. However, complementary approaches measuring trait information at specimen level (e.g. Mauffrey et al., 2020;Cappelatti et al., 2019) are useful, especially for quantitative traits, to fully capture intraspecific trait variability and allow more in-depth analysis.

Conclusions
We provided a consolidated database of important traits of European seaweeds that is distinct in its completeness and taxonomic coverage. Because the database is standardised and fits within the broader framework of WoRMS that aims to include all taxa described, it serves not only as a significant resource for phycological research focusing on trait-based ecology or evolution, but also for general macro-ecological and macroevolutionary research in general. The database can help to explore ecological questions about relations among traits or help to unravel taxonomic and evolutionary patterns of traits in seaweeds. Because some trait values vary geographically and some traits values were documented at the genus level, the database may not always provide the highest variation at the species level. However, an active thematic editorial community is currently committed to further updating and improving the quality of this European database and expanding it to a global level to facilitate thorough and broadscale trait-based analysis.
Filamentous -Existing out of a branched or unbranched row of cells joined end to end (Womersley, 1987).

Haliptilon
Prostrate -Trailing on the ground or lying closely along a surface (Lawrence, 2005).

Valonia
Stoloniferous -Forming a prostrate axis, lying on or in the substrate, from which erect branches arise (Womersley, 1984).

Tubular -
In the form of a tube, having tubes, consisting of tubes (Lawrence, 2005). Calcified articulated -Algal thallus that is encrusted or impregnated with lime with noncalcified joints rendering the thallus a segmented nature.

Hydrolithon
Non-calcified -Algal thallus not encrusted or impregnated with lime.

Cladophora
Unreported -So far it has not been reported in the literature whether the thallus is calcified.
Cytomorphology Unicellular -Having only one cell or consisting of one cell.

Unicellular Siphonous
An algal growth form that is filamentous, tubular, multinucleate, and with a few cross-walls, if any (Brodie et al., 2007).

Nonunicellular
Having more than one cell or consisting of more than one cell (adapted from Lawrence, 2005).  (Lincoln et al., 1998).

Nonunicellular
Endophytic -Living within a plant tissue or macroalgal thallus and not deriving nourishment from it.

Callocolax
Epilithic -Growing on rocks or other hard inorganic or organic substrata (Lincoln et al., 1998).

Acanthophora
Epiphytic -Living on the surface of a plant or alga and not deriving nourishment from it.

Vertebrata lanosa
Epipsammic -Growing in or on sand or other soft inorganic or organic substratum -Epizoic -Living attached to the body of an animal used for a non-parasitic organism that lives attached to the outer surface of an animal.

Polysiphonia carettia
Unattached -Growing without attachment to any type of substrate. -

Summer -
The organism can be observed in summer. -

Autumn -
The organism can be observed in autumn. -

Winter -
The organism can be observed in winter. Genus or species example Semiexposed -Occurring in habitats that are subject to moderate energy wave forces.

Semiexposed
Moderate energy rock Occurring on rocky substrate subject to moderate energy wave forces.

Himanthalia elongata
Sheltered -Occurring in habitats that are subject to low-energy wave forces.

Sheltered Coarse sediments
Occurring on coarse sediments such gravel, pebbles, shingles, and cobbles occurring at sheltered locations.

Sheltered Low-energy rock
Occurring on rocky substrate subject to low-energy wave forces.

Macrophytedominated sediments
Sediments with a high cover of macroalgae or seagrasses.

Sanddominated sediments
Sediments composed of a mixture of sand particles (0.074-4.75 mm) typically deposited in a low-energy environment.

Codium fragile
Unreported -Not been reported in the literature in what type of wave exposure the species occurs.
-Zonation Intertidal -The part of the shore between high and low tide.

Littoral zone
The part of the shore covering the intertidal and the splash zone, with the upper limit marked by the top of the lichen zone and the lower limit marked by the top of the laminarian kelp zone.

Subtidal -
The part of the shore continuously covered by water, below the intertidal zone.
Lower infralittoral zone The part of the infralittoral zone that supports scattered kelp plants. Dominated by animals with sparse foliose algae except where grazed. The part of the circalittoral subzone on hard substrata distinguished by the presence of scattered foliose algae amongst the dominating animals; its lower limit is the maximum limit of depth for foliose algae

Corallina officinalis
Upper infralittoral zone The region of the sublittoral which extends from the lower limit of the infralittoral to the maximum depth at which photosynthesis is still possible.

Life cycle
Life cycle Diplontic -Having a life cycle in which the direct products of meiosis act as gametes; only the gametes are haploid (Lincoln et al., 1998).
Haplodiplontic Heteromorphic Organisms with the gametophyte and sporophyte of different morphology and size (Womersley, 1987).

Laminaria ochroleuca
Haplodiplontic Isomorphic Organisms with the gametophyte and sporophyte of similar morphology and size (Womersley, 1987).

Haplodiplontic Not applicable
It is not applicable whether this taxon is heteromorphic or isomorphic.

Parvocaulis
Haplodiplontic Unknown According to the literature it is unknown whether this organism is heteromorphic or isomorphic.
Haplodiplontic Unreported So far it has not been reported in the literature whether this taxon is heteromorphic or isomorphic.

Haplontic -
Having a life cycle in which meiosis occurs in the zygote to produce the haploid phase; only the zygote is diploid (Lincoln et al., 1998).

Tolypella
Unknown -According to the literature it is unknown what life cycle this organism has.

Asexual reproduction
Asexual reproduction -Reproduction which does not involve fusion of gametes or meiosis and usually results in progeny with an identical genetic constitution to the parent and to each other. Asexual reproduction may amongst others occur by binary fission, budding, asexual spore formation or vegetative propagation.

Asexual reproduction
Asexual reproduction by an unknown mechanism Asexual reproduction is detected, but the underpinning mechanism is unknown.

Asexual reproduction
Asexual reproduction by fragmentation Type of asexual reproduction in which the organism breaks up into smaller pieces, each of which can develop into a new individual, as in some algae (Lawrence, 2005).

Asexual reproduction
Asexual reproduction by parthenogenesis Direct development of a new individual from an unfused gamete.

Ulva prolifera
Asexual reproduction Asexual reproduction by vegetative propagules Asexual reproduction by development of a new individual from a vegetative propagule.

Prasiola crispa
No asexual reproduction - The taxon does not reproduce asexually, which is reproduction which does not involve formation and fusion of gametes and results in progeny with an identical genetic constitution to the parent and to each other. Reproduction may occur by binary fission, budding, asexual spore formation or vegetative propagation. In asexual division in eukaryotic organisms, all cell divisions are by mitosis.  (Lincoln et al., 1998).

Ectocarpus
Oogamous -Having a reproduction involving a large, non-motile female gamete (egg cell) and a small, motile male gamete (sperm cell or equivalent), except for red algae in which the male gamete is also non-motile (Womersley, 1987).

Cryptonemia, Dictyota, Fucus
Not applicable - The attribute gamete type is not applicable for this taxon.

Halothrix
Unknown -According to the literature it is unknown which gamete type this organism has.

Fosliea
Unreported -So far it has not been reported in the literature what the gamete type is for this taxon.

Rosenvingea
Gametophyte arrangement Dioecious -When male and female reproductive structures are formed on the separate individual and the sex is determined in diploid phase.

Alaria esculenta
Dioicous -When male and female reproductive structures are formed on the separate individual and the sex is determined in haploid phase (Beukeboom and Perrin, 2014).

Dasya elongata
Mixed -With individuals bearing only male or female reproductive structures and individuals bearing both male and female reproductive structures in one species.

Monoecious
-When male and female reproductive structures are formed on same individuals and the sex is determined in diploid phase.

Fucus distichus
Monoicous -When male and female reproductive structures are formed on same individuals and the sex is determined in haploid phase.

Hydrolithon
Not applicable - The attribute gametophyte arrangement is not applicable for this taxon.

Pedobesia
Unknown -According to the literature it is unknown which gametophyte arrangement this organism has.

Chaetosiphon
Spawning Fertilisation in the water column -Fertilisation with gametes released in the water column.

Batophora
Fertilisation on the female gametophyte -Female gamete retained on the female gametophyte (e.g. Rhodophyta).

Atractophora hypnoides
Not applicable - The attribute spawning is not applicable for this taxon.

Porphyridium purpureum
Unknown -According to the literature it is unknown which spawning mechanism this organism has.

Chordaria
Unreported -So far it has not been reported in the literature whether there is spawning for this taxon.

Spongonema
Life history Generation time 1 to 3 months -Life cycle completed in 1 to 3 months in the lab.
Ulva 3 to 12 months -Life cycle completed in 3 to 12 months in the lab.

Cystoseira barbata
1 to 3 years -Life cycle completed in 1 to 3 years in the lab.

Saccharina latissima
More than 3 years -Life cycle completed in more than 3 years in the lab.

Furcellaria lumbricalis
Life span Annual -(1) Applicable to structures or growth features that are marked off or completed yearly; (2) living for a year only; (3) completing life cycle in a year from germination; (4) plant that completes its life cycle in a year. (Lawrence, 2005). Thallus which survives only one growing season (less than 1 year) (Womersley, 1987).

Perennial
Long perennial Thallus or part thereof with a lifespan exceeding 3 years (more than 3 years)

Cystoseira barbata
Perennial Short perennial Thallus or part thereof with a lifespan exceeding 1 year but under 3 years.

Undaria pinnatifida
Unreported -So far it has not been reported in the literature which life span this organism has.

Reproductive frequency
Throughout the year -Fertile individuals observed throughout the year.

Codium fragile
One long period a year -A unique fertility period lasting more than 1 month.

Pelvetia canaliculata
One short period a year -A unique fertility period lasting less than 1 month.

Desmarestia viridis
Several long periods a year -Several distinct fertility periods lasting more than 1 month each.

Ceramium virgatum
Several short periods a year -Several distinct fertility periods lasting less than 1 month each.
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Review statement. This paper was edited by Giuseppe M.R. Manzella and reviewed by Annalisa Minelli and one anonymous referee.